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9v323c | How do fiber optic cables carry the internet? | Engineering | explainlikeimfive | {
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"The cables are like tubes for light. Shine light in one end and it illuminates the other. Using this, you can turn a light on and off on one end and have a sensor on the other to read it. By using pre-determined binary sequences of ons and offs, it can transmit data in the same way that computers and electronics do.",
"Everything is converted to ones and zeros then it is transmitted as a series of light pulses which represent the ones and zeroes. Once received, it is converted back to it's original form. Better explanations will be coming.",
"It's really as simple as turning electrical signals into light. This improves the speed of signals, and allows for a more responsive internet experience as it minimizes delays from travel time significantly. It also reduces problems caused by sources of interference, like radio waves. [Basics of Fiber Optic Cable]( URL_0 )"
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9v3uze | How thermal shirts are warmer than regular shirts. | \-Why is thermal material better at obtaining warmth? | Engineering | explainlikeimfive | {
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"When the air is cold, there's always a thin layer of warm air covering the skin, which acts as a sort of blanket called the \"boundary layer\". When the air is perfectly still, the boundary layer is thick, and we don't freeze easily - but wind will blow the boundary layer away, which is why wind feels so cold. Clothes don't just provide insulation, they also keep this warm air around your skin and prevent wind from blowing it away. Skin tight clothes work much better than loose clothes for that purpose, since they don't flop around and have any opening to let the wind in. That's why they're much warmer outside than a regular shirt. In addition to that, these thermal shirts are usually made from a material that doesn't soak in sweat, which is really important to prevent wind chill."
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9v553k | M: How do barcode scanners work? | Engineering | explainlikeimfive | {
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"A typical barcode consists of 95 columns of the same thickness. When a reader scans the barcode it reads the columns left to right, it reads a black column as a 1 and a while column as a 0. When you see black (or white) columns that are thicker than others, that’s a series of 1’s (or 0’s). It reads this by shooting a laser at the barcode and testing the reflection. Not a lot of laser reflected back? It’s a 1. Lots of laser reflected back? 0. Those 1’s and 0’s together create some kind of information (typically an item code in the store’s database that the register can reference for information such as price). This is called Binary, which is a totally different subject.",
"They shoot a laser at the barcode. There is a sensor that detects light. The laser hits the light part, it gets reflected into the sensor and reads it as a one. The laser hits the black part, it gets absorbed and the sensor detects nothing and reads it as a zero. There's a chip that puts all the ones and zeroes together and gives it to the thing the scanner is plugged into."
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9v7oxd | Why do americans like to build their houses out of wood when they live in natural disaster-prone areas? | Engineering | explainlikeimfive | {
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"You have to look at the kinds of disasters that we have. With Tornadoes the winds are so powerful that brick and stone homes are not much more durable than wood. You can make it out of reinforced concrete, but that is much much more expensive than wood homes and concrete is much uglier. With hurricanes winds can be damaging, but those winds are much slower than in a tornado and most damage is not from winds. The damage from hurricanes is from the flooding. Both rain and storm surges flood buildings damaging the building and washing out the foundations. Concrete does not provide much resistance to black mold and none to foundations being washed out. With earthquakes wood is superior to concrete, brick, or stone because it is able to bend and shift with the movements rather than crack and break. With wildfires wood is weaker. So here it would make sense to have more durable materials, save that the regions prone to wildfire are also prone to earthquakes. Then you have costs, which I commented on earlier. Wood is extremely cheap in the US, but properly build concrete, brick and stone homes are not. Most people are willing to put up with the small amount of risk for fire in order to have a larger home.",
"I know in California it’s because you need more flexible materials for when earthquakes come"
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9va442 | What is really stopping science from make commercial planes fly higher than presently possible to shorten travel times? | Engineering | explainlikeimfive | {
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"I suspect the answer isn't science, but economics. It's possible to make commercial planes to use different technology to shorten travel times. However, the costs for it would not be commensurate with the advantages, so people wouldn't be willing to pay enough to make it worthwhile. For example, look at the Concorde."
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9vdkrh | How do stealth planes work? What makes them stealthy? | Engineering | explainlikeimfive | {
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"When they talk about stealth, they mean via RADAR. A quick breakdown of RADAR: Pretend it works like a dolphin's or bat's echolocation: we shoot radio waves in a direction and then those waves bounce off stuff and if the waves bounce back toward us, we can \"see\" whatever the radio wave bounced off of. Stealth planes use a combination of angular shapes and reflective materials to bounce RADAR off of the plane, but not back towards the \"listening\" location. Imaging a dolphin trying to look for fish with clicks, but every time a sound hits the fish, the sound bounces up or down instead back to the dolphin, making it so the dolphin doesn't detect that particular fish.",
"Most commonly, stealth refers to avoiding detection by radar. Radar works by sending out a signal and then measuring what bounces back. You send out a pulse with a power of of 10, and a big plane might bounce back 1 and a small plane bounces back 0.1; your goal is to reduce the amount of power that returns to the radar tower even further than that so that the radar basically thinks it's just getting random noise back instead of being able to identify that it's seeing a plane, or by making it misunderstand what kind of a plane it is actually seeing. This is primarily done by designing the plane's shape so that the radar that bounces off the plane bounces away from the radar tower and by coating the plane in material that absorbs the energy from the radar.",
"So as many have said, geometry is a big part of how to bounce RADAR in any direction OTHER than back at the RADAR receiver. It also helps to use materials that reflect these radio signals. Other strategies involve using materials the RADAR passes right through, or to build inside the hull of the aircraft microwave traps, which use RADAR frequency reflecting metals and geometry to bounce the waves INTO the trap where they are absorbed - there's no going back out. Another method is to paint the plane in metallic paint, typically an iron based paint, as this will absorb many of these radio waves. Other forms of being stealthy involve making less noise, and dissipating heat. You can make less noise by flying slower, which is why the B2 can't go supersonic, there's no point. The geometry of the exhaust also try to spread out the heat so it cools faster and leaves less of a thermal streak in the atmosphere. Another means is by reducing, well, really any emission. They go radio silent, because you can pinpoint where a radio transmission comes from, they don't use signal lights, a \"duh\" sort of thing. And another clever trick is to fly at night, which should hopefully be obvious as well. These countermeasures don't make these craft impervious to RADAR, it just reduces it's effectiveness. When you use RADAR, you're broadcasting where your equipment is. So the thing to do, then, is to take a path that follows the greatest gaps in their coverage, where they are furthest away, where the signals are weakest. This path isn't always higher and higher... Another thing you can do is use support and have another craft flood the air with RADAR noise to confuse and blind their systems. This craft is itself not stealthy, because if RADAR were audible to humans, it'd be some sort of transmitter SCREAMING from here to the horizon. It's also a reason to believe you're about to get bombed.",
"Stealth planes either scatter radar waves randomly, or absorb (let pass through) radar waves, depending on the model of plane, reducing their radar signature, (the amount of waves reflected back to the radar receiver) or eliminating it entirely. If they do show up on radar displays, they don't register as planes.",
"Imagine you're standing in the middle of a field blindfolded, and can't move. Somewhere around you is an object you need to find, but all you have to help you locate that object is a bucket of ping pong balls (work with me here...). In order to find the object you start throwing ping pong balls in every direction until you feel one bounce back and hit you. Congratulations, you now know where the object is. This is essentially what radar does. It shoots out radar waves (ping pong balls) in every direction, and when those waves hit an object, they bounce off that object directly back to the source, registering a \"hit\". What \"stealth\" planes do is basically make it so those radar waves *can't* bounce back. This is achieved by a number of different methods that others have already mentioned (surface coatings or creative uses of geometry to capture waves) So, imagine that same object in the field is now covered in glue. Your ping pong ball hits it, but now it can't bounce back. As far as you know, you didn't hit anything with your ping pong ball, so you have no way to tell that something is actually there."
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9vgomn | If aircraft cabins are pressurized, why does it still make our ears pop? | Engineering | explainlikeimfive | {
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"Because they are not *fully* pressurized. Typically they are only pressurized to the equivalent of 1 mile altitude.",
"They’re not pressurized to sea-level conditions. Doing so would exert too much stress on the airframe. Typically planes have the equivalent pressure of about 8-10k feet. Enough to relieve the strain on the plane’s frame, but enough oxygen so that passengers don’t need oxygen masks.",
"Your ears pop when there is a pressure change, even a small one. The equivalent cabin altitude starts at sea level when you take off and slowly climbs to a normal range of 5000 to 7000 feet while the airplane climbs to a cruise altitude usually in the 30-38k range. The cabin is kept at this higher pressure by diverting high pressure air from inside the engines into the cabin. If the engines are brought to low power to start a descent the air coming in is less so there is a small pressure change and then another small pressure change when power is brought back up. Some airplanes are “leakier” than others because the seals around the doors and emergency exits are worn a little meaning the max cabin altitude is a little higher and the pressure changes are greater. The largest pressure change is when the plane descends to land, the cabin altitude descends to meet the actual altitude somewhere around a couple thousand feet above the airport.",
"They are not fully pressurised; the aircraft climbs/changes pressure at a different rate then the pressure can escape (or enter) our ears.",
"I’m a certified flight instructor. Air is continually compressed and pumped into the cabin. If the pressure differential between outside and inside were to grow too great the plane would burst like an over inflated ballon. That’s why it can’t be pressurized to sea level when at altitude. The fix is a check valve that continually releases pressure once it gets to a certain level. So your cabin pressure is constantly changing based on the pressure differential that your aircraft is designed to withstand. As you climb, air outside gets thinner so there is a greater pressure differential. The pressurized air in the cabin pushes against the inside of the aircraft with a greater force. This greater force opens the check valve which vents air and allow cabin pressure to drop without damaging the aircraft. This causes your ears to pop as the aircraft changes altitude."
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9vl677 | Why are some roads or highways concrete and some asphalt? | Engineering | explainlikeimfive | {
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"It depends on volume the roadway is expected to handle, size, scope and cost of the project, availability of resources, service life expectancy of the road, climate, complication of shutting down an existing freeway for maintenance or complete replacement, how the road authority cares for the road, just to name a few. I work for an asphalt and aggregate company, I prefer concrete because it has a much longer service life but the cost is far greater than asphalt. I drive on a road everyday my company laid the concrete for in the 1960’s and it is still in service in Minnesota where the salt and freeze/thaw cycle is very hard on any road surface."
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9vlxsa | ; Why are public toilets so much harder to clog than private toilets? | Engineering | explainlikeimfive | {
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"Public toilets usually have access to much higher volume water supplies than at homes. This is due to the amount of people they are expected to be able to service; your home unit likely has a tank where water is stored for the next flush and is slowly replenished. But public toilets may need to be flushed often and without the failure points of the tank mechanism. Instead the public toilets can flush with a blast of a larger amount of water than the domestic piping can handle, and this also serves to blast stuff down the drain with more force. Also they really want to avoid clogs so they can make the drains bigger too."
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9vqi4m | With all windows and doors shut, how is it possible to perfectly breath inside? | Engineering | explainlikeimfive | {
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"Your house is nowhere near air tight and there is a ton of air inside it even if it was. The issue in air tight spaces is not lack of oxygen anyhow- it is an increase in carbon dioxide that happens when you exhale. But even in an air tight house it would take a very long time to be an issue."
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9w4n5i | Why are buses still mostly box shaped? Should they not have a more aerodynamic shape? | Engineering | explainlikeimfive | {
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"Aerodynamic drag is essentially inconsequential below 40-45 mph, so since most buses operate below this limit, aerodynamics are a low priority. The other issue is that you need to make *both* ends of the bus aerodynamic in order to truly mitigate drag because of how pressure drag works (it's also the same reason you see those semi trucks with the inward-facing wings on the back of the trailer). However, on the back of the bus that basically turns into wasted space that you can't jam people into, and at the *front* of the bus it harms visibility for the driver.",
"I think busses are more concerned with fitting as many passengers as possible, they generally don't need to travel at high speeds like trains. Boxes fit more seats and safe standing areas than tubes.",
"At the low speeds that buses operate at, aerodynamics are not important. What is important is maximizing the number of passengers relative to the footprint of the bus. Boxes happen to be excellent at that."
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9w6zx5 | how do jet engines compress air? | how exactly does the spinning of the blades compress the air? | Engineering | explainlikeimfive | {
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"When air first passes through the engine, it goes through the compressor stage which is a series of fans that vary in angle and diameter forcing the air to compress and go into the second part where it is mixed with fuel and ignited. The expanding gasses are then forced out the opposite side of the engine where they pass through more fans which are connect to the same shaft as the compressor fans. This keeps the fans turning once they are started, but to start them they normally need an external supply of compressed air.",
"It's not obvious unless you really think about the whole jet engine, but they do it same way you compress anything else: by cramming lots of stuff into a confined space. The fans push air down the throat of the engine, which narrows a bit behind the fans. This narrowing space steers the airflow inwards from all sides. The same amount of air is now occupying a smaller space, the definition of compression. The air can't simply stop because the fanblades are pushing more air in. Even without the narrowing, there will be some compression simply because the fans are pushing air IN faster than it can go OUT. Picture a hallway that narrows down to a single open doorway. If there is only a few people moving down the hallway they can go through the door without slowing down very much. If you get too many people moving down that hallway, they'll tend to pile up near that doorway. Add something behind the people that's pushing them forward (e.g. other people desperate to escape a burning building) and people at the front will end up getting crushed.",
"A jet engine is a turbomachine which begins with a compressor stage which compresses the incoming air to the nacelle into a combustion chamber where fuel is combusted in the presence of that compressed air, and the exhaust gases from that combustion drive a turbine which shares a shaft with the compressor blades. The faster the engine runs, the more air is compressed.",
"Turn on your room fan to the highest setting. Put your face right in front of it. & #x200B; Do you feel the air press against your face, pushing your hair back? & #x200B; That's compression, and the pressure will build up if the air cannot escape fast enough."
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9wabwe | How did the synchronisation gear in WWI fighter planes work? | Engineering | explainlikeimfive | {
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"The axle of the propeller had a bump on it behind each propeller blade. The bump would push up a rod, and the rod would then push up another rod that would interrupt the firing mechanism on the machine gun so it couldn’t shoot when there was a propeller blade in the way. Overall it was a fairly simple mechanism."
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9we0kh | Why are train tracks covered with stones? | Engineering | explainlikeimfive | {
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"It's known as a ballast - it's job is (in Eli5 terms) to hold the wooden crossbeams in place (which in turn hold the tracks in place) Stones are ideal because they are heavy enough to hold the track in place while having enough give to be able to be put under pressure (i.e by a train weighing a few hundred tonnes), limit weed growth, can handle rain,snow, etc.",
"Tracks need to remain level and straight for long periods of time. They are also exposed to tremendous weights and stress. The stones placed under the wood ties called sleepers are to ensure that the weight of the train is distributed evenly over the ground. Without the stones, the earth beneath would settle and sink, making the tracks unsafe.",
"Lots of reasons! Water makes steel rust and wood rot. Rocks don't hold water. Think of a lake vs gravel bed. If you were to flood an area, rocks would drain almost perfectly, where clay and soil would hold the water until it evaporates. By draining the water away from the train tracks, they last way longer. Keeping the area dry also cuts down on vegetation. No green means that the sun hits any water directly, helping evaporate away whatever didn't drain. It also makes the immediate area uninhabitable for larger vegetation since there's not enough water. Maybe some weeds, but bushes and trees that could do some real damage aren't going to get started in the rock, and there's not enough sun hitting the soil beneath the rock if they got down there. Rock is also ballast. Trains shake train tracks ALOT. Engineers use the same technology to dampen Earthquakes, Guns, and Trains: Be heavier than the shaking. We can anchor the tracks to wood beams that don't break when you bend (shake) them, and then anchor the beams in the rock. This system lets things move around in little bits without actually moving in big bits. If you anchor everything rigidly in concrete and steel, the shaking will eventually start breaking up, failing completely. We can design for this, but it will need to be replaced with some regularity. That means shutdown the line, rip out the block, and pour a new one. Kinda like doing highway repair. And that's Gravel's true benefit; It's cheap! If the RR company sees the gravel is falling away from the track, but the track itself is still fine, they can just dump more gravel on it and let the vibration of the track settle it into place. Maybe 30min of shutdown. Not 30 days.",
"The gravel is used to keep the ground level, and stop it from settling. Even when it settles it generally settles all across, because the ties disperse the weight so well.",
"If they were just on top of grass/ground vegetation would grow which would have to be cut and maintained to allow access to the tracks at all times and to make sure nothing gets ON the tracks themselves. Stones stop anything from growing there and make the tracks easily accessible for maintenance."
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9wifcz | Why do elevators need to close the door after just seconds of inactivity even if no buttons have been pushed? | Engineering | explainlikeimfive | {
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"If the doors are assumed to close after use, people can expect it and move out of the way. It will also move to the next floor faster if someone presses a button on a different floor if the doors are closed downstairs. I imagine it would make users more nervous and hesitant if an elevator door left open on their floor suddenly closed while they were walking up to it if someone called the elevator upstairs at exactly the right moment.",
"Some elevators that have to cover a wider span of floors will sometimes reposition themselves so that, based on time of day and whatnot, they're on a floor closer to where they'll get called to. But more often than not its just to save a few seconds to close the door when they DO get summoned.",
"The do not need to do that but there is a good reason for that behaviour. If there is someone in it the will soon press a button and it can start faster and usable for someone else faster. If it is empty it can move toward the floor where a button is pressed directly without closing the door."
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9wlfj4 | Why do pens sometimes not work in the one spot you want them to? | Engineering | explainlikeimfive | {
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"Typically this means that spot on the paper is slick — and it gets even slicker from the polishing effect when you run the pen over it — so the ball point won't roll because there's not enough friction."
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9wozc2 | If light switches work by breaking the circuit when you switch them, how do lights that are controlled by multiple switches that change direction based on each other work? | Engineering | explainlikeimfive | {
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"A normal light switch is either open or close A three way light switch connects hot to either wire A or wire B. If both switches are set to A then the light turns on, if they're both set to B then the light turns on. If they're in different positions then the circuit is open and the light is off. See the rough wiring diagram below Hot---Switch < === > Switch----Light----Neutral",
"Two wires, from switch to switch. If both switches are closed on wire a, or wire b the circuit is complete. Light on. If one switch is closed on wire a, and the other is closed on wire b, the circuit is broken twice, no light."
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9wsheq | How do turbines create electricity | Engineering | explainlikeimfive | {
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"To do that you really need to explain generators first. When you run electricity through a wire, a magnetic field is formed around it. If you coil it, that field \"stacks\" and gets stronger. You can wrap wire around a nail, hook it to a battery and make an electomagnet. Lots of kids have done this experiment. Now the trick is that people forget, you can do this in reverse. If you push a magnet against a wire coil, it makes electricity. It actually resists you slightly as the electricity is made just for a moment. A generator is just a bunch of coils and magnets efficiently deigned to do that over and over continually. It turns physical energy into electrical. A turbine is the fan part that turns the generator.",
"the spinning from the turbine spins some magnets in a generator. moving magnets near wires makes electricity happen in the wires.",
"Short answer: they don't actually create electricity, the alternators attached to them do. Long answer: A change in a magnetic field induces a current in a wire. An alternator is a bunch of coils of wire on an armature, with a bunch of coils of wire on a field spool that spins around. The field spool is only used to create and sustain a magnetic field, ie to excite the armature. As the field spool is spun around by force driving the turbine, a rotating magnetic field is formed which causes current to flow in the armature coils, and which causes an AC current to be generated. Now, the reason that the armature is outside the field spool, ie why an alternator is \"inside out\" is one of power. The amount of power that an alternator produces is gargantuan, which makes it difficult to tap it off using brushes - so instead, they are used for the lower powered excitation current that's put into the generator."
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9wwhul | How is uranium mined? | Engineering | explainlikeimfive | {
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"Pitchblende, or now renamed Uraninite, is a steel coloured mineral which contains high amounts of uranium oxide (UO2 or U3O8). This is found in many parts of the US, South America, some parts of Europe, South Africa and Australia. Other minerals exist but this is the most common. The mineral can be crushed, then acids, alkalines and peroxides are added sequentially to leech out almost pure uranium oxide. The uranium oxide can be combined with fluorine gas to produce Uranium Fluoride (UF6), which is a gas. The gas can then separated by centrifugation. Enriched uranium-235 (which is lower density than uranium-238) can be extracted this way. The uranium fluoride can be reduced to uranium metal, then placed into nuclear reactors / weapons. The whole process to produce viable uranium fuel requires insane amounts of resources, produces lots of radioactive waste, and uses hundreds of specialised gas centrifuges. This makes it impossible for any nation to hide that they have nuclear capabilities."
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9wxq5h | engine liters and how they define the size of an engine | I was just watching a YouTube video of chevys new 2.7 liter turbo charged 4 cylinder in their Silverado trucks. And I remembered ford has a 2.7 liter v6 and was confused how this actually works. I know most v8’s by their liters. 4.7, 5.7, 6.0 etc etc and googled if a 4.7 or 5.7 could be a v6 and nothing was produced. So How is it possible to have a 2.7 liter in a 4 cylinder and a v6 variant but not the bigger liters in smaller engine variants? | Engineering | explainlikeimfive | {
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"Just to cover it in case you don't know. V8, V6, V4, these refer to the number of cylinders in the engine and their configuration. V6, 6 cylinders in a V shape. The liters refers to the total volume of the cylinders. The reason why 2.7 liter V4 is a new thing, even though there are 2.7L V6s. Because each cylinder needs to be bigger, each cylinder is 0.675L to total 2.7. With a V6 each cylinder would only be 0.45L.",
"Imagine you need to provide 12 litres of water to someone. You could give them 8 jugs of 1.5L (8 cylinder) Or you could give them 6 jugs of 2L (6 cylinder) Or you could give them 4 jugs of 3L (4 cylinder) At the end of the day, they get 12L but it comes in different configurations. Generally it doesn’t scale like this but you get the picture. Litres is the amount of space inside each cylinder and the V8/V6/4-cyl is the number of cylinders and the configuration of the engine.",
"Engine capacity is cylinder bore times piston stroke times number of cylinders. So a 6 litre 4 cylinder engine needs each individual cylinder to be 1.5 litres. That's a very powerful bang each time the mixture ignites. So you need strong components, particularly the connecting rod and crankshaft, to handle that force. Better to divide it up between six or even eight cylinders of lighter weight. It's an example of the square-cube law in action, strength goes up with cross section (scale factor squared) while weight increases with volume ( scale factor cubed)",
"Engine displacement is the total swept volume of all the cylinders, the surface area of the pistons times the length of the piston stroke. The combustion chamber (the space remaining when the piston is at the top of its stroke) is not included. So, a 2.7 liter 4 cylinder has larger pistons, a longer stroke, or both, than a 2.7 liter V6. The world's [largest piston engine]( URL_0 ) is 14 cylinders at 1810 liters per cylinder.",
"Everything is theoretical possible. However things are the way you see, mostly due to historical reasons. Developing and manufacturing a new engine is extremely expensive. So automakers use engine blocks that were developed before with new cars. It is very possible that Chevy or Ford has such engine and they installed it in their new model. Traditionally trucks in America had 8 cyl. But in Europe engine volume was smaller and they had 6cyl. A large number were Diesel while America ran on gasoline. Is the question why aren’t there 5L engines with only 4cyl? Because with more cylinders the engine is smoother. And they can afford the extra cost in a larger, more expensive block. Engines with less cylinders are cheaper to manufacture.",
"General rules: 1) the bigger the total engine displacement, the more power you get. 2) the more cylinders you have, the more everything costs because you need more of each part. 3) the more cylinders across which you spread out your displacement, the less power you lose reversing direction of the pistons as they go up and down, because everything weighs less, and you can also rev higher for the same reason. Car manufacturers run the numbers on these and many other variables they can tweak, but mostly cost vs power, which dictates passenger cars tend to be 4-cylinders up to around 2.5 litres, 6 cylinders up to about 4 litres, and 8, 10, or 12 cylinders for anything bigger. For a big engine, it’s too inefficient to use a 4 cylinder, and for a smaller engine it costs too much to use more. Which is why you see these rules bend (more cylinders per litre) where cost doesn’t matter so much, such as f1 racing and Supercars."
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9x3rto | What does higher grade gas actually do to your car? Should I enter 93 octane into my Honda Civic, or stick to 87? Does it really matter? | Engineering | explainlikeimfive | {
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"Higher octane ratings burn slower and are used to prevent engine knock in cars that reach higher RPMs. You shouldn't have any adverse effects on your car by using high-octane fuel, but if the owner's manual doesn't call for high octane fuel, it won't actually help any either, and you're essentially just wasting money.",
"If your car doesn't specifically need the higher octane fuel, you won't get much use out of putting it in your car. You shouldn't cause any negative effects either, but you're basically throwing away an extra 10-20c per gallon of gas. Some cars are tuned specifically to use higher octane gasoline, because they can compress more before they'll ignite and can be used to increase performance in very fast, high RPM-cars or in heavier luxury cars that can make use of that extra power. If you put lower-octane fuels in those cars, the fuel can detonate prematurely and cause \"engine knock,\" which causes the engine to shudder and is bad for the engine mechanically. But as long as your car isn't tuned to need premium fuel, you'll get no real benefit or harm from using any type of gas. Your Honda Civic will perform exactly the same no matter which pump you pick, so you might as well use the cheapest one.",
"Imagine that each cylinder inside your car engine is filled with blueberries instead of gasoline. As the cylinder compresses the blueberries, they might explode if you have too much pressure, leading to broken, crushed blueberries. Now imagine that your cylinder is filled with Haribo gummy bears. This time, when you compress the cylinders, they don’t explode under high pressure, and no mess is made. An 87 octane rating is like the blueberry, and a 93 octane rating is like the gummy bear, if your car has a high cylinder pressure. High pressures come from turbocharging. Higher rated gasoline’s are able to better resist spontaneous combustion from high pressure. Most consumer cars do not come with turbochargers, and will normally run with pressures that aren’t high enough to crush / explode the blueberries. This makes 87 octane a sufficient rating for most economy cars. Of course, even with a normal pressure, some blueberries may still be crushed, so in the end you have less blueberries to work with (but almost all of them are intact). Filling with gummy bears ensure that none of them are crushed, leading to higher efficiency in your engine. Non ELI5 portion: 87 for your Honda Civic is perfectly fine, but you May see slight increases in fuel efficiency using 93. This increase is likely negligible compared to the price jump for 93. However, if you’re in a car with a turbocharger, or a performance / sports car that is designed to run the cylinders at high pressure, using 87 octane will cause a premature combustion in the cylinders, resulting in abnormal cylinder firing pattern (this is called engine knock), and reducing your power. This is why all BMWs require premium 91+ octane fuel."
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9xate1 | Why is there a gap on a normal mirror when you put your finger to it? | I stay in hotels all the time and one phrase that occurs in my head every week is "if theres no space then leave the place", referring to two way mirrors for peeps (you put your finger on the mirror and there should be a small gap between you and the reflection), but what I dont understand is how the gap is there in the first place. And why is it not there on a two way mirror? | Engineering | explainlikeimfive | {
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"An ordinary mirror is a thin layer of metal covered by a layer of glass. The \"gap\" is the layer of glass. The metal is what gives the reflection. A one-way mirror is a glass window, covered with a partially transparent, partially reflective coating. The window itself gives the reflection."
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9xayjh | If I turn on the hot water tap, why does it slowly warm up instead of being cold for a bit and then instantly hot? | Engineering | explainlikeimfive | {
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"along with the water in the pipes being cold, the pipes themselves are also cold. As the cold water clears out, the hot water comes in, and the cold pipes steal some of the heat from the water, hence the first of the hot water comes out just warm, with it taking a little while before the pipes are warmed up enough to not steal most of the heat from the water. & #x200B; There's also a small amount of mixing of the hot and cold water at the point where they meet, but mostly it's the pipes thing."
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9xmsfr | The really simple version of windowing | So my trainwreck of a DSP class hasn't really been going well for me, it seems like the professor doesn't provide any context for what we are learning and it would be a better use of my time to just copy articles from wikipedia. That being said, I have a homework problem asking me to design a hamming or rectangular window to show the peaks of two distinct sine waves. I've fairly lost on the high level purpose of a window and where to begin applying it in DSP. A lot of the documentation I found online weren't high level enough for me, so hoping I could get some help here with the basic concepts of windowing in DSP & #x200B; Thanks! | Engineering | explainlikeimfive | {
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"A 'window' is just the subset of your time-series you'll be evaluating with your DSP. Everything outside of the window is considered to be zero. So let's say you have 1 second of signal. You want to look at it in 10 ms lumps. This means you'd need to 'window' each 10 ms lump, ignoring the other 990 ms of data. The easiest way to do this is with a rectangular window. If I'm looking at 45 ms to 55 ms of my signal, I simply take those samples at full value and ignore all others. However, consider what happens when I look at the frequency domain of my windowed signal. If my original signal was DC, I would expect my frequency domain to be a delta at 0. But once I've windowed it, I've created a rectangular pulse so my frequency domain looks like a sinc - the sharp edges of my rectangular window have created all sorts of high frequencies that didn't exist in the original signal. A Hamming Window is meant to ameliorate that flaw, by creating a sinusoidal-type tapering at the edges. It still won't construct the perfect delta at 0 you'd expect from looking at the frequency domain of your DC signal. But it won't have nearly the same amount of high level noise created by your rectangular window. For simple signals and DSP, windows are unnecessary. If you're building a low-pass filter, you'd normally just use the difference equation and calculate the output in real-time based on the entire data set you received (your window is rectangular, but every time you get a new data point it expands to include all past and current data). DSP chips are designed to accommodate this sort of approach, with fast multiply-and-accumulate (MAC) instructions. Where you really need windows is with time-varying frequencies. Think of speech recognition. If you talk to me for a minute, I can't just take a look at a minute-long frequency domain representation of your speech. While it will contain all the frequencies you spoke, there won't be any order to them. The first phoneme you spoke and the last phoneme you spoke will be treated identically (and may indeed overlap, obscuring one from the other). Instead, I need to take 'windows' of your speech so I can preserve the order and distinctiveness of each phoneme you uttered over that time period. With 10 ms windows (resolving down to 100 Hz), I can take a look at what frequencies were in use at any given time, with the order of those frequencies preserved. If you use the same phoneme at 3 seconds and at 37 seconds, I can separate those two easily. The choice of window I use determines the balance between 'slurring' my processing (how much I change my data by overlapping into other windows) and 'corrupting' my processing (adding artifacts from the signal processing itself). For example, in the speech recognition example, I probably want to err on the side of 'slurring' because speech-over-time tends to rise and fall in frequency - the information from the previous window is somewhat correlated with the information in the current window. But if I'm analyzing mechanical transients, this is not the case. Those transients do not flow smoothly from one to the other, but are sharp discontinuities. As a result, including a previous mechanical transient will (probably) confuse my results more than simply generating artifacts of the signal processing would. Note: I recognize the above is not strictly 'ELI5' since it's doubtful anyone without any background in signal processing would understand much of what I'm talking about. However, the OP indicated they had some familiarity with the basics."
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9xoa4r | Why aren't there standardized tire sizes? | Engineering | explainlikeimfive | {
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"Because different vehicles have different needs, different load levels, different speed ratios, perform different task. Your Toyota Yaris doesn't need the same tires as my SS Camaro."
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9xrfcv | Car Air Recirculation | In light of my school district finally canceling school due to the Camp Fire smoke being at a dangerous high. How does a car recirculating air work and how effective is it at protecting our lungs from the smoke? | Engineering | explainlikeimfive | {
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"Cars are really leaky, and not at all airtight, so the effect is minimal when you're surrounded by smoke. There's a small filter for incoming air typically between the hood and windshield (by the wiper arms) that lets cabin air in and filters it (older cars may not have a filter) So when you turn on recirculate, a door moves and sucks air from the front passenger floor/kick area rather than the outside filter. I usually use this unless I need strong fan movement or am on a long road trip, as it doesn't cycle the air well and it gets stagnant."
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9xxuex | How do you run a wire to a LED on a rotating bearing? | A few moments ago I saw a gif of someone spinning LED's with a electric motor. I thought it was really cool but then I wondered: How do those LED's get power? It can't be trough a regular wire because it would be wound up in no time. So how did those LED's get power? Are there special types of bearings that allow electricity to run to the rotating shaft without problems? And what are those bearings called? | Engineering | explainlikeimfive | {
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"I have not seen the gif you’re referencing, and you didn’t provide a link. But my guess is that they are battery powered. It takes very little power to light an LED, so we could be talking very small batteries.",
"This video has several examples of brushes/slip rings on a large generator. Scaled down, they still perform the same function. [ URL_0 ]( URL_0 ) In an alternating-current (AC) generator, sometimes called an alternator, the brushes and slip rings carry direct current (DC) to coils in the rotor (the moving part of the generator) which turn it into a giant set of electromagnets. The magnetism along with the motion of the rotor causes current to flow in heavier coils of wire in the stationary portion of the generator (called a stator). These wires connect to the load. In a direct-current generator (shown about 1:48 into the video), the magnets are on the stationary portion and a slip ring broken into sections (called a commutator) contacts brushes which pull the generated current off the rotating member and supply it to the load. As each set of broken sections connects to a different set of coils, the net effect is the polarity does not change, hence the generated current is DC. As one would imagine, in a DC generator, the brushes handle a far greater amount of current, emit sparks, and burn up faster, which is why most generators today (including those in cars) are of the AC variety. & #x200B;"
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9y1crh | Why is it so difficult for a plane pilot to do a water landing? | Hello, I just saw the movie Sully and I don't quite understand why it was such an accomplishment, I really want to understand so I can appreciate it. Thanks a lot :) | Engineering | explainlikeimfive | {
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"The engines usually hang below the wings. If one engine hits the water first it will dig into the water causing the plane to spin to that side and then roll in the water.",
"Water will stop the plane very fast, so the plane needs to be going as slow as possible. The slower the plane is going, the less lift it has, so the faster it falls.",
"For a commercial jet a typical landing speed is in the ballpark of 150 knots (173 mph). At that speed, hitting water is extremely violent. The jet also has limited control at that speed. It's hard to avoid getting out of that straight, flat line. Even the Sully landing was hard, taking an engine off IIRC. URL_0"
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9y588l | Would a prank shock gum/shock pen still work if the victim was ungrounded? | Engineering | explainlikeimfive | {
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"To answer the title question: yes. When you stick a fork in an outlet the electricity flows from the outlet to the earth through you. When you grab a gum shocker thing, there are usually two leads on the toy, one out and one in, so the circuit is made going into your finger and back out into the gum packet again.",
"Grounding i.e. path of least resistance. Helicopter line repair men can hop onto an active power line (after using a metal rod to \"sync\" the differences between the helicopter and it). URL_0 When you're on a line suspended, there is no path to \"ground\", touching the single line, feel nothing. However touch that line and the one next to it and you get fried. Electrical outlets will shock you because you are \"grounded\"."
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9y5em4 | How can the Voyager and other craft go to such extreme distance? How does it have enough fuel? Why can't we send humans like that instead? | I'm curious about these pictures that I see all the time. Voyager or some other spacecraft sends pictures of distant planets, being some light years away. Even pictures of Earth looking like a speck of dust. How did it travel so far? How is this possible? For humans, I understand it would take tons of more specialized equipment but surely we can send them a great distance than the Moon. Most of what I say might sound very uninformed and downright wrong. So anything and everything related to this will be very much appreciated. Thank you for your replies! | Engineering | explainlikeimfive | {
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"Nobody mentioned this, but the Voyager missions were created to take advantage of an unusual planetary alignment. They gravity whipped around several planets as they went outward, both saving on fuel, and getting great close-ups of them. Normally a mission like that would only fly by one, maybe two, and never have enough fuel to break solar orbit.",
"Just to offer one correction, Voyager does not use solar power. Had it been dependant on this it would long ago have shut down due to its distance from the Sun. Voyager uses a unique nuclear power supply where the heat from slowly decaying radioactive material is converted to electricity. In the 20+ years the Voyager probes have been in space the power generated has dropped by about a third.",
"Also just to be clear, neither Voyager is \"some light years\" away. It's about 14 billion miles away, but it'll take another 20,000 years at its current rate to get 1 light year away from us. Space is huge.",
"The great thing about space travel is that there isn't anything to slow you down. This means that we can leave voyager doing its own thing and it will just keep going until it hits something. Voyager is one a one way ticket away from earth and one day it will stop sending us data, but it will still be out there. As for why we can't do it with people is simple. Humans generally want to come back home or would at least like to know where they are going.",
"Voyager and other unmanned spacecraft which are designed for long distances have the ability to shutdown. So awesome thing of space is there is no friction or well anything.. so when you launch one of those craft, once it separated from it's boosters, it will keep going in that direction at that speed until it hits something (planet, astroid, etc). Due to that, Voyager can. Shutdown it's systems and conserve energy. (Think of turning off a phone, going on a trip, then turning it back on, it will have the same amount of battery as when you left) In addition we use solar technology and other ways to recharge the batteries., With humans, it's a different story. Food, air circulation, gravity, all things we have to continually power and keep working. Making sending humans vs robots different. Tldr: Voyager shuts down during journey and only powers on when it HAS to, also it uses super battery saver mode. Humans have no such capability (yet)",
"For starters, humans need to eat...the Voyager probes have been out there for 40 years, so if you add up all the food that a 40 year old has ever eaten, storage space becomes an issue.",
"Think of Voyager like an arrow. Once fired it doesn't need fuel to keep moving. We aimed it in a way that made it pass by large planets the craft would \"fall\" towards and using a little fuel can sling shot out from. The Earth appeared the size of 1 pixel while Voyager was 3.7 billion miles /6 billion km and was taken in 1990. Voyager is now 11.7 billion miles/18.8 billion km away moving at 38,000mph/61,000kph. A light year is 5,879,000,000,000 miles/9.461x10^12 km so Voyager is only 0.2% of a light year away. The amazing part is that we are able to pick up the signal it is sending. The transmitter uses less power than the light bulb in your microwave. By the time the signal reaches Earth it is only one-tenth of a billion-trillionth of a watt. Human's can't leave Earth like that for a few major reasons. 1 safety. There is no way to shield astronauts from radiation or avoid the side effects of weightlessness. 2 weight. Humans and everything we need to stay alive weigh a lot. 3 space is huge. The Moon is a 3 day trip, Mars when closest is months. From there you're talking years of travel. And when you get to these places you have to survive them, and they are as extreme as you can imagine. E: a digit",
"You’re asking why we don’t just send humans on a one-way trip? This isn’t Kerbal Space Program",
"Funny, I was watching the documentary about this on Netflix just yesterday. They launched two of these guys. Oddly enough, Voyager 2 launched before Voyager 1 (the press was annoyed about this). But it was because Voyager 1 traveled faster than Voyager 2 and would catch up, then passed it. Both are launched and use other planet's gravity to \"sling shot\" them and pick up speed. They got some crazy cool pictures of a few planets and their moons. And after it passed the last planet Carl Sagan asked for them to turn it around and have it take picture of the solar system from the outside looking in. The various scientists thought this was dumb b/c there was essentially no scientific reason to do it. But Sagan though of it as commentary on the Earth. The Earth was a tiny dot, not even one pixel on the picture that was taken. Every human that exists, that had ever existed was on what appeared to be this tiny spec in the solar system and that we should take care of this planet.",
"We can in theory send humans on a deep space journey with today's technology but they won't survive long due to inadequate life support technology. To get an idea of how space travel works try playing Kerbal space program. It looks a bit silly but surprisingly good at explaining simple concepts of science used for space travel.",
"I don't know how to eli5 a gravity assit but I'll try. & #x200B; first thing you need to know is there's something called conservation of energy, it means if you don't exert any force on an object it'll have the same amount of energy. in this case we'll look at kinetic and potential energy, kinetic energy increases when the body's velocity (you can call it speed) increases and potential energy increases when that same body get's away from the body it's pulling it (in your daily life it's earth, in voyager's case it's the sun). So what does it mean? it means, when voyager get's closer to sun it speeds up and when it get's further it get's slower because it's potential energy is also decreasing and increasing at the same time. & #x200B; now, let's throw voyager in an orbit with varying speed-height, like it get's close to sun and get's further. and if you can calculate it really good, to make when you're close to sun you're also passing behind one of the planets you'll be stealing some potential and kinetic energy from that planet (it's really complicated to explain the actual thing happening so just accept it like that). now you just earned more speed only because you passed behind a planet without burning any fuel. Let's do the same thing with other planets on the way. and there are some massive ones on the way like jupiter and saturn, they're so massive (or heavy if it fits you better) the amount of kinetic and potential energy you'll take from them is HUGE, so huge it's enough to leave the solar system. & #x200B; but making all these maneuvers takes too long, getting to mercury, venus, then to saturn and jupiter and finally leaving the solar system, sending a human on this journey would be very boring for that human and it'd take too much food to stay alive. so we just send the probe without a human.",
"A bit of an oversimplification on most of this, but it's a few things. First I'll answer your questions about Voyager. First is the fuel. Voyager uses hydrazine, a fuel that can burn for MUCH longer, but produces less thrust per second. We don't use it for human travel because it takes a very long time to pick up any notable speed (when compared to the vastness of space). The second is trajectory. Voyager was launched at just the right time so it could be \"slingshot\" by multiple planets. This is called a Gravity Assist. Imagine a spaceship flying towards a planet very very quickly, but it's aimed *just* next to the planet. Once you enter that planet's gravity, you will start to sort of orbit a bit. But, because you're going so fast, you'll never complete a full orbit around it. Instead, you leave the orbit with extra speed and in a different direction. Of course, you still need to use fuel to make sure the spaceship is going at exactly the right speed and will get launched in the right direction. What made Voyager so great is that it did this on Jupiter and Saturn, while Voyager II went around Jupiter, Saturn, Uranus, AND Neptune. It was perfect timing. Another thing is... the lack of people. People are heavy, and the things we need are heavy. Voyager was about 1800 pounds at launch, and is now under 1650 pounds, which is really really light for a spacecraft. Add on a 170 pound man, plus his food, water, clothes, spacesuit, exercise equipment, hygiene products, and all the technology that comes with manned spacecraft, and you added at *least* 300 pounds to that vessel. That's just one person as well! Now, why can't we send humans that far? Well, with our current technology, we can't fit all the necessary things on one ship to go that far. Humans are heavy, as mentioned before, which means we need more fuel to launch. We also don't want them on that ship for 50+ years because they would start to suffer from symptoms of old age, and they would almost assuredly go insane. Mars is a doable target though. It would be very difficult to launch from Earth to Mars, but it is possible. It would take a long time, but again, it's still possible. However, the next promising project would make space travel MUCH easier. Our eyes are set on going to the Moon to set up a Moon base. Literally. This Moon base would be built up so that it could provide for itself (with some help from Earth of course). There is a debate over whether or not we should mine on the Moon, but if we did, we may discover materials we could use to build more spacecrafts *on the Moon*. If we can launch a spacecraft the size of Apollo 11 from the Moon, we could reach many more distant planets. This is because the Moon as significant lower Gravity, which means getting off the Moon would be incredibly easy compared to Earth. On Earth, we use different fuels and engines to launch vs when we're in space. The fuels and engines used to launch are *very* inefficient, but provide ALOT of thrust. The exact opposite of a nuclear engine. In space we use *more* efficient fuels that still provide meaningful thrust. We do this because we have to overcome Earth's Gravity. So, on the moon, we can use more efficient fuels and engines all the way through, which gives us more distance to travel. The other part is air resistance. The moon has no air like Earth does, so a lot of the aerodynamic parts we put in Earth made spaceships would be unnecessary, which makes the ship lighter, and again, makes it easier to take off. TL;DR: Voyager was a masterpiece, and we could absolutely travel further if we launched from the Moon, which is our next big project.",
"Think about it, will be the only thing left of the human race. Space is so vast and empty that its calculated not to hit anything for billions of years.",
"In addition, at least in the general case, humans expect to return. That by itself doubles the need for energy for propulsion.",
"Well, once a rocket flies into space it can keep flying without losing speed or using any fuel bc there is no gravity or air to cause it to slow down. They have been in space for 40+ years so if humans were to use this mode of transportation it would need to be a one-way trip supplied with enough resources to last several generations.",
"I’ll be honest I initially thought this was about the U.S.S. Voyager from Star Trek until I got to the last line. The answer there being the Bussard collectors allow underway replenishment of the ship. For the IRL probe the answer is simple, Newton’s First law: any object in motion will continue to move in a straight line until acted upon by an external force. There is no air or other sources of friction in space.",
"There's been mention of nuclear generators, how space has no friction, turning off and on power to save energy, and humans needing stuff (food, gravity, oxygen, ect.). One thing I haven't seen mentioned is radiation. Cosmic radiation is very bad for humans (and sometimes even for computers, but less so). It's such a problem that even going to Mars posses a significant risk of cancer. Going beyond that with as little shielding as probes have would only increase the risk.",
"It doesn't take fuel to follow an orbit. It takes fuel to reach one. Think of an orbit as a type of trajectory. A trajectory is a line you can draw which will show the entirety of the movement of a thing from the moment it starts moving to the moment it stops. If the start point and end point meet, you have got yourself a never ending travel, an orbit. The ISS doesn't burn fuel to go round and round the earth (ok it does sometimes but that's because the atmosphere is still present and it degrades the spacecrafts orbit slowly) If you start burning fuel, you can start calculating the entire trajectory. The more fuel you burn towards a proper direction, the longer the trajectory becomes. If it becomes long enough that you never fall back to earth, you can then stop burning fuel and you will be eternally* be tracing that trajectory. If you spend enough fuel at specific points, you can draw a trajectory that can even reach out of the solar system, and you will be stuck at tracing that trajectory eternally* * unless you hit something or you enter an area that has some kind of matter, that can slow you down due to friction and will alter your trajectory.",
"At what distance or time will Voyager no longer be able transmit back data?",
"If you're talking about rocket fuel, it doesn't need any. Once it reaches a certain velocity, it will coast more or less forever, it doesn't need to be continuously thrusted. There is little dust outside the solar system to slow it down. The escape velocity is where the Sun's gravity weakens with distance too quickly to ever pull it back. That was achieved by a combination of the original rocket thrust plus some planetary slingshot manoeuvres. As other answers, electrical usage for operating the equipment is minimised to prolong it's operational life."
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9ya246 | What does quenching do? | Why do blacksmiths and such “quench” the metal after hearing it with water or oil? What does it actually do? | Engineering | explainlikeimfive | {
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"There is something called a phase diagram for materials. For steel for example, depending on how you treat it, you end up with different forms of steel, by the size of the grains it forms. Quenching is typically done to make a hard steel, martensite. It works because after the steel is heated, the liquid is able to cool the steel much quicker than air, reducing the amount of time the steel is in other temperature ranges that other forms of steel could form."
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9yhgdt | When you send your old engine core back for a replacement, and they fix it to resell it, what do they actually do to fix it up? | Engineering | explainlikeimfive | {
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"Depends on the specifics of the situation. If you're talking about a car engine, an old shortblock can be refurbished. Re-bore the cylinders (slightly larger, but nice and flat again), replace things like journal bearings and piston rings, and you can have a refreshed motor ready for a new life. For electric motors (like a heater blower motor, or a forklift motor), quite often, they can be repaired or refurbished. Generally speaking, the windings/core/stator/magnets/housing last forever. It's the brushes and bearings that wear out, but those are serviceable. Swap in some new bearings and brushes and the motor is as good as new."
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9ytcyt | ]The difference between a Motor and an Engine. | Engineering | explainlikeimfive | {
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"In modern language they are synonymous except that you wouldn't say \"an electric engine.\" If it runs on electricity, it's definitely a \"motor.\" If it runs on fuel (wood, coal, gas, propane, etc etc) it's properly called an engine, but motor could also be correct. The differences become distinct if you look into the [history of the words]( URL_0 ).",
"In the past, a motor powered something that moved (like a boat) and an engine powered something that didn't (like a factory). The two words quickly started mean the same thing, though, and began to be used interchangeably. Nowadays, generally, an engine works on combustion and a motor works on electricity (a gasoline engine and an electric motor.)",
"Motor is a very old term. Its original meaning, from late middle English, is someone that moves things. If you're job was to help people move stuff, you were a motor. As we started making things that move other things, the term motor was stolen for that and the original use became less used. Engine on the other hand, comes from the Latin *ingenium* which means talent or device. An engine is a device made by a talented person that does a job. Think \"siege engine\", a machine made by as siege engineer (the same root applies to engineer) that broke walls and gates. With the advent of steam power, talented people built devices which converted fuel into mechanical work. Steam engine were eventually replaced by internal combustion engines which we now use and simplified the name to engine.",
"A engine runs on internal combustion of a fuel(including steam engines) a motor relies on electricity to move",
"They're kind of synonymous; engines typically refer to combustion engines, whereas motors will either refer to relatively small combustion engines, or specifically to electric motors."
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9yu4ic | Why does the shower knob sometimes need larger twists to change the temperature then other times when just a tap does the trick? | Engineering | explainlikeimfive | {
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"It can be a lot of factors. Where I live, we have water heating. When it's hot outdoors, and no one has used up the hot water, the stuff coming out is near boiling. In the winter, it's both harder to heat the water, and it's being pumped away to the water heaters constantly. Therefore, in times of heavy use and extreme cold, the water from the \"hot\" side can be just hot, not burn-your-face-off hot. In summer, a small adjustment gets big results. In winter, you can turn it almost all hot."
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9yu81e | how does an altimeter work? | Engineering | explainlikeimfive | {
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"There are two types of altimeters in use today: 1) Pressure Altimeters measure the air pressure outside the plane. In order for this to be accurate, you need to know the ground pressure of the airport you are landing at. This led to airports having accurate weather stations and recording the pressure change regularly - > the source of weather forecasting data. However, the pilot has to remember to ask for the pressure and dial it in correctly, so errors can occur. 2) Radar Altimeters measure the distance from the bottom of the aircraft to the ground. This takes much more size, weight, power, and cost (~$12K) - put it's the standard flight instrument in commercial airliners. Sure, they have a backup altimeter that's pressure driven, but it's just a backup."
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9yur6f | How do electronics manufacturers make chips as small as a few nanometers wide when we can hardly even see things much bigger than that? | Engineering | explainlikeimfive | {
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"Simplistic answer: they design them big-sized, then every step that follows is how to recreate the big design at the small scale. The original manufacturing took shapes drawn on transparent sheets & did a reverse microscope thing to shrink the images down to the size of the chips. As we approach shapes so small that you can actually count the individual atoms in some of the features, there are all kinds of funky physical effects used to do the shrinking, but the basic concept still remains. How are those shrunken shapes used? Every set of shapes is used to cover the surface of the chip with a protective coating, but with holes where the shapes are (or opposite). Once you've got everything covered except the bits you wanted, you can 1) etch away the exposed bits, 2) contaminate(dope) the exposed bits, or 3) cover the exposed bits with some additional material. Then remove the protective coating. By doing the choices 1-3 over and over with 100s of shrunken images, and many different exposed chemicals, metals or other materials, you create many different layers that form all of the devices on the chip & all their connections with each other. Of course, the precision of the engineering required to do all this reliably on a mass scale is mind-boggling. There are reasons why the most sophisticated chip fabrication plants cost billions to construct, and why designing each new chip is a multi-million dollar decision.",
"They use custom-made machines, of course. But more specifically, they don't build each component on the chip separately — they actually *print* the electronics onto the chip, slightly similar to how a photocopier prints toner onto paper.",
"photo lithography, we design it quite large then it is printed using lenses to focus the image smaller [wiki link]( URL_0 )"
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9z59l8 | Why is the switch that controls the rear defroster almost universally on a timer rather than an on/off switch? | As someone who lives in a colder climate, when driving in snow/sleet the fact that the rear defroster automatically turns off after X minutes is maddening as the window begins to fill with ice and snow. I had an old car, a 92 Ford Escort and the rear defroster was a mechanical switch. It was awesome! We left it on pretty much all year which did not seem to cause any issues. Why do most cars have a switch that runs on a timer? | Engineering | explainlikeimfive | {
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"Heating tempered glass too hot while surrounded by such cold might cause an imbalance in the forces of compression and tension within the glass. Tempered glass is strong until it isn't, and then it just explodes practically.",
"They pull a fair amount of power, and in almost all climates once the fog is gone, your normal HVAC should keep the window clear. Climates where ice continues to accumulate are few and far between, and the rear glass heater is primarily designed for fog, not ice and snow. Hence, it's set to kick off a minute after the ice or fog has been melted. Also, yes, concerns over excessively heating the rear window. If you super heat tempered glass then it cools quickly, bad things can happen. Again not a concern when constantly melting ice and snow, but that's not an issue for the overwhelming majority of people who need that button. It wouldnt happen, but it could, so it kicks off. Not like it bakes the glass but you know, better safe than sorry. The real reason for the timer? The hotter the element gets the quicker it wears down, can fracture (the copper not the glass). They do NOT last a long time always running in warmer climates dealing with fog over actual frost. Fun fact, your rear window defrost pulls about double the power of your highbeams. Around 220w versus 120w."
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9z7m52 | The older the house gets the more it starts randomly making creaking sounds, why? | Engineering | explainlikeimfive | {
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"Most houses are built with wooden floors and other elements made of wood. Wood will expand and shrink due to moisture and heat and cold. Over time wood expands. This will cause wooden planks for example to run against each other and against nails. Also pipes get older and clogged up."
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9z8rz6 | How do "radar absorbent" coatings on stealth airplanes work? | For instance the B-2 Spirit airplane has a radar absorbent outer coating to avoid detection. What materials absorb radio waves and how? | Engineering | explainlikeimfive | {
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"Radar sends out electromagnetic waves that reflect from airplane and return back to the radar. Radar then calculates where the airplane is by how long it took for those waves to return. Radar absorbent coating just absorbs the waves and turns them into heat, with only few of them returning back, and shape of the aircraft is designed to reflect them into different direction (that's why stealth planes look so \"weird\"). Note that stealth airplanes are in fact not totally invisible to radar, their reflection is just smaller, so some technically advanced radars can detect them."
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9z9lv6 | Why can't you just remove the wires from a bomb in order to disarm it? | Engineering | explainlikeimfive | {
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"Some bombs instead of being signaled to go off are instead constantly being told not to go off. If you remove that signal then they explode. This is how the abort explosives on rockets work. They are constantly receiving a signal from the ground saying please don't blow up. That way if ground control ever loses control of the rocket it automatically explodes and doesn't plow into some random city.",
"I am not a bomb tech but; it is possible to rig something to go off when it detects a lost of power. It would actually be pretty simple- have a wire that is always energized feeding into some sort of control unit. When that control unit detects loss of power, it triggers the bomb. If your talking about IEDs and such overseas, they often just blow them up in place because it is much less risk to human life. They can send in a robot, drop some C4, retrieve the robot, then detonate the whole thing. Sure there are probably bomb techs there with the skills to go diffuse it by hand. But they don’t know how the bomb will be set off. It could be a timer, a pressure plate, or some dude hiding with a remote detonator. So the just risk the robot, not the human.",
"Read about this bomb a self-taught bomb designer built that destroyed a Lake Tahoe casino. URL_0 That’s how you make a bomb that can’t be disarmed. Even nuclear weapons designers from Lawrence Livermore were consulted.",
"Bombs can be made in many ways, some use electricity to explode, and for those you can in principle magically teleport away all the wires and the bomb won't go off. But for the sake of illustration we can also design a bomb that uses electricity to not explode. Start with some chemicals that explode when you mix them, say, ANFO and high test peroxide. ANFO is an incredible explosive by itself, but it's fairly stable, so we want to use the peroxide to set it off. For that to work the peroxide wants a fuel source, say some diesel or perhaps a wax, but the two must be kept separate or they'll explode of their own accord. So the idea is that you suspend the diesel over the HTP using an electromagnent to prevent it from falling. In this not-that-hard-to-make scenario, if ever the current to the electromagnet is cut the fuel falls into the HTP, which immediately explodes with enough vigor to set off the ANFO... which then explodes with enough vigor to take out an office building. The tricky part is, the bomb tech doesn't start out knowing how the bomb is constructed.",
"I know someone who used to work with a military bomb squad, and he said that they usually just shoot the bomb to get rid of it",
"Depending on how the bomb is wired, maybe you can. But bomb-makers who are really committed will build defenses into their bomb, such that pulling a wire will cause a redundant trigger to fire.",
"Short answer - any IED (improvised explosive device) may be so shoddy that simply touching it could set it off. Ditto un-exploded ordinance (UXO). A guy in El Paso drove over an old WWII mortar round that was covered in sand - blew the front off his jeep. & #x200B; Military ordinance or an IED from a skilled bomb maker has anti-tampering. The device must be disarmed a certain way or else it goes off. & #x200B; And when you see a bomb, you may not see the whole thing. For all you know, there is a wire you just missed. & #x200B; And that is assuming that a wire is what will set it off. & #x200B; It could be command detonated, have a photo sensor, a pressure plate or even be a decoy. & #x200B;",
"I've thought the \"cut the ___ wire\" was more about dropped military bombs that didn't go off. They would be standard rather than the random wires someone would do in their garage"
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9zdhnh | what makes the indicator in a car's dashboard tick? | When you're indicating to turn, the dashboard ticks to indicated to the driver too, and I was wondering what makes that sound, is it the bulbs, or a little switch in the dash? | Engineering | explainlikeimfive | {
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"There’s a mechanical switch called a relay, which has an electromagnet in it to switch the bulb on and off. The relay is designed to be loud so you can hear it. On some newer cars, it doesn’t use a relay, therefore the circuit doesn’t produce the sound. But for safety reasons, they need to somehow generate the sound, so there is a speaker that makes the sound."
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9zewgq | Why does low water main pressure ( < 20 psi) pose a health risk? | A main broke near me, and my whole road is under a 30-hour boiled water advisory due to the water pressure dropping below 20 psi. Why does water pressure matter in this case? I looked online and couldn't find any real explanations. | Engineering | explainlikeimfive | {
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"Because if there's a leak somewhere, there's a chance that dirty water could be pushed in.",
"Think of a long-assed straw that has tiny pin-pricks in it - as long as there's sufficient pressure (clean) water will leak out of those holes into the surrounding area, but if there isn't sufficient pressure (unclean) water might leak into the straw - and you now have contamination.",
"there ALWAYS are leaks, the problem is that once the pressure gets low enough bad stuff can move back into the system making everyone sick. they can mitigate this somewhat by upping chlorine concentrations.",
"The water in the pipes is assumed to be clean and safe to drink because it was treated at the source. Now, leeks can happen anywhere and it's fine as long as the leek is in a single direction, water leaking from inside the pipe to outside the pipe. Because in that situation the water inside the pipe is still assumed to be clean, you've just lost some of it. It's wasteful but not unhealthy. If the water pressure in the pipe gets too low, it's possible for a leek to happen in the other direction. Outside water getting into the pipe. That's bad because now you have unclean water into the system and no idea what water is clean or what water is unclean. This has to do with pressure because water will always seek to move from a high-pressure place to a low-pressure place. If the high pressure is inside the pipe then the water will only ever move out of the pipe and not into the pipe. On the other hand, if the pressure inside the pipe gets to low then the outside water might be higher pressure than inside water and the flow will reverse, water into the pipe rather than out of it. So when water pressure in the system drops, they need to issue a boil water order to prevent people possibly getting sick.",
"That only needs about 40 foot head to force water back the wrong way, may be a roof tank in a building on a small rise.You can also get back siphonage. Either way it can get contaminated water into the supply pipes.",
"I work in the water industry. Low water main pressure can be a risk because of back-siphon or back-pressure. If a customer's service side has more pressure than the supply side, water can be pushed back into the water main. That is back pressure. Now, back-siphoning is when a vacuum, basically, is created by much more water flow happening as compared to the actual water volume. This can cause a vacuum effect to happen and, for example, say a customer leaves a water hose in a dog bowl, the water from that bowl can be sucked back into the water main, just like a vacuum. To answer the question, 20 psi is pretty low as it is, and a pressure like that is not hard to overcome without suitable backflow prevention. Basically, once the water is treated, distributed throughout the system, and supplied through a meter or any other means, we do not want that water back into the system because of so many variables that can happen beyond the water purveyors control. Low water main pressure is a huge cause for concern because of this."
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9zmsdx | How would wirelessly charging a device work? | Engineering | explainlikeimfive | {
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"If you wave a magnet back and forth near a wire, you create an electrical current in the wire. This is how a generator works. Turns out, you don't even need an actual magnet, because an electrical current creates a magnetic field, and that magnetic field, if it is changing, acts like a moving magnet and creates an electrical current in any wire that happens to be nearby. This is how transformers work. The two sides of the transformer are not actually connected electrically, they are just linked by the magnetic field created by the changing electrical current in the primary coil of wire. You can use the same principle to charge a phone or whatever: the phone and the charger are effectively the two parts of a transformer.",
"Do you mean aside from wireless phone chargers?"
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9zo0ii | why are there controlled engine braking zones? | Engineering | explainlikeimfive | {
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"Noise abatement mostly. In 12 years OTR driving I've only ever seen engine brake restrictions in or near populated areas",
"In large commercial trucks and vehicles, engine braking preserves the life of the brakes but is also very noisy. Then they make it illegal to engine brake in residential neighborhoods/populated areas or between certain hours to cut down on noise pollution."
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9zvhwl | How do molded dice with depressed dimples (where 6 dimples takes out greater mass on a side than one dimple) get balanced so that they are completely unweighted? | Engineering | explainlikeimfive | {
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"There are dice out there that are engineered specifically to avoid this issue. They're typically casino dice, but the pips (the dots or dimples on dice) on casino dice are filled with a different color to keep them balanced. The general consensus on this issue is that imperfectly weighted dice are *random enough* for most purposes. Meaning that unless you measure each individual die and test it enough to determine which number it will land on the most, it doesn't matter. Most people don't use dice for anything remotely serious, so the general outcome of the rolls isn't that important. Edit: I get it, we all take board games seriously, but when I say important, I mean that most people don't have thousands of dollars riding on their dice.",
"With a typical six sided die, the fact is that they are not engineered and manufactured to be perfectly fair - they are made to be 'fair enough' for use playing things like board games that require some level of randomness, but ultimately are not too strict. As a thought, if your die has perhaps a 5% bias to a particular face, even if you roll it 100 times over the course of a game, you will never pick up on that bias when on the confines of a game and all of the other variables that brings to hide it. The one place where this statistic can make a difference is in the big money gambling, which is why unlike generic board game dice, casino grade dice are made to a vastly higher tolerance to ensure fairness, to note some of the main rules: They must be exact cubes, manufactured to a tolerance of a thousandth of an inch. This is why they are all have sharp edges to ensure this and show any tampering To create the pips, the material removed to form the pip is refilled with an opaque version of the same material to ensure no change in the weight distribution (which answers the initial question here). The pips must also be drilled and filled rather than painted, to ensure the paint cannot be worn off or altered during play. The dice must be partially translucent, so that you can visually check for things like air bubbles or weights they would influence a roll. The dice are all produced in sets of five, which are all given matching serial numbers - so the dealer knows the set of dice in play and they cannot be substituted. Lots of rules that make sense when gambling for high stakes, but if all I am doing is playing a board game with friends or determining my characters stats in D & D, then the tolerances won't really influence everything enough to make any meaningful difference (though that will never stop posters believing they have 'lucky dice').",
"A lot of people have answered this question generally (i.e., most dice you buy that aren't casino dice aren't fair). But, they haven't talked whether the cause is actually the pips. There's one cool study out there that actually suggests it's not the carved out pips that causes the unfairness, but rather, uneven faces. Zachariah Labby threw a set of 12 dice 26,306 times and found that 1's and 6's were more common. This is important because the opposite sides of dice always add up to 7. If it were the pips, then the 1 and 2 side would be heavier, and thus more likely to land face down (meaning that 5's and 6's would be rolled on the dice more frequently). When he measured the dice he used, he found that the 1-6 axis of the dice had smaller faces. Thus, they were less likely to land face down, which increases the chance of getting a 1 or 6 on the dice. So, dice are unfair but it might be because they aren't perfect cubes rather than the hollowed out pips. Of course, this is only 12 dice so it's always possible that the pips make a difference sometimes too (i.e., if by chance, the dice was close to a perfect cube)! Each dice varies slightly from others due to various imperfections, so not all dice are necessarily biased in quite the same way. Original Source (open-access) URL_1 Video of his dice rolling machine (he's not a crazy person who rolled dice 26,306 times by hand!) URL_0",
"They don’t, really — the slight effect of the difference in each side isn’t great. That’s part of why the opposite sides add up to seven — the pairs of sides would balance the central mass of the die. That’s likely why casino dice are printed rather than dimpled — the weight fluctuations would be minimized with the printed dice.",
"The simple fact is that, if we are talking about cheap dice (non-casino dice, or anything that costs you 1-digit prices), then the dices are NOT unweighted at all, far from it. -------------------------------- This study : URL_2 Rolled dice 144,000 times. The results are as follows : * Casino Dice : 16.7% 1's * Cheapish dice (Squared edges) : 19% 1's * Cheap Chessex Dice (rounded edges) : 29% 1's Keeping in mind that any number should have a 16.7% chance of being rolled, we can see that cheap dice are simply not random at all, but heavily weighted to rolling ones. --------------------------------------- If we are going to talk about cheap dice, then what you are asking is hardly even a valid question. Cheap dice are just composite, and the composite used often has little (or none usually) Quality Control. The weight of the composite is often inconsistent, leading to some dice weighing more, or worse yet, some SIDES of one dice being heavier than the other side of the same dice. Not to mention AIR pockets! Check out the Reddit article here for a picture of a die with a huge air pocket. It was cut open because it rolled inconsistently, and voila! air bubbles. URL_1 This article probably covers many of people's questions ----------------------------------------- Also, a lot of us who frequent game conventions know Lou, the crotchety old dice guy. He's always willing to talk to you, even when you don't have time :) He's a seasoned source, and his lecture here : URL_0 covers a LOT of the issues with dice, most of which stem from the manufacturing process",
"We have a machine at work, next to the vending machine, that pays out 100% every time. I believe it’s made by Change.",
"I have to believe you could engineer a way around that to within acceptable tolerances. Using more paint on the 6 dimple side, or maybe the dimples go deeper the fewer dimples there are?",
"Relevant: URL_0 Probability expert Persi Diaconis speaks on the fairness of standard dimpled dice and does a really good job explaining it.",
"You won't know the unevenness of the die unless it's measured through experiment so therefore it is in a sense completely random your its use.",
"I've wondered the same thing about lottery balls. How is the paint for 2 digit numbers not give an advantage to the ones with paint for only 1 digit?",
"I’m not sure of the math, but I’d be surprised if the fact that all opposite sides of a die add up to 7 pips doesn’t have anything to do with it"
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a00rl8 | What is this thing on the bridge and what is it for? (Imgur link in text) | URL_0 The thing in the upper right corner. They were added a few years ago onto the Brooklyn and Manhattan bridges. Sorry couldn't get the whole thing. Car was moving too fast | Engineering | explainlikeimfive | {
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"Those large cables on the suspension bridge allow workers to walk up to the tops of the main support pillars. (You can see the cables on either side acting as guard rails for where they would walk up) The thing you're questioning is a gate to prevent everyone else from walking up there too without a key to unlock the gate because they're being foolish, suicidal, etc. It wraps completely around the suspension cable to prevent people from easily getting around the side, or below it."
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a01vwy | How does the iRobot Braava NorthStar Navigation Cube work? | Engineering | explainlikeimfive | {
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"It projects an IR grid on the ceiling. The Robot looks up and uses the lines to navigate. IR bounces difusely off white paint, just like ordinary light."
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a02wai | How does shipping mass amounts of helium work? What significant impact on weight occurs with liquid refrigeration in metal tanks? | Engineering | explainlikeimfive | {
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"Compressed helium is heavier than air, it's only when allowed to expand inside a container to near atmospheric pressure does bouyancy come into play"
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a04qg9 | Does sunlight coming through a windowpane onto skin provide any benefits at all? | Always wondered if you still got anything beneficial from sunlight if its through a window Is it a feasible way to get sunlight you miss when working night shifts? | Engineering | explainlikeimfive | {
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"It would depend on the type of glass — with just straight window glass with no coatings, yeah, there’d be some vitamin D benefit. With highly tinted or UV-coated glass, probably significantly less benefit, if any. There are mood benefits to being able to see the sun, even through a window, so there’s that, at least...,",
"That warm feeling like a cat napping in the sun?.........."
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a05fwm | How do computers' hardware understand anything? | In the end they're just metal pieces, right? How are they able to understand any logic or store data? Or is there something specific about the alloys they use (which means I could possibly use other metal -- although a thousand times inefficiently). I understand how low-level coding works. But still, they communicate only to the basic program on the computers (like BIOS, for example). How do the most basic communicate to the hardware of the system? | Engineering | explainlikeimfive | {
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"Well, you have a transistor. Depending on the transistor, you apply a current or remove the current, and that allows current to flow or blocks the current from flowing, depends on the transistor. Maybe think of this like a water pipe with a shutoff value. Water builds up waiting to flow or is forced down a different path, then you turn the valve and water begins to flow or flows down an easier path. Thats, what a transistor is doing. Then, you chain a couple together to get something like an SR Flipflop. (Checkout the wikipedia graphic URL_0 ) that's two transistors put together to store 1 bit. So you could effectively chain a ton of them together to make various kinds of memory or ripple counters and what not. Next, you would want to be able to make decisions with logic, so you can configure different kinds of gates, like AND OR XOR NOR and so on. Here is a NOR Gate, [ URL_2 ]( URL_1 ) And with NOR gate you typically make AND and OR gates. But stringing them together let you make decisions. Then you would want to ADD something so you can take a look at the half adder and the full adder, again this is all just putting more and more transistors in more complex configurations. & #x200B; You need to start with the flip flop to see why this all would work in the first place and extrapolate that. And yes you can make an incredibly inefficient transistor, that would probably cost your more then the few pennies to just buy some gates or transistors. There is probably a video using cellophane and aluminum somewhere."
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a05zbt | What is the mechanism or device in a car that turns the wheel back around automatically as you're exiting a turn? | I'm not sure quite how to explain it but when you're driving and you go to make a turn of course you're going to turn the wheel in either direction left or right. however when you let go of the wheel momentarily as you're exiting the turn in the wheels aligned them self straight back onto the road, what is the device or mechanism that allows the steering to do this and how does it work? | Engineering | explainlikeimfive | {
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"No mechanism, its physics. The wheels are not locked in place, and the car wants to travel the path of least resistance, forward momentum forces the wheels to return to a roughly straight position, unless you are holding the wheel to prevent it. Its easy to see with a model car. If you turn the wheels, the car will turn initially when you push it, but then friction from the floor forces the wheels back to their starting position.",
"The wheels themselves! One of the alignment angles is called caster, and it is typical for cars to have a slight caster to do just that. Sorta like shopping cart wheels though not as pronounced, caster makes the wheels themselves \"follow\" their connection point and when you combine that with good old Newtonian physics (an object in motion will remain in motion until acted on by an unbalanced force), this is what you get, the car wants to continue moving forward and the wheels fall in line.",
"It's a part of suspension design called [caster]( URL_0 ). Basically, the car wheels are oriented behind what the steering wheel is connected to. This makes them 'drag' back to being straight, just like the front wheels on a shopping cart."
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a067hp | How does velcro actually work? | Engineering | explainlikeimfive | {
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"If you look at the *rough* end of velcro closely, you will notice bunches of tiny hooks. The fuzzy end is bunches of tiny loops. When you put the fuzzy end of velcro against the rough end, enough hooks catch onto enough loops to stay in place."
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a06k5u | Harder or softer shoes base (cushioning/sole) is good for shock absorption? | Few days ago, i tried a pair of volleyball shoes from asics and i found the base was quite hard compare to a pair of running shoes Ultraboost from Adidas, which is very soft and comfortable. In terms of shock absorption should the base of a pair of sports shoes for badminton or basketball or volleyball that involves a lot of jumping be soft or hard? 🤔 | Engineering | explainlikeimfive | {
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"Neither/Both. The property for absorbing shock is “viscoelasticity”. Such materials are “stretchy” but “slow”, like hard gels, rubber & wood. If you put a weight on the material and it doesn’t instantly go to the final shape, and it can recover back to the original shape, it’s viscoelastic. So materials that seems “hard” like wood and “soft” like gels are both great at absorbing shock. Rubber can be very soft or very hard, and will still absorb shocks well."
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a07l26 | Could we repurpose something like the Hubble Space Telescope into a sorts of over engineered macro lens? | Engineering | explainlikeimfive | {
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"No, for several reasons. One, it's hideously expensive just to get to orbit. Repurposing something that's already in orbit for a different purpose would require multiple servicing/engineering missions, which would be prohibitively expensive. Second, large telescopes like Hubble don't use lenses--they use mirrors. Reflecting telescopes work completely differently than refractors. Third, if you're talking about using Hubble to project a spot of highly magnified light on the ground, forget about it. Hubble has instruments blocking its light-path, but even if it didn't, its focal length would be way too short. A telescope is designed to project a magnified image into a detector (camera/spectrometer/human eye). That detector is seldom more than a few inches from the output of the telescope. Hubble orbits at an altitude of 353 miles."
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a0e8lk | How can a half built, completely open plywood house frame be fine after sitting open during terrible weather? | Engineering | explainlikeimfive | {
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"The materials are weather resistant to a degree, as long as the exposure is not too prolonged. And they are allowed to dry thoroughly before being sealed up.",
"Most houses here in the Pacific Northwest are sheathed in OSB (oriented strand board). OSB can be exposed for a certain amount of time based on its exposure rating without suffering any degradation. The edges of the sheets are gapped 1/8th of an inch to allow them to swell without bowing. Once the house is wrapped in a membrane like Tyvek it will start to dry out. This is usually through the rough in phase, electrical, HVAC and plumbing. By then the moisture content should be less than 14%. If its not then you can run dehumidifiers and heaters before insulating and drywall. Its also a good idea to check for mold and spray if necessary before insulating.",
"By the time the house is ready for insulation it will have dried. If it's the winter heaters are used. There isn't enough exposure for rotting. Plus the wood has already been sitting outdoors for a while, tarped but still has high moisture in northern climates",
"If a tree branch falls in the woods, it will rot and turn to dirt. But has to be wet enough for long enough, over and over for years. A few rainstorms aren't enough to rot wood."
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a0kj2n | What makes a phone vibrate? | Engineering | explainlikeimfive | {
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"A small rotating motor with a weight attached to it. Example: URL_0",
"There's a motor with an off-balanced weight in a phone (and PlayStation controller and just about any other vibrating device). As it spins, it causes back-and-forth motion that makes the phone vibrate.",
"There's a little electric battery powered motor inside the phone (they're really little). Attached to the motor's shaft is a weight that isn't centered on the shaft. When the motor spins, the weight wobbles back and forth, and the whole phone vibrates in response. When I was a kid, I put an index card on a motor's shaft (it wasn't a teeny motor, but--still) and when powered up, the whole dang motor would vibrate like crazy and eventually fall off the desk. The off-center weight is the key."
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a0qsdp | How is stainless steel made, and why is everything metal not made of it? | If it does not rust, then why are cars, tools, everything else that may rust not made of it? Is it a cost issue? | Engineering | explainlikeimfive | {
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"Stainless steel is a group of steel alloys with high chromium and low carbon. This makes them significantly more resistant against corrosion but the low carbon content means that it's flexible and not very strong We often use high carbon steel for items that we want to heat treat and make extremely hard. There are other alloys that are really magnetic or not magnetic at all. There are about a thousand alloys of steel, each excels in certain areas and is preferred for certain tasks. With the variety of alloys out there, why pick one to use for everything rather than picking the perfect one for your application?",
"Stainless steel ms are alloys of iron with a minimum of 10.5% Chromium as well as other minor constituents. This does mean that it doesn’t rust under normal conditions (or at least not as quickly) but it will affect its structural prosperities - strength, hardness, elasticity, heat conductance, fatigue resistance, etc. The result being that it isn’t suitable for every application. That being said various types of stainless steels are used in a huge variety of applications. This includes structural and mechanical uses as well as things like cutlery, jewellery, architectural materials, etc.",
"A 300-series stainless is typically used in car exhausts. But even then, stainless does not guarantee it will not corrode. Things are still exposed to crap like road salt, and plenty of people don't regularly do an underside wash to remove dirt that can trap moisture for extended periods. Plus, it's more expensive as you mentioned, and the majority of stainless produced is what is called Austenitic, which cannot be heat-treated and is barely (if at all) magnetic. I use 304 stainless screws at work and it's like butter under an air driver (though 410 seems reasonably tough).",
"Stainless steel is made of iron alloyed with chromium and a couple other metals, depending on the exact alloy. Making stainless steel is much more expensive than normal steel because these extra alloying elements are much more expensive than just using iron, and because making stainless steel that is actually stainless requires much more care and expensive equipment than just normal steel. Another factor is that many applications require metals with specific properties like strength, hardness or flexibility that are hard to meet with a stainless steel, because the elements like carbon which are added to give these properties also make the steel less stainless. Most of the time it's easier to make a component out of some normal steel and coat it with something that prevents rust, or use aluminum alloys which are cheaper and lighter than stainless steel at a lower cost and weight.",
"Right now, a very large amount of the steel made in the USA is part or all recycled metal. The vast majority of the recycled metal is regular carbon steel, so it's pretty easy to adjust your melt to make your desired carbon steel alloy. BUT, turning this recycled carbon steel into stainless is expensive, both because of the large amounts of alloying metals like chromium, but also because of the process of removing the carbon for the \"ferritic\" or non-magnetic stainless. The process of removing the carbon is called oxy-argon decarburization. You melt the steel, then you bubble an argon/oxygen mix through it, at about 95% argon. So with these bubbles you get a lot of surface area, but not a lot of oxygen, and that oxygen is going to prefer to latch onto the carbon and pull it out as CO2 gas. This process uses 100's of tons of argon per batch, and argon is crazy expensive.",
"Stainless steel isn't the easiest steel to weld. You have to make sure you have the right carbon content to be able to weld, and the welder has to learn specific processes for each type of steel."
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a0zd1f | Why are wood baseball bats in MLB milled to have a concave bore at the tip? | Engineering | explainlikeimfive | {
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"There are several reasons, the biggest of which are removing weight to increase swing speed, and improving the overall balance of the bat. Source: Catcher from 2nd place 1992 Little League team."
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a1efad | Why Did the SR-71 Replace the A-12? | From what I've read, the claim is that there were no longer missions for the A-12 because flying over Russia was too dangerous...and that's really the extent of information about why this craft was retired only after a year of service. However, the SR-71 was essentially the exact same design, but was slower (mach 3.3 vs the A-12's mach 3.35) it flew lower, and yet it stayed in service for 30 years. Why? That makes no sense at all. Also, the SR-71 still has the world record for fastest plane, yet the A-12 was faster...lol. Why did the A-12 quietly get swept under the carpet while the slower SR-71 took its place? | Engineering | explainlikeimfive | {
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"The mission of the A-12 was deemed too dangerous because a U2 had been shot down and made clear that due to recent advances in missile technology the Soviet Union could track and shoot at the A-12 before it even made it's first flight over the Soviet union. The project was cancelled due to budget concerns which included possible fighter and bomber variants, but by that point the SR-71 had already been ordered. The SR-71 had 2 seats and a higher fuel load making it better suited to the role for which the A-12 had been intended. So they used them for the role instead. The CIA soon proved that the aircraft was exceptional in the role and the value of it's reconnaissance convinced the White House to keep them in service for decades to come. With a fleet of SR-71's available the A-12 simply didn't have a role to play anymore so they put the highly classified aircraft into storage. It's not like the US military has ever been known for saving money...",
"If you're getting these top end speeds 3.3/3.35 from Wikipedia, then they're wrong. These aircraft use Turboramjets, where the only limiting factor of speed is is how much heat the engines and the airframe can handle until it starts to disintegrate. In reality, they can go faster than these publicly listed speeds, but we may never know how fast they have actually been flown or how fast until they disintegrate. TL;DR specs on wikipedia are not accurate"
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a1ftjg | How come when your draw cold water from one tap, it cause another tap drawing hot water to become even hotter? | Engineering | explainlikeimfive | {
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"The other tap is presumably using a mix of hot and cold water adjusted so that the mix is at the desired warm temperature. Turning on another cold tap elsewhere tends to reduce the cold water pressure which will reduce the amount of cold water in the mix, thus increasing the temperature.",
"The first tap is drawing both hot and cold water. When you open the second tap and draw cold water, you are lowering the pressure in the cold water pipes and less cold water comes out of the first tap. Since less cold water is coming out of the first tap, the hot water makes up a higher proportion of the water coming out, and the overall temperature of rises. When I went to college every freshman in the dorm got burned the first few times they took a shower because any time someone flushed a toilet the cold water pressure at the shower would drop to almost zero. (These were tankless commercial toilets.) After the first few times you learned to listen carefully for a flush while showering and you'd quickly move out of the water stream briefly. Although it would have been common courtesy to announce a flush, it was much more fun to hear guys yelling and cursing, so warnings never caught on. After a few weeks of this it just became automatic, you could sense a flush without it even registering and you'd hop out of the way without thinking. You'd go home for a visit and do the same thing at home without even realizing it."
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a1fxgu | Could someone please explain what a sensor differential response pattern is (zero-order, first-order, second order,...) and why an inherently omnidirectional sensor by definition has no first-order components? | Engineering | explainlikeimfive | {
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"A zero order response means that the sensor output is related to the actual property being measured at that instant. For example, a strain gauge sensor produces a signal which is proportional to the strain at that instant. A first order response means that sensor output is related to the property being measured with a time lag. For example, a thermometer measures its own temperature not the substance it is in. If you drop a thermometer in boiling water, there is a lag as heat flows from the water into the thermometer - so the reading smoothly rises over a few seconds until it stabilises at the water temperature. Mathematically, this forms a 1st order differential equation, hence this is called a 1st order response. Higher orders of response also cause delay but can cause other issues. 2nd order responses can cause overshoots in some circumstances or change the shape of the delay curve in other ways. An example of a 2nd order sensor is a thermometer in a protective cover. There is a lag fir heat to pendtrate the cover and another lag to penetrate the thermometer. Another example is an accelerometer measuring the position of a mass on a spring - acceleration and displacement are related by a 2nd order differential equation so this is a 2nd order system."
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a1girw | How does data take up physical space? | Engineering | explainlikeimfive | {
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"depends on type of memory - in flash, rom, ram etc data is stored as electrical charge in one cell (transistor) (ones and zeros: has charge - one, doesn't have charge - zero). Optical disks, like CDs store it by \"printing\" that one or zero on a surface that can be read by laser. Then there are magnetic disks, that store data as magnetic field. To summarize, all these mediums have cells that represent one or zero and that cell has some non zero physical size",
"If you want to save some specific data you have to put it onto a medium and that medium is what takes up physical space. How much physical space that is, is dependends on what type of medium you use. For computer data you have several options. Lets take flashdrives as an example. Image them as a series of little switches you can flip up and down to store your data. Those little switches are actual physical object a certain size. Over time we learned to make those switches smaller so we can pack more data in less physical space. In reality it looks like [THIS]( URL_0 ). Those structures are super super small, so you need a special microsope to see, but they still take up space and you need billions of them to have enough little switches so you can save what colour every pixel in your movie is at every second."
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a1ht67 | How do nuclear fusion reactors generate electricity | In a nuclear fission reactor I know they heat water into steam which turns a turbine but how are they planning on making a fusion reactor work? | Engineering | explainlikeimfive | {
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"text": [
"There are no commercial scale fusion processes that have yielded useful energy yet. In theory, once they are perfected they'd operate like a regular Fisson/Coal/Gas plant and use the heat to power a steam turbine. While there are other more sophisticated ways to get electricity from fusion, the tried-and-true steam turbine has long been the champ for low cost and high reliability.",
"A fusion reactor would, probably, generate electricity by the same method as a fission reactor - that is by heating water and turning a turbine. & #x200B; I say probably, because they're not in active use yet. There are fusion reactors running but they currently take more power to run than they generate, so are not generating any power yet. I don't expect there's any even trying to generate power, they're all experimental at this point."
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a1iozj | Why do Diesel engines sound different than gas engines? | Engineering | explainlikeimfive | {
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"To understand the difference you must first understand the basics of how an internal combustion engine works. An internal combustion engine draws in air and mixes it with a fuel, in this case diesel or gasoline and then injected into the cylinder where it is compressed, ignited, exploded and exhausted. Harnessing the power of these explosions with the crankshaft is what allows the internal combustion engine to do its work whether it be powering a transmission in a car to move the vehicle or spin an electric generating turbine for any number of things. Diesel engines produce a different noise than gasoline engines because they run a different way. The Diesel/ Air mix is compressed and explodes naturally creating a different sound whereas gasoline engines need to light the gasoline/air mixture on fire with the spark plug to create the explosion. The diesel air mixture being compressed and exploding is what creates that distinct diesel \" knocking\" sound that is just not there on a gasoline engine."
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7
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a1k0oj | why do aircraft carriers not tilt to one side in the water? They have big control towers on one side of the boat and nothing to counterbalance? | Engineering | explainlikeimfive | {
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"That superstructure isn’t that heavy relative to everything that goes into the construction of an aircraft carrier, plus they can balance the load by filling ballast tanks.",
"They do have things to counterbalance that weight, but they just aren't on the outside of the ship. Shifting heavy internal equipment toward the other side can easily counterbalance the added weight of the tower, and there is no way you can tell that by looking at the outside.",
"They do have things to counterbalance it , including things inside the hull below the flight deck that you don’t see from the outside as well as the shape of the hull.",
"It's an interesting question, most ships tend to be symmetrical, so not an issue. A lot of the weight of the ship will be below decks, in the engines and presumably tanks of fuel and water. Whereas the tower is mostly empty space, where people work Presumably these are balancing each other."
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a1stn4 | When cops or people in movies say a car is heading “East on Elm St.” or “West on 42nd”, or that a person is on the “Northeast corner of the building” how are they determining this? | I’m assuming they’re not busting out a compass and figuring it out. Is it as simple as that the streets run E/W or N/S, and it’s just determined from that? What about for buildings? How is that determined on the fly? | Engineering | explainlikeimfive | {
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"Many people have a constant working idea of which direction is which. It's not a movie thing, it's situational awareness. Especially with emergency services and military, it's used to avoid confusion. If you pay attention and consciously think about it, you'll always be able to orient yourself while driving, hiking, etc. This comes in handy when giving directions or describing a location. West is always west, but left is not always right.",
"In the US most cities are built on a grid that roughly aligns North-South and East-West. It is easy for humans to learn which direction is which if you live in the area and get to know it, and humans have done that for virtually all of their existence until everyone started to depend on their phones for stuff like this. It is a very basic skill that they develop through part of their police training if they do not already have it.",
"If you're in a well laid out city like NYC or Chicago it's easy. The streets in NYC run roughly East-West, and the avenues run roughly North-South. If you're on 42nd Street in West Midtown and headed away from the water then you're going east It also helps in cities with one ways that run in opposite directions so you can know which way you're facing just based off which way the one way streets you can see are going and what they're numbers are. If you're in a city like Boston ¯\\\\\\_(ツ)_/¯",
"Probably not accurate, but some/most people have a good general sense of direction. Especially when you've grown up in the city, some streets just become ingrained into your memory, along with certain landmarks (buildings, unique statues, parks, etc)",
"Cardinal directions are common knowledge. And some people are very good about it. I memorize common directions to know so I always know the directions in a familiar place. If I am outside I'll use the sun and time to give me a rough idea. Also modern cars typically have a compass built in I would assume cops have that. Also I would say good sense of direction is a valuable skill for cops since they have to communicate accurately to each other to meet together or find someone or something so they probably work on it so they could know.",
"If they are a cop of the town they probably know the area super well and they probably know what direction they are heading in. Also, some roads or highways have labeling on them so you would know the direction, like \"He is heading south on route one\"."
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a1uts2 | why do batteries have multiple lower voltage cells in series instead of just one big cell at the rated voltage of the battery? | Engineering | explainlikeimfive | {
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"To explain what cowboysauce means, each chemical reaction pair has a specific voltage that is dependent on the chemicals. You cannot change it by making it into a bigger container or larger surface area. That could change the current but not the voltage. the only way to get more volts is to place the batteries in a row (in sequence), so each one adds a potential (a voltage), until you have the target total voltage. Kind of like why a ladder has many steps. Our legs can only go so high with each lift (just like a single chemical cell can), but if we put lifts in a row (cells in sequence), we can get up pretty high.",
"The voltage of a battery depends on the chemical reactions that the battery uses. You can’t change the voltage without changing the cell chemistry so your only option is to put cells in series.",
"\\+1 to the previous guys comments on chemistry. Also it's partly economics. If you sell a battery with 1.5 Volts it can be used in devices needing 1.5, 3, 6, 18 volts etc by cramming loads of AA Batteries in the back of a device. (I was going to say into the back of a Game Gear but didn't that take 6 C batteries?)",
"There's [a funny quote answering this question]( URL_0 ) from a tech site that I like: > **You don't just call up the battery fairy and ask her to set your battery to a certain voltage**. Instead, a battery's terminal voltage is largely determined by the different compounds that you use as the anode and cathode, hence chemistry. There are only so many materials available. You have to work with what exists.",
"Electrons like being on some materials more than others. The amount that electrons like being on a material has a limited range. Imagine the range going from 0 to 5. Now call the scale units voltage. The scale has to end somewhere and that end specifies the maximum voltage that a single battery can have. Other limitations with reactivity and cost limit what we decide to use, but there doesn't exist a pair of materials where electrons prefer one material 244 V more than the other material. We're limited."
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a1z797 | What is a catalytic converter? | Are there any health risks from driving a car with a failing one around? Or are the risks mostly car-related (decreased fuel economy, shortened engine life, etc) | Engineering | explainlikeimfive | {
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"Depends if you're inside or outside of the car. If you're outside it's definitely not so good, the catalytic converter converts harmful substances into less harmful ones. So it is a pretty important part of the car, not to mention that it's usually (at least in most countries) not allowed to drive with a non-working one. But you also have to remember that a catalytic converter generally only works under high temperatures, on cold days or when you only drive short distances the emissions pretty much exit unconverted because it takes a while to heat up. It's also the reason why cars generally need more fuel in the first few minutes, because the software is programmed to heat up the catalytic converter as quickly as possible."
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a214i4 | Afer WW2, US and Britain pursued an "intellectual reparations" program harvesting all technological and scientific know-how as well as all patents in Germany. How were the Germans able to recover with such superior engineering when other countries have a huge head start at the industrial rev? | Engineering | explainlikeimfive | {
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"text": [
"The industrial revolution is generally reckoned to have taken place between 1760 and 1840. Germany wasn't even a unified country until 1871, but the various proto-German states were major players in the early days of industry. Germany's quick post-war recovery after 1945 was largely due to absolutely massive American investment in rebuilding and rearming Germany under the Marshall Plan. The US fed about $100 billion in rebuilding funds into Western Europe in 4 years."
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10
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a24qw7 | What happens during laser eye surgery? | Engineering | explainlikeimfive | {
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"eav3li9"
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"text": [
"Well, there are a lot of different kinds of laser eye surgery. The most common is to correct nearsightedness or farsightedness. The way they do this is to change the shape of your cornea. They actually remove a small bit of your cornea which changes the way that light passes through it. They make this change in such a way that causes the light to focus directly onto your retina, which makes your vision clearer."
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a25y3s | What is tge difference between a normal differential in a car and a limited slip differential | Out of curiosity really | Engineering | explainlikeimfive | {
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"text": [
"Your two simplest types of differential are locked and open In a locked differential, both wheels that are connected to it *must* spin at the same speed, they're physically connected. This is good for slippery situations as both wheels will always be spinning but bad for situations where you need the inside and outside wheel to spin at different speeds(aka cornering) In an open differential the power goes to the wheel with the least resistance. If one wheel has zero traction then it will get all the power and just spin while the other wheel gets no power and just sits there. This is terrible for slippery situations as you lose all power once one wheel begins to slip, but its really simple. A limited slip differential hopes to find a happy medium. It puts a bit of load on both so that even when one wheel is on a slippery surface and spinning there will be some resistance so the other wheel will get some torque to work with and hopefully move you out of the slippery situation. There are various ways to implement the limited slip portion but the goal for all is that even when one wheel is spinning, there should be some power going to both wheels. This is better for slippery situations and works for turning, but its more complicated(and therefore expensive) than an open differential These days there are all sorts of hydraulic and electronic differentials out there that are *active* and will change the torque distribution based on a variety of factors. Its not just about keeping power going to both wheels, it can be about moving power forward or backwards to make the vehicle handling more comfortable or fun."
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a26h5f | How did they fix the Martian Rover without having to send someone to actually fix it when it broke down a few years ago? | Engineering | explainlikeimfive | {
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"text": [
"When one of the front wheels broke? I believe that they tested solutions here on earth with another rover and a simulated Mars surface. They decided the best solution was to drive it backwards, dragging the broken front wheel.",
"I am going to extend this to any spacecraft. The different martian rovers have all broken down multiple times and still continued with their mission. A lot of things you would normally do to something when it is not working can be done remotely. For example turning off power and turning it on again. Letting it cool down. Jiggle it. Run the motor in reverse. And so forth. So even though you have an issue with a component and you can not visit it to fix it you can still try out different methods and see if any of them works. And if you can not fix the component then you might find a workaround. I suspect you are talking about the wheel motor on the Opportunity rover which seized, likely due to dust. They tried several different things which had worked in the past to get the dust out. However in the end they had to do with the five other wheels as they had enough power to drive it along with reduced speed."
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a28br8 | How does a Tesla motor work so efficiently? | Engineering | explainlikeimfive | {
"a_id": [
"eavze50"
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"text": [
"It's just an electric motor, there's no fanciness in there. They still (or formerly did) outsource motors with General Electric being their choice for the AC motors. Technically it is less efficient to run them off of a battery than through an AC power source as they require inverters which induce losses. But the technical efficiency comes from the fact that electric motors can see efficiencies of 80% or more, meaning for every 100 watts of power used, you get at least 80 watts of power at the output. Contrast that to internal combustion engines which are lower than 50% efficiency typically, meaning a lot of potential power is going right out the exhaust or being used to heat the engine block up rather than turn the output shaft."
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8
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a28xae | is there a reason the reverse gear is before drive in an automatic car rather than all the way down. | Engineering | explainlikeimfive | {
"a_id": [
"eaw4w2x"
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"text": [
"It actually goes back to Ralph Nader, who wrote a famous book called *Unsafe at Any Speed* about various deficiencies in American cars. Among them was the arrangement of settings in an automatic transmission car. If a driver *used to* a stick shift car habitually manipulated an automatic's gearshift at highway speeds, the previous arrangement had a tendency to put it in a gear that would cause a violent accident. He said that PRND(L) was a safer arrangement, and as many of his suggestions, that was implemented. Even though technological improvements *might* have reduced the risk anyway, the arrangement has become a *de facto* standard."
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12
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a29n61 | How do parking brakes work? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"I work in the auto industry, more specifically I started out building cars and have worked my way up to a quality inspector /test driver for cars before they are shipped to our dealers so I can tell you about a specific American made light truck class of vehicles. I can explain disc parking brakes, drums are before my time . Anyway they are mechanical , that's why you hear the ticking or ratchet sound when you push them . They can be partially or fully engaged . Basically what happens is when you push the pedal or pull the handle a cable forces the caliper (where the brake pads are located onto the rotor. They are cable operated and do not require the hydraulics like your other brakes do. In order for a vehicle made at my factory to pass the brake test it has to withstand 3400lbs of force on the parking brake alone while the car is in neutral and the other brakes are not engaged . Some call them emergency brakes , but under no circumstance ever would I ever say it's a good idea to use it as a brake trying to stop . Im a driver at said auto company and I test the cars . We are trained if a throttle sticks or brakes fail you shift the car into neutral and control your speed . Your transmission also has a parking pawl test meaning the car must be able to stand several thousand pounds of force in the park gear without moving ."
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a3843l | two of my bedroom has ground wire but no ground detected. Is is okay to leave it like that? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"Either it wasn't run in the first place(if its an older building) or it broke somewhere along the line The ground connection is a redundant safety connection, ideally it should never be used but its there just in case. If you only use two pronged devices in your bedroom(like most people) then it doesn't matter, but I wouldn't plug a computer or anything with a ground prong on its plug into those outlets It'd be good to fix, but not like urgently critical"
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a38oy4 | How noise cancelling headphones work, and why some headphones cancel noise when extremely similar ones don’t. | Engineering | explainlikeimfive | {
"a_id": [
"eb49m9c"
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"text": [
"Identical sound waves cancel each other out. Noise cancelling headphones have microphones that pick up outside noise and feed it back on itself so it's cancelled out and you only hear the different waves coming from your player inside the headphones."
],
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5
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a3mxu2 | why do the frets on a guitar get smaller toward the body? | Engineering | explainlikeimfive | {
"a_id": [
"eb7f59r"
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"text": [
"The first fret is exactly 1/24th of the entire length of the string from bridge to nut. The second fret is exactly 1/24th of the entire length of the string from bridge to the 1st fret, and so on. This is because the change in pitch relative to the change in string length is non-linear, meaning the shorter the string, the smaller the change in length that is required to achieve the same change in pitch as a longer string."
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a3ncwz | The Falcon 9 just took off, how can this spacecraft keep on accelerating while they already reached 22000 mph? Do you need loads of fuel? what kind of motor does this have or how does it go? | I'm just impressed by this take off. It's actually insane if you think about it. & #x200B; EDIT: space rocket\* | Engineering | explainlikeimfive | {
"a_id": [
"eb7i9fw"
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"text": [
"It doesn't have a motor, it's a ~~jet~~ rocket engine. Meaning propulsion comes straight from burning the fuel. The higher up the rocket goes, the less friction it has. Friction is what keeps speeds on earth down. Once out of the atmosphere there's no more friction, only gravity, and that's reduced from what we on earth experience. So once you're out of the atmosphere and are escaping gravity, in the words of Jeremy Clarkson: POWERRRRRRR"
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6
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a3w7j4 | Why do different electronic devices download and install software updates at different speeds? | I'm asking about a situation where a software update is similar in size. Why would two different devices with similar software updates sizes take different amount of time to download and install if they are using the same internet connection? For example if I have an app on my smartphone that is approximately 100mb for an update, why is that faster than an equivalent 100mb firmware update to a video game console OS? | Engineering | explainlikeimfive | {
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"text": [
"There are many factors that can come into play and speed depends on both ends of the connection - yours and the company you are downloading the update from. Since you mentioned a console I'm going to guess that it's a Playstation. Complaints about Sony's slow network have been around for years since a lack of investment on Sony's part means that console owners get slow download speeds for updates despite having a fast internet connection on their end."
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a445v7 | Why do diesel locomotives use their Diesel engines to produce electric to drive electric motors rather than just use the power from the diesel engine to move the train? | Engineering | explainlikeimfive | {
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"text": [
"So, the alternative you're talking about is a direct drive or mechanical drive locomotive. There's a few problems: - the power being transmitted would require ridiculously large transmissions and gearing which would be breaking all the time. - torque. Combustion engines deliver peak torque at a specific designed rpm range. Outside of that range the torque is not optimal... pulling heavy trains requires massive torque. Whereas: the torque provided by an electric motor is constant regardless of RPM. So the ability of the motor to turn the wheels is the same no matter how fast the motor is turning. To deliver a comparable torque curve from a combustion engine you'd need an overly complex and contrived transmission which would, again, be breaking all the time.",
"Torque: an electric motor is at full torque as soon as it starts. An internal combustion engine achieves maximum torque at a certain RPM range and transmissions are used to keep the engine in that range as consistently as possible.",
"1) Electric motors can produce absolutely silly amounts of torque at low speeds (and from 0 RPM), which would be murder on a normal drive train and which even diesels will have issues with. This is very useful in trains due to just how heavy they are. 2) All engines have a particular speed and torque that allows them to produce power at peak efficiency. So, instead of letting the engine dance all over the place with respect to speed and torque depending on demand, the engine can just park itself at that peak speed/load and produce power very efficiently.",
"With diesel electric you can have a motor and a break on every wheel on the train. That's far more efficient than just using the wheels on the locomotive",
"That big diesel engine makes lots of power. It would need a big transmission and big gear boxes to handle the power. That's more maintenance and more moving parts to break, and more weight which takes more power to move. Also, electricity is routed through wires, which can go anywhere easily. Mechanical drive needs room for drive shafts, which impacts the rest of the locomotive's design. With today's electronics, things like anti-lock brakes and traction control can be done with electric motors, but would be difficult or impossible with mechanical drive systems. Even many of the giant trucks that work in mines use electric motors, for much the same reasons."
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a44xsz | Why are electric toothbrushes so much more effective than manual ones? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"Put a toothbrush in your mouth and brush as fast as you can for 10 seconds, without pushing too hard. Now imagine doing it 200 times faster, for 3 minutes. Without getting bleeding gums. That."
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17
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a47je1 | With how many satellites we have in space with different orbits, how do they avoid crashing into each other while in orbit? | Just watching the current spacex launch and they have 64 satellites as payload. How do they program them to avoid crashing into other satellites and other space debris? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"> Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space. There are tens of thousands of satellites up in orbit but they're spread in orbits between hundreds to tens of thousands of km up above the surface, this gives them a ludicrous volume to be spread among. Consider that there are billions of cars on the surface and there's still plenty of room for more cars, and we can't even use the 70% of the surface that is water. If you put something into a random orbit, the odds of it hitting anything else are actually extremely low.",
"collisions occur between satellites and space debris in orbit quite often. it's at the expense of the party responsible for the debris or collision, however the laws are quite different for outer space, and evidence of fault can be nearly impossible. it's hard to prove a chunk of broken off equitment that damaged something belonged to one specific party."
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a4be1t | How do trains stay on their tracks in the snow? | Engineering | explainlikeimfive | {
"a_id": [
"ebcyfu2"
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"text": [
"The sheer weight of the train on the small surface area of the wheels easily compacts and melts the few inches of snow on the track left behind from the plow on the front of the train. The wheels are also cone shaped which forces the wheels towards the inside of the rails even if there is some ice built up. But usually, the trains run frequently enough to keep ice from building up to much on the rails."
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5
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a4du4y | How do refrigerators work? | Engineering | explainlikeimfive | {
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"text": [
"You know the cold feeling when you spray alcohol on your hands? Or when your sweat evaporates? That’s the same way refrigerators work. There is a fluid called refrigerant which evaporates in the fridge wall, cooling it down. You use a compressor to pull in the evaporated gas, and pressurize it so it heats up. The hot gas then flows through a condenser and dumps heat outside the fridge, so it cools down. Then a metering device causes the pressure to drop back to normal, so the cool gas becomes a liquid again. The liquid is then pumped back into the fridge to complete the cycle. Essentially, the refrigerator transfers heat energy from the inside to the outside, using evaporation and condensation to change the form of refrigerant between gas and liquid. Edit: Fixed! I forgot how compressors worked. And the fluid is called refrigerant.",
"Refrigerators operate on the principles that fluids behave in a certain manner at different temperatures and pressures. Here are the sequences of events in a typical Freon based refrigeration system: 1) Compressor compresses the Freon gas to a pressure of about 250 psi. The temperature of the Freon at this point might be 250F. 2) The compressed gas passes through a condenser, where a fan blowing over the condenser causes the high pressure gas to \"condense\" into a high pressure liquid. The temperature is reduced a little. 3) The high pressure liquid Freon proceeds through a small line/hose to a device called an expansion valve (this goes by other names, but the result is the same). While going through the expansion valve, the high pressure liquid \"expands\" and turns into a low pressure liquid (~ 40 psi). When liquids or gases expand they lose energy and the temperature is reduced. Try letting out a little of the air in your tire and you will notice that as the air goes from the high pressure (~35 psi) confines of the tire into the lower pressure confines of the atmosphere, the temperature is reduced. Freon acts the same way, but when going from 250 psi to 40 psi, the temperature difference is more significant. The temperature leaving the expansion valve is usually about 35F. 4) The Freon leaves the expansion valve and immediately enters the evaporator. The evaporator core (looks like a small radiator) has a fan blowing across the finned exterior that causes the low pressure/low temperature liquid to \"evaporate\" into a low pressure/low temperature vapor. The fan pushes the now cold air into the area that you want to be cooled. 5) Once the low temperature vapor leaves the evaporator core, it proceeds to the compressor where the cycle starts all over.",
"You know how when you heat water, it turns to a gas (steam)? The liquid water has absorbed heat energy and turned into steam. The heat has been transferred from the heat source and is now in the steam. Same thing with the fridge. It uses a compound called a refrigerant that is liquid at very low temps, and turns into a gas at a higher temperature. But this refrigerant turns to gas at temps much lower than what you are used to. So to the refrigerant, room temperature is actually hot enough to turn the refrigerant into gas. At this point, the heat from your room temp objects have been transferred to the refrigerant, and the refrigerant turns to gas. So, remember heat transfers from a hotter object to a colder object, and when the heat is transferred, the colder object gets hotter, and the hotter object gets colder. Your refrigerant is now in gas form and is relatively hot compared to its liquid form. If you don't manage to get the heat out of the gas-state refrigerant, eventually the amount of heat in the gas-state refrigerant will be the same as the heat in your objects. At this point, there will be no net heat transfer from your objects to the refrigerant because both are the same temperature, and now your objects can't get any colder. So... what to do? Well, we know we need something colder than the gas-state refrigerant to dump away the heat from the gas-state refrigerant. We also know that if you compress a gas by a certain amount, the gas becomes hotter! We also know that if you decompress the gas that same amount, the gas becomes cooler to the temperature before it was compressed. So, here's what we do. We compress the gas-state refrigerant so that it gets EVEN hotter. In fact, the goal is to make it hotter than the air inside your house. This way, the heat from the even hotter gas-state refrigerant is able to dump some heat into the air in your house. Cool. But even after this, the refrigerant is still hotter than the objects in the fridge (even after the refrigerant dumped some heat into the air), meaning there will be heat going INTO our objects! What to do? We DECOMPRESS the gas refrigerant! Remember, decompressing the gas-refrigerant will cool down the gas! Now, hopefully, the refrigerant is back in liquid form and is able to grab more heat from our objects. The whole goal of compressing the gas-refrigerant is to boost the heat inside it so that it can dump the heat abosrbed from the objects to the air in your house. Then we want to restore the heat brought on by the compression by decompressing the refrigerant. Now, the refrigerant back to its liquid state so it can grab more heat from the objects. Eventually, the amount of heat in your objects will be too little to allow the liquid-refrigerant to get into the gas state. ------------------------------------------------------------------- So if you're still confused about why we need to compress/decompress the gas, think of it this way: You have a cup and two containers of water: red and blue water, and imagine the colors don't mix so it is easier to visualize. You start with an empty cup. You add in some blue water, but only a little. The cup is not overflowing (yet). This blue water will represent the heat absorbed from the objects and into the refrigerant. Now the refrigerant is in the gas-state. Now, add in some red water. Add enough red water so that only all of the blue water overflows out of the cup and onto the floor. The red water represents the heat added when the gas-state refrigerant is compressed. The overflow of the blue water represents the dumping of the heat, which was absorbed by the refrigerant from the objects, into the air. Notice here that the red water does not overflow out of the cup. This is the same as saying none of the heat created by the compression of the gas-state refrigerant is dumped to the the air outside. Now, remove the red water from the cup by placing it back to the red water container. This is the same as decompressing the gas-state refrigerant. The refrigerant is now back to liquid form. The blue water (heat absorbed from object) is lost to the outside air, and the red water (heat from compression) is restored back to the red-water container (lowering the temperature of the gas-state refrigerant by the same amount it gained from the compression). As you can see, the amount of red water in the red container is the same as it was in the very start. The purpose of the red water was to boost the water level by a certain amount so that the blue water overflows, then to restore the red water back to the container."
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a4dxtv | How does a car with an automatic transmission not stall when coming to a stop in gear, unlike a manual transmission? | Engineering | explainlikeimfive | {
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"Generally because the clutch mechanism is designed to be able to 'slip' when one end is locked. For example, many auto transmissions use a tank of oil with a propeller at one end and a turbine at the other end. Normally the propeller swirls the oil hard enough to turn the turbine which drives the car. But when you lock the turbine, the propeller and oil can still swirl without damage. In a manual transmission, the clutch design usually isn't able to slip like this without causing damage."
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a4lzlo | What are those long metal handles that are fused to the end of metal that allow blacksmiths to handle the ingots when forging swords? | So like when a blacksmith is forging a sword, sometimes he holds the ingot with pliars but other times the ingot is fused to a long 3 foot metal stem (not the tang). Are both the ingot and the stem heated and fused? [here's an example]( URL_0 ) | Engineering | explainlikeimfive | {
"a_id": [
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"Many modern blacksmiths use an arc welder to attach something like a piece of rebar to the end of the metal that they're working. Having a bar like that is much easier than using tongs, because tongs require the smith to be constantly squeezing hard enough to keep a solid grip on a heavy piece of metal. Historically any sort of extra bar would have either been forge welded, which is much more difficult and time consuming than using a welding machine, or the smith would have a just used tongs"
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a4we9n | How the heck to spaceships dissipate heat? | Engineering | explainlikeimfive | {
"a_id": [
"ebhz5wi",
"ebi2uaq"
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"text": [
"Radiators pointed away from the sun. If the earth is shielding them from the sun they are typically cold. Edit: removed really shiny",
"Heat can be transferred by three methods: conduction (solid-solid), convection (solid-fluid/fluid-fluid) and radiation (no medium required). In space, large amount of heat is generally due to radiation, i.e., the sun shining on the object. Imagine- if we feel warm when standing in the sun after all the clouds, and other things (I won’t sidetrack into), what would happen in space? Point is, if it gets warm here, it will get crazy levels of hot in space. So what do you do? Use super reflective surfaces. Why? Funny thing about radiation is it has to be absorbed by the object to show up as heat. If the object does not absorb it, it will not heat up. That’s the principle behind using super reflective surfaces on space-equipment. As for heat generated internally due to electronics, it is dissipated using radiation fins. I’m gonna try to ELI5 this (bear with me). When objects heat up, they want to give out heat through all three modes of heat transfer. If conduction and convection cannot take place, radiation has to occur. Now to make a simple analogy: have you ever seen a red hot block of metal? Do you feel the heat coming off it? That’s radiation right (with convective effects, I know)? Similarly, the heat from the electronics is taken to a body that is heated up. This heated body now radiates the heat away into space, thus cooling the electronics systems onboard."
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a4y0xl | Can a mirror on the parker solar probe stop the heat from destroying it? | Hi. I just watched a documentary about the parker solar probe. Fascinating stuff. Anyway I want to know if a mirror, put in front of the probe, can shield it from the heat coming of off light from the sun? Maybe there's a reason why a heatshield is needed instead of a mirror and I'm just ignorant, but please help me. I really want to know. | Engineering | explainlikeimfive | {
"a_id": [
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"> I want to know if a mirror, put in front of the probe, can shield it from the heat coming of off light from the sun? \"The solar shield is hexagonal, mounted on the Sun-facing side of the spacecraft, 2.3 m (7.5 ft) in diameter,[30] 11.4 cm (4.5 in) thick, and is made of reinforced carbon–carbon composite, which is designed to withstand temperatures outside the spacecraft of about 1,370 °C (2,500 °F).[1] **A white reflective alumina surface layer minimizes absorption.**\" [Wikipedia]( URL_0 ) The reflective alumina layer is basically a mirror in this context. The difference between a reflective surface and a mirror is that the mirror keeps the reflected light moving at the same angle as its arrival, allowing a visible image to be formed. But a visible image is irrelevant in this context, the aim is just to reflect the most light and the direction isn't important."
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a51sds | What makes a NASCAR so LOUD? | This weekend I got to see a NASCAR do some demonstrations at a local race track. I couldn't believe how loud it was. Even when it was only going slow (like 50mph) it hurt my ears. How is it I can be on the interstate and barely hear cars running but one NASCAR is so loud? | Engineering | explainlikeimfive | {
"a_id": [
"ebj6rqt",
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"text": [
"700-800 horsepower engines. High octane racing fuel. No muffler. No catalytic converter. That is an arcane but working formula for a crap-ton of noise.",
"Because they don't run with mufflers. Running pretty much any car without a muffler at response is stupidly loud."
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a54ug2 | why, when travelling at speed, does it look like wheels are going backwards? | Engineering | explainlikeimfive | {
"a_id": [
"ebjvq97",
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"text": [
"Look up \"wagon wheel effect (it's 2 a.m. local and I'm too lazy to source) Basically, the \"spokes\" on the hubcap are slightly offset from when our eyes last saw them (because it takes time for our eyes to register each position), giving the impression the wheel is spinning backwards. You can get a similar effect staring at a ceiling fan. If you stare past the blades the fan will start spinning backwards, until your eyes adjust to the fan blades, then it will spin normally. This sudden shift is easier to see than on car wheels because fans spin slower.",
"Basically, your eye can only see so many images per second. Sometimes, if the wheel is spinning at a certain speed, your eye will only catch sparse images of it and it happens that the wheel branches are doing a little less than a complete rotation between each image, giving your brain the illusion that it's going backwards... while it's actually going forward superfast. Not sure if I explained it well, this is my first ELI5. If you want to read more on it, this phenomenon is called the stroboscopic effect (also called the wagon-wheel effect) and there is a lot of info about it on google. [ URL_1 ]( URL_0 )"
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a55x1k | Why do Russian/Soviet fighter jets have a distinctive look over their western counterparts? | Do they have different understandings or knowledges of engineering? | Engineering | explainlikeimfive | {
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"ebkc0pt",
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"text": [
"I mean, \"distinctive look\" is in the eye of the beholder, is it not? But more to the point about different designs: both US and Soviet jets were developed in very different cultural/military/industrial contexts. The US jets were developed in a context that encourage \"bleeding-edge\" technological superiority, at great expense and cost. This was a consequence both of US military strategy (which emphasized technological superiority, a holdover from how they felt they won World War II), and the so-called military-industrial complex, which made it very easy for vast funds to be secured for this kind of work. If there is a stereotype of US jet development, it is that the US jets had slightly better statistics than the Soviet ones (flight ceilings, max speeds, etc.), but at much greater cost, not only to build them, but to maintain and field them. The Soviet jets, by contrast, needed of course to compete with the US ones, but unlike the US case, they tended to prioritize manufacturing efficiency, interoperable parts and maintenance, and durability over their lifetime. This was a product of their own version of the military-industrial complex, which was dominated by state-controlled design bureaus (like Tupolev), and where cost was a much more important factor. In general the Soviet approach to military technology could be summarized as, \"it isn't nearly as bleeding-edge as the USA, but it is cheaper and we have more of them, and they can all be repaired about as easily as a tractor can.\" This same dynamic applies to a lot of different types of weapons, e.g., the M-16 vs. the AK-47; the Atlas vs. the R-7 rocket; and so on. I have not looked into the post-Soviet Russian approach; my vague understanding is that they are now more similar to the US design philosophy (fewer units, higher-tech) than the older Soviet model. There are ups and downs to each design philosophy. The US model occasionally leads to fantastic overspending, and to the deployment of systems that sometimes end up getting cancelled in a few years because they are not adaptable to multiple war-fighting environments. The Soviet model can lead to lots of sub-par deployments that, if they fall too behind the bleeding edge, are essentially compromised against better tech. Both nations went through several \"generations,\" and many different models, of jet technology. At the core, the US and Soviets had essentially the same knowledge of engineering. But they had very different production systems that adapted that engineering to different goals, and that led to somewhat different systems being developed.",
"They serve similar functions but were designed by different people with different design philosophies in isolation. You’d think they’d become more similar over time, even if that isolation continued, but aerospace is different. Change is expensive and risky so remnants of old designs remain because they work."
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a55zld | How does a water tap work? How can water flow so fast from the moment you open it? | Engineering | explainlikeimfive | {
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"There is always water in the pipes that is under constant pressure (either from a pump or because it is coming from a water tower, so more water is above the pipe and kind of presses down, creating pressure). So when you open the tap, the water immediately gets pushed out due to the pressure inside the pipes. The reason there is no delay in the water flowing out is that the water you see coming out in the beginning has been sitting in the pipes right behind the tap before you opened it so it doesn't have far to go."
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a5efwu | how does water get to my sink or shower? | Engineering | explainlikeimfive | {
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"Water flows into pipes in the walls and under the house from pressurized pipes under the street, or from a pump in a well if you live in the country. These pipes run through the walls, bringing water to the various sinks ands toilets and baths. One pipe also goes to a hot water heater, which uses electricity or fuels to heat the water, which is then also piped to your sinks and such. It takes a while for the water to get hot, because the water in the pipes in the walls cools off when not flowing, and all that water needs to flow out first, before hot water from the heater reaches you."
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a5ieg2 | Why do car windshields not shatter like regular glass? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"Windshields don't shatter into multiple pieces as they have a sticky rubber layer in the middle to hold them together Side and back windows shatter into tiny pieces because they're tempered. This stores stress in the glass which makes it harder to break, but when it does break all that stored energy is released and it explodes into a million tiny pieces Strong windows that turn into tiny pieces are good because they're harder to break and if they do break in an accident they won't cut you to pieces",
"Because they're not regular glass. Windshields are made from laminated glass which is made by taking two sheets of glass, sticking something between them (commonly vinyl) and pressing them together into one sheet. When the glass breaks, the vinyl keeps it stuck together as a single sheet. The tiny pieces you're thinking of may be the side and rear windows. They're generally made from tempered glass. Tempering (heating it up and rapidly cooling it) make glass strong and also causes it to break differently so you gets lots of tiny pieces instead of large shards.",
"Side windows and rear windows are tempered (tiny pieces) so if it breaks you don't get stabbed by large pieces, and the most you will get is sratches or cuts. The windshield is laminated ( breaks but stays together after breaking) because any rock or other debris that could break it would other wise cause the glass be flown into the cabin, if the vehicle is moving fast when it breaks. And it is assumed that the seat belt and air bags will keep you from flying through it."
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a5ifnx | how an organ (instrument) works | Also, how would an organ from 300 years ago differ compared to today? I notice in a lot of churches today that the keyboard where the organ is physically played is at the front of the church, but the pipes the sound comes from is generally at the back of the church? Is everything “wireless” now? Or are there pipes and wires running through the walls or under the floor? In the past, would the organ pipes and keyboard have to be closer together? | Engineering | explainlikeimfive | {
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"In earlier instruments there was a physical, mechanical connection between the keyboards, or 'manuals' as they're known, and the wind chest, which is the unit that delivers air to the pipes via valves. There is a set of controls in the middle called the Stops, they determine what sets of pipes (or 'ranks') are sounded when a particular key, or pedal is pressed. In a modern organ, as you've noted, the main change that has happened is that the interface between the manual, and the actual valves that drive the pipes has been made electrical as opposed to mechanical. This has a few advantages. Mainly, again as you've noted, the unit where the manuals are, called the Console, no longer has to be physically attached to the rest of the organ, it can be separate. This gives a lot more flexibility on how an organ can be installed - the pipes can be positioned where they'll sound best, the console can be positioned in a place that looks visually attractive, or out the way, whichever is most appropriate. The second advantage is down to the fact the keys are now working electrical switches, as opposed to physical linkages. What happened on older organs was that if you had more stops pulled; more pipes being sounded, the physical effort it would take to press the keys increased, because you're physically moving more linkages, each key-press was actuating more valves. So playing expressively took a lot more skill, as you might want to adjust the sound of the organ mid performance, but you also had to be ready for the fact that it would change the physical key-feel of the instrument with each change. This isn't the case on a modern electromechanical organ. There was an intermediate system that used an air system between the keys and the rest of the organ. That gave a greater flexibility than direct mechanical action in every sense; keyfeel and location of the organ, but it wasn't as free and flexible as the modern purely electrical system, as well as introducing a noticeable lag between the press of a key and the sounding of the pipe. The other main difference is how the organ is supplied with air. On older organs, the way they were supplied with air was via a big old pair of bellows that some church-hand had to operate while the organ was being played. Nowadays that's done using an electric compressor."
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a5jy4z | How does Cruise control prevent you from going over your set limit when going downhill? | I’m not a car person in the slightest or even pretend to be, but how does cruise control work like this? | Engineering | explainlikeimfive | {
"a_id": [
"ebn2a3t",
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"text": [
"It doesn't, at least not in my car. But it will, once you get to the bottom, just coast until you reach the speed you set it at before (that is, it doesn't reset itself to the highest speed you reached on your descent).",
"Cruise control in older cars only uses the throttle to maintain speed, and can’t do anything to slow you down if you are going down hill. Modern cruise control is more computerized and can control things like gear selection, applying breaks, it can communicate with the ABS, and even communicate with the main computer to adjust fuel flow to the engine and shut down cylinders. This also allows room for more problems from computer errors, improper coding, and hardware failure"
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a5kvz3 | How are large cranes in cities transported to construction sites and taken down when finished? | Engineering | explainlikeimfive | {
"a_id": [
"ebnaesf"
],
"text": [
"What type of cranes are we talking about? [Tower cranes]( URL_1 ) are pretty common in cities. They can be assembled from parts that fit onto trucks. [HERE]( URL_2 ) is how tower cranes 'erect themselves' [Other styles]( URL_0 ) of cranes will also come in truck sized pieces to be assembled on site. [Mobile cranes]( URL_3 ) are sized so that they can legally travel on roads, so they usually don't have a problem getting into places in a city."
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"https://d1evx2irsqd9h8.cloudfront.net/Pictures//web/l/q/j/71807-skeyes_test.gif",
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"https://imgur.com/gallery/dTSHCNY",
"https://5.imimg.com/data5/UH/BA/MY-3375855/mobile-crane-500x500.jpg"
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a5rkqg | What is the difference between $5 department store headphones and $500 headphones? Do they fundamentally have the same components inside or made with different materials? | I saw Bose Headphones from $300 to $500 but feel like I would be paying more due to the brand name | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"The size, weight, shape, material, and electrical properties of the transducer inside the headphones make a big difference in the quality of the sound. Better made headphones will sound much closer to the actual recording than cheap department store headphones. Professional headphones that faithfully reproduce music and also won't fall apart can be had for between $75-$100 USD. Source: Part of my day job is to record video and audio of large conference sessions. I need headphones that are very durable and also very accurate.",
"Build quality is a major factor, but really once you get past the $100-$200 range, any extra you'd be paying is for brand over quality. The \"cheap\" $5 headphones are probably fine for listening to streaming music from your smartphone, but would definitely fall into the \"you get what you pay for\" saying, as they're likely to not last very long. If you're particulary rough on your electronics, or lose them often, and constantly have to replace your headphones, you'd be better off finding a pair for less than $25 that you like than paying hundreds of dollars on a pair of headphones. Additionally, the higher-quality headphones tend to have built-in subwoofers, and some even have a 3-speaker design with subwoofer and tweeter. Another contributing factor for price is active noise cancellation. Some headphones now are starting to include Dolby Atmos support, which provides not only left/right surround sound, but height as well."
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a5rvxy | How is boat speed measured? | Car speed can be measured by rotation of tires. But how is speed measured for engines with thrust - boat, airplane etc. Isn't same thrust would result in different speed based on current load of the boat/airplane? & #x200B; | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"Some boats have a tiny paddle wheel or unpowered propeller which measure the flow rate of water past them. Some boats (the ones owned by people with money to burn) use Doppler shifted sound or radar to measure the speed of water (or even the sea floor, if the water isn't too deep) at a distance. Most boats measure their speed by looking at where they are every so often and guesstimating or using a GPS app on their phone. Airplanes have a small tube sticking out, at a 90° angle to their direction of travel. As air flows past it's opening it creates a vacuum, and by comparing the pressure caused by this to ambient pressure they can calculate their air speed. For ground speed they use radar (often ground-based) and GPS.",
"It's measured in knots, which historically was literally a piece of rope with knots tied in it. You'd let it out into the water and count how fast the rope was pulled out. Airplanes take airspeed measurements through something called a pitot tube (which is different from their ground speed), boats now use a similar thing called a pitometer.",
"In times past, a log line. A float that is dropped in the water and paid out on a line for a certain time. Counting the number of knots tied at fixed separations gives the speed, hence the term for boat speed. The time and separation of the knots is related to the nautical mile, 1 minute of latitude. Nowadays it is usually derived from GPS."
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