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73z0is | Why does such an important feature as a planes transponder not work like the planes flight data recorder and be much harder to turn off and have it's own backup power supply? | I understand the FDR can be removed from the planes power supply by removing the circuit breaker, but it will then use it's own local power supply. Why does the transponder seem have a less strict "up time" policy applied to it by it being able to be turned off and having no localised power supply? | Engineering | explainlikeimfive | {
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"Because the transponder isn't as critical to the operation of an airplane. The main purpose of a transponder is to make it easier for air traffic control to tell which airplane is which, and to relay altitude information. If the transponder fails, the pilot has to verbally relay that information, which slows things down. But a plane can land just fine without a transponder. A transponder is not designed for locating a plane after a crash. It can be used that way in an emergency if it survives the crash, but that is not what it is for. Finally, if a transponder becomes faulty, it can broadcast bad information and interfere with other aircraft, so it is important for a pilot be able to turn it off as necessary."
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741eth | How do we get so well-lit pictures from Juno and New Horizons for example from places where the sun does not provide as much light? | Engineering | explainlikeimfive | {
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"They are very long exposures. The camera lens (shutter) is held open for many seconds. If the vehicle is zooming by, they actually pivot the vehicle while the shutter is open, to keep the shot steady.",
"As Concise_Pirate said, it is a long exposure. If you own an amateur telescope and you attach a camera to it you could take cool photos too but you would need to make really long exposures (from 5 to 15 minutes) to get a clear image."
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743skh | What causes 'house noises', and when does it become dangerous? | Engineering | explainlikeimfive | {
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"Homes heat and cool over the course of the day, and the structure of most wood frame houses will creak a bit because the heating and cooling tends to be uneven (it's typically warmer in the upper floors of a house). It's only dangerous if the house is actually improperly built and is structurally unsound.",
"Bad sounds in a brick house sound like someone hitting two bricks together. Its loud enough to identify it as “yup, that’s bricks” and not the regular creaking and cracking you might hear. This indicates the brick walls are shifting significantly and cracking. This usually is caused by the foundation sinking unevenly. If you notice this frequently, and you’ve got uneven floors, doors won’t close that used to, cracking plaster, you need to get a pro in to evaluate. It’s very unlikely to collapse around you, but the damage is going to continue to get worse, and more expensive the longer you wait.",
"The same is true in plumbing/heating systems which, especially if made with copper pipes, will expand and contract with heat. Also any air in water pipes will bang like a drum. If the materials used in house building had no plastic/elastic tendencies then the whole thing could eventually become brittle. Think in terms of the wing of an airliner that is designed to flex and yet remains incredibly strong."
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745v72 | Back in 1936 the Nazis managed to broadcast the olympics to many different countries. Why can my local radio station signal only manage a range of 100-200 km? | Engineering | explainlikeimfive | {
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"> Back in 1936 the Nazis managed to broadcast the olympics to many different countries. The 1936 Olympics were only broadcast live to 25 special viewing booths in Berlin. Tapes were sent out to other countries after the fact.",
"At the time what was used for long distance transmission was \"shortwave\" radio, now technically called \"High Frequency\" or \"HF.\" The wavelength of shortwave signal causes it to have the inherent characteristic to \"skip\" off of the D layer of the atmosphere, giving it the ability to travel an extremely long distance. That coupled with the much higher amount of power used to transmit on shortwave frequencies versus VHF (FM) makes it capable of travelling even further. Regular FM radio stations exist on a limited bandwidth. Since there are so many stations on the FM band taking up a limited space, they have to share their frequencies with other stations that exist in other places. Think of it like this; there is a station in one county that transmits on 107.9 MHz. In an adjacent county, there is another station that transmits on that same frequency. In order to keep from interfering with each other, they are both limited to 1,000 watts of transmitting power each. Now on the shortwave side; there's a station that transmits on 14.00 MHz., and there is no other station in the world that transmits on that frequency. So therefore, it's not limited in wattage. It could use a million watts, maybe two million watts, to create a much more powerful signal. Shortwave radio was pretty much the 1920s-1940s primitive version of the internet. And it worked, so much so that it still exists, and so much so that it is on occasion the only source of communication available. The CB radio band is a shortwave band. It's the shortwave radio's version of /b/. Edit: changed my archaic \"MC\" to \"MHz.\""
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74dguu | What is a bump stock? Does it make a gun full auto? | Engineering | explainlikeimfive | {
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"[Here's a video of one in use]( URL_0 ). A semi-auto rifle can only fire one round every time you pull the trigger but many modern weapons can fire as fast as you can pull. The bump stock causes the recoil of the weapon to push the gun back slightly and disengage the trigger so that when the weapon returns forward, you trigger the trigger again. This gives you the same results of a full-auto weapon (at a slight decrease in accuracy) without meeting the legal definition of being an \"automatic weapon\".",
"Okay, so when you shoot a gun there is the part behind the trigger that your hand holds and the buttstock that your shoulder rests on. Essentially what a bump fire stock does is the shoot firmly grabs the stock and holds it tight the the shoulder while holding their finger very rigid. As they pull the trigger, the rest of the run recoils into the bump fire stock that the shooter is still holding, and then bounces (or bumps) back forward causing the trigger to hit the shooters rigid finger which cause the gun to fire again and the process repeats. Did I explain it alright or am I clear as mud?",
"A semi-automatic weapon will fire as fast as you can pull the trigger. If you hold it just right, where your finger remains stationary and the weapon is braced with your other hand/arm, every time it kicks it with bounce back into your trigger finger and fire again and again until you lose your grip or you run out of ammo. It wildly inaccurate, and mostly is an expensive, noisy, slightly dangerous but kind of fun trick. You can buy a special stock, the bump stock, which makes it easier to pull off. Still pretty inaccurate, but if all you want to do is fire into a crowd...:("
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74emn5 | Every month there’s a new micro SD card with more space than the last. We’re fitting gigs of data when a decade ago we could only fit megabytes. But what’s the theoretical limit of how much data we can store in a micro SD card? How can we keep fitting more data in smaller spaces? | Engineering | explainlikeimfive | {
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"Moore's law states that the performance of semiconductor technology improves by a factor of two roughly every 18 months. For something like an sd card, the data is stored by effectively etching out a physical space on the semiconductor material for each bit of data to be stored. These chips are created by a die (think of it like a printing press or vinyl press except at a really small scale, like nanometers) and draw out the circuits onto the semiconductor material. As the die technology improves, the resolution that the circuitry can be created gets better and better at a smaller and smaller scale. The theoretical limit of this particular technology breaks down once you can draw out circuits down to the scale of a single atom thick. If you try splitting that atom to go smaller, that's called nuclear fission and would probably be bad for anyone in a several mile radius. I'm not sure when that projection is scheduled to be here, but there's a good chance that quantum computing will pick up before that limit becomes the commercially viable option. Quantum technology has the potential to provide an even greater exponential increase in storage capacity and computational speed. No idea on what that limit could be, but there's a chance we hit the next technology before that limit too... that is unless the robots win or we blow ourselves up with our godlike computational prowess :)",
"Last I read, the SDXC standard put the maximum at 2TB of file space per card, though capacities haven't reached that yet. Physical limitations on cell size my not allow this, and newer standards for SD cards are shifted to throughput speeds instead of capacity. So right now, pending a new file standard, the upper limit is 2TB.",
"Every time a new and smaller/denser chip technology is made, the size (eg 7nm) is not evenly spread. This isn't quite answer to your question but it's also interesting. Due to quantum tunneling and similar things we can't really go much smaller than that. This way of making only certain parts super small is a clever way of pushing the limits of current technology a bit further. But we can't really go much further than this.",
"[This]( URL_0 ) video from SciShow explains it pretty well.",
"Not so much every month for space, rather for reliability and bandwidth. For instance I remember only a few years ago when solid state drives were not reliable at all. I'm also a bit different, I would prefer having a 256GiB microSD over a 512GiB regular SD. I thought that whole 200GiB gimmick was silly and ingenious at the same time. SD cards are more arbitrary, so it would probably be better to work in centimeters, I mean you wouldn't mind a 1 cm by 1 cm (square) by 10 mm thick card if it hit the theoretical limit, would you? It would also be easy to relate to the size of an SD card. Even better is they use something called nanometer or nm gauged setups, which you get a million per centimeter. So 1,000,000 * 1,000,000 is 1,000,000,000,000 or a trillion bits. Since we use bytes instead of bits you divide by 8 and round to the nearest computing magnitude yielding 128 GiB. Why not 125? Because in computing the numbers work on doubling rather than incrementing like in decimal. This is called binary, or hexadecimal (meaning 6 and 10, or 16). Anyways, you're probably going to be looking at something like 2^40 or 1,099,511,627,776 bits rather than an even trillion. So wait, we have cards that have 256 GiB that are much smaller than a centimeter and we're not even to a 1 nm process yet? What evil magic is this?! It isn't. It's called layers. I'm sure you don't want a card only a nanometer thick! All that itty bitty (to you and me) yet massive (to the switches) extra space lets you stack these cards! [Notice here it says Samsung's microSD cards are made out of what? **3D** V-NAND]( URL_2 ). They are saying they can make this up to 100 layers thick, and they are using a [19 nanometer process (PDF file)]( URL_1 ) to do it. Yes, that's 20x less capacity than we've been working with here, but it doesn't matter when you have 100x more room. If they ever do get that down to 1 nm you'll have 100 trillion little switches! Not only that but what if that allows them to increase the amount of layers by 20x as well? Then you're looking at quadrillions, or 15 zeroes. Those would be petabytes or PB. [I can't wait until we are working at the DNA scale]( URL_0 ), 10 base pairs at 3.4 nm. If we're that close to DNA already I'm actually quite a bit impressed."
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74g7f3 | Why do they make 'normal' cars that go faster than the speed limits? e.g. Hyundai i10 that goes up to 150mph | Engineering | explainlikeimfive | {
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"There are several things which affect this, some of which have already been mentioned: * The first is selling point (mentioned by @PM_ME_YOUR_AIRFOIL). Not so much so for your base models but for higher end cars Top Speed and how quickly a car gets there may affect which car someone buys. * The second is engine size. the larger an engine in terms of capacity and the number of cylinders makes a car smoother, hence why a Bentley may have a 12 cylinder engine, but may only produce as much power as a 6 cylinder, more sporty car. A by-product of this may be that the top speed is higher, and will certainly be higher than the top speed of any speed limit. * A sub point of the one above is engine wear. Larger engines tend to take the stress of mileage a lot better. This would obviously depend who and how they drive it, but an engine working at less stress is generally a less troublesome one. A by product of this is a higher top speed (in some cases) * The next is top speeds of roads. In Germany they have speed limitless motorways/freeways called Autobahns. This means in certain markets it is appealing to have a car that can far exceed a speed limit that may appear in another country and it would not be cost-efficient for a manufacturer to tailor a car just for one market (we're talking speed here, not other factors such as which hand drive the car is) * If a state/country was to make law that cars had to have a speed limiter, it would only apply to new cars and would cause a gigantic list of issues and cost for very little gain. It would also mean that governments could not earn revenue from fining motorists for breaking the limits (like it or not, it is a fact the government earns money from people using their car above the road limits). * The most important thing about the top speed is related to a cars gear ratios and where a car will sit in terms of speed and revolutions (RPM) of the engine. A combustion engine car (a car that takes petrol or diesel) will be most efficient sitting in RPM and speed terms towards the lower to middle end of its RPM scale (if a car red lines at 8,000 rpm, 2,500-4,000 rpm will likely be its most efficient). This satisfies two things, one is fuel consumption, which affects why someone may buy a car, less fuel consumption means less money spent on fuel, and the next is emissions standards, which is particularly important in countries such as Britain, where the amount of tax on a vehicle is based on the amount of emissions the car kicks out. To satisfy this, the very top of the highest most gear would need to be well above the local speed limits in order for that speed limit to achieve good fuel consumption and limit emissions. The points above generally fit under a limiter fitted to a car to limit its top speed. It would be far too expensive to carry out an operation like this, and my cynical view is governments would not earn revenue from people breaking the speed limit and instead would have to spend money enforcing limiters. Manufacturers don't limit speeds because they are competing against each other, who in turn are all trying to get the most in every way out of a car. The next points relate to having engines small enough so that they can only reach 70. This would be incredibly impractical for everyday use as I will explain: * Hills. Going up a hill is harder work for an engine. A small engine would struggle with a hill. * People and weight. Cars and vehicles are used to transport things, limiting an engine size limits a cars ability to transport more things. A phenomenon which every young man with a car will experience is their first car. Traditionally a young British male's first car will be a small, cheap and old hatchback with a tiny little engine as this is the only form of motoring he can afford. Carrying on tradition, you will at some point ferry yourself and 4 of your near-adult weight friends around to your destination (a lot of the time there is no destination and you are just simply driving your mates around). These small engines struggle greatly and performance is hindered, and if you add a hill, the problem gets twice as bad. I will now list these quickly: * Acceleration/Performance of a small engine would be terrible, trying to keep up with traffic would be a constant task/worry * Fuel consumption would be terrible * Engine wear would happen a lot quicker as parts are having to work harder. The list is endless, but another side point is, for everyone that enjoys their cars - if someone made you limit your car to a speed, would you enjoy it as much? The answer is probably not, and the economy surrounding cars is too great for one government to lose over another (think of tourism Germany gets for individuals wanting to take their cars to the Nurburgring and on their Autobahns.) Note - I feel this was very long winded. Feel free to pick at anything. Honestly. Edit - Thank you for your comments. As Kkrit pointed out - most of Germany's Autobahn's have top speed limits now, but there are still a few without. It is without doubt the most famous and unique example of where speed limits aren't in place. Edit 2 - I tidied the formatting up!",
"Because otherwise it's not very sympathetic to the car's mechanical parts. For instance if you say \"Well my highway has speed limit of 70mph so I'll buy a car that has a top speed of 70mph\". Well great, but in order to do highway speeds you then have to be absolutely hammering the engine, with your foot plastered to the floor just to keep up with the traffic. A car that can do 150 will be just cruising nicely at highway speeds, while your 70mph top speed car will be screaming, with the exhaust pipes glowing, smoke pouring out of the bonnet, and everyone in the car being shaken to death. Just because it can do it, doesn't mean you will, it just means that at the speeds you're *meant* to travel at, it won't be overstressing the engine.",
"Because top speed is still a selling point, mostly. As long as people are willing to pay for a fast car, companies will make them. Also, speed limits vary worldwide, and some places such as racetracks or the Autobahn have no limits at all. Finally, a car with a weak engine that can just maintain the speed limit will have dangerously bad acceleration, and will be too slow against a headwind or up hill.",
"There's no compelling reason to artificially limit a car's top speed. If you just made the engine so small that the car couldn't go past, say, 70 mph, it would accelerate extremely slowly, probably to the point of being dangerous.",
"Think about us humans. Our top speed is when sprinting. We get tired very quickly when doing that and it stresses out our bodies. But for our normal every day pace when walking, it's very easy to do that. If our max speed was our walking pace, we would get tired fast, since that would as difficult as sprinting. A higher top speed makes the lower speeds that you experience every day a lot easier to handle, whether for walking or at highway speeds for cars.",
"it's about the horsepower. in order to make a 1000kg car to go with 90km/s on a straight road, you need about ~20hp. but going above 90 is problematic because of the wind resistance(drag) increases way to much(in the formula it has ^3). imagine using a i10 with 20 hp. it can't really go uphill, you can't really load it and it will have a painful acceleration. you can avoid this by making it 90hp, it will give you a comfortable ride under the speed limit, and it can reach around 180km/s as a side effect."
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74h9ff | Why do soldiers need to break step on bridges so it doesn't collapse but people in a club can dance to the beat of the music with no such consequence to the building? | Engineering | explainlikeimfive | {
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"Bridges have long spans of not having anything underneath them, so they can be very susceptible to resonance. Buildings are smaller than bridges and the floor is often build right on the ground. That said, nightclub collapses are not unheard of.",
"Fun fact. This is an urban myth. It's not possible to create constructive waves strong enough to destroy a bridge by marching in step. Even if you got everyone in step you'd have to match the harmonic frequency of the bridge to create large enough oscillations. Then if you even managed that the waving of the bridge would be enough to throw people out of step or knock them down before it failed. Look up the Tacoma narrows bridge on YouTube. Also know as galloping gurdy (sp?) That failed because of wind it took an incredibly amount to make it fail over time.",
"On a related note, I think it's interesting learning about these tragedies and how they occured. [Versailles Wedding Hall Collapse]( URL_2 ) [Hyatt Regency Collapse]( URL_0 ) [The Iron Ring]( URL_1 )"
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74hpju | What's the internal difference between a flashdrive, an SSD, and an SD card if none of them have moving internal parts? | Engineering | explainlikeimfive | {
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"They're differentiated by speed, reliability, and cost, these come from their usages A flash drive is limited by power provided by USB and the max speed of a USB port, it doesn't need to be stupid fast, but cost is important An SD card may need to record live 4k video so write speed is very important, it's small and easy to swap so capacity isn't a huge problem, price is still important but less so, and it won't see a huge number of writes so it can be decently reliable without great hardware SSDs need to be big, fast, and reliable. They'll see petabytes written to them over their lives. Write speed can be traded for read speed which is what computing applications need. Cost isn't nearly as important as for flash drives so they can use better chips Not all flash chips are created equal, they are created and binned for specific applications and use cases"
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74i3fa | How are the weight of massive structures such as the Great Pyramids measured? | Engineering | explainlikeimfive | {
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"Math and measurements. You measure how much the type of stone used to build the pyramids weighs for a given volume, measure the dimensions of the pyramid, use the dimensions to calculate the volume, then use the scale measured in the first step by the calculated volume to arrive at the approximate weight.",
"For the pyramids in particular, it's super easy. We know what the pyramids are made of (sandstone), and we know how much a cubic meter of sandstone weighs. And it's a simple matter of measurement and geometry to figure out the size of the pyramids. After that, it's just a matter of multiplication."
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74jy7q | How do weather radars generate a top-down view of clouds when they're on the ground? | I think of a radar like a flashlight, where it shoots out a beam and whatever it hits, the beam bounces back. But with a flashlight, I get huge shadows behind the things I'm looking at and can't extrapolate this view to a top-down view. I can't see the burglar hiding behind my ottoman no matter how hard I squint, unless I move my position. So then how does a fixed-position radar build this top-down view? How does it discriminate between the front side of a cloud, the back side of a cloud, the sides of the cloud, the sides of the cloud behind that, etc.? Right now in my area there are like hundreds of layers of clouds just in one direction. I look at the map top-down and I can see how a radar could figure it out from my earth-orbit POV, but the radar is on the ground, not in space. I can't imagine every beam is able to report its complete itinerary back to the station like "I bounced from position A to B ... to Z and then finally back to you for good"...so how does this work? | Engineering | explainlikeimfive | {
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"> I can't imagine every beam is able to report its complete itinerary back to the station like \"I bounced from position A to B ... to Z and then finally back to you for good\"...so how does this work? And yet it does! Or, more exactly, the radar shoots out a pulse. Then it opens its eyes and starts looking for reflections. Every nanosecond it sees how much of a reflection it gets. And because it knows how fast light travels, and of course it knows what direction it's looking in, it knows where that reflection came from. (It knows that it must be a direct reflection because it looks at the strongest signal it got, which is practically speaking almost impossible to be a multipath reflection). If it gets a strong reflection 50,000 nanoseconds after it emits the pulse, it knows it's hitting a cloud at about 25,000 feet away (light travels about 1 foot in 1 nanosecond)."
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74negc | what in the audio jack’s design makes the iPhone more prone to water damage than the lightning port? | One reason that’s cited for getting rid of the audio jack on smartphones is that the audio jack makes them especially vulnerable to water damage. But there are other orifices on smartphones, too. On the iPhone there’s the lightning port millimetres from the audio jack. How is the lightning port doing better when it comes to water damage? | Engineering | explainlikeimfive | {
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"> But there are other orifices on smartphones, too. Whenever you have a port in a container such as a smartphone the efforts to secure it against water intrusion are going to have a certain rate of failure. Whatever the rate is the fewer ports the lower the risk to the device as a whole. Obviously they can't get rid of the lightning port but if lightning can do everything the 3.5 mm audio jack can do then removing it would be expected to lower rates of water damage. Note that this holds true regardless of if the audio port is any easier to secure against water or not. I think there are more compelling arguments in favor of the move though. For example the lightning port has the potential to provide superior sound quality compared to a normal audio jack. Why? The lightning port can provide external power to connected devices by drawing from the internal battery. This can power amplifiers and various audio processing hardware in the connected devices, as well as things like active noise cancelation which would otherwise require power storage integrated into the devices themselves at the cost of increased weight, the cost of complexity, and the need to charge more devices. Obviously something like USB-C might be an even better solution but the lightning cable has been around for four years now while USB-C is still picking up steam."
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74ox5k | Why are we not sending more people to the bottom of the Mariana trench? | Engineering | explainlikeimfive | {
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"Well, it's hard to get there, takes very specialized equipment to do so, and there's a lot of ocean to explore besides just the bottom of a particular trench.",
"Why should we? We've been there before, seen sand studied it. Its expensive and dangerous to get there as well. better to study other parts of the ocean we haven't yet."
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74rdbu | Why are huge buildings concentrated in dense central areas of almost all major American cities and not more spread out? | They are almost always concentrated in a downtown sort of area. Why is that? Why aren't they more spread out? Wouldn't spacial competition and property taxes make more of a case to locate larger structures away from one another? Not to mention cause less congestion traffic and commuting wise since less people work in the same area. | Engineering | explainlikeimfive | {
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"City planners divide the city into zones where specific types of buildings and other structures are allowed. They try to act in the best interest of the public and city. Having a sky scraper in the suburbs would likely mean increased traffic as that big ol' building likely employs many people. Suburb moms and dads often enjoy quiet roadways and little traffic. If you are curious about zoning laws you should look into how Houston basically let the housing industry go crazy and build wherever they want. This freedom for private industry to do what ever they makes them the most money made Houston one of the fastest growing cities in the country. It also resulted in builders throwing up housing units in known flood planes and spill ways. And hurricane Harvey just us why that's a bad idea.",
"High real estate prices are why those tall buildings exist. Land is very expensive in downtown cores so buildings have to go up rather than out. There's no reason to build a skyscraper on cheap land, because a lower and wider building is much less expensive to build. Downtown areas also offer convenient locations near public transit and prestigious addresses for the occupants of those buildings."
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74v7kf | How do we navigate on other planets? | I'm been thinking about this question for some time. Today on earth we rely heavily on GPS to navigate around or via maps created with satellites or drawn by hand (ancient). But as we (hopefully) get to explore space and visit other planets, I guess we will need some new tool to navigate around. I guess putting satellites around the planet is one way to do it but imagine it won't be cost effective or great way to spend resources if we intend to get around ;-). Will we use the stars (like when first went out to sail) or will we send probes to map the planet or what? How do the rovers navigate on Mars? | Engineering | explainlikeimfive | {
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"The effort to get people to other planets greatly exceeds the effort put a GPS system around them. That is likely one of the first things we will do. > How do the rovers navigate on Mars? They don't, not in any real sense. Their top speed is about 0.1 mph, and in normal operation it is stopped for 2 seconds for every second it is moving. They average about 2 miles a year. Navigation largely consists of \"look at that weird looking rock, let's go over there!\""
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74vgm4 | What's counter torque and why is it needed to make a helicopter fly? | I once asked someone about how propellers can make an object fly. He said that without counter torque, no helicopter can fly no matter how fast the propeller was spinning. So what exactly is counter torque? | Engineering | explainlikeimfive | {
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"A helicopter flies by using an engine to spin a primary rotor, but that rotor has inertia so when trying to spin it there is a force pushing back that tries to spin the helicopter body too. Counter torque is provided by the tail rotor and ensures that only the main rotor rotates around the shaft and the body of the helicopter isn't also spinning around the shaft. This is why a helicopter that loses the tail rotor starts spinning out of control",
"Remember Newton's third law of motion? Each action has an equal and opposite reaction. Imagine a helicopter with no tail rotor, and the main rotor spins counter clockwise. The helicopter's body will spin opposite to the helicopter's blades. So without a tail rotor, the body will just keep turning clockwise. In order to stabilize the body from turning in circles constantly, the tail rotor provides a force that pushes the helicopter's tail counter-clockwise, balancing out the torque provided by the main rotor. [Here's a video on YouTube that provides a visual example and explains the mechanics behind a tail rotor.]( URL_0 ) There are helicopters without tail rotors, but they have some sort of mechanism at the tail that produces the same effect a tail rotor does: provide a force at the tail opposite the helicopter's spin.",
"People here are mixing up the air resistance of the rotor with the torque from the engine. The torque that makes the helicopter spin comes from the engine spinning the rotor. If the rotor spins and is braked by air resistance no torque is applied to the body. This is actually very important, as it allows you to do an autorotation (landing without engine power) if the tail rotor fails. . Also there are helicopter designs that do fly without any sort of tail rotor or torque compensation. E. G. You can pump gas through the blades, that comes out at the tip of the blade pointing backwards. This induces no torque on the body URL_0",
"The main rotor is constantly cutting into the air in one direction. If there was nothing bracing the body of the helicopter it would start turning in the other direction. Just like if you try to push something heavy while standing on a slippery surface you could go backwards, instead of pushing the object forward. A helicopter has a vertical rotor on the back to counter the spinning of the main rotor."
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750eve | How does the accelerator pedal in cars work? | I have two theories in my mind but they can be both wrong. 1st theory: When the pedal is pushed a little, the car accelerates and then remain at a certain constant speed if the pedal is maintained at the same position. If the pedal is pushed further, the car accelerates and then remain at a higher constant speed as long as the pedal is maintained at the same position. 2nd theory: When the pedal is pushed a little, the car keeps accelerating at a slow rate and speed keeps changing at a slow rate. If the pedal is pressed further, the car keeps accelerating at a faster rate and speed keeps changing at a fast rate. | Engineering | explainlikeimfive | {
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"In most cars, pushing the accellerator (or gas ) pedal increases the flow of fuel to the engine. This means the engine will output more power, causing the car to accellerate. However, this accelleration doesn't continue forever. Mainly because as a car's speed increases, the amount of wind resistance increases as well, that's like a force pushing against the car forcing it to slow down. So when you press the accellerator, the engine will push the car harder, and as it speeds up, the air will push harder against it. Once they reach an equilibrium, the car stops accellerating and instead maintains a constant speed.",
"In new cars it's electronic. You push it and it tells the engine how much gas and oxygen to let in (for combustion). Older cars have a cable attached to the pedal that connects to the throttle body (lets air in the engine). My pushing the pedal it in return pulls the cable and opens the throttle body butterfly valve. More oxygen = more power, so the more you push the pedal the more oxygen gets sucked in which in turn accelerates the car."
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751fic | How does the smoke detector know the difference betweem smoke and steam? | Engineering | explainlikeimfive | {
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"There is many different types of smoke detectors out there. Some can't see the difference between smoke and steam, but the more complex ones does a good job of separating the two. The ones I use at work now is: * optical. (Think LED in one end of chamber and a photo-resistor in the other end. Mesures the amount of light that get through.) * double optical and thermal. (Uses two different angels on the chamber for the optical part to guesstimate the difference in smoke and steam based on different reflections.) * optical / thermal / gas. (Reads Co2. Works like a champ in theaters, clubs etc. that uses theatrical smoke on stage.) * only thermal. (Works on the principle of fire is hot. Heat rises and sets of the detector at a set temperature or if the temperature is rising very fast.) Double optical is very good at separating steam and smoke. (Use it in kitchens and locker rooms close too the showers. No problem with false alarms.)",
"There are photoelectric and ionization sensors. One checks for flame and one checks for smoke. You have to be car because not all detectors have both. Photoelectric uses infrared light to see of there is smoke in the distance. Ionization sensors have two little plates that sense a break in ion flow, cause by smoldering of a flame.",
"Smoke is technically a colloid of solid particles in an air medium. That makes it slightly more likely than steam to set off the sensors in a smoke detector, but if you shove a boiling pot of water right up against a detector, you have a small chance of setting it off. It would be more likely for dust to set it off. For example if you're removing drywall or if the detector is mounted on the wall and you dust it off after a long time.",
"Smoke detectors work by having a small amount of Americium-241, a radioisotope, and a radiation detector. Smoke contains particulate matter which absorbs some of the radiation, the detector picks up on this and triggers the alarm. Steam doesn't absorb radiation as well so it doesn't trigger the alarm."
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753e52 | What determines the type of batteries a toy or electronic would need? Why not have one universal? | Engineering | explainlikeimfive | {
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"Each device has different demands on size, voltage needed, and total charge desired. Most things you buy batteries for use alkaline batteries, or were made originally to use alkaline batteries. The chemistry of them means one cell generates 1.5V. If you need more voltage, you need more cells. If you've got the space and want it to last as long as possible, you'd want to use a larger battery (like a D battery in a flashlight). If you've got less space, you need to use a smaller one. AA in remote controls, even though C batteries (while huge) would last for a very long time. If you need more voltage, you need multiple batteries. It's pointless to produce an alkaline, 4-cell, 6-volt battery because it'd be about the same size as four 1.5V batteries, and would require manufacturing a whole other kind of battery with no real benefit. Far easier to sell only single-cell batteries, and just have some devices use more than one at a time. Between all these factors, there simply isn't a single kind/size battery that meets the needs of every device."
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759hhf | How does a jet engine provide reverse thrust when landing? | I searched but didnt really find a proper explanation (though i may be using the wrong terms) I look out the window and I see that the shrouding of the engine kind of folds back to the outlet of the jet and I guess maybe redirects it forward... Ok... but isnt all of that air its redirecting being sucked in from the front to begin with? So even if the redirection were 100% efficient (surely not) it would be a net zero thrust? So where does the reverse thrust actually come from? bonus question, how much good does it actually do? I mean... seems like it must be a fair amount of complexity and weight on the aircraft, plus the token amount of fuel, to shave a few feet off the landing distance? Wouldnt it be better to just let the existing brakes do the job if the runways can contain it? (or cant they?) | Engineering | explainlikeimfive | {
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"Jet engines work by dramatically increasing the speed of air flowing through them, using a fairly simple design. Air flowing into the front of the engine exits the back at a much increased speed. This is why engines can generate thrust from a standstill. When \"target type\" thrust reversers are deployed, they redirect the flow of the gas exiting the engine to point it in the direction of travel, which results in reverse thrust and slowing of the aircraft. Regarding why they are used - several reasons. Firstly to reduce the wear on brakes, thus avoiding potential failures; secondly to reduce the landing distance; and thirdly to compensate for the negative effect of poor weather on braking efficiency."
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75atlb | Why not construct concrete homes in the tornado alley instead of rebuilding every time? | Engineering | explainlikeimfive | {
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"Cost, odds, and aesthetics. The odds of being hit by a tornado are very small - they aren't that wide and only touch down for a few miles, usually. So odds are a house isn't going to get a direct hit by a tornado. Then it is a question of cost and aesthetics. Since home builders and buyers know their house is very likely not going to be hit, they'll go with the house that is much cheaper - it means they can afford a bigger and nicer house (or just save on money). Also, while there are plenty of good looking, modern, high end concrete homes, if you don't have skilled concrete tradesmen, a concrete home can look ugly, or you'll just need more materials as a facade.",
"It is more expensive to build a building capable of withstanding a tornado than it is to rebuild. Not for an individual house of course, but overall it balances out. Imagine every year 1% of houses are blown down by tornadoes and to build a house capable of withstanding such immense forces would make it cost 40% more. If you built every house to withstand a tornado you would spend about as much as simply replacing the houses which blow down every year for 40 years. Is the average lifespan of a house in that area more or less than 40 years? What if you took that 40% you might have spent on the house and invested it for 20 years, would you be able to rebuild the house that much more easily? So you see it isn't clear that sinking money into building for a relatively rare event is really that wise of a choice. Rare disasters are better dealt with through insurance rather than trying to make every structure \"everything-proof\".",
"1. It is statistically-unlikely that your house will suffer a direct hit from a tornado. So what's the point of constructing it out of more expensive materials specifically rated to withstand a statistically-unlikely event? 2. Building tornado-proof homes is more expensive than building weaker, wooden houses that insurance takes care of. 3. In most cases, it costs more to repair a damaged concrete house than it does to rebuild a destroyed wooden house. Most of the damage suffered by a house made primarily out of concrete is caused by destruction of windows, doors, and its roof. You ever see a house with a concrete roof? Me neither; they're more commonly called *bunkers*, and they're normally underground. Sometimes, you end up having to demolish the concrete structure anyways.",
"Water does a lot of damage. Also damaging concrete buildings so you end up rebuilding a more costly home",
"I still wonder why they put trailer parks in tornado zones. I understand that not everyone can afford a traditional house, but why allow them at all in places like Missouri and Oklahoma?",
"It's not the same homes blowing down every year. You can guarantee you are safe by building a home that costs three times as much, or hedge your bets, and most likely come out ahead."
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75jtbf | How do sailboats / ancient ships steer if the wind isn't blowing in the direction they want to go? | Forgive me if my terminology is off. But I've always wondered how long voyages were made given the unpredictable pattern of winds. How are ships able to go where they want if the wind is, say, blowing in the wrong direction? | Engineering | explainlikeimfive | {
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"A sail can catch wind and push the boat forward from just about any angle greater than 70 degrees from directly in to the wind. A sail boat has a big fin on the bottom that keeps it from being pushed sideways, and the sail is at an angle so the wind is redirected, which all together pushes the boat forward. So as long as the boat isn't going directly into the wind, it can often just sail straight forwards in the direction it wants to go. If the boat wants to travel in to the wind, then what the boat does is it picks up speed going across the wind, and then turns into the wind. While the boat is facing in to the wind, the sail is not catching the wind, so there is much less force trying to slow the boat down as it turns. AS the boat loses speed, it will turn far enough that it is going sideways to the wind again, but in the other direction- and once it picks up enough speed again, it will turn back in to the wind again, which creates a zig-zag pattern that slowly moves them in the direction they want to go. This is known as Tacking in to the wind. Depending on how close to directly in to the wind they want to go, they could just go in mostly the right direction, hoping that the wind will shift in their favor and then tack in to the wind to correct their position once in a while if the wind doesn't change.",
"It's called tacking, and basically you zig zag using a portion of the wind to move in the direction you want... so if you were sailing directly into the wind you'd angle the sail so that it moves you up at a 45 degree angle, then switch and have to move you down at a 45 degree angle",
"No one yet seems to have mentioned the keel. The keel runs from the front to the back of a boat. The wind, when from the side, just wants to blow the boat sideways, it is the keel that stops that from happening and thus the boat is sort of \"squeezed\" forwards a bit like when you squeeze a toothpaste tube. The toothpaste is channelled by the sides into coming out the front. Same idea."
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75m77v | Why did drones start exploding in popularity ~2 years ago? What technological advancement happened that allowed them to be accessible to consumers? | Engineering | explainlikeimfive | {
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"The thing that made them explode was auto stabilization, specifically the prevalence of easy to program PID controllers. Before then you couldn't just hook 4 motors up to a magic box and download one file to have it work. You had to custom design each drone and then custom design a stabilizing algorithm. So simply put drones took off because they became very easy to program and design.",
"solid state gyroscopes and inexpensive use of PID loops to allow single chip/board solutions to complex mechanics like balancing or stabilizing. If you think back to the world before things like the Segway or hoverboards, solid state stability controls used to be rarer in consumer goods. But as they got smaller, cheaper, and lower powered, you started seeing them pop up in laptops to park the hard disk when dropped, cellphones and game controllers as input devices, even in toys. Prior to these electronic solutions, products often had mechanical solutions such as helicopter flybars. Back around 2000 radio control helis mostly used weighted bars to stabilize them, and tail gyroscopes were just coming on the market in mass quantities. Today you find inexpensive boards with 3 axis gyro stabilization such as what you would need to operate a quad copter. PID loops could have their own ELI5 writeup that would be exhausting to read but those are the broad strokes."
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75p20j | If 10% of accidents are caused by wet roadways, why can't we create a tire type that doesnt hydroplane and is easily fitted for all vehicle types? | Engineering | explainlikeimfive | {
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"Because we need to have tyres that work in all weather conditions, including completely dry roads. Producing a rubber compound that was super effective in wet conditions would likely involve a fairly soft tyre surface with loads of grooves to disperse water - if that tyre were driven on dry roads, it would introduce a lot of vibration from the grooves, and the rubber compound would break down, dramatically shortening the life of the tyre.",
"/u/togoboldly has the right idea with his response, however I'd like to go a bit deeper. Keep in mind, the contact patch (area that connects tire and ground) is roughly just a tad bit bigger than a grown mans hand laid flat. **What causes a tire to lose effectiveness or hydroplane in wet conditions?** When it's raining, water forms on roads. And some of it stays on top of it. Now, you remember as a kid when you were playing in a tub or pool, pushing water on the surface to create tiny waves? That's what an ungrooved tire surface is basically doing. It's pushing water away from it, and by rotating also pushing it under it. Water is a nasty bugger because it doesn't compress, and many a wonders of modern world use this to their advantage. **Alright great, so we just gotta do grooved tires, right? Can't be that hard!** Well, yes and no. There is a lot of research going on all the time by the top manufacturers, on how to optimise the tire performance. They come up with shapes and compounds which they test through. Very popular way of shaping the tires has been the upsidedown V shape. It catches the water that is supposed to go under the tire inside the groove, and pushes it to the sides and out.**THIS IS ALSO THE REASON WHY ITS EXTREMELY IMPORTANT TO PAY ATTENTION TO THE ROLL DIRECTION, WHEN YOU INSTALL NEW TIRES**. If you'd mount up a tire with the V going wrong direction, it would CATCH that water and push it under the tire. Not good. Unless you want to turn your car into a sled. The thing with grooves is, it adds edges. And edges adds friction. It isn't bad to have friction on tire, but too much of it and you get noise (annoying as hell), vibration (also annoying, but wears your car a lot), decreased fuel economy (Which is a big factor in consumer tires) and tire wear. Some of these can be battled with changing the tire compound a bit, but there is a limit. Too soft, it wears out and is noisy. Too hard, it has no grip, dry or wet. So believe me, there are tons of engineers working on the best possible solution. Here in Finland, our roads are very coarse, and we have snow and ice at least 3-4 months a year. Maybe 2 dry months and the rest is wet as fuck. Police are super strict on the tires and you can get a fine that is a % of your income, which can rack up to hundreds of euros. The reason is simple, public safety. Here is a great image about how the tire wear affects the amount of grip: [Link]( URL_0 ) It is in Finnish, sorry for that, but it gives you the idea. Leftside column is speed, top column is the amount of millimeters in groove depth. Minimum required depth in Finland is 5/32 inches or 4mm. Quite fucking scary, when you think that that's the only thing trying to get your car where you want it, instead of sliding around.",
"There are such tyres already, but they are unsuitable for dry conditions. A standard tyre is a compromise - its not the most performant in the dry, nor the wet. It would be up to the driver to adjust their driving to the conditions - which I suspect is the real culprit behind your '10% of accidents are caused by wet roadways' statistic",
"We can't fix the fleshy thing behind the wheel that either chooses to ignore the necessity to replace it, or is too ignorant to know otherwise. I just drove around the other day in a car where the back wheel locks up because the slave cylinder is malfunctioning. I didn't crash into anything. People fail to take appropriate action, and caution, when driving. They fail to learn what is required to remain safe. They fail to plan ahead to avoid issues. It's not the tire, or the engineer, or the material, or anything else but the fault of the driver."
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75pwi9 | Why are the seats on public toilets often a horseshoe shape, whereas home toilet seats are complete circles? | Engineering | explainlikeimfive | {
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"They're cheaper to purchase (less material) and easier to keep clean because there is less surface area to contaminate. They also last longer because they crack less than seats that go all the way around. It also makes it easier for women to wipe themselves after they are done and avoid unnecessary skin contact with the toilet itself.",
"At home you tend to not piss on your own seat. In public many people just don't care about others and won't bother lifting the seat to pee. So you pee between the gap",
"It reportedly was originally for women to wipe, but every time I mention that everyone says that nobody would wipe from the front side of the seat. Regardless, that is what the International Association of Plumbing and Mechanical Officials states that they should be U shaped for this reason, and while that isn't a law, many local gov'ts might enforce the code regardless. URL_0"
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75sx3m | What is codeless/semi-codeless GPS and how will turning it off affect current GPS devices and service? | Engineering | explainlikeimfive | {
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"GPS works by transmitting a very precise time signal for multiple satellites. Because the distance from each satellite to the receiver is different, the signals come in at different times, and the receiver can use the time discrepancies between the multiple signals to work out its position. The GPS signal from each satellite consists of a repeated code - this is a long string of binary numbers that look random. The advantage of this is that most digits of the code provide a clear timing signal - so when a 0 in the code is followed by a 1, there is a big change in the signal. Because the code is about 1000 digits long and repeats about 1000 times per second, the receiver can average the timing signal from each digit and each repetition, to get a very clear measure of the timing. As well as the civilian signal transmitted on the L1 band, GPS also transmits an encrypted military signal on a separate frequency. The 2nd frequency (L2) is useful because it allows the receiver to calculate the effect of atmospheric conditions on the signal for improved accuracy. (Atmospheric delay affects one frequency more than the other, so if you can measure the time difference between the two signals from one satellite, you can calculate the effect of the atmosphere). However, dual frequency receivers have a problem: the military signal has an encrypted code, and because the receiver doesn't know the code, the receiver can't just average the signal to try to extract the timing. Because using dual frequency signals improved accuracy, very high end GPS systems intended for scientific use or surveying, have developed some clever tricks to try to extract timing information from the military signal. The original technique for this was \"codeless\". It used signal processing techniques without any knowledge of the code. This is difficult, because the code identifies each satellite, so with no code information, it's a problem telling one satellite from another, so the system may not always work. The codeless timing signal is also very weak and therefore the extra accuracy over regular GPS is often dissapointing, and this technique has been completely obsolete for about 10 years. A more advanced \"semicodeless\" system was developed which takes advantage of the fact that a copy of the same military signal is also transmitted on the L1 band. By using the L1 copy of the military signal as a reference, it becomes possible to measure the time difference to the L2 signal, even if you can't actually read the code itself. This time delay is all that is needed for atmosphere correction. The GPS signals are being modernised. One of the key upgrades is to have fully open dual and triple frequency signals for civilian use. There will be no need for semicodeless trickery to get atmosphere correction, because the updated signals will have a public code allowing them to be directly decoded. A public L2 (called L2C) signal is already partially active, and should be fully active by 2020. There is also a public L5 signal coming giving a 3rd signal choice. Satellites with L5 signal will be launched starting next year, and the signal should be fully active by 2024. The new signals won't be received by old receivers using codeless/semicodeless processing of the old L2 military signal, and when the old military signal is switched off (once the new civilian GPS signals are fully active), then these GPS units will not be able to operate in codeless/semicodeless mode. Users of old codeless/semicodeless systems are being given advanced warning to start planning upgrades to a new modern unit capable of receiving the new dual/triple frequency civilian signals."
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75t0qk | How does a hazmat suit and gas mask work? how do they filter the toxins out and how long do they last for? | Engineering | explainlikeimfive | {
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"Gas masks work by passing outside air over a series of filters before it is allowed to enter the mask. The simplest ones use particulate filters that prevent large particles floating in the air (like dust, chemical powders, or smoke) from getting through. There are also chemical filters that form chemical bonds with harmful compounds and trap them in the filter. The most common chemical filtering agent is activated charcoal, which is effective against most organic molecules. Some masks also utilize filters that directly react with airborne chemicals and render them inert. What kind of mask you decide to use depends on what you're trying to filter out. Masks designed to deal with a wide range of NBC (nuclear, biological, chemical) threats use filter canisters that contain several different kinds of filter, but tend to be quite restrictive of airflow. Chemical hazmat suits are a chemical infused (usually synthetic) fabric tightly woven and sealed to prevent entry of chemical gases or absorption of liquid chemicals. They are absolutely miserable to wear. How long filters and suits last depends on what chemicals are present in the atmosphere and how rough you are with them. It is generally not recommended to use either for more than 24 hours."
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75t27i | If a nuclear bomb is dropped on other nuclear bombs that are idle on the ground, will it create a double explosion or do these weapons need to become 'activated' in order for them to be able to detonate? | Engineering | explainlikeimfive | {
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"Depends on a lot of things. Nuclear bombs work by changing the critical mass of the nuclear fuel. \"Critical mass\" is the amount of nuclear material you need to have a sustained nuclear reaction. You can artificially make a smaller-than-critical mass into a critical mass in several ways. One way is to cover it with a material that reflects neutrons (which would cause all the neutrons that would've escaped outwards from the material to reflect back inwards, generally used in nuclear reactors and research), but another way is to change the temperature and pressure of the material (which is done in nuclear weapons by using \"explosive lenses\" which is a fancy way of saying you surround it with conventional explosives). Without those explosives going off (and a few other things I'd guess) that nuclear weapon isn't actually fissile (able to undergo fission). A nuclear explosion above the silo where the bombs are stored is just as likely to vaporize the exposive as anything else, not to mention that unless that explosion is able to actually trigger the explosive correctly it's not going to explode. (C4 and TNT for example are completely safe to burn, and only explode with specific stimulii.)",
"Detonating a nuclear bomb is a very precise process, a lot of complicated things have to happen in just the right order. Even the most primitive bombs would be unlikely to go off in a high order explosion just because of a nearby explosion, nuclear or not. Modern nuclear weapons are actually deliberately designed so this is impossible, as a safety measure.",
"Nuclear weapons are conceptually simple, you smash together enough U-235 (a 'supercritical mass') and it goes boom. The hard part is making it go boom when you want it to, and NOT melt when it's sitting in a silo. There are two kinds of 'explosions' that occur in a nuclear weapon. The most obvious one is the nuclear fission chain reaction that makes a nuke what it is, which I'll call 'nuclear detonation'. The second kind of explosion (technically the first to happen) is what I'll call a 'primary detonation', which is a bunch of high explosives rigged precisely to trigger or 'ignite' the big boom. There are two main ways in which these parts are put together, the 'Gun' and the 'implosion' methods. In the gun device, a big slug of U-235 is shot at high speed into another mass of U-235 that fits it like a glove, bringing together a supercritical mass. This device was constructed during the Manhattan Project by essentially strapping a bunch of expensive equipment and U-235 to an artillery piece and firing it. The second method uses a hollow sphere (also known as a pit) made of Uranium. In its hollow shape, the mass is not supercritical, but when it is compressed by the primary detonation into a solid ball, the mass becomes supercritical. It's a lot like crushing a soda can, but your hands are TNT and the soda can is going to blow your block party off the map. In both methods of detonation, the critical mass must be brought together very quickly and very precisely. Otherwise, instead of the desired nuclear ignition, a 'premature detonation' will occur, severely reducing the weapon's power (loads better for the world than premature ejaculation ;). So, to answer the question, if a nuclear warhead were dropped on a warehouse full of nukes, it would NOT cause nuclear detonations in the other weapons. It would, however, cause lots of bad shit, including: -Big Boom from the original warhead -All of the nuclear materials in the bombs is now volatile nuclear waste that may be in various states of criticality and may or may not fissioning and creating more hazardous waste. To get an idea of what this could develop into in the worst case scenario, read up on the elephant's foot at Chernobyl. Its not exactly the same situation, but Chernobyl gives us an idea of how difficult it is to move and protect ourselves from uncontained fission materials. -Detonation of high explosives from primary detonation systems of other bombs. This is unlikely to cause any nuclear detonations because the precision of the detonation is completely overwhelmed by the initial warhead, but explosives are explosives. TBH the size of these explosions is nothing compared to the initial weapon's power, and amounts to something like a mosquito bite on an arm that a bear just tore off of you. -WWIII (Assuming some head-ass didn't bomb their own country, which almost happened once in North Carolina I think) TL;DR: No, it won't cause a 'Double Explosion', but it's still a nuke, and it's gonna kill the heck out of you. Side note: For similar reasons, nuking or crashing a plane into a nuclear power plant does not cause a nuclear detonation. Nuclear weapons are devices carefully orchestrated and calibrated to 'make the stars align' so to speak, and create the very narrow conditions that make a nuclear explosion possible. On the other hand, a power plant is designed to generate electricity in a sustained and controlled fashion, which inherently precludes the possibility of a nuclear detonation, simply because the specifications on how to trigger a nuclear detonation are so tight."
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75usmw | In the first Kingsman Movie, how do they shoot/edit the Church scene to make it look so awesome? | Engineering | explainlikeimfive | {
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"Part of it is the perception of a single take (which had already been explained as several cuts put together cleverly). Another part is that it appears that frames have been removed from the scene, making things appear much more frantic. The biggest thing, however, is that the scene is shot in a way that makes you feel like you're Harry. The camera focuses on Harry, only showing you threats as they become relevant to Harry. At least, that's the best way I can describe it. Hopefully this helps/actually makes sense. EDIT: Your > You're. Words are hard :c",
"If you look closely there's a lot of hidden cuts that make it seem like it's all one take. The cuts are hidden by zooming or people, blocking the camera and a lot of editing tricks.",
"This YouTube video by Aaron Field explains what makes the fight scene so badass. It's the same as what others said, but a watching the scene at the same time does help! URL_0",
"It is my understanding and opinion that it has to do with it looking like a single take. In most movies when the fighting happens they tend to cut at or before the attack hits and switch to a different angle to make the attack looks like it hit but it actually didn't in real life. But because of the single cut tricks the fight feels more real and that the attacks actually do hurt.",
"Is it just me or does this scene show the issue with streaming movies? Streaming services and even youtube don't really \"buffer\" anymore, everything is loaded on demand. Cheaper and more efficient I'm sure, but it cannot keep up when this scene comes on at all. The bitrate must suddenly jump a whole lot - completely ruins the scene though. Why in the world do I even own this on Amazon?"
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75x06t | Why do some new jet engines have "teeth"? | Engineering | explainlikeimfive | {
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"They reduce the noise from the jet engine, [by making a smoother blend of hot air from the jet and cool air from the atmosphere]( URL_0 )."
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760zeg | Is it more efficient to leave the AC running on auto at 74F (in 85 degree whether) or turning it off when leaving the house, and turning it back on when returning when the ambient temp in the apartment is 85? | Engineering | explainlikeimfive | {
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"Turn it off, you will use less power. Thermodynamics tells us that heat loss is proportional to temperature difference, so if you let your house warm up the heat from outside enters more slowly. Essentially the product of time and temperature difference is your cooling energy. There is a minor subtlety with maintenance and life cycle, your AC unit may not be designed for continuous duty, so long cool down cycles could be hard on it. Not sure if that is the case in your unit, seems like a bad way to design anything but it could be. Edit: one non-thermodynamic factor is humidity and mold, which will be different at a constant temperature vs a cycling temperature.",
"Turning it off would be more energy efficient. The reason why this is, is that running your A/C at a cooler temp doesn't necessarily use \"more power\". As an example, suppose you have an 1100w microwave. That microwave will on average, always use 1100w no matter if you're cooking for 10 seconds or 5 minutes. If you cook for 1 hour, you will have used 1.1 kWh. If you multiply that by 24 you will have the kilowatt per day, supposing you ran it for 24 hours. Multiply that by 1/4th of the year, 91 days, and you'd have used ~2049kWh of round the clock microwaving (Many home owners use between 3,000kWh and 20,000kWh when yearly home electrical use is totaled). The same concept applies to your A/C (can usually use 500w to > 2000w (2kWh)). So, your A/C will just run longer because your thermostat will be trying to keep your house at that temp. Depending on the placement of your thermostat, it may be in an abnormally warmer part of your house and just be unable to detect the temperature properly. Meanwhile, all the bed rooms may be a comfortable or colder temp. Not to sell a product, but getting something like a smart thermostat would probably help a lot. They are a step up from a normal thermostat in that you can control some of them via your phone. You could leave the house at your usual time, A/C off, and then if you remember, can turn it on via phone before you're expected to come home so that your house will be chilled a little bit when you get there. Some of them have really robust means of scheduling them but anytime you don't have to run your A/C, you'd be more efficient than just running it all the time at a consistent temperature. You would save more money by turning it off when possible. Additionally, if you're paying a lot for A/C it may be a good idea to look into reducing your electrical bill by replacing it with a solar bill or see if there are more efficient AC* units. Final Edit: Clarity.",
"For what you describe, assuming an apartment or small home, it is more efficient and safe on the equipment to turn it off through the day. Be sure you close the blinds or curtains to help keep it from warming too much during the day. Buy a programmable thermostat so it can cool your home only when you are there, and only down to the temperatures you need. Good devices ($50 range) can calculate how early to start cooling when you return home and run for multiple cooling cycles so it can minimize both cost and wear on the equipment. (No need for the very expensive learning-style devices unless you are truly incompetent at following on-screen instructions, although they seem to make neat toys for people who have money for them. Programming when you're up and about isn't hard. ) For an outdoor temperature of 85F -- which really is not very hot outside -- there is only a small temperature drop needed. Blocking light through windows and double-checking the insulation and weather seals will be your biggest help both for efficiency and cost. Assuming you closed the blinds and curtains during the day the weather you describe your home is unlikely to hit high temperatures, likely never hitting the same 85F you see outdoors. You're probably only going to cool the home 5F or so. In the general case it is a more complex answer, and it depends on the type of equipment, the difference between indoor temperature ranges and the outdoor temperature. It depends on the volume of air, a 30000 cubic foot larger home is quite different from an 7000 cubic foot apartment. It depends on how ducts are situated, if they are whole-home or zoned. There are some newer systems designed to run more continuously with slower fan speeds, and there are higher capacity HVAC systems for bigger volumes and large homes, but neither sound like your setup. Also zoned systems can require less power, but they're uncommon. Most homes are set up for whole-home cooling, so if the room opens up to the main area the duct vents and doors should remain open to prevent frosty-cold vents with hot rooms, or inconsistent temps between rooms. Most AC units work well at cooling for a block of time but not continuous duty. Running for extended times has hazards like the lines freezing up which can lead to expensive repairs, but that mostly happens for bigger temperature drops or high volumes of air to be cooled, or when there is a small temperature differential with the outside air. Efficient systems will run for five or ten minutes, long enough to get the refrigerant flowing in a strong pattern through the various systems to deliver cold air to the blower, and the blower will take that very cold air and blow it powerfully through the entire home. Then they'll shut off before the systems start to overheat or before any of the lines approach freezing temperatures. But really, most of the mid-grade or better programmable systems have all the brains to work this stuff out. You can tell it you're out for the day, and return at 6:00 PM, and it will sort everything out.",
"Also, 74 degrees is way too cold. My home is set at 78 or 79 most of the summer. Your bill will go down drastically if you set it higher.",
"It depends. If it is slow to pull down, leave it on. If it isn’t, then turn it on when you need it or have it come on before you come home with a programmable thermostat or remote controller like a Trane Nexia.",
"Keep it on. Temperature is not the only issue here. Humidity in the air is so crucial. If you turn it off, the AC unit has to constantly pull the humidity out of the air and uses a lot of energy. It also stresses the equipment.",
"I once read a comment by an air conditioning repair man that was very flustered by the idea of turning it off to save energy, because AC units are not designed to run for long periods of time (85F - > 74F), and are instead designed to maintain the temperature by running for short periods of time.",
"Yes, you will save on electricity to turn it off during the day but a better option would to be to get a programmable thermostat and have that installed. You can set it to go up when you leave and to be nice and cool when you get home. It would be more convenient and better for the A/C. They're designed to run a certain amount of time and if they don't you could get mold all in your ductwork.",
"Hope this will be okay to post as it is slightly related. If you have a 2 story house or more rotate your vents for the different seasons. What I mean by this is during the summer close all downstairs forcing the cooler air upstairs and allowing it to move downstairs this will create a more steady temperature throughout the entire house and upstairs will feel cooler so you aren't trying to run the AC all the time to try to cool the upstairs where you most likely sleep. Alternatively during the winter close your upstairs vents to push all the warm air downstairs again allowing it to move upstairs maintaining a more steady temperature. If you don't do this you may notice that in the summer upstairs is always hot and in the winter downstairs is always cold. Forcing you to run your AC and furnace longer and more often.",
"Everything in your home is at a certain temperature. When you turn off your AC, not just the air warms and gains humidity, but also all of your building materials, finishes, and furniture. In order to recool your home, the heat energy and humidity has to be removed from all of this mass as well as the air. If you live in a wood framed structure, temperature and humidity are going to shrink and swell the skeleton of the building. This causes problems with Sheetrock, door swelling and not shutting correctly, window frames may even move and slightly break seals. This ruins the efficiency of the structure. Your HVAC system is just one part of the entire dwelling. It prevents nail pops, creaking floors, mold or mildew, and improves air quality. Turning the AC up a few degrees while you are away is not detrimental, but off will have a negative effect on all of the other systems in the structure."
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7630f8 | why does it take 4 years for repairs on Big Ben clock in London? | Seems a very long time. | Engineering | explainlikeimfive | {
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"They're doing a lot of work. Work, especially work on an old listed masonry building, takes time. They're not just working on the clock face and mechanism—though they are doing that—they're also adding a lift to a building that was never designed to have one, retrofitting and repairing the masonry, replacing the lighting, and more. And all of that is being done to what is probably the most recognizable structure in the UK, which means it needs to be done carefully and thus relatively slowly."
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76941f | How does Dolby Atmosphere "Place" sound? | If the soundbar is next to the TV, how does it make sound appear as if it's behind you? Surely you have to hear the sound travel past your ears before it bounces off the walls? | Engineering | explainlikeimfive | {
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"The first idea to think about is, how can you tell what direction a sound is coming from? Snap your fingers to the right of your head. Now snap your fingers to the left of your head. [Even if your eyes were closed]( URL_1 ), you would be able to tell which direction the sound was coming from. This is due to **time of arrival differences** of the sound waves. The sound hits one ear or the other milliseconds before the other. Your brain interprets those time of arrival differences and figures out which direction the sound is coming from. With two speakers, or headphones, this is used to make sound appear to come from the far left, panning all the way through the center to the far right. There is no effect to produce \"depth\" using only two speakers. In the 70's, you began to see quadraphonic systems being sold: 4 speakers. This lets the audio engineers pan forward/back in addition to left/right. Quadraphonic setups are better known today as surround sound, where it's not 4 speakers but 5 plus a subwoofer: 5.1. Or 7.1. So, if it's one speaker, you cannot produce a depth effect, except for something like [reverb]( URL_0 ) or echo, which is a different perception."
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76b5xy | Why do big lorries/trucks have wheels that are an inch off the road and don't do anything? | Engineering | explainlikeimfive | {
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"Typically large dump trucks and dump trailers have what we call drop axles. They can be raised or lowered depending on whether the truck is loaded or not. When it’s unloaded, most truckers raise those wheels up, as it makes it easier to turn the truck around corners. When the truck is loaded, they lower the axle so it takes a share of the overall weight of the load. Many states have laws governing how much weight can be supported by each axle. Using the drop axle when loaded serves to distribute the weight over 3 axles, instead of 2 (or 4 axles, instead of 3, or 5 axles...etc.)",
"The extra tires (or, well, axles, actually) are not spare. They are there for when the trailer carries it full load capacity. You see, road taxes are paid for heavy machinery and trucks according to the axle pressure. (which is a fancy way to say \"for how heavy one axle can be\") And added to that, every road has a stated maximum allowed axle pressure. Which means that if you want to be able to load to maximum capacity and still be allowed to use the road you have to have a minimum number of axles to even out the weight between. And it could have stopped there, if it wasn't for the less practicality of having all axles in the ground. All those axles being in the ground at the same time makes it harder to negotiate a turn. The trailer will kind of prefer to carry on forward. The wheels will leave ugly rubber marks (that, if you want to be blunt, remove life span from the tires) and it's just not possible to make the same narrow turn that you would have wanted. So for that reason, you can lift up the extra axles. The axles that remain in the ground permanently are only those that are needed to legally and practically carry the weight of the empty trailer. The machine transport trailers that carries excavator machinery also sometimes have to lift an axle or two to be able to pass rural railroad crossings, but that is mainly because the trailer is very low and the axles are on the front and back end of the trailer. But that is probably beyond the scope of your question. Load the trailer, and you'll have to drop an axle. Load more and you'll have to drop another. And so on. But lift them again when you don't need them, and it will be a bit easier to drive the damn thing. It might be worth noting that even if you have hydraulic steering on one of these extra axles, and thus get extra maneuvering help in a tight spot, you would still probably typically want to lift the extra axle when you are without load to reduce wear on the tires and gain fuel economy when driving forward.",
"Generally, these trucks are empty. When they are full and heavier, the trailers sit lower and the floating wheels touch the ground, providing more support for the truck."
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76ez6r | How do spiders get their strands of silk sometimes 20 feet connecting high structures? Do they really walk one end that far across the ground? | I see the top of a tree to the corner of a house, or the top of one tree to another 30 feet away. It looks too far for a little spider to pull a web, but it's there. | Engineering | explainlikeimfive | {
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"If it's a small spider, it can float across using the thermals to stay up. I'm not so sure about a larger spider, but the smaller ones will make a loop of web and use it as a parachute to get enough drag to be lifted by the small currents caused by sunlight",
"Yes, they do. There's some species of spiders that have been know to accidentally suspend rocks from their web by attaching to them and building their web. They have different types of silk they can spin, some sticky, some not, to build their web.",
"Usually they stand on the upwind side of the gap and let the wind carry their silk to the opposite side, where it sticks to something. Then they climb along it to make the rest of the web. URL_0"
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76gzkf | How big ships made out of thousands of tons of metal can float? | Engineering | explainlikeimfive | {
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"Because the area inside the water that the boat takes up - including the volume of air *inside* the boat - weighs less than a volume of water of that same size. The air that's inside the boat weighs less than water, so it counters the metal parts of the boat that weighs more than the same volume of water. If you just crumpled a boat down into a ball of metal, that wouldn't float, because then it would be denser than water on average and thus would sink. The boat *plus the air* isn't denser than water on average, which is why it floats. This is why when a boat fills up with water, it usually sinks - because then the metal frame of the boat plus the water inside of it overall is denser than an amount of water equal to the size of the boat, since the metal parts are denser than water while the water inside the boat is just as dense as water (obviously).",
"This is due to displacement and bouyancy. Something will float if it weighs less than an equivalent amount of water - so a block of wood or polystyrene weighs less than a block of water of the same size and it floats, while a block of steel or concrete weighs far more than the same size block of water and it sinks. With vista we figured out that the material does not need to be solid - a hollow cube of steel weighs far less than a solid one, and boats work on this principle. If you add up the weight of the hull of a boat and all of the air inside it, this will weigh less in total than the same volume of water, because the really light air offsets the heavier steel when you average then out. It does sound crazy that this works when you talk about supertankers and cargo ships weighing hundreds of thousands of tonnes, but because they are so huge and displace so much water, the weight of the equivalent volume of water is even more and they still float."
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76jkqm | How exactly do arm prosthetics work? | If a person has an amputated arm and only have a stub, how exactly do they move a prosthetic hand to turn and grab things? | Engineering | explainlikeimfive | {
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"Not sure of the science behind it, but you manipulate the prosthetic through tensing the remaining muscles in the arm. The prosthetic senses these electric impulses or contractions and operates the prosthetic."
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76paye | How does the power grid work? Specifically I understand power plants largely need to run at 100% production-how does that work in practice and where does all the extra power go and does this mean there are brownouts all the time while others are spooling up? | Engineering | explainlikeimfive | {
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"There are many different kinds of power plants with different performance characteristics, and they're used in combination to ensure that the supply of electricity matches the demand as demand changes throughout the day. In one group, you have the \"base load\" power plants. These are typically things like large coal and nuclear plants that produce electricity very cheaply, but need to run at 100% production all the time, and take a long time to turn on or off (on the order of several hours up to several days). These just run at 100% production all the time. Next you have the \"peaking\" power plants. These are typically natural gas plants that produce electricity more expensively, but can be turned on and off quite quickly (sometimes within a few minutes). These are used to handle the times of highest demand, and only run for a few hours a day, if that. Finally, you have \"load following\" power plants. These are plants that can modulate their output to ensure that the right amount of power is available when it's needed. There are a lot of different kinds of plants that can do this, though sometimes they have to have some modifications from the standard design to allow load following operation. These are used to handle the fluctuations in demand outside of peak hours, when they'll be running at full output and the peaking plants will be used to produce the extra supply necessary."
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76wvjw | Why does every university have their own referencing style, why has there not been an effort to standardise referencing? | For example there exists something like 20 different Harvard styles which seems insane, if you are going to copy another style why change it? | Engineering | explainlikeimfive | {
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"Because it's not that important. The important thing is that you actually reference everything. The format you put it in is up to you.",
"I have a BS and MS in psych and only ever had to reference APA style. The first time I was introduced to other styles of referencing was when I started my job in research."
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76xnq6 | - Are speed bumps designed to damage your car if you go over them too fast? | Engineering | explainlikeimfive | {
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"Speed bumps are intended to force drivers to slow their speed. Most are designed in such a way to make it uncomfortable for the driver to take them too fast, but are not intended to damage their vehicle. Others are poorly designed and *will* damage the vehicle, even at a reasonable speed. Even properly designed speed bumps can damage a vehicle if the vehicle is going much too fast, though. The wear and tear you experience going over a speed bump is negligible. You'll experience more wear and tear through simple road driving because you do more road driving than you spend time driving over speed bumps. Thing to remember about speed bumps is this: the speed that it is safest to driver over the speed bump is the speed you are supposed to be driving on the road even where there are no speed bumps. If you are going 45 mph, then slow down to 25 mph to go over the speed bump, accelerate back up to 45, slow down to 25, again to go over the next speed bump, you are doing it wrong. You should be traveling 25 up to the speed bump, over the speed bump and between the speed bump. The city likely placed the speed bump on that road because drivers are travelling that road too fast.",
"Some are, yes. We have the normal speed bumps but there is a whole road nearby that uses ~~his~~ these rubber things bolted down and even if you are going really slowly the jolt they give you is enormous.",
"In portuguese, speed bumps are called \"quebra-molas\". roughly translated, it means \"suspension-breaker\". So yes, I think so.",
"every time you drive your car, yes, the suspension will suffer wear & tear. If you hit bumps like they weren't there, you will just wear out the suspension a bit sooner.",
"If you hit them too fast, the suspension will pack down and you risk smacking the front plastics of the car into the ground. Properly shaped and sized speed bumps shouldn't do any damage as long as you slow down enough. Most speed bumps should be driven over at less than 20mph.",
"No, speed bumps are designed so that if *you* drive over them too fast, *you* will damage your car.",
"Square edge bumps (rough roads, potholes) will wear your suspension out faster than speed bumps because these are felt as impacts that are transmitted to the bushings in your suspension. Speed bumps are smooth, they are soaked up by the normal travel of suspension and are not jarring impacts. As long as you are not bottoming out your suspension by going over them too fast (felt as a clunk) they will not hurt your car."
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770i61 | Why is the record on voyager plated in gold? | Engineering | explainlikeimfive | {
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"To add, the record is copper, plated in gold, and then plated in U-238. The reason to make it all metal is that vinyl couldn't possibly survive the harsh environment of space without quickly decaying, and the uranium, with a half-life of 4.468 *BILLION years*, is there so optimistically, a civilization who discovers the record can date it's age.",
"For the same reason connectors on computers are better when they are gold: Gold is a noble metal. This means that it is resistant to corrosion and oxidization."
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776q63 | How does “cracking” a safe actually work? The kind where headphones are worn and every turn of the knob is carefully listened to. | Engineering | explainlikeimfive | {
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"When you turn the knob of the safe, you can hear the mechanism \"tick\" when the wheels touch. This enables you to use the dial to measure how far you are turning each wheel. By putting torque on the opening handle, you can measure or hear (depending on the design) the gap in the wheel that allows the safe to open. The technique is specific to the design of the lock mechanism, and it can easily take hours. [This cool animation]( URL_0 ) shows how the mechanism works.",
"[Here's some pictures of a lock I have]( URL_1 ), a S & G 6730-MP. A lock specifically designed to resist the type of attack you're talking about. A safe lock contains a pack of 4+ wheels, each with a slot cut in them. The last wheel (topmost silver one, called a cam) has more of a sloped cut in it called a cam gate. These are mounted on bearings that fit over the rod the dial connects to. There's a piece of metal in there--a bar with a hooked end. This is the lever and the bar is called a fence. This is connected to the latch that locks the safe bolts in place. The safe bolts are the giant locking bars that stick out of the edges of the door. If you dial the correct combination, slots in the wheels align. Then you rotate the dial until the cam gate engages the hooked end of the lever. It can only do this completely if the bar can fall all the way through the slots cut in the wheels. Further rotation of the dial will then pull the lever and retract the latch, freeing the bolt. The cam is directly connected to the dial and the cam gate is at a known position, usually near zero. So the procedure to open a safe will be to dial your combination thus aligning the regular wheels. Then you turn the dial past zero, the location of the cam gate. If the combination is correct, the lever engages in the cam gate and pulls open the latch. Lock manipulation consists of dialling a combination, then rotating the dial back to where the cam gate is. Because the wheelpack, bearing, and fence are not perfectly machined, there will always be one wheel that sticks out more than the others. If the wheel that sticks out has a slot under the fence, the lever will dip and partially engage the cam. Then when you rotate the dial back and forth, the hooked end of the lever will tap the edges of the cam gate making a click on either edge. These are called contact points. Usually you go by feel instead of sound, but sound works as well. Since the cam gate is sloped, as you align more slots under the fence, corresponding to correct combinations, the lever will fall deeper into the cam gate. This causes the pairs (from tapping the left and right edge of the cam gate) of contact points to get closer together. This is a very precise process and so a lot of safe manipulators will attach a precision dial indicator with a magnifying glass so they can read off tenths of numbers. Slop in the mechanism is a big issue as is dirt. A safe manipulator will go through all the dial numbers in one direction and [graph the contact points]( URL_0 ) to find one of the slots. There's a procedure for finding the other slots, then figuring out which order they go in when you dial the final combination. As you can see in the photos, my lock has a cover which slides over the cam gate. You have to turn another knob in the center of the dial to open the cover. If you open the cover, you can't move the dial. There are also serrations on the wheels which screw up the contact points. This makes manipulation much more difficult. Edit: So many edits. Sorry.",
"Turning the dial makes a klicking sound. With an old and/or worn lock, the \"correct\" position might produce a slightly different click. Source: Successfully cracked an uncles safe as a kid - he told me if I actually got it open, I could keep whats inside. I made him pay for his cigars! But I have to admit, the mechanism was so worn, I could nearly hear the difference without the toy stethoscope."
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778cbk | how do floating bridges deal with tide fluctuations in the saltwater, or water level drops in lakes? | I drive across the hood canal bridge often - URL_0 - With tidal swings of up to 16.5 feet, how does the bridge deal with the rise and fall of the tide? Are there mechanisms that extend the bridge? | Engineering | explainlikeimfive | {
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"Floating bridges can be thought of as upside down suspension bridges. The hollow or foam filled concrete sections want to float quite high in the water. Cables attached to anchors pull the sections down below where buoyancy alone would want them to sit. Adding a heavy load of cars doesn't drop the level of the bridge, it just reduces the tension in the wires holding the bridge down. Similarly, if the water level drops, the level of the bridge doesn't go down, the tension in the wires is reduced. tldr: The bridge wants to float higher than you ever see it, even when the water level is low."
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778jm4 | Why can't current be defined as co-directional with the flow of electrons? | Engineering | explainlikeimfive | {
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"text": [
"Because current is the flow of charge, not electrons. The particles that allow for the flow of charge are called \"charge carriers\" and electrons happen to have a negative charge, so current flows in the opposite direction of electrons. Electrons aren't the only charge carriers. In semiconductors for example, you have both free electrons and places for electrons to go (called \"holes\") which can act as charge carriers. In your body, positively charged potassium ions act as charge carriers. Essentially electric current is defined without caring about what carries the charge, just the direction the charge moves."
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779iii | Why do car batteries die from not being used? | Engineering | explainlikeimfive | {
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"text": [
"Everyone is answering the wrong question hehe. A battery that sits is still being consumed. All kinds of batteries lose power as it sits. This is called self discharge. Car batteries are made from a lead grid submerged in a sulfuric acid bath. Once the charge gets below 9v or so, the battery is no longer able to be charged even though the components are still physically there. Another cause of failure is the lead plate is connected to the outside terminals by a small arm. If this arm breaks due to corrosion, it doesn't matter how much charge is left since the path of current is broken.",
"Vehicles have a number of electrical systems that are always on or otherwise requiring power (good example is the clock). These slowly draw power from the battery and reduce the charge in the battery. If left too long, the battery won't have the amps to power the starter motor."
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77b1tn | What is the difference between CDMA and TDMA/FDMA? What are the advantages and disadvantages of CDMA? | Engineering | explainlikeimfive | {
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"TDMA - time domain multiple access. Each unit gets a time slot in which to transmit. So if you’ve got 4 transmitters, you tell unit one to transmit for 15 seconds at the the exact :00 seconds mark, unit two transmits for 15 seconds starting at :15, etc. then if you add another unit, then each one can only transmit for 12.5 seconds on their scheduled interval. Advantage: share the same piece of spectrum. Disadvantages: requires tight time synchronization between all units and can be highly latent with lots of transmitters sharing the space FDMA - you have a big chunk of radio spectrum and then you assign each unit their own smaller piece out of your big chunk. Advantage: simple, no synchronization required. Disadvantage: unless you can dynamically control all of the units and resize their signals, you waste unused spectrum if a unit is off the air CDMA - uses a random code combined with the digital signal to spread the transmitted signal over a wide bandwidth while being able to use a energy efficient signal. Main disadvantage: very easily jammable with a simple high power transmitter."
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77g9ib | What is this sideways slot in this outlet? | Engineering | explainlikeimfive | {
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"text": [
"It means the outlet is rated for 20 amps instead of the normal 15 and can accept plugs of [this shape]( URL_0 ) for appliances that need the higher amperage.",
"it is a 20amp recepticle. Its pretty rare to find a 20amp appliance since few outlets can safely support it (or need it). you might see it on some power tools or portable AC units. Maybe a microwave... for the most part, they try to keep things under 15amps so they dont exclude many buyers (and confuse even more). 20amp plug for reference URL_0"
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77h9b6 | How do arcade tokens work and can generic ones be used at any arcade? | Go to an arcade, buy the tokens, and magically they only work at that arcade. Ok, I know it’s not magic, but how do the machines know what tokens are theirs? Also those generic eagle tokens that a lot of arcades use, can they be used at any arcade that uses them? This all assuming you can even find an arcade that still uses generic tokens, or tokens at all, these days. Same question with slot tokens, but I assume the answer will be the same. | Engineering | explainlikeimfive | {
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"Some machines have the ability to weigh an inserted token (this is why arcade tokens are so heavy) to make sure it is viable. Also, others judge the tokens by their size (this is why some arcade tokens are huge). Some machines implement both of these systems to ensure maximum security. I saw a youtube video on this somewhere but I can't seem to find it right now"
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77mwtv | Cell phones conversations are crystal clear, but high-stakes air traffic control conversations are done on crackly-ass radios | Why can't ATC communicate with cellphone-level clarity with airplanes? Shit's important. | Engineering | explainlikeimfive | {
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"Cell phone radios transmit over small distances, aircraft radios transmit over long distances. Cell phones are digital, meaning that a damaged packet gives no sound, Aircraft radios are analog, so that even if a lightning bolt interferes with the signal, the pilot can hear something. Super-clear, sometimes not working, radio isn't something the aviation community would be interested in spending a lot of money for a new radio system in order to get.",
"ATC needs to communicate to *all* planes listening on a frequency. Cellphones are clear in part because there is a direct connection between the two phones that's being relayed by cell towers and then through land lines; the over-the-air portion is as short as possible and the rest is grounded, whereas ATC is necessarily entirely over the air, and thus more prone to random noise interference. EDIT: Clarity near the end; I had like 3 ideas of how to format the last bit, and I used the wrong bits of all 3."
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77nkwm | Why do you not feel like you're going fast while in an airplane, when in fact you're traveling 600+ MPH? | Engineering | explainlikeimfive | {
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"There aren't any forces acting on your body that would make you feel that way, except for when you are accelerating to/from that speed. Every part of the airplane -- your seat, the floor, the air in the cabin -- is moving at the same speed as you are, so there is no way for your body to tell.",
"The Earth is hurtling through the air at many times that speed. We feel acceleration and deceleration, a change in speed, and you definitely feel the acceleration for the first 20 minutes of the flight or so as you take off and ascend. That's why you stay in your seat! Your seat is attached to the plane so it's *pushing* you up to the speed. They get to that speed gradually so it's not too uncomfortable. But once you're at cruising speed and altitude, everything in the plane is moving at the same speed as the plane, so it's like your not moving at all. Speed is relative.",
"Also parallax; all the items moving relative to you are either so foreign (clouds) or so far away (ground), it's difficult to judge how fast you're moving relative to them.",
"The human body is not great at sensing speed in general, only acceleration. We have a bunch of senses that all work together to let us know when we're changing speed or direction, but we have to rely on context clues to get a sense of constant speed. The next time you're in a car (as a passenger) close your eyes while the driver is going a constant speed on a straight road. You'll notice that it won't feel different from sitting in a parked car. Your sense of speed in a car is largely dependent on seeing objects in the distance and seeing how you move relative to those objects. When you're in a plane, you don't get that kind of feedback. Even with a window seat, objects are so far away that we don't have a good concept of how fast we're traveling. When you're landing and taking off, you start to gain a sense of speed.",
"Because you don't feel velocity. You could be going a million miles an hour and you wouldn't know it. What you feel is acceleration, a change in velocity. On an ordinary plane flight, this occurs at the beginning (during takeoff) and at the end (during landing). You also feel it when the plane changes direction, which also requires a chance in velocity.",
"You and the air inside the craft all accelerate with the aircraft. As long as the aircraft maintains a relatively steady speed, there are no forces acting on you that would indicate you are travelling fast. Then looking out the window at altitude, objects on the ground are so distant that you don't have a good reference to know how fast you are travelling. The best time to see how fast you are travelling as at takeoff and landing when you have a better reference as to your speed with objects on the ground."
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77q4jf | if silk is as stretchy as elastic and stronger than steel, why isn't it used more often for... everything? | Engineering | explainlikeimfive | {
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"People have been working on this for probably around 20 years! The problem is, spiders can't be farmed like silkworms. They tried to solve this by taking the gene for making spider silk and putting it in another organism. Now we have yeast and bacteria that make spider silk, and goats whose milk contains spider silk. The problem with this is that it's not in silk form! It's more like a jello, instead of being usable fibers.",
"Spiders typically produces several kinds of silk of varying properties and compositions. The strongest type is the dragline silk they use to dangle from. This typically has an *Ultimate Tensile Strength\" that is about half that of grades of steel used for wire ropes. (Note that the strength of steel can vary significantly based on the composition and the processes used to strengthen it.) The difference is that the density of steel is about 7.8 gram/ cubic cm while silk is about 1.2. So, overall, silk has a much better *Strength-to-Weight ratio* than steel. However, currently the material with the best strength to weight ratio is carbon fiber. CF is common for applications that require extreme strength, vibration resistance, and minimum weight such as aircraft, drones, car body parts, The reason that synthetic silk hasn't been a commercial success, is complex. Apparently, the properties of spider dragline silk involves some sophisticated chemical processing tricks, performed by cells in the spider's silk gland. This yields a fairly complex, well ordered molecular structure. Moreover, the individual silk fibers themselves have a sophisticated multilayer structure. The processes spiders use to produce silk aren't well understood. Laboratory made silk tends to have disappointing properties, and a molecular structure that is rather disordered compared to spider silk. Silk is biodegradable. In fact some spiders eat their old webs. But this is a problem in many applications where you'd want to use something like silk. For example, you don't want your clothes to start decomposing in the wash. You don't want to go climbing and discover that your ropes are moldy and smell terrible. Now, considering that there are synthetic fibers already on the market that have equal or superior properties such as aramid or high modulus polyethylene, and require much more straightforward and simple processing methods, silk is a bit of a non-starter. Unless it can be made more cheaply and the demand for biodegradable fibers grows."
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77tj48 | what actually happens when you set a microwave oven to different power levels? | Engineering | explainlikeimfive | {
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"text": [
"Microwave ovens only have one power setting. On...and off. So why do they give you a number of different power settings and cooking modes on it's control panel? Well what actually happens is these modes do not actually alter how much power the microwave emits. Instead it changes how frequently it applies this power over a set amount of time. A microwave works by using radio waves to agitate water molecules in food. Changing the amount of power these radio waves are emitted at would do nothing to cook the food faster or slower as the molecules would get agitated in the exact same way. So instead of altering the power the radio waves are emitted at, we alter the amount of time the food is exposed to this \"radiation\" in an on and off cycle. Cooking a food item on lower power mode for 30 seconds could make the microwave apply the radiation to excite the water molecules in the food for 1 second followed by gaps of 3 seconds where no radiation is applied. So the food was actually exposed to cooking for 8 seconds. Cooking the same item on high power mode might expose the food to cooking for the entire 30 seconds. As you can see, cooking the food on low power mode for 30 seconds is the exact same thing as cooking the food on high power mode for just 8 seconds. So why do we not just do this? Because it's about uniform cooking. Or making sure all parts of the food is cooked. Unfortunately microwaves do not emit the radio waves in a uniform pattern. So to insure we do not get uncooked spots of food, the microwave moves the food around using it's rotating plate to make sure the radio waves travel through all parts of the food. By alternating the power, the microwave ensures all parts of the food gets heated without outright overcooking the food. It still gets it wrong sometimes, as you might have experienced when you sometimes eat some microwaved food and find an overcooked or undercooked spot (This is common with microwavable pizza). TL;DR: Microwaves only have one power setting. The power modes on its controls merely change how frequently it alternates the on/off cycle of applying the cooking process to the food in order to ensure it is uniformly heated without ruining it.",
"To add to the excellent answers here: listen to your microwave next time you use it on 50% power. You'll hear fans running all the time, but there will be a deep humming noise that turns on and off periodically: that's the microwave generator."
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77xrwy | Why are motorcycles so loud compared to cars? | Engineering | explainlikeimfive | {
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"1, Not all of them are. 2, Some motorcyclists prefer a loud sound because 2a, they think it's cool 2b, the vibration feels powerful 2c, they believe that \"loud pipes save lives\" (that is, that when other motorists *hear* a loud motorcycle, they're not likely to accidentally hit it) 3, different muffling systems.",
"motorcycles are loud for 2 reasons. One; the engine is right there. it isn't shielded or dampened by layers of metal, so you hear any noises it's making. Secondly, the exhaust pipes. They're usually stock made for a certain sound, but can be modded to be either louder or quieter. I am of the belief that having a bit of rumble is necessary for a good biking experience; and can certainly make you more noticeable in some situations. Though i always reccommend getting a loud aftermarket horn. the meep meep horns dont do jack.",
"There's nothing inherently loud about motorcycle engines, I have a 1200cc engine motorcycle with a stock muffler and it's not loud. People put inefficient noise reduction mufflers on motorcycles or cars for two reasons, because they think it reduces the flow of air through the mufflers thereby increasing engine performance, and because they think it sounds good.",
"They don't *have* to be, some motorcycles are really quiet. An example of a very quiet motorcycle is the Honda Goldwing, some police forces used Honda Goldwings as [police bikes]( URL_0 ) and they stopped because pedestrians kept stepping right in front of them because they don't see or hear them coming.",
"Apart from necessarily smaller/shorter exhaust systems and air boxes a lot of the reason is the higher revs bike engines run at. I use a bike engine in a car so it has a decent length exhaust run and a big silencer on it but it's still very loud because the usable rev range is so high. I've actually been trying to get it quieter to meet race track noise limits and it's practically impossible to get it as quiet as a normal car. Induction noise is as big a problem as exhaust noise too by the way...they're sucking in a lot of air at 13,000 rpm."
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77y8tw | How do recycling plants process liquids? | Like if a water bottle still has some water in it, or a soda can still has some soda in it. | Engineering | explainlikeimfive | {
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"text": [
"Yay! Something I can finally answer! I work as a chemist at a waste disposal facility. At our plant we sort liquids into various catergories depending on what we think is in the containers. These containers are then shipped off to a machine which essentially crushes all the liquid out of the containers making cubes of plastic or metal. The liquid itself is gathered in large 1000L drums and shipped off to another facility to be chemically treated to be safe for release into the environment. If this can't be achieved the liquid gets incinerated in small batches. Hope this helped!"
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77ygsl | Do you have to heat LNG to decompressing it so it can be transported to individual homes? | Engineering | explainlikeimfive | {
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"text": [
"No, you can expand it, which makes it very cold, and pump it into pipes in the ground. The ground will slowly warm it to a useful temperature.",
"You do have to add heat to it, but when something is at -160°C, getting heat into it isn't the problem. In addition, the amount of heat energy required, the \"latent heat of vaporization\" isn't that high (510 kJ/kg), especially not compared with water (2257 kJ/kg)."
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77zuh8 | Why would a person downshift and use a lower gear to accelerate (or obtain more power)? | Engineering | explainlikeimfive | {
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"text": [
"It's all about at what RPM (revolutions per minute) your engine produces the most power. Let's assume that when you're driving along in 4th at 80 kph your engine is turning over at 2,000 RPM and is producing 80 HP. However, maximum power is produced at 5,000 RPM when your engine produces 200 HP. Now, you need to pass and accelerate quickly from 80 kph to 130 kph. If you simply, 'put your foot to the floor,' you have only 80 HP to call on immediately that will somewhat slowly rise as your RPM slowly increases. However, if you gear down to 3rd or even 2nd as necessary so that your RPM immediately rises to 5,000 RPM and peak HP, you'll have immediate access to 200 HP and your acceleration will be much quicker. Downshifting on a manual is exactly what an automatic transmission is doing when it shifts into 'passing gear' when you 'floor it.' It's just downshifting.",
"Lower gears produce more rotational force (torque) because a bigger gear is spinning the output. It's all about leverage. Here's a great video that explains it: URL_0",
"This is for motorcycles, but will work for most vehicles: Depending on the make and model of the car, there is something called a \"power band\" in the rpm. RPM is revolutions per minute, which refers to the revolutions of the engine. At the upper part of the rpm's there's a power band, several degrees of rpm that provide the more torque per revolution. When you downshift into a lower gear (with the intent to accelerate) you throw the RPM (the engine) into that power band, resulting in faster acceleration. There's an old saying: \"drop a gear and disappear\". When you drop that gear and accelerate, you'll blast past anyone at speed."
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789ehz | Why does concrete from 40/50 years ago seem better than the concrete of today? | Engineering | explainlikeimfive | {
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"Concrete is pretty much rock paste (mortar or cement) and rocks (aggregate). Having more and bigger rocks (to an extent) might result in a stronger concrete, but it also makes it more difficult to mix and pour the concrete, and it won't be as consistent. So, it is possible that the older concrete is stronger but the size of the rocks in it makes it too difficult to pour from a modern cement truck. It is also very possible that you are experiencing survivor bias, meaning that the only old sidewalks you see were the very best, all the rest were torn up and replaced with newer concrete more recently.",
"Concrete will never fully cure; it continually hardens forever, although for all practical purposes, it reaches a point where further hardening will be so slow as to be unnoticeable. So, maybe the old concrete you've seen was left alone long enough, and newer stuff was not.",
"That \"rough rocky look\" is actually just the aggregate having come to the surface because the old cement and sand part has washed or worn away. The smooth pasty look would have been closer to what the \"old\" concrete looked like when it was freshly poured. It isn't stronger - not by any means - just go check out some very old barns and see how thier very old foundations are doing. If anything today's concrete is significantly stronger due to exact measurements, extra chemicals (depending on specific use) and superior finishing techniques.",
"You've gotten some good answers here so far, but add to it that you've likely only seen concrete from 40-50 years ago that is good. The rest has deteriorated. As with anything that needs to be mixed, it's not just the quality of the mix but the accuracy that can make a difference in longevity. Add to that differences in use and soil stability under the concrete and you'll see some big differences no matter when it was made. As a commercial real estate appraiser I've seen tons of 80-90 year old buildings with solid concrete, and I've seen 40-50 year old buildings with concrete that has crumbled nearly to dust. You're just more likely to see the concrete that has lasted well, the weaker stuff is already gone.",
"Every single concrete mix is a little different and some are a lot different, depending on the specifics of the material. Various concretes have different PSI (pounds per square inch) strengths; you can buy at Home Depot concrete mixes at 4,000, 5,000, and 6,000 psi - and the lower psi mix is cheaper. Commercial concrete can easily be at 10,000 psi or more. So if someone did some cheap sidewalk work with a lower final psi, and the columns or floors in your building are a high psi, that could be the difference. The builder can change the mix of cement and rock to get different strengths, and also to change how fast it will reach those strengths. Also, concrete gets harder as it ages. Usually it gets pretty tough by 28 days - maybe 90% of its maximum hardness. And then takes much longer to get from 90% to 95% maximum hardness -but it does keep getting harder. Also, you also only see old concrete that is good and has lasted. The cheap concrete from 1965 has mostly been replaced by 2017. That's a selection bias - if you only look at 200 year old houses, you conclude \"all 200 year old houses are pretty well built.\" But that's because only the exceptionally well built houses or the ones with worthwhile features lasted 200 years. All the crappy ones were torn down a long time ago, or so extensively remodeled over the years that no original portions are left."
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78icv0 | How does a rigid airship go up and down withput releasing helium? | Engineering | explainlikeimfive | {
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"Many airships have [ballonets]( URL_0 ): the main rigid structure holds lifting gas (hydrogen or helium), but there are little balloons inside the main balloon which hold air. By pumping air into or out of the ballonets, you can change the volume of the lifting gas without affecting the shape of the outside of the airship: this changes the buoyancy. URL_1",
"The other answers seem like they are talking to an engineering student. Okay, so when you fill a helium balloon, youre filling it with metal tanks that just sit on the ground. Those metal tanks have pressurized helium in them, which make them heavy enough not to lift off the ground (not that they would, but for the sake of argument). So if you put a bunch of these tanks inside a rigid airship, you can fill up rubber balloons inside the shell of the airship to create the lift. Not only are you decompressing the air in the tanks, making the tanks lighter, but now youre replacing the breathable air in the chamber with lifting balloons. The rigid airship is just a protective shell for the balloons, and a surface to store the tanks. TLDR: The important fact of helium based vehicles is being able to compress the helium to make it heavy, so you can descend without wasting helium. Inflatable balloons are within the airship and can be filled, which increases their volume and forces breatheable air out of the ship. This is why you see catwalks within airships, which can be used by people without air-masks."
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78jb3w | What exactly makes CANDU reactors more safe than other nuclear reactors? | Engineering | explainlikeimfive | {
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"Candu reactors aren't necessarily safer. They are different. In terms of probabilistic risk assessment, the core damage frequency for candu plants better than PWR plants and on par with generation 3 BWR plants. In terms of the reactor itself, it can still melt in a loss of decay heat removal event like Fukushima. It also has a runaway condition during loss of coolant accidents that can damage fuel, but still maintain coolable core configurations and keep release rates within accident limits. It's not more safe. It's different safe."
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78o2sa | How are flood plain maps drawn up? Is there anything that goes into them other than scientific data? | Looking into buying a home and studying flood plain maps, it seems that the streets are in the 100 year plain, while the homesite is in the 500 year plain, for blocks and blocks... This creates a funny pattern that just doesn't seem right. Last time I checked, water doesn't discriminate on where it flows. | Engineering | explainlikeimfive | {
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"A lot of that is elevation, and they are counting inches. Homes are usually built a couple inches above the road (maybe a foot?), this gives you a lawn that slopes away from the home, and the road itself slopes to the ditch/drains on the side. It reduces flooding in the road and home (prevents heavy rain from creating streams into your home). Those flood maps are determining if your lawn is going to flood. If you had heavy rains making a flood, it would be normal to see the ditch in the road fill up first, and then slowly spread over the road, and then it would come up your lawn closer and closer to your door. Those lines are how likely it is to get that far."
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78ppdo | How does a database handle 1 billion users? | I'm really interested in how companies store such HUGE amounts of data but somehow retrieve and display the information in < 1s. I was looking at my Facebook activity for 2010 and it has the thousands upon thousands of posts I liked. You can't possibly store this in some sort of pivot table database, could you? Facebook would have literally 800 billion rows. And my actual question is, how does a company, like Facebook, store more than 1 billion credentials? Is it split into regions like Users_GB, Users_US? I work as a web developer. But as I work for a small company, I just can't fathom data this big. Any insider knowledge, anecdotes and discussion from anyone who works on large databases would be appreciated. Edit: Went to bed, woke up and the blew up. Thanks for all the great discussion, and thanks for the gold. | Engineering | explainlikeimfive | {
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"The good news is that data isn't stored in one giant table. It is actually made up of multiple tables. Tools exist that then link those tables and read the information in them fairly effeciently. Oracle, MySQL, etc handle massive amounts of data pretty easily. Because these tools don't actually care what the data is, they just need the reference points, they can quickly retrieve the data. Think of how RAM works: fast switching pointers. This is how you get a user's data out of billions in a table. The software doesn't need to know that the person's first name is JOHN, it just needs to know that that user's pointer is here and it points to that table and then that table points to the next connected bit of data, etc. It is a chain of tables. Again, most of the retreival of the data isn't getting ALL of the data, it is just getting the pointer. That pointer leads to the next pointer, and so forth. Just fast swapping pointers and then one DISPLAY of the data. Big Data as it is termed, is a huge industry specifically for this challenge. It isn't perfect, but an entire industry exists simply to find better ways to store data so that that pointer chain can be processed faster and faster.",
"The simplest answer to your question is that the billions of users (and trillions of other pieces of data associated with those users) are stored on a database split across thousands of machines. Every piece of information has an ID associated with it, and each ID maps to a particular portion of the database hosted by a particular set of machines responsible for that portion of the database. Software abstractions allow you to fetch the data associated with various IDs efficiently without worrying about the details of how the data are stored across the machines. If you want more details on Facebook in particular, a good starting point is to look into things they've written about TAO, a system they built on top of MySQL and memcache to manage their distributed data store.",
"Background: I work with data for a living. You mentioned the word \"big\", and that's really appropriate here because some data can't be stored on a single logical volume (disk drive). Think of a standard database like a file system on your hard drive. Each element of the database has an index, and you can query the disk via index to retrieve records, kind of like accessing a file on your computer. The database engine keeps a record of where each bit of data is stored, so it can fetch them on request. What happens when you have so many records that indexing them becomes impossible? For example, you might have far more data than can fit on a single drive. Or you might have so many dimensions to your data that indexing it properly becomes too computationally expensive. This is where we transition into \"big data\". Big Data refers to database storage systems that help manage very large or very dimensional data structures. This happens by storing the data itself across multiple drives / volumes, and creating aggregated indexes or \"nodes\" to slice the data for retrieval. The actual mechanics of this can be pretty complicated, but there are drop-in solutions like Hadoop to streamline the process. Depending on how quickly you want the data retrieved, how deep the data is, how many simultaneous calls you expect, and a host of other factors, you'll set the number of nodes accordingly to balance cost (each new aggregation requires disk space and horsepower) and function (more nodes = faster retrieval). You might be surprised by what data actually qualifies as \"big\" data. With proper indexing and partitioning, a SQL DB can handle millions upon millions of records without flinching. On the other hand, a poorly optimized DB can result in slow read times and clunky operation with just a few million transactions. Data is neat!",
"Big Data Engineer here. The ELI5 answer is that your data is replicated to multiple machines in easier to handle chunks. Say you have a table with 100 rows. What Facebook does is splits these tables into 5 machines each holding 60 rows. \"But wait, that's 300 rows. Why did my data increase?\" Well because if we had a unit of data in just one place, then you create a bottleneck. If everyone was viewing Elon Musk's FB page, then the one machine with Elon's page wouldn't be able to handle traffic for the other 19 people. By duplicating the data, Elon's FB page can be stored on three different machines. Finally, you have one more machine that routes incoming requests for data. \"Looking for Elon Musk's page? Go to machine A. TheNak1123's page? Go to Machine B. Elon again? Go to Machine C.\" The key point to remember is that storage is cheap. A 1 TB hard disk costs a consumer like you or me 50 bucks. The key resource for these companies is time. By throwing money to buy more computers and splitting up the data, Facebook can efficiently organize and use large quantities of data on the scale of Exabytes (Millions of Terabytes).",
"My area of interest also. They have new types of databases that do not store data in row format(NoSQL) this removes the performance constraints on storing such data in a relational database. They also do not use one database but what you get is api's pulling data from many sources e.g there could be a Ugandan facebook user db only. Facebook is the creator of new frameworks such as presto which enable speedy access to data.",
"Recognize that 1 billion users is really just a Google and Facebook problem and maybe a handful of others. The basic pieces of relational databases, if used properly, ensure that most of the time you don't actually need to store whole records. You store the bits that are unique and then just keep links to the bits. Then you make sure that you never ask questions that require you to scan the entire database. Think about a library. You don't have to look at every single book to find something specific. You look up the book in the catalogue, get the books call number and then go right to the specific shelf you need. Databases work this way too. The library catalogue is a database index. For those rare cases when you do want to browse in a library, you don't pick random shelves, you arrange the shelves in sections that have books of the same type. So you can browse, but not the whole library. For databases this is partitioning or sharding. Lastly, modern databases can do some special things with indexes when you're not looking for individual records but you want to count things. Most analytics queries aren't actually for individual records but for aggregates. You can define views and indices on particular types of aggregates so that asking those kinds of questions is more like going to one specific shelf in a library instead of scanning all the books one by one. You need good database design, good database admin, and in very rare circumstances brand new tech.",
"Layers upon layers of caching, prefetching and denormalized views. Also data centres near the physical location of the user combined with really clever precompiled queries. Basically magic at that point. It's vastly interesting but also incredibly complicated.",
"There are a whole lot of techniques, but to keep it ELI5 -- it's *conceptually* one big database, but under the covers it's broken up into a bunch of pieces. Let's say it's 1000 pieces for a given table; those 1000 pieces are actually on 1000 different servers. Some kinds of systems query this by having a map of which categories are stored on which servers. For example, your username begins with \"abc\", users with \"abc\" are on server database-a. Then you can query database-a for the actual result. This only works with some kinds of data. Some kinds of systems instead send the query to _all 1000 servers_. Since each server only has a small set of data to look through, they can answer relatively quickly, and then the querying software can further comb through the returned results, which leads to getting useful results very quickly. A very common way to do this is [MapReduce]( URL_0 ).",
"Hey! I can answer this question. So large data sets, like you see at Facebook or Netflix, are typically stored in a *horizontally scaling* database like Cassandra. Horizontal scaling means that to increase the performance of the database you just add more computers (which I'll call machines from here on out). How does this work? Well, Cassandra is a distributed database system. This means the entire data set is contained on a bunch of different machines, not just one. This is called a cluster. All the machines in the cluster talk to each other to make sure they're all in sync. And when somebody requests data, they can do so from any one of the machines in the cluster. Every time data is saved to the database, we specify what's called a *primary key*. This tells Cassandra exactly which one of the many machines to save the data to. This makes searching faster because we not only break up the large sets of data, but we know exactly where that data sits in the cluster. Think of a primary key as your zip code. We also specify a *clustering key*, which is used to tell Cassandra how to sort all the data on an individual machine. Think of the clustering key as your home address. With the primary key and the clustering key, Cassandra can very quickly find the data you're looking for. To make searching faster, all you have to do is add another machine to the cluster. This decreases the amount of work each machine has to do individually. This is the key concept behind horizontal scaling. Apple, for instance, has about 1000 machines in their cluster. Hope I didn't butcher my explanation. Source: am a software developer who has worked with big data",
"Nobody is mentioning the most basic concept that makes all of this possible while seeming impossible: humans think linearly, but computers access data logarithmicly. When data is properly indexed, finding 1 thing in a billion is more like finding 1 thing in 30 (logarithm base 2 of 1 billion).",
"The truth is: it doesn’t. At scales like you’re talking data is replicated amongst many independent databases which might not even be traditional RDBMS, they could even be memory based no-sql type databases. systems like these also have multiple layers of caching. It is a complex problem requiring teams of developers to solve. Ultimately the system gives a user the illusion that the entire world is connected to a single shared database but that’s not the truth.",
"Checkout this for an overview of high scalability systems with some details: URL_1 Basically what all other said: partitioning (ala divide algorithms). Ensuring that data is consistent between partitions, geographical replicas - is a whole other beast: do you prefer to do it quickly but risk having some inconsistencies (weak/eventual consistency) OR do you prefer slowing down but ensuring everything is consistent between clusters. Caching is another strategy. The fact that you went to your 2010 records probably made some literal hard drives spinning somewhere. But if you are browsing your wall, that was served from fast RAM in a memcache or similar process. In some fancy pants edge rare cases data may go into painstakingly slow tape drives with robotic arms and that could take seconds, even minutes. Check out Amazon Glacier for example and YouTube videos of robotic arms taking tape drives. To see a real architecture all the way down to the hw level, check out StackOverflows posts on their architecture: URL_0 Sources: studying for SRE interview for the past couple of months :)",
"The same way that a filing cabinet would hold thousands upon thousands of files for, say, medical patients: Have separate folders for last name letters, so that instead of searching through every single folder, you just take the patient's last name's first letter and look through that folder first.",
"Media Storage ^ CNS -- > Content Delivery Provider -- > Load Balancer -- > Web Cluster Servers & DB cluster Make sure the load balancer has a high ability to take on concurrent users Media storage should be a Data Lake of sorts for large data deposits Web/app and DB servers need to have the ability to auto scale Redundancy is important by having multiple availability zones and regions in the chance that individual servers fail It really just relies on planning. Massive quantities of data can be processed and used as long as the architecture is made correctly. Edit: the media storage section should be above the content delivery provider",
"Lot of good answers here. Lots of detail and specifics, but none Is consider ELI5 - so I'm going to take a shot. There are lots of kinds of databases, so the exact solution varies a lot whether you use what's commonly referred to as a relational database (Oracle, PostgreSQL, MySQL, MS SQL Server), to document databases (MongoDB, CouchDB), graph databases (Neo4J, Atlas), to triple stores (Redland, AllegroGraph, DB2, Jena). But regardless of the kind of database, and the amount of data stored - the trick to storing vast amounts of data is having an efficient way to organize the data in a way you plan to access it. Say you have huge quantities of LEGOS. They come in lots of shapes, sizes, and colors. In order to effectively build something quickly - you need to organize the blocks in a way that makes it easy for you to find. Say you want to build a pile of parts in single color? Well you might just create bins of legos only organized by color since you only care about the color of the part. But then let's say you want to build a tower that uses only 2x6 bricks. You then might organize by shape instead. However what if you needed to build an entire structure that used a specific \"recipe\" calling for specific size, color, shape and quantity of bricks? You might then have a bin for each style of brick, sorting bricks that are of the same shape, color, and size in the same pile. Then organize the piles in a way that you could quickly locate any specific part in quantity very fast. This might be keeping all the same colored bricks close together and organizing those piles from small to large shapes. Big Data is handled no differently. One creates an index into the data in a manner that is equivalent to the way you will search. So regardless of the type of DB system used, if you are searching using color, size, and shape, you create an index of only those things color, size, and quantity with a reference to where the entire piece of data resides.",
"High traffic sites are not a simple client-server-database architecture, you have many more components that help deal with the massive amounts of requests that are sent their way. The most useful tools for scaling are ( listed alphabetically ). 1. Caching - avoid hitting the computes as much as possible let the cache handle it 2. CDN (content delivery network) - let the CDN take as much load from their origin and let the edge nodes take take care of it 3. Layering - Put in as many layers as necessary to have resiliency for example, have a layer of servers that detect where the request is coming from ; if request comes from mobile send it to a different service stack than if it comes from desktop or if it as API request etc. Also consider caching at each layer of the stack 4. Load balancing - use as many load balancers as necessary. It is always a good idea to put your servers and databases behind a load balancer When it comes specifically to the database layer, data in database is typically put in different databases based on logical usage. For example, there could it be database hosted on a bunch of linux hosts that stores only user authentication information. There would be another database hosted on many hosts that stores all the posts user has created. This is done for multiple reasons, one of which is that there are limits to the number of connections a database can open before it runs out of connections another reason can be that the type of database used for one usecase might not be the best solution for another type. For example, RDBMS is not a great for transactional data but not so great at analytical data like analyzing social networks etc, you would want to use a graph database for that. And ofcourse, within each logical separation, you can will have the data in the database optimized using partitioning, normalization and indexing etc. Other redditors here have explained this in enough detail, so I am not going to repeat. You can also have multiple logical instance (where each logical instance is a pool (a pool is a collection of single machines or hosts) of database servers on a Solaris/AIX/Linux/Windows etc server), where you might have 10 instances that are read-only and only 1 instance where new data gets written and synced read only instances of the database - how often does a user update their facebook password or username vs how often do they login ? Using the above mentioned tools, architects come up with systems that serve massive traffic. How you use these depends on what your usecase is, analyzing your user behavior on your website and your server/data center capacity etc, which a lot of times boils down to how much will you be reading and how much will you be writing at it logical layer. And if your abstract all these ideas high enough, the main idea here is to to *divide and conquer*. The key is to figure out *what you need conquering* and *how you want to divide* in order to conquer... So I guess that's the ELI5 answer ? ***divide and conquer***. Would a 5 year old understand division ? **I it would be really interested to hear a Reddit engineer's take on how Reddit does their scaling... A reddit engineer AMA ?**",
"Databases can really just epicly be big and Oracle and what not can handle that stuff. Every once in a while I get a call in the middle of the night to add another 1TB to a 227TB database... The real answer is indexing, how you organize your data, and technologies like Oracle text, hadoop, and other stuff like that. 8 Billion rows in a table is not out of this world. The best way is to break the data down into pieces (tables) to the point that it's almost annoying. Everything get's an ID and that ID is indexed in it's respective table. Retrieving a ID which is a unique number, is very easy and almost instantaneous. Consider the following case: You are a company that receives computer files and extracts all of the details about each file into a searchable database so that you can easily determine which files are import and which ones are not. If you think about a file, there are lots of important properties you can keep track of and the data is called Metadata. You have: File Size, File Extension, Date Modified, Date Created, Mime-Type, Original File Path, File Name, Directory Name, etc... then you start getting into file type specific attributes... for Microsoft Office files you have Author, Document Title, A flag for Encrypted or not, Office has it's own Date Created and Modified values, version of the document format... I could go on and on and on... when you add emails you can literally have hundreds and hundreds of potential metadata fields. When you have a database of several hundred million files, that can add up to several billion rows of metadata very quickly. So how do you get all the metadata for a file? Let's Look: First you start off with a really simple table: DocumentID| Bates -----|---------- 23459872345 | Some formatted number a person can easily keep track of 23459872346 | PEIGHT-3548-3405 Then you have a table with all of the metadata fields to ID's MID | Description ---|--- 1 | FileName 2 | File EXT 3 | DateMod 4 | DateCreated Now.... how do you organize the actual metadata? Let's look at a table with the metadata for 4 files ID | DocumentID | MID | Value --|----------|---|----- 1 | 23459872345 | 1 | Awwwyeah.gif 2 | 23459872345 | 2 | gif 3 | 23459872345 | 3 | 5/27/2017 02:33 4 | 23459872345 | 4 | 5/27/2017 02:33 5 | 23459872346 | 1 | 06 - the world is mine - david guetta.mp3 6 | 23459872346 | 2 | mp3 7 | 23459872346 | 3 | 12/28/2016 03:12 8 | 23459872346 | 4 | 5/27/2017 03:12 9 | 23459872347 | 1 | IP schema.xlsx 10 | 23459872347 | 2 | xlsx 11 | 23459872347 | 3 | 9/18/2017 01:00 12 | 23459872347 | 4 | 1/1/2017 18:36 13 | 23459872348 | 1 | patch panel plan.xlsx 14 | 23459872348 | 2 | xlsx 15 | 23459872348 | 3 | 10/9/2016 03:41 16 | 23459872348 | 4 | 10/8/2016 23:55 So how do you get all the metadata from a specific file? In the large table the ID column and the document ID column are indexed. So you would search that table by document ID and 4 rows would be returned. If you just wanted the file extension you would throw an AND in there and you would want MID = 2. This is a very simplistic search and explination, but that's how it would work. The real question is.... how do you backup a 227TB database? You don't... you keep it in triplicate using different technologies...."
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78wyzv | Why are GPUs faster than CPUs? | Engineering | explainlikeimfive | {
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"Imagine you're a house painting company. CPU is your top employee. She can handle masking, mixing, interior, exterior, edging, wall texturing, pretty much anything you want. She's super fast, but she can't be in two places at once so for a large project it's still going to take her a while to finish it. GPU is your team of 20 unskilled laborers. They have no idea how to paint, but they're very good at following directions if you set them up and tell them exactly what to do. They always work as a team, they have to be doing the exact same thing at all times. CPU and GPU work really well together and they can finish a house in half the time. While it's true that GPU is 20x faster than CPU at really simple tasks like painting one very large blank wall a solid color, most of the time you need a combination of CPU and GPU to get the job done. For example, CPU will come into a room and mix the paint, mask the edges, and prepare the brushes. Then she gives each GPU laborer a section of the wall and tells them what to do. While they're busy painting she goes to set up the next room. When she gets back, they're finished and she can do the remaining touch-up work. When you're playing a game on your computer, your CPU is in charge. It does all of the AI and coordinates sound, graphics, input, networking, and everything else. When it's time to render each frame, it figures out exactly what to draw, then hands it off to the GPU with very specific instructions on what to do. The GPU executes those really quickly, freeing up the CPU to spend more time on other things that the GPU isn't capable of.",
"GPUs are not faster then CPUs in a general sense. GPUs are 'fast' because they are highly parallel when doing identical operations - this is called SIMD or Single Instruction, Multiple Data. For example if you asked the question \"What is 1+1?\" a CPU can answer it faster. If you asked the question \"What is 1+1 and 2+2 and 3+3 and 4+4\" then the GPU could answer it faster, assuming all those numbers where nicely lined up in advance. But if you asked the question \"What is 1+1 and 2-2 and 3*4 and 4/4\" then the CPU would again be faster. If we think of them as people then a 4 core CPU is 4 really smart people in an office. A GPU might be 4 big rooms each containing 100 monkeys and a trainer who shouts out commands."
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78zftu | What makes a rocket go? | I was wondering about how a rocket takes off, is it a controlled explosion? What is it? And how does it work? | Engineering | explainlikeimfive | {
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"You may have heard Newton's third law, \"for every action, there is an equal and opposite reaction.\" In this case, the rocket is shooting out a whole bunch of particles one way with a lot of force, thus providing thrust and moving the rocket the opposite way.",
"It is the rapid expansion of hot gases, and it works the same way that the recoil from a gun works when you fire a bullet. Think of it as a controlled firing of many bullets, causing a persistent force in the opposite direction, which moves the rocket.",
"A rocket is powered by a very energetic chemical reaction causing a whole lot of gases to forced out of a nozzle on the bottom of the rocket. Newton's 3rd Law states that for every action there is an equal an opposite reaction. So a whole lot of mass being pushed out the bottom of the rocket at high speed produces an equal and opposite force that pushes the rocket forward. A controlled explosion isn't a bad way to think about how rockets work."
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790x36 | How does digital clock work? | How does digital clock counting time? Not display i wonder how they can know how much time pass? | Engineering | explainlikeimfive | {
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"There is a special elements called a quartz. It is the same thing that beach sand is made of. It has a special property of converting different Energies. With pressure it turns into heat, with light it splits light, and with electricity it creates a vibration. Some quartz crystals make a very stable vibration. Imagine one of these quartz could do 1000 times the second perfectly. We learned how to measure that has had to turn that into a clock. You may be familiar with counting seconds as one one thousand or one alligator. Running electricity through a piece of quartz crystal splits one second into to one thousand beats per second. By making a device that can separate every 1000 beats we can create a stable clock.",
"Typical household digital clocks do not rely on quartz crystals for timekeeping. These clocks sample the AC voltage from the wall outlet. The frequency of this voltage is very well controlled at 50 or 60 Hz (varies by country) The clock measures the passage of time by counting how many times the electricity cycles. After 50 or 60 cycles, one second has gone by and the clock updates the display accordingly. The power distribution company strives to keep the frequency correct since varying frequency has a negative impact on the performance of the grid. The clock is incredibly accurate because of this. Interestingly, old style electric motor clocks also exhibited high accuracy for the same reason. In this case, though, a synchronous motor is used that spins once for each voltage cycle. 50 or 60 revolutions then equated to one second, so time could be mechanically displayed very accurately."
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791tz7 | How does car's speedometer & tachometer work? | Like how do they actually "get" the speed & RPM readings? (Older analogue, modern analogue, modern digital) TYIA. | Engineering | explainlikeimfive | {
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"Older analogue: have a small gearing system that manually winds around at a known ratio to the wheel, and thus the wheel diameter. From this the speed is known Newer digital: have a beam (most likely light) being broken. If you know what time it is, and how long since the light was last broken _and_ you know what is breaking the light you can work out the speed of the object.",
"Just wanted to add that the speedometers are calibrated to the specific size tires that your vehicle comes with from the factory. If you change the size of your tires then your speed will be off. Larger tires will cause your speed to be faster than your speedometer says, and smaller tires will cause you to be slower.",
"The old analog gauges used a cable from the transmission to the speedometer. The cable was a long flexible wire inside a housing. The cable would connect to a small gear adapter that connected to the transmission. As you drive the transmission would spin the gears in the adapter this would spin the cable which would spin the speedometer. On modern cars there is a speed sensor on the transmission. It consists of a magnet and a metal toothed ring .As the ring spins it creates a current in the magnetic sensor the speedometer uses this signal to display the correct speed. Tachometers used a cable attached to a rotating part on the engine such as the camshaft. Then they used an electrical signal from the ignition coil. Now they use the same type of sensor as the speedometer. There are usually two sensors that tell the engine computer how fast the engine is spinning. The camshaft and crankshaft sensors. The tachometer uses the signal from one or both of these sensors to display the rpm."
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796e88 | How do bowling alleys (lanes?) not dented/cracked? Hundreds of weighted balls get dropped on them daily | Engineering | explainlikeimfive | {
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"text": [
"[Maple is extremely hard, dense, and shock-absorbent. The lanes are also refinished frequently to protect them.]( URL_0 ) Apparently they're about 2 or 3 inches thick."
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798x5q | Why does one train need to stop when meeting another? | Engineering | explainlikeimfive | {
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"text": [
"It's not clear what you mean by \"meeting another\". Obviously not on the same track, because crunch. So I'm guessing that your train stops, the other train goes whooshing by, then your train proceed? It is likely that your train has moved to a siding (a short section of additional track) to let the oncoming train pass. A little further up the tracks merge to a single track, and if your train kept moving, there'd be that crunch."
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79bcqu | The Otto Cycle | I get the 4 strokes of an engine, but don't really understand the p Vs V diagram URL_0 : Can anyone explain what the graph is showing and how it relates to an engine? | Engineering | explainlikeimfive | {
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"A [wiki gif]( URL_0 ) will help with this You start at 1, that is cylinder up with no fuel in it. 1-2 the intake valve opens and cylinder fills with the air gas mixture. The pressure is always atmospheric pressure so it is a horizontal line on the graph(1 in gif) 2-3 the intake valve closes and the piston pushes up compressing the mixture. The shrinking volume and increasing pressure creates the lower curved line.(2 in gif) 3-4 the spark plug fires causing the gas to combust. The piston doesn't move so volume stays constant while pressure increases significantly(you see the spark in the gif) 4-5 is the power stroke where the high pressure gas pushes the piston down to increase the volume and reduce the pressure, this makes the upper curved line on the graph(3 in gif) 5-6 is shown in the graph to get you back to your starting pressure and temperature. In reality we don't have an infinite ideal heatsink to put on the cylinder head to get all the energy out which results in less efficient engines and hot exhaust. This is not shown in the gif. 6-1 would be the exhaust stroke where the valve opens and the used air/gas mix is expelled from the cylinder at standard pressure, only the volume of the cylinder decreases so you get a horizontal line again. (4 in gif) And now that you're back at 1 you open the intake valve and go again"
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79pbwu | how do sites like Reddit manage to store all the content they do? | Engineering | explainlikeimfive | {
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"text": [
"The thing about Reddit is that it is all text. It doesn't host images (you're linking those from other websites). Also, text compresses nicely (either as open text or like an Oracle Database). Because of that, my guess is that you could store the entirety of everything that has ever been written on Reddit using a single deduplicating array of about 200TB (which costs about $25,000 + the dedupe software). One TB is holding one trillion characters in a text-based system. You could easily get 10:1 compression due to repetition, cut/paste on a system like Reddit, so that single array could realistically be able to hold 3 PetaBytes. I don't know reddit's actual statistics, but That's a LOT of Bytes. The thing that would be pertinent to Reddit's IT staff is the web services and maintaining that access to everyone, and the speed of searching the data."
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79rxoc | How're elevators replaced? | Engineering | explainlikeimfive | {
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"text": [
"Removing a old one can be done by taking it apart at the bottom level. Either disassembly the parts or just cutting it apart. The new elevators are build from parts that fit trough the door on the lowest level. The harder part to install is likely guide rails etc in the shaft Here is a video of presentation of how a large manufacturer replace a old elevator with a new URL_0"
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79uhvh | Why do you lose less power to resistance when transmitting high voltages than when transmitting high currents? | According to [Mouser Electronics]( URL_0 )), transmitting high voltages is more efficient because it reduces your current, which in turn reduce your P=I^2 *R product. However, Ohm's Law goes both ways: power can also be expressed as P=V^2 /R, so power losses should be equivalent, no? Why is it that when talking about resistance losses, formulas always frame current as the independent variable, but not voltage? | Engineering | explainlikeimfive | {
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"The `V` in `P = V^2 / R` isn't the line voltage, it's the voltage drop across the line. Very important detail. The higher you can drive the voltage, the lower the current. The lower the current, the lower the voltage drop. > Why is it that when talking about resistance losses, formulas always frame current as the independent variable, but not voltage? Largely to avoid the mistake many people make by assuming resistive loss uses voltage and not the voltage drop across the component.",
"If you transfer at a higher voltage then the current flowing is lower. If you transfer at a lower voltage then the current is higher. So, say for instance for every 100 amps you lose 1 volt. If you have a high voltage and 100 amps flowing then you have only list 1 volt. But if our voltage was lower so that now we need to flow 10,000 amps to get the same amount of power, we have just lost 100 volts in our wires. Either way the voltage drop for a given cable will be the same no matter what voltage is present. It is the current r flowing which will determine the voltage drop. And therefore power lost."
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79vm65 | What is 'cold working' & 'hot working'? | In terms of metal works, etc. I've also read the term 'the rivet is driven cold'. What is it? How is it done? | Engineering | explainlikeimfive | {
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"It very simply refers to the temperature a metal is worked at. Cold working means simply working the metal as it is, hot working means working it when it's hot. Hot working obviously requires the metal to be heated, but makes it softer, as well as leaving less residual internal stresses behind (by heating the metal, internal stresses can be allowed to resolve). Cold working typically creates internal stresses.",
"Previous answers are correct. If you mash a piece of metal that's cold, you create microscopic weak points in it, as you are basically ripping apart tiny crystals (extra-strong solid regions) that form within the metal. If you mash a piece of metal that's hot, new crystals tend to form as it cools off in the new shape, so you don't get those tiny stress fractures in the final product. Depending on how you heated and cooled it, the metal can end up harder or softer than it started.",
"Metal is really hard, but not so hard you can't bend or mold it. Sometimes you can change it's shape by hitting it with a hammer, sometimes you can bend it by hand. Doing this is called working the metal. Metal becomes softer and easier to bend if you heat it, so you can hit hot metal and change it's shape, hot working. If you don't need to heat it, such as bending a paperclip that's cold working. Each changes the inside of the metal differently leaving it more or less likely to bend or break, how brittle it is."
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79wpfk | Why do electronics crash when overclocked too much? | Engineering | explainlikeimfive | {
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"If it doesn't outright overheat, the way a computer works is by organizing its action into clock cycles. Everything in the computer is designed to work within those cycles, so that when the next cycle starts, each part has finished it's share of the work and is ready to pass its result on and start on the next chunk of computation. At some point, speeding things up will cause some component to be unable to finish in time, and this can often lead to a catastrophic failure of the logic of the computer... i.e. it crashes.",
"As /u/1_km_coke_line indicated, one reason is overheating. Running integrated circuits at unusually high frequencies or voltages can cause them to get hotter than they were designed/tested for. Ideally when this happens, the OS or BIOS on the computer sees that things are getting too hot and throttles back or shuts down. Sometimes the heat rise happens too fast or too locally for it to get recognized in time and the computer crashes. In a worst-case scenario, there can be permanent physical damage due to the excessive heat. But that's not the only possibility. Integrated circuits (such as a microprocessor) are generally designed to work in a kind of cadence, with the clock setting the beat. This avoids situations where different signals arriving into a sub-circuit at slightly different times cause it to change state when it shouldn't. The clock acts as a gatekeeper, basically saying \"OK, now is the time to evaluate the inputs and react accordingly\". This is called \"synchronous circuit design\". The clock gets split up into different \"[phases]( URL_0 )\", and different parts of the circuit are hooked up to different ones. That way, when one set of circuits feeds into another set, they don't all try to change at once and get confused. When you overclock, you risk screwing this up. Things start happening faster than the circuits were designed for, and incorrect operation can happen. This is made even worse by the fact that overclocking heats things up, and circuits slow down as they get hotter. This incorrect operation can cause data corruption and all other kinds of bad behavior. Including causing a crash. A third, much less likely scenario is that the clock circuit itself will misbehave when overclocked. The clock signal being fed into a microprocessor is fairly high frequency (although not as high as the internal clock frequency). If the overclocking is done by telling the motherboard to speed up the clock, the quality of the signal can degrade to the point that the processor isn't getting a \"clean\" enough clock signal to recognize it. But a lot of time this isn't how overclocking is even done.",
"They overheat. resistance in the various circuits becomes too high with increased temperature, causing them to get even hotter during continued use. Eventually the boards/circuits will be damaged by the high temperatures and stop working.",
"In transistors, there is something called propogation delay. It is the time it takes from the switching signal being sent to the time the transistor actually activates or deactivates. This is a very short period of time, but processors tend to act near the margin of stability. Overclocking does 2 things. It shortens the period of each clock cycle, which may outright exceed the propogation delay of the transistors (sort of like if you only left yourself 5 minutes to making a connecting flight, the signal won't make it from the first transistor set to the 2nd set). The other is the increased cycle rate increases the power consumption and therefore the heat of the processor. Increased heat leads to longer propogation delays. In our plane example, this would be like having to fly around a storm, lengthening the flight time. This is why you can overclock more at lower temperatures, things like liquid nitrogen shorten the propagation delay of the transistors."
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79ydxk | How does a Torque converter, convert torque? | Engineering | explainlikeimfive | {
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"text": [
"The simplest way to explain it is it's two fans facing each other. Power is applied to one fan, that blows fluid by the other fan and it spins. In real life they are very close and tightly located next to each other so the fluid can only flow through the fan blades.",
"The unsaid part of the name is that the torque converter converts horsepower into torque. The input shaft is spinning faster, making more horsepower and the fluid couples it to the output shaft, which is turning more slowly, but with more torque. It works identically to a using a higher gear ratio (lower gear), but with the benefit of being a variable conversion ratio but with the downside of being much less efficient."
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7a0you | How do they light rockets in space? | I am aware that in rockets, both the oxidizer and the fuel is carried. However, I don't understand how, when they start a second stage, they start the reaction. On the ground, they use big sparks.. don't know the details, but it makes sense. When I watch videos of SpaceX launches for instance, the rockets in the second stage just seem to go. | Engineering | explainlikeimfive | {
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"Hypergolic ignition! A hypergolic fuel mix is two chemicals that will ignite when they come into contact with each other without the need for an outside ignition source. But the Falcon-9's Merlin-1D Vacuum 2nd stage engine uses the same Kerosene/LOX (liquid oxygen) fuel as the first stage, and that isn't a hypergolic mix. So how do? In the second stage's kerosene injector line there is a small plug of TEA-TEB (triethlyaluminum-triethylboron). When the pumps come on the TEA-TEB gets forced down the line until it meets the LOX in the combustion chamber. TEA-TEB is *strongly* hypergolic with oxygen. So the instant the two substances meet they ignite. The kerosene fuel follows right behind the TEA-TEB, hits that fireball, and it's off to the stars! Chemistry is fun, hypergolic chemistry is *really fun*, and then you go a little too far and make [chlorine trifluoride]( URL_0 )"
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7a2tvy | What kind of debris shielding is on the ISS that can stop high energy objects from penetrating the hull? | Engineering | explainlikeimfive | {
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"text": [
"The debris shields aren't made from something special, they just have multiple layers with space between them -micrometeors get through the outer layer but shatter in the process, so the inner layer can stop the slower and smaller parts from getting through."
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7a5qri | Why older houses are built with a traditional wooden door along with a glass/plastic secondary door | The secondary door usually being on the outside of the house | Engineering | explainlikeimfive | {
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"text": [
"The secondary door is a storm door. It's so you can open your front door and get some sunlight/ventilation in your house. It also protects your door. It was not uncommon for older houses not to have storm doors, as well. A lot of older homes have them because previous owners installed them over the years. If I had to take an educated guess as to why homes don't come with them, it is because they are polarizing (some people don't like them) and not providing them is cheaper for home builders.",
"Do you mean storm doors? They basically act as a large screened window allowing in light and airflow while keeping bugs out."
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7a7dk1 | why is a tail wing on an aircraft necessary? I’m talking about the physically vertical one, like can’t a pilot just use the roll to steer and keep the plane stabilized? | Engineering | explainlikeimfive | {
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"The vertical stabilizer is used for yaw stability. The airplane can rotate around three different axes. The rotation where the nose goes up or down is called pitch. The rotation where the wings go up or down is called roll. The (much less common) rotation where the whole plane spins in a circle around the middle of the fuselage is called yaw. You probably didn't even really think about yaw because it's not a big part of usual aircraft operation. But without the vertical stabilizer, if a gust of wind hit the front or back of the plane, it could start spinning like a Frisbee very easily, and that can be [very difficult to come out of, and very difficult to survive.]( URL_0 )",
"The tail stabilizer is just the easiest and best way of keeping the plane stable. Look at a B-2 bomber. No tail stabilizer. It can be done, but its more complicated, and if the computers fail, its a lot harder to fly. For a military plane, where the budget isn't too important, stealth is very important, and there are only 2-3 people, its a good trade off. On a civilian airplane, there really are no advantages (perhaps a bit more fuel economy, depending on how you designed it), so although it can be done, its not worth the trade off."
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7a96vu | Why does using the air conditioner in your car use up gas, but using the heater doesn't? | Engineering | explainlikeimfive | {
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"text": [
"Your engine generates a bunch of heat just from being on. In fact this is a major problem for engines and oil and water systems pull the heat away. The heater in your car uses this to heat air and blow it into the cabin. The AC has to run a compressor which takes energy."
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7agy0r | What are some of the factors that led to a noticeable drop in quality in American automobiles? | Engineering | explainlikeimfive | {
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"text": [
"They got cocky and fell way too hard into the \"gas is cheap everyone wants big cars\" fad. However, they pivoted very quickly after they found out they were in the wrong. Most modern US cars are quality and much more fuel efficient. Just check the rating for Ford in 2007 vs. now."
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7anovt | Why can't quieter leaf blowers be engineered? | Engineering | explainlikeimfive | {
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"text": [
"Your ears detect changes in pressure, a leaf blower moving air at x mph creates a rather large change in pressure and therefore a lot of noise. A car creates large changes in pressure as well, but there contained within each cylinder which is why we can design muffler systems to make them quieter. As for other lawn equipment, it’s simply because muffflers are pretty big and thus most small engines don’t have mufflers that are as effective. Though there do exist muffled generators that have complete mufflers.",
"They actually have rather quiet leaf blowers. They are newer and correct me if I am wrong, but electric powered. I couldn’t answer to how they compare in wind power and efficiency unfortunately.",
"Lawn equipment, especially leaf-blowers, are designed to be light enough to be comfortably operated by one person. It is much easier and lighter to slap ear protection on the operator than it is to build a system that muffles the noise of the engine. Electric leafblowers are much quieter than gas-powered leafblowers because they aren't powered by explosions. You wouldn't be able to muffle the fan, however, without defeating it's whole purpose to blow out a lot of air."
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7apmyb | how do countries import or export electricity ? | Engineering | explainlikeimfive | {
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"The high voltage power grids are usually connected over borders, so that they can simply send power over the wires as needed and then pay for the power afterwards.",
"Just like the power meter in your house can measure the electricity going in to your house, you can do the exact same thing (but, you know, with heavier measuring instruments) on a high voltage cable that runs across a border. So, basically. At every point where a new buyer is connected, you add a power meter and measure how much they use. That the cable runs across a border is typically completely worthless information. No-one cares about that. The important bit is that when one grid owner takes over from another, the new grid owner has to cough up for what they import into their own grid. I have an example, I'll try to simplify it so that it's easy to follow it. In Sweden, power grid above a certain voltage is owned by a power company that is controlled by the government. Their purpose is to ensure that all the power companies, large power producers and other nations national grids can buy and sell to each other in realtime. They don't care particular much about how the trade happens, but their job is to ensure that everyone can trade. The actual balancing (that is, the calculations that ensure that enough production happens to meet demand) is done by another entity and how they cooperate there is beyond the scope of this reasoning. For historical reasons, a lot of the production takes place in hydro plants in rivers far up north in the country while a lot of the consumption happens in the more urbanized areas in the south. This means that there is a constant need for producers up north to transmit to the south. So, the grid company has a power meter at each entry point. Measuring how much is put in and measuring how much is withdrawn. Electricity is damn hard to store, and sometimes there is a grid failure. And a plant has to close down for maintenance real quick. You know, things that make it important that you can grab electricity from someplace else. For that reason, we got connections to Norway, Finland, Denmark and Germany. I think there is a baltic connection too, but I'm too lazy to look it up. So, if we need more electricity than we are able to produce, we contact producers in Norway and ask for more production. Or Finland. Or Denmark. Or, in a worst scenario from Germany where electricity is usually a lot more expensive. The germans have a historical habit of buying all of our surplus, because our electricity is a whole lot cheaper than theirs. (or, well. it was anyway. I'm not sure now.) The day after, THEY found have a plant that stops and have to buy from us. But, well. The general idea is that if you connect everything to the same backbone and measure in which direction the flow goes and charge accordingly, it unifies the buyers and the sellers into an international market. A market that, at least in theory, functions better because there are more players on it that all understand how they rely on each other.",
"There's a sort of secondary way to do this. If a country can generate more electricity than it needs, it can use the excess to smelt aluminum and then export the aluminum.",
"Electricity doesn't decide where to go, no one can tell a current flow to go in a certain place or in another place. The only thing that can be done is decide how much a certain country's power plants need to produce. Then when countries want to buy electricity from other countries they buy it from stock market. Let's make an easy example: Mexico wants to buy electricity from USA Let's say that U.S.A. and Mexico want to turn on a 100 W to light bulb; U.S.A. has power plants capable to produce 300 W and Mexico can produce 50 W. One day before, Mexico opens the stock market app and buys 50 watts of electricity from USA for a certain price (intersection between demand and offer curves) to be supplied in a specific interval of time (let's say during the night). The TNO (transmission network operator) will then check the stock transaction to determine if electrical lines interested in the exchanges can be safely used (current in a line must be below a threshold), let's say it is for simplicity, at this point the USA power plant can produce the extra amount to sell it to Mexico. At the end of the day, the power plants interested in the transaction will raise their production rate to fulfill the established exchange of the stock market. USA will produce 150 watts and Mexico will produce 50 watts, both USA and Mexico light bulbs will be lit on!"
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7as36d | Why stapling papers only works with a quick movement, but if we do it slowly it just creates a twisted mess? | This question has haunted me since... this last Halloween, I think | Engineering | explainlikeimfive | {
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"Doing it slowly gives the paper time to deform in response which is what causes the tangled mess. Paper can deform while the staple is much more resistant to deformation, so if you do it quickly, the stapler can just rip through the paper since it is moving with more force than the paper can contain. If you push too slowly, it doesn't tear through the paper right away so the paper has a chance to deform and stretch around the pressure, distributing the force over a larger surface area.",
"The same way a bullet wouldn't go through a wall if I were to just throw it. You need a strong enough force."
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7at6i7 | Li-Fi and it’s relationship to 5G? | Engineering | explainlikeimfive | {
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"5G is separate from Li-Fi all together. Li-Fi is effectively WiFi using flashing LEDs instead of radio waves. It is very short range and exclusively line of sight which makes it useless for a broad area wireless communication system as you wouldn't be able to communicate with a tower unless you could see it. 5G is still poorly defined. It will use 28 GHz, 37 GHz and 39 GHz(aka radio waves not light) but how it will use them and what protocols it will use are still up for debate. Last month Qualcomm demo'd the first 28 GHz modem using their interpretation of what 5G will be, but there is still no real standard for it."
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7au018 | how does a car turbo work? | Engineering | explainlikeimfive | {
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"Yer not alone in askin', and kind strangers have explained: 1. [ELI5: How does a car turbo work? ]( URL_5 ) 1. [ELI5: What does a turbocharger actually do? ]( URL_6 ) 1. [ELI5: How does a turbo charger in a car work? ]( URL_2 ) 1. [ELI5: how a turbo works in your car. ]( URL_7 ) 1. [ELI5:How does a turbo work?Why do people use it? Is it legal? ]( URL_4 ) 1. [ELI5: How does a turbo work in a car, and why does it 'make a car go faster' than a non-turbo car of the same model? ]( URL_0 ) 1. [ELI5: How does turbo in a car actually work? ]( URL_3 ) 1. [ELI5: How a turbo works in a car. ]( URL_1 )",
"Take a fan and put it in the exhaust of the car. As air passes out of the exhaust, it pushes through the fan and spins it Connect that fan to another fan, so that when one spins, it spins the other. Point that second fan into the car engine intake, so that when it spins it blows more air into the engine. More air (specifically, more oxygen) means a bigger explosion when the fuel in the cylinder is ignited. That's basically it. Although the fans are actually turbine/impeller type bladed systems, they work in basically the same way",
"exhaust gasses spin a turbine in the exhaust, hat turbine is connected to another turbine that compresses air into the intake. The gas and explosion is the source of energy that makes this circular loop a positive sum. more air, more gas, more exhaust, more air, more gas, more exhaust, repeat."
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"https://www.reddit.com/r/explainlikeimfive/comments/3fypco/eli5_how_does_a_turbo_work_in_a_car_and_why_does/",
"https://www.reddit.com/r/explainlikeimfive/comments/l8lgi/eli5_how_a_turbo_works_in_a_car/",
"https://www.reddit.com/r/explainlikeimfive/comments/2rllpz/eli5_how_does_a_turbo_charger_in_a_car_work/",
"https://www.reddit.com/r/explainlikeimfive/comments/2c0hoc/eli5_how_does_turbo_in_a_car_actually_work/",
"https://www.reddit.com/r/explainlikeimfive/comments/3ireuo/eli5how_does_a_turbo_workwhy_do_people_use_it_is/",
"https://www.reddit.com/r/explainlikeimfive/comments/6sp5gx/eli5_how_does_a_car_turbo_work/",
"https://www.reddit.com/r/explainlikeimfive/comments/1iusxy/eli5_what_does_a_turbocharger_actually_do/",
"https://www.reddit.com/r/explainlikeimfive/comments/1p1x5p/eli5_how_a_turbo_works_in_your_car/"
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7b0z7i | How do studfinders work? | Engineering | explainlikeimfive | {
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"They have an electronic oscillator circuit which changes frequency in the presence of matter. The wall has capacitance which changes with density. You put the finder against the wall, press the button, and it does a calibration based on the drywall or paneling. When you move it across the wall, it senses changes in the oscillator's frequency and converts the deviation into a few distinct levels to light some LEDs."
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7b5akl | How is water managed and supplied in Trains ? | You can use well explained technical terms and please talk about normal trains that are used for daily commute not the rather hi-tech ones ! | Engineering | explainlikeimfive | {
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"Pretty much like in an RV. There are tanks to hold clean water, typically refilled with a hose at the ends of the line. These are usually mounted above the sinks and toilets, so gravity delivers the water. The drains all connect to storage tanks under the cars, which are pumped out at the end of the line.",
"I can chime in on this as I maintain trains for a living. Trains are filled on either end of the train set via a potable fill inlet. The water is then pressurized (for our train sets it's set at 4.5 bar) . To fill you purge the water system of air via cutout cocks or valves . Fill the water until it is spitting out the bottom much like an rv and an overfill hole. Then you return the cutout cocks to position to bring the system back to a pressurized 4.5bar supplying the trainset with water",
"Trains, like planes, have storage tanks for potable water. These are refilled either at major stops, or back at the yard at the end of the day. They also have waste storage tanks to captures the waste water, and that is pumped out, again at designated stops or back at the yard. There would be a pair of tanks like that at each car with a restroom in it. As far as \"management\", the fixtures are often low-flow types to use as little water as possible.",
"From a summer maintaining railcars: every individual car has a potable water tank for sinks and bars and blackwater tank for toilets. The Grey water (soapy water or rinse water) was fed in a drain system that emptied onto the track below. All of the cars were outfitted with valves on either end to hook up to a flexible food grade hose that would sit slack between the cars. We would have to bleach all fittings before hooking up hoses to the fill ports and when connecting two cars. There is another pump on each tank that allows for movement from one car to another in the event that one car (or more) ran out of water. Water would be pressurized throughout the railcar by activating a pump. All of these features were controlled on a switch board mounted on a door next to the entrance of the rail car that also controlled all light, generator usage, head end power Control (half of the diesel electric generator on the engine is used to power up to ten rail cars.) black water would be pumped out at the end of the day and all tanks would be filled with fresh water."
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7bdj02 | Bilge Pumps | Engineering | explainlikeimfive | {
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"It's probably ballast water, not bilge water. When the ship is empty they pump water into ballast tanks to lower the ship into the water deeper and keep the ship more stable. As they load the ship they pump that water out."
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7bdndt | When connecting high voltage batteries in parallel using connecting wires, is it better to start connecting from the positive terminal or the negative terminal? | Engineering | explainlikeimfive | {
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"From a safety standpoint its best to connect the ground first, so as soon as the circuit goes \"hot\" it has a ground ready to go. As opposed to connecting the hot terminal and then messing around trying to connect a live wire."
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7bfgvh | Why the 125cc Honda Grom engine produces more HP and torque than the Honda GC 160 dispite the 160 being larger? | Engineering | explainlikeimfive | {
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"To be a little more specific, the Grom's engine is higher compression, and it revs almost twice as fast. The tighter you squeeze the fuel-air mixture together before ignition, the more power it will make when it burns. And, of course, the faster the engine spins, the more power cycles it's going through. But the Grom's higher compression high-speed engine undergoes greater stress. Everything's a tradeoff.",
"Because displacement doesn't necessarily equate to performance. Engine performance is a balancing act between production cost, peak power, usable power, noise, pollution, fuel efficiency, service lifetime, and gov regulations for engines emissions, tax implications etcetera",
"CC is a measurement of engine displacement. Think of all these performance products that say they increase the performance of your engine. None of those products are changing the size of your pistons, they are how ever doing thing to increase performance, more air intake, better exhaust flow etc. So in your example a smaller cc engine designed for a street vehicle was engineered to perform better than a larger cc engine that was designed for a lawn mower."
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7bh7m3 | How does pressing down on the gas-pedal result in faster piston movement? | So when we accelerate, what happens in the engine, to move the pistons faster, is it brake related?, physics related? fuel related? | Engineering | explainlikeimfive | {
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"Fuel. Pressing the throttle opens a valve, allowing more fuel which creates a bigger combustion inside the engine, pushing the pistons harder and resulting higher rpms. (Agreed, more fuel and air).",
"The throttle controls the [Butterfly Valve]( URL_0 ) which allows more air to move past it the more it is opened. Or in other words, pushing down on the throttle causes the valve to open more, allowing more air to the engine. The way more fuel is added is either a fuel injection system which measure the airflow rate and adjusts the fuel flow accordingly or, in the case of old cars, a carburator which uses a creates a vacuum with the intake air that sucks fuel from a nozzle."
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7bpb8o | What happens when DC current is applied to an AC induction motor? | Engineering | explainlikeimfive | {
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"If you have a big DC source supplying the power that can supply enough power then your AC motor will suffer a \"thermal event\" Without the changing of the AC signal then the motor won't spin, if the motor isn't spinning then it doesn't push back on the current by creating \"Back EMF\", and without it spinning the motor just looks like a long wire, aka a short which will lead to the motor acting like a toaster not a motor."
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7buzbs | What are neural networks? Specifically RNNs. | Engineering | explainlikeimfive | {
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"The little league team you coach just won the big game, and you ask them if they want to go out for pizza or for burgers. Each kid starts screaming their preference, and you go with whatever was the loudest. This is basically how a neural net works but on multiple levels. The top-level nodes get some input, each detects a certain property and screams when it sees it...the more intense the property, the louder they scream. Now you have a bunch of nodes screaming \"it's dark!\", \"it's has red!\", \"it's roundish!\" as various volumes. The next level listens and based on what they hear they start screaming about more complex features. \"It has a face!\", \"It has fur\", until finally get to a level where it is screaming \"It's a kitty!\". The magic part is no one tells them when to scream, it is based on feedback. Your little league team went for burgers, and some of them got sick. Next week, they might not scream for burgers, or might not scream as loudly. They have collectively learned that burgers might not have been a great choice, and are more likely to lean away from the option. A neural net gets training in much the same way. You feed it a bunch of kitty and non-kitty pictures. If the net gets it right, the nodes are reinforced so they are more likely to do the same thing in similar situations. If it is wrong, they get disincentivized. Initially, its results will be near random, but if you have designed it correctly, it will get better and better as the nodes adjust. You often have neural nets that work without any human understanding exactly how.",
"The current top analogy is so unrelated to neural networks that it doesn't help, so let me try expand on it: Imagine someone is looking at an object, like a cat. They write down lots of traits that the object has - for example, \"four legs\", \"furry\", \"brown\", \"has whiskers\", etc. Now let's say you want to make a machine that, when given that list, will figure out what the object is. The simplest way to make that machine is obvious: make a list of qualities for every object in the world, and then have the machine check which of those lists matches the one you just wrote for that cat. It'd work, but obviously this is far too much work to do. So you think \"Hey, a lot of these objects have a lot in common - why do I need to make separate lists for each one?\" So instead, you have lots of smaller machines that only asks one question. For example, a machine that checks \"Is this an animal?\", and it'll see if \"is breathing\" or \"has a heartbeat\" or such are on the list, and say \"Yes, this is an animal\". And then there's another machine that checks \"Is this a mammal\", and that'll ask the animal-checking machine for if it's an animal and then check the list for \"has hair\". Some machines would only check the list, and some would ask many other machines for their answers, and some would do both. And eventually, just from machines-asking-machines-asking-machines, you have a final machine that answers with \"Yes, this is a cat\". ...Of course, even making those smaller machines is still too much work for categorising every object in the world, so instead you try have it build itself - using random guesses for what the categories should be - until you end up with a working system. This can result in crazy smaller machines, like one that might ask \"Does it have two legs, two arms, and nose hair longer than 3.5cm?\", but it should overall work fairly similar to the cat-detecting model I just talked about. Right, now as for Recurrent Neural Networks, it's pretty simple: it's exactly the same as what I just said, but where smaller machines can also ask questions from the *previous* list's answers. For example, in voice recognition, one machine might go the \"It is/isn't an 'ow' sound\" machine and instead ask \"Was the *previous* thing he said an 'ow' sound?\". (The one thing I didn't mention is that most small machines would actually have answers in a probability rather than yes/no, but that's not true for all neural networks.)",
"Let me give this a try. Neural networks are a computing architecture inspired by biological brains, although they are not an exact replica. The brain is a network of connected cells called neurons. Each neuron takes input from other neurons. If the signal from all of the input neurons is strong enough, then it fires and sends its own signal to downstream neurons. Brains learn by creating and destroying connections between neurons, and altering the strength of existing connections. Neural networks are simpler than biological neurons, but they are inspired by the same principle. A neural network takes input in the form of numerical data. It passes that input through multiple layers of neurons. Each neuron adds up the input from the layer above it, and sends its own output to the layer below. Eventually the last layer in the stack produces an output. The network learns by a process called back-propagation. To train a network, you show it samples of input, and the matching samples of output. Back-propagation alters the strength of connections between individual neurons so as to reduce the error between the sample output (\"what the output should have been\") and the actual output that the network produced when it saw the sample input. After many, many such training iterations, the network may have configured its connections (or \"weights\") so that it is able to make meaningful correspondences between inputs and outputs. As a simple example, a neural network might learn to recognize cows by looking at a series of pictures. Some of those pictures are cows and some are not. The pictures are turned into numbers (pixel by pixel) and passed into the top layer. The output from the bottom layer will have a signal strength that is interpreted as \"yes, cow\" or \"no, not cow\". If the network got it right or wrong, the connections that helped/hurt the conclusion are strengthened/weakened accordingly. A recurrent neural network (RNN) is the same concept, with one extension. The neurons don't just process the input coming from the layer above, but also connect back to themselves so that they have a way to \"remember\" their prior states and prior input. There are various specialized neurons such as long short-term memories (LSTMs), gated recurrent units (GRUs), etc that accomplish this in fairly sophisticated ways. Hope this helps? Happy to explain in vastly more detail any part that you like. I realize this answer isn't literally meant for a five year old but I hope it's accessible to most non-technical adults.",
"The insight behind neural networks is that if you take a bunch of simple equations that each do a tiny little bit of processing (like adding up the results of other equations and tweaking the value based on its size), and you stack enough of them together, they can do pretty much anything you want. You just need to find the right \"settings\" or \"weights\" for them so they do the specific thing you want instead of something else. We've discovered special rules that let us take the output values we want and the input values we want and adjust the math in between to make the whole network more likely to produce the desired output when it's fed the desired input. Repeating this over many input-output examples eventually leads the network to \"generalize\" - i.e. to capture the structure of the information so well that it can work on inputs it hasn't seen before. A \"neural network\" is just a big stack of these simple equations that have been tuned using one of these special rules to map a particular set of input and output examples together. Once it's \"trained\" in this manner, it can be used on new examples to do useful work without needing human judgement. An RNN (or recurrent neural network) is simply an extension of this, where the network is solving a problem that takes place over many steps, so many copies of the network are initialized in sequence, each being fed some information from the past copy like a colossal game of telephone, letting it preserve some \"memories\" from the past and make multiple outputs before stopping. As an example, you can use an RNN to generate text. If you feed it text one letter at a time, and train it to predict the next letter of the text, it'll eventually get pretty good at it: it'll \"remember\" some information about the letters that came before, and use that context to make a guess at the next letter. Once it's trained, you can feed it its own output as input (basically telling it \"you were right\" after each guess) and it'll happily spit out line after line of text that structurally resembles the text it was trained on.",
"I'll try and start from a real simple overview-explanation and work my way down to more and more specifics. Basically, a Neural Network is a system that is able to learn a complex function from a large set of examples. Let's say you have a couple of thousand pictures of cats and another couple thousand pictures of dogs. Each image has a label, e.g. 'cat' or 'dog', although that would be represented by a number, so cats are -1 and dogs are 1 or whatever. You feed these pictures through the network, which for now is just a black box for us, and it gives you an estimate of what the picture shows. (It spits out a number between -1 and 1, in this simple case.) In the beginning of the training process, the result is going to be random. But the network is punished every time it gives a wrong answer and changes some of its parameters, and gradually, over time, the accuracy improves. After a couple of thousand training iterations (that is, feeding an image in, receiving an answer, punishing/rewarding the network, adjusting parameters) the network has learned to distinguish between images of cats and dogs. Now, how does that work? The smallest part of a network is a neuron. A neuron is a really basic thing, it takes in a couple of inputs, sums over them and pushes that sum through a nice little function, a sigmoid for example or a ReLU. (You might wanna google these to look at a graph, a sigmoid is just a function that is shaped like an S. It squishes inputs from the real numbers to the interval between 0 and 1, for example) So, for example, five numbers go in and one number comes out. The simplest network you could construct contains only one neuron. This is where the magic happens: before the inputs are summed up, they are weighted, that is, multiplied with some real number. So, for example, our network receives the inputs 4, 5 and 6. Those might be the values of pixels in an image. They might be the height and length of the animal we are trying to classify. They might be < insert other example here > , doesn't matter, its just data. 4 is multiplied by -1.3, 5 is multiplied by 2.1, 6 is multiplied by 0.4. (You might be asking where those weights come from, I'll get to that in a minute) Now, we sum over those weighted inputs and push that through a sigmoid, out comes another number. In a really simple network with only one neuron, that number would already be the networks output: something close to 1 for a dog, something close to -1 for a cat. In more complex networks, the output of this neuron would be the input to the next neuron, in the next layer. There can be millions of neurons in large, complex state-of-the-art networks. The important point to take home is: numbers are multiplied and summed up, the result is squished and then fed forward to the next layer. This is why this process is called feed forward. But I promised to explain where the weights come from. Truth is: In the beginning, those are random numbers. Which explains why the output of those networks in the early stages is pure garbage. The interesting thing is how those weights are adapted, and for that we use an algorithm that is called backpropagation. What basically happens is that the output of the network is compared to the actual label of the image (or data point, to be more general). So, we calculate the error that the system made. That error is propagated back through the layers, and those weights that are responsible for the error are adjusted. (To be even more specific, ELIlikemath or so: The weights span a vector space called the weight surface. We can use calculus to relate the error that the system makes to the constellation of weights. There is a combination of weights that leads to the smallest possible error, and that combination of weights corresponds to a valley in the high dimensional vector space. We can calculate the gradient of the network function to walk downhill in that vector space) Depending on how the neurons are connected in the network, we give it a different name. What I just described is just a Multilayer Perceptron, MLP for short, the vanilla version. More complex version are Convolutional Neural Networks, CNNs, and Recurrent Neural Networks, RNNs. I am no expert on RNNs, the basic idea is that it is possible for information to flow through the network backwards as well, I think. Edit: added paragraphs, was not aware of the fact that you have to add a blank line",
"**Actual ELI5:** You know those stupid captchas? They have you select boxes--which ones have signs, which ones have trees, etc. By looking at them, you know which ones to select. Even if you could only see what's in each box individually, you would be able to figure out pretty well whether or not there's a tree there because we've seen trees before (training data). So, let's say we have an image and we know what trees look like, even when we can only see a little box of the image. Now, we have a new picture. We start off with a teeny tiny box--not sure, but we've learned something. Then, we get bigger boxes over the entire image--we've learned a little more. There's something that looks textured like bark, something that could be a leaf. Even a larger box now--okay, we can tell that those are clusters of leaves and here's an entire branch. Now we know it's a tree. Let's say that now, we have a video. We figured out that the picture is of a tree, but now we want to know if the next frame also has a tree. If you're smart, you think \"of course!\" not that much can change from frame to frame. So we look at the next picture in the video and do the process over again, except this time, we know, \"hey, this box said it had bark texture or a leaf shape last time\" and we can figure out if it's the same this time. . . . If you want the tedious explanation: **Neural Nets:** an input (images, a sentence, etc.) goes into a series of nodes in hidden layers, which output what you want (yes/no, things that are discrete - classification, a regression - possibilities, various values, etc.). What happens in the hidden layers, broadly, is that in the first layers, features are made by some mathematical process. Further layers would generalize upon features, getting more and more abstract. A NN can be as small as 3 layers (input -- > hidden -- > output) or larger like what you see with CNNs. CNNs are a specific kind of NN that use convolutions of different sizes (matrix size) and strides (how far each convolution occurs from one another). Imagine a convolution as a box going over an image--it can be 5x5 pixels big or 25x25 pixels big or 2x2 pixels big and move over 1 pixel at a time or 20 pixels at a time. Each of these decisions end up affecting what features are output. There are other parameters to tune like learning rate (how fast things are learned--too fast and one bad training example can screw you up, too slow and it just takes forever to get a functioning CNN), momentum, weights, etc. In networks, everything is initialized randomly. Then, as training data goes in, each layer of nodes gets their numbers changed by these mathematical processes. Epochs are how many times you run your training data through, you do it until you reach a plateau, which you can determine by the validation accuracy plateau-ing (95% would be good, but if you plateau at 30%, you know you need to fix something--you don't just keep training and hope it gets better). **Reccurent Neural Networks:** These are particularly useful for things like sentences and videos, where what comes before and after are important. This is a broad area, so I'm not going to explain each one. RNNs are basically just NNs where the input data is not only your training data, but also what the output of previous/posterior nodes has been. There's a feedback loop connecting it to past decisions so that those are carried forward. The issue with these are that there are so many operations--you know how 2^10 = 1024, but 2^20 = 1048576. Imagine that, but on a huge scale, where the values of these nodes can quickly explode to huge numbers or vanish to near-zero. The following is supposed to solve that issue. LSTMs are a specific RNN that can learn long-term dependencies. We have a list (cell): they figure out which information we want to throw away from the list (forget gate) and what we want to add based on input data (input gate), and then update the list. As you run through it, some old bullet points of the list still make it through and some new ones are there too. But, how much the new items influence your list depends on a parameter you set. The gates start to learn how much data is supposed to flow and what should flow the way CNNs learn feature detectors. How does this solve numbers exploding or vanishing? It does so by adding functions instead of multiplying. So if one of your numbers is smaller or larger, it's no(t as big of a) biggie. Source: PhD student, this is my area. I can expand on more, but I figure things would get too long and I skipped over things like backpropagation and gradient because I figured the layperson wouldn't care. I got lazier and lazier...so the latter is a lot less specific, sorry!",
"I'm going to try for an actual ELI5-level answer... artificial neural networks (or ANNs) are magic boxes that are full of magic numbers. These boxes have the following properties: 1.) They take some numerical inputs and give some numerical outputs 2.) They know how wrong their output is (\"error\") 3.) Based on their error, they know roughly which direction each of their magic numbers should be adjusted to be less wrong Although these properties are actually the result of fairly straightforward algebra and calculus, neural networks can be surprisingly powerful for certain problems, especially when a bunch of them are stacked on top of one another (this is a \"deep\" neural network and does \"deep learning\"). RNNs (recurrent neural networks) are the same as vanilla ANNs, except that they care about the order and context of their inputs. This makes them good for things like text processing (a regular ANN wouldn't care about the difference between \"the quick brown fox jumped over the lazy dog\" and \"the quick brown dog jumped over the lazy fox\"). The name and \"biologically-inspired\" label are sort of misleading... ANNs used to be called weighted matrices (and a lot of other things) a long time before they were associated with anything biological. It was only after we found out that they were particularly good at many of the same kinds of problems brains are (particularly vision and speech-related tasks) that we started calling them \"neural networks\". Also because it sounds cool.",
"A neural network is a set of mathematical operations that maps a set of inputs to a set of outputs. They are useful because they can map _any_ set of inputs to _any_ set of outputs. The really interesting thing is that the \"weights\" of the network, which define how the inputs get transformed as they move through the set of computations, are adjustable. This means that you can take the outputs predicted by a network with one set of weights, compare them to the outputs it _should_ have given you, and then intelligently adjust the weights to get closer to the right answer next time. With enough repetitions of that process, you can \"train\" a neural network to do pretty incredible things, simply by showing it enough of the right data. An RNN is a special type of neural network called a \"Recurrent Neural Network.\" A regular neural network can map one set of inputs to one set of outputs, and then it is done. An RNN takes the outputs from one \"time step,\" or one prediction, and feeds it back into the network along with the data for the next prediction. This gives it the ability to \"remember\" things it has seen recently in the context of new inputs. In other words, a regular neural network might be able to look at a picture and tell you whether there is a cat in it or not. An RNN could look at a series of pictures from a movie and tell you what the cat is doing in them.",
"[These videos provide a decent introduction to neural nets in general]( URL_0 ) (I'm not sure if the series is complete or if he'll go into further details in future videos)",
"ill try and eli5 this. basically neural networks are ways to solve problems by recognizing patterns. so suppose i want to solve an addition problem. i can write a series of steps like you may you have learned. write the numbers one on top of each other. start at the right and add down. carry 1s if the result is more than 10... that is a definite way to solve it and you get the exact answer. now some problems are really hard to write down such an exact method. things like identifying things in pictures. so you can use a neural network to figure out a pattern which can give you an answer. it maybe not be correct. but it tries. so you train the neural net with a bunch of inputs and correct outputs and it tries to learn the pattern. going back to the addition example. youd feed it data like. 1+1 = 2 3+4 =7 100 + 200 = 300 ... the more samples you give it to learn the better chance it has of a good answer. also the bigger the neural net (nodes) the better chance of a good answer suppose i just gave it the 3 values above to train on. then i asked it what is 50 + 10 it might come up with the answer 100. its not correct, but its not a bad estimate."
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7bw9rb | ; Why aren’t MotoGP bikes lower to the ground and longer | Engineering | explainlikeimfive | {
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"motogp bike specifications are heavily regulated. the bike designers are free to do whatever they want within those guidelines. generally a longer wheelbase would be better for top speed (it would make the bike more stable), but it would have worse handling (it would be harder to turn). the same goes for being lower to the ground (lowering the center of gravity). every design change is a balance between top speed, handling, accelerating, braking, and other factors such as tires, road type, weather conditions, etc. since each race track is different, there's no one design that's best. it's up to the designers and riders to carefully balance all aspects of their performance."
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7c1j6o | How does hit detection work in first-person shooters? | Engineering | explainlikeimfive | {
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"[check out this video by Gameranx that talk about bullet physics in games]( URL_0 ) the player basicly have a lazer pinting straight ahead, if a enemy is touching the lazer when the player press the trigger, the enemy will get hit. some realistic games have a delay from the player pulling the trigger to the enemy getting hit to give an effect of bullet travel time or bullet drop.",
"There are many solutions to this, but usually there is one computer that acts as the server (if it's P2P then it's the hosting player, if it's dedicated servers then it's the actual server itself). The server is the only computer that calculates hit detection and has a complete picture of the whole game. Everyone else just sends and receives updates from it. So if you shoot someone, you don't actually do any calculations on your computer. Hell, you probably just tell the server that you pulled the trigger and it will create the bullet for himself and do all the necessary calculations without asking you for anything. And then, when the bullet collides (or not), it will just tell you that it collided (or not) and then your computer would draw the blood splash, or ragdoll the dead target, or whatever. The way ping factors into all of this is that: A) If you lag then it will receive you input message late and will spawn the bullet late, thus possibly missing your target. Nothing can be done about that. B) If your target lags, then it will need to do some smoothing to somewhat predict where they would be realistically, if they didn't lag at this very moment. Technically that is not hit detection, but should be mentioned anyway.",
"In terms of ping, hits are calculated on the server, not on the client, which is how come you might see rubber banding on your game but still get hit by someone. The most basic games just simply draw a straight line from you to the target to see if you hit them. Some games take into account some physically effects. Star Citizen is the only game that I know of (though I'm sure there are others) that actually creates bullets as physical game objects and calculates physics on the bullets in that way. Pretty cool."
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7c1ka4 | What is Big O, Big Omega, and Big Theta Notation? | Engineering | explainlikeimfive | {
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"When you have two sequences or functions, you are often interested in how fast they grow compared to one another. For example, suppose you have two functions f(n) and g(n) that tell you how many operations two different algorithms use to sort a list with n elements. As n gets bigger, if the number of operations f(n) gets bigger faster than the number of operations g(n) gets bigger, then you would say that g(n) is a more efficient algorithm. There are a variety of ways to make that idea more precise, and Big O/Omega/Theta are one of them. \"f(n) is Big O of g(n)\" means that the rate of growth of f(n) is at most the rate of growth of g(n). \"f(n) is Big Omega of g(n)\" means the rate of growth of f(n) is at least the rate of growth of g(n). \"f(n) is Big Theta of g(n)\" means f(n) and g(n) have the same rate of growth.",
"/u/BigSerene is very close to correct, but I want to rephrase what they said an add some clarification. Buckle up, because this is knee-deep in college-level computer science. Big-Oh notation is used to compare algorithms and determine which ones take more time than others to complete. But what is an algorithm? Well, it's a finite series of steps to solve a problem. Very handy in computing. Some algorithms are really slow, though. Some are fast. We want to find the fast ones and avoid the slow ones (assuming both of them get the right answer in the first place). In order to do that, we need to have a standard to compare them by. That's Big-Oh. Say I want to find a pair of matching socks. Once thing I could do is, for every sock I own, compare it to every other sock I own (at a rate of one sock per minute, for simplicity) until I find a match. In the worst case, I'd need to go through every sock I own, _for every sock I own_, to find a match. Let's write it in Big-Oh. The notation is written relative to the size of your input. Usually this size is represented as _n_, and in this case, it represents the number of socks I own, which can vary over time. We want our algorithms to run reasonably well regardless of the size of the inputs. This one runs in O( n^2 ). One nice thing Big-Oh does for us is get rid of clutter. I'm going to avoid the actual definitions because this isn't /r/askscience, but the jist of it is that we can ignore any _constant-time processing_ that isn't related to how many socks we're looking through. Say I putzed around on my phone for an hour for no reason before I picked my socks. We could speed up by _not_ doing that, but it won't change or Big-Oh expression, because O( 60 + n^2 ) = O( n^2 ). Same story if I wanted to count up all my socks beforehand. O( n + n^2 ) = O( n^2 ). Or if I wanted to double-check something about each pair of socks I looked at. O( 2n^2 ) = O( n^2 ). It's really nifty and helps us compare algorithms without having to know the specifics of how the data is handled or preprocessed. Obviously the algorithms with the additional expressions will run a tad slower, but when you're processing a hundred million socks, those expressions are eclipsed (known as the asymptotic running time). These next few explanations are a distillation of the definitions, to keep things a little simpler. Unless you want to get in deeper, just take it at face value that constants are ignored, and that all members of an addition expression _except the one with the greatest magnitude_ are also ignored, a la O( 1000 + n + 1000n + n^2 ) = O( n^2 ). Big-Oh says that, _at worst_, the approximated running time of an algorithm (how long it will take me to find my matching pair of socks) will be _no higher than_ the approximation given in parentheses (in this case, n^2 ). Big-Omega is similar and says that, _at best_, the approximated running running time of an algorithm will be _no better than_ the approximation given in parentheses. For an algorithm to be Big-Theta( n^2 ), it has to be Big-Oh( n^2 ) _and_ Big-Omega( n^2 ). Those last two are mainly used in analysis, or to find the Big-Oh notation of algorithms that don't play quite as nicely as my socks example (say, algorithms that recurse). Hope that helps!",
"I'd highly recommend reading the article \"A Gentle Introduction to Algorithm Complexity Analysis\". It really helped me understand the concepts and how to analyze the complexity of an algorithm, along with the notation. URL_0"
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7c1sdc | Why are the heating vents placed over windows in homes and heaters over the entry doors in big stores? It seems like it would be inefficient as all that warmth gets lost to cold outside temperatures. | Engineering | explainlikeimfive | {
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"The reason is sort of basically to counteract the worst areas of weather differentiation. Window glass is the coldest part of a wall. When warm room air hits it, the air cools, and cool air sinks. The movement of cool air creates floor drafts that most people find uncomfortable. The placement of forced-air heat registers or baseboard heating units under the windows counteracts this process by sending up warm air to mix with the cool. The end result is that the room feels more comfortable. The reason is because the heat loss occurs mostly in the windows and the fenestration. The idea is that you would like the incoming air to be heated up. Also, it creates an air curtain that prevents more heat from being lost through these exposed areas. Finally, it makes the temperature of the room more or less uniform. If the heaters were placed at the center of the room, you would create a large temperature gradient, resulting in drafts and discomfort for the occupant. source: URL_1 URL_0 and that I have a minor in architecture",
"It creates a kind of wall of hot air. If the window in open it obviously doesn't work as well but any air that hits the window and is cooled, when it comes back it gets warmed by the air in front of the window. It also prevents the imbalance if temperature u can get sometimes. Ever been able to feel the temperature difference when walking across a room, radiators by the window prevent that to an extent.",
"Those blowers over the doors of stores and places like that, aren't heaters but just air blowers designed to create a kind of \"air barrier\" to prevent the hot/cold air from inside to scape outside",
"In homes, heating vents and radiators are placed next to windows so that the rising hot air immediately moderates the cold air by the window. By convection, the now coolish-warm goes up, across the ceiling and then comes down on the other side of the room, from where it comes back to the window to repeat the cycle, driven by the convection currents. By this second and subsequent go-arounds, the room temperature is raised to a comfortable level all around. If the radiator had been placed away from the window, yes there will be a hot corner there but the convection currents then bring this warm air across the ceiling and then, already cooler, down the inside of the cold window. You now have a stream of cold air creeping back across the room, with the effect that in that far corner, you’d have an uncomfortable temperature gradient between the upper half and lower half of the room."
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7c5jp4 | Why do different engines require different viscosity lubricating oils? | Why do some engines use 5w-20 as opposed to 10w-20 or 20w-50? | Engineering | explainlikeimfive | {
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"Engine are built using \"tolerances\" between parts. those numbers simply means that they define a space between each moving components. Engine Oil is simply filling up the space between those parts to reduce the amount of friction. The wider the space , the more viscous oil will be required to keep things from rubbing too much. On the opposite, the tighter space will require a thinner oil to be able to seep thru. So when a engine is built , the maker will do some testing to see which oil viscosity is required to enable those hundreds of friction points to be lubricated and stay lubricated."
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7c7yqz | why do we have an International Space Station but not an International Underwater Station? | Engineering | explainlikeimfive | {
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"We do have an international underwater station, of sorts. While it's primarily funded by universities and government agencies in the US, [Aquarius]( URL_1 ) has been home to scientists from all over the world. It's located just off the Florida Keys nestled in a reef. The scientists use a technique called [saturation diving]( URL_0 ) so that they may stay and do research for days to a couple of weeks at a time and can then decompress and resurface appropriately.",
"We wouldn't be able to. The pressure of water above you would be too high. The Earth's oceans are sometimes more of a mystery than our universe."
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7calzd | Some of online programs are open source and have its source code on GitHub (for example bitwarden, a password manager). Doesn't knowledge of the source code allow hackers and user to break and abuse the program easier? | Engineering | explainlikeimfive | {
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"If knowing how your software is written lets me break your security then you have terrible security Security through obscurity is no security at all A well done security algorithm is still secure even if you know all the math behind it, having that open so that people can check it ensures that you have done the math right and haven't accidentally(or intentionally) introduced weaknesses into the security based off of how you implemented it. Not having the source code public doesn't stop hackers from analyzing it and finding weaknesses, they can still decompile it, but it does stop the public and researches from finding and patching weaknesses before they're exploited.",
"And many others can look up the code to get rid of flaws and edges. Open source has the oportunity that the whole community can contribute to make these programs save and secure.",
"Its balanced by the fact that since the code is public, anyone who wants can check the code and identify or fix any security flaws present. If you want to exploit a flaw in Open Source software, you either have to be the first one to find it or you need to be a lot smarter than the people developing and checking it.",
"As a rule of thumb, if knowing how a program works poses a security risk, you don't want to be using that program because of the security risks. Programs aren't supposed to come with bunch of errors, security flaws and bugs. If you ship one anyway and just trust these flaws are hidden well enough, you're essentially just waiting until one of your employees turns into the dark side and sells these flaws, or someone just stumbles on them anyway and starts using them. You as a user then should ask yourself, if someone **doesn't** have their source code available to you, how secure can you be really? NSA actually had leaks some years back which revealed that they considered it a significant problem that they couldn't easily introduce flaws into open source programs that they could later use to attack users of those programs. With closed source, their opinion was that introducing these flaws was much easier.",
"Well, it may... or it may not. Ideally, there should not be any security flaws at all, but in the real world bugs that can cause security holes are pretty much unavoidable. So it really all comes down to an arms race: who finds the flaw first, a responsible user or a malicious hacker? Whether the software is open source or closed source probably doesn't make a lot of difference to the arms race, in that it's going to be a race between the good guys and the bad guys one way or the other. Making it open source makes things easier for the bad guys, but it also makes things easier for the good guys. A problem with closed-source software is that some software companies are very reluctant to fix the flaws when they are found, instead preferring to deny that a problem even exists: with open-source software, if the flaw isn't fixed by the original creator, somebody else can fix it. A problem with open-source software is that you may not know whether the patch you're downloading for it comes from a reliable source: it's possible for a malicious user to release a patch which contains a deliberate security flaw, effectively making it a Trojan. Whether closed-source or open-source, the biggest issue is actually a reluctance by software administrators and private users to keep the software properly updated. There have been some massive security issues involving open-source software (the Equifax breach was one), but usually because somebody has seriously dropped the ball and the victim hasn't kept the software patched. The Wannacry virus was an example of a security flaw involving closed-source software. It affected outdated versions of Windows, which many companies and organisations were still using, but Microsoft had long since dropped support for it -- meaning that no patch for that particular flaw was issued for those platforms before it was exploited. In this case, though, it's not entirely the companies' fault: it is hugely expensive for a large company to switch to new (closed-source) operating systems every few years, something that often entails buying new hardware as well, and there's no guarantee that the software your company relies on won't break when the system is upgraded. Had the various versions of Windows been open-source, it's possible that somebody might have released patches for all of them before Wannacry became an issue. Basically, using open-source software is probably at least as safe as closed-source software, but you really do have to ensure that all your software is always properly patched and up to date otherwise you're going to be in trouble whatever kind of software you use.",
"So there are some decent explanations but I think they are missing a bit. The big thing being that knowing the code does not allow a hacker to \"create\" a flaw. An exploit either exists or it doesn't. Hackers are attempting to find ways to do things that weren't intended. To keep this an ELI5 lets create a simple physical example. Let's say I buy gold for a living. In my shop I create a scale that converts the weight to a dollar value I will pay the customers. I have an attendant who will handle the transactions and he does only exactly what I tell him. In my instructions I tell him that he will take what the customer hands him, put it on the scale and pay the customer what the read out says. In creating my instructions I assume that because I only buy gold that customers will only give my attendant. I don't tell him not to weigh things that aren't gold. Well for a few months things go great until one day a customer hands my employee a rock instead and the attendant still weighs it and pays him. The flaw was always there but now it has been discovered. In an open source environment I would have posted the attendants instructions on the wall and someone might have pointed out the flaw to me before a malicious user found it."
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