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6rm702 | How do magnet factories make magnets without everything sticking together? | Engineering | explainlikeimfive | {
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"Just watched a \"how its made\" video on youtube. The magnets are cast in bulk, but the metal isnt magnetized yet. Then they have a process to slightly magnetize them. They become magnetic, but super weak. They then run them individually through a machine that strengthens the magnetic field. The worker places them far enough from one another so that they dont stick to one another. The machine that magnetizes them is electric so it can be turned off."
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6rmyr8 | Why are the blades in a guillotine shown as being slanted? Does it really matter if the blade is straight or not? | [History] | Engineering | explainlikeimfive | {
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"Yes. A blade that strikes at an angle gets all its weight and momentum into the portion of the edge that first touches. If a blade hits flat, that momentum is spread over the whole edge, and it doesn't cut as well. Guillotines were notorious for not always cutting nice and cleanly.",
"think of it this way, when you cut a tomato do you slice it or just take the knife and push it directly through it?",
"[For reference from someone smarter than me]( URL_0 ) They are shown that way because *they were that way*. The angle means that the cutting force of the blade is hitting a small area since the blade is impacting different areas in sequence. It makes the cut cleaner, more reliable and perhaps most importantly *faster*."
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6ros5i | Why are tank barrels not rifled when almost all other guns are | Engineering | explainlikeimfive | {
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"Modern tanks can fire a pretty wide variety of ammunition. With a rifled barrel you're limited to rounds that expect to be rotated by the rifling. That makes things like fin-stabilized rounds harder to fire. The other big disadvantage of rifled barrels is that they wear out quickly. This rate of wear is faster the bigger the barrel is. A handheld .22 rifle may fire tens of thousands of rounds before the rifling is worn out, while on the highest end--say, the ~~14~~ 16 inch diameter guns of an Iowa class battleship--you can only get a few hundred before servicing is required.",
"Some are rifled. Rifling depends entirely on what sort of ammunition is expected to be used. IIRC, the current sexy in tank ammunition is armor piercing sabot fin stabilized. It's basically a skinny dart with fins on it to keep it stable in flight encased in plastic. Since there is no need to spin, and the rifling could catch on the plastic sabot, they don't put rifling in the barrel.",
"Short explanation: Rifling is useful for one kind of rare and somewhat outdated ammunition type, kind of useful for another, and actually bad for two of the main types of ammunition in use today. Long explanation: The main forms of ammunition in tanks today are Armor-Piercing, Fin-Stabilized, Discarding Sabot (APFSDS), High-Explosive Anti-Tank (HEAT), and High-Explosive (HE). As others have noted, APFSDS is a thin but very dense dart with fins for stabilization. The spin imparted by rifling wouldn't help stabilize it any more, and would just reduce velocity. HEAT works by means of a shaped charge, which funnels the explosion into a copper or other metal-lined cone that creates a focused hypersonic jet of metal which can push (not melt) right through the target. If the round is spinning from rifling on impact, the jet will be dispersed and less effective. There was a French HEAT round designed for rifled guns which worked by isolating the charge from the rifling by means of an outer shell and ball bearings, but this increased drag and dead weight and reduced the charge's diameter, making is less effective than an equivalent round designed for a smoothbore. HE may benefit somewhat from rifling but not enough to justify it, especially since modern tanks have excellent fire control systems that can very effectively aim the weapon. There *is* at least one kind of round that truly benefits from rifling: High Explosive Squash Head (HESH). HESH is filled with plastic explosive, kind of like an explosive dough or clay, and works by squashing that plastic explosive against the target surface like a pancake and then detonating. This concussive force knocks off chunks of the target material on the inside, which is supposed to mean that shards of metal or concrete or whatever it's made of fly around inside the tank or bunker and shred everyone and everything inside. The spin from rifling helps the explosive pancake spread out more to knock off a greater amount of shrapnel on the inside. Only Britain and India bother with HESH anymore--spaced armor, which has gaps of air between layers, is very effective against HESH, and most modern tanks have it now.",
"Modern tanks fire long, skinny projectiles with fins on them to provide stability as it is fired into the target. Rifling these rounds would only decrease this stability.",
"The primary mission of a modern tank like the US M1 Abrams is to destroy enemy tanks and the most effective way of doing so is by using a special type of ammunition called an armour-piercing fin-stabilized discarding-sabot (APFSDS). This is a type of kinetic energy penetrator ammunition that relies not on explosive power but on the mass and speed of the weapon to \"punch\" through enemy armour. It's basically a extremely dense, fast moving metal dart. While traveling through the tank barrel this dart is surrounded by a multi-part housing (the sabot) which falls away once the round exits the muzzle (hence the \"discarding\" terminology). Once in flight, fins stabilize the round. This technology makes the use of rifling unnecessary."
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6rozwn | How does the choke work on a small engine? | What purpose does the choke serve on a small engine like a power washer or a snow thrower? Why can't that be built into the engine? | Engineering | explainlikeimfive | {
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"By restricting the amount of air mixing with the fuel in the carb being it reaches the cylinder. It's literally choking the engine.",
"Under normal operating conditions, an engine will target an air:fuel ratio known as the stoiciometric ratio. If there's too much air relative to fuel, the engine is running \"lean\"; if there's too much fuel relative to air, the engine is running \"rich\". Large modern engines use a large number of sensors to electronically control fuel injectors. Compact engines such as those found in pressure washers and lawn mowers lack any sort of electronic control and rely entirely on a mechanical throttle and carburetor to control engine power. When an engine is first started, it is cold. The oil is viscous, there's no fuel in the intake, and there's no momentum in the flywheel. Thus, it's necessary to run the engine a bit rich in order to ensure that enough fuel gets where it needs to go. Once the engine warms up and gets moving, it will operate under its own power. What the choke does is constrict the flow of air entering the carburetor, creating a rich fuel mixture. This ensures a reliable start.",
"A chokes job is not to limit the amount of air coming into an engine. It is to increase the amount of manifold vacuum seen on the main venturi to draw in additional fuel. The additional fuel is required because the fuel vapor condenses on a cold manifold and doesn't reach your cylinder in sufficient quantity needed for an easy start. A carburetor has 3 paths for fuel to enter the manifold. An idle circuit, a transfer circuit and a main circuit. The idle circuit is just a tiny hole below the butterfly plates you see when you look down into a carburetor, and sees full vacuum with the plates closed. The transfer circuit is basically a tiny slot that is uncovered when the plates start to move off their seated position. At this point there isn't sufficient fuel coming from the idle circuit and not enough air flow to start the main jets flowing. This supplies the additional fuel required to allow a smooth transfer from idle to main circuits. A choke circumvents the throttle plates some what by closing off some of the air flow before the main venturi. This is how the main circuit can see vacuum to supply the additional fuel. Is not readily there to limit air, but rather increase fuel entering the manifold. Clear as mud?",
"So, you will notice that on engines with a choke, there is no primer ball. The choke is there to create a stronger vacuum to kick start the suction of gas from the gas tank/fuel bell."
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6rv07p | Why are there not grates or filters on the outside of airplane propellers to protect against birds flying into them? | Engineering | explainlikeimfive | {
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"While birds are an issue, a much more serious one is ice. Planes regularly fly through clouds where the drops of water are 'supercooled', just waiting for some reason to freeze into ice. A grid in front of the engine would be a perfect place for ice to accumulate, slowing down airflow into the engines, before breaking off and damaging the blades.",
"At the speeds airliners travel, a grate will do two things with regards to bird strikes: Jack and shit. Any bird that hits the grate will just be squeezed through and damage the engine anyways",
"When you're traveling at the speed an airplane does, it's likely that anything that hits the grate will be pushed through the grate in many pieces and/or dent the grate which could then impact the props.",
"Weight and airflow obstructions caused by exterior grates and cowls can massively reduce engine performance, cooling and serviceability. Additionally a light, less restrictive grate could deform or fragment on birdstrikes, causing much more catastrophic damages to engines and props now not designed for birdstrike survivability."
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6rv4ls | Why do sink and other faucets only go out 10-25% over the basin making it awkward to use them rather than going 50-75% where it would be much easier to use them? | Engineering | explainlikeimfive | {
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"> Why do sink and other faucets only go out 10-25% over the basin making it awkward to use them rather than going 50-75% where it would be much easier to use them? If the faucet was directly over the center of the basin then getting something into the sink itself would be difficult; suppose you wanted to put a pot into your kitchen sink but the spigot was over the center and wouldn't rotate? Suppose you wanted to dunk your head or face into your bathroom sink but the spigot was dead center? It just wouldn't work, it would get in the way constantly."
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6rx0c2 | Why can't we use the heat from the Earth's core to power steam turbines and generate clean energy? | Engineering | explainlikeimfive | {
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"We can, much of Iceland is powered via geothermic energy, the problem is that most places aren't as suited as Iceland. You'd have to drill too deep to be able to efficiently harness the energy.",
"We do use geothermal power in regions that the magma is close enough to the surface to do so. Iceland is primarily powered in such manners. But we do not actually have the technology to dig very deep into the earth so we can only set up this type of tech in volcanic regions."
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6rxh9j | why 0db is the loudest sound that can go in a system without distortion? Why zero and why quieter sounds are negative dB? | Engineering | explainlikeimfive | {
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"Decibels are *relative* measurements, not *absolute* ones. To be more precise, a decibel is a way to measure the ratio of two values. One value is a reference value, the other is the one you're comparing it to. And these can be values of anything you like. When talking about sound pressure level, the reference value is the quietest sound that can be detected by an average human -- about 20 micropascals of pressure, which is called 0 dB. A sound one trillion times louder than that can cause permanent damage; because decibels are on a logarithmic scale, it's convenient to take the base 10 logarithm of one trillion, which is 12, to represent that sound -- that's 12 bels, and a decibel is one-tenth of a bel, so that makes 120 dB. But decibels can be used in whatever way you want. It's not, remember, a unit of measurement exactly, but a way of saying how much more or less something is from whatever you say is \"the norm\". And that's the clue: 0 dB means \"the norm, however you define it\", and then negative values mean \"less than whatever you said the norm was\" and positive values mean \"more than whatever you said the norm was\". In professional audio, 0 dB is the root mean square voltage which delivers 1 milliwatt of power across a 600-ohm resistor (because in early telephone systems, the impedance of a standard telephone circuit was 600 ohms) -- this is more accurately referred to as \"dBu\". But 0 dB can be anything you want it to be.",
"This comes from the way an analog mixer works and I think understanding this helps understand a good mix too. Each channel on an analog desk has an input, be it a mic (typically) or something else. These feed into the desk, there is no amplification at this point, just the microphone signals going straight in. The only thing a desk can do is reduce the signal from those sources. So it says 0db because I haven't reduced the signal, if I need that particular guitar to be quieter I can pull it down to - 6db so it works with everything else. So if I have a band setup, a mix that has the full signal of every instrument is going to sound bad, so I reduce the signal (negative dB) for instruments until they sound balanced. Once you have everything balanced it goes out to amplifiers and speakers (be it live or studio)."
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6s1pm3 | Why do some some toilets go whoosh, while others take longer and go WHOOOOOOOOOOOSSSSSHHHHHHHHH? | What causes toilets to flush at different speeds and the power behind certain ones? | Engineering | explainlikeimfive | {
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"Most older toilets are gravity fed (the tank drains into the bowl). Newer toilets are pressure fed. They use less water, under pressure, to work."
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6s1tci | why is it more complicated to land on mars than on the moon or titan? | Engineering | explainlikeimfive | {
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"One big reason is the atmosphere. With the moon, there is (for all intents and purposes) no atmosphere so you only need rockets to slow down and land. On Titan, you have a very thick atmosphere, so you can forgo rockets and use a heat shield + parachute instead. Mars is the worst of both worlds. The atmosphere is thick enough so that you need to have a heat shield, but it's thin enough that using a parachute for the final descent is impractical, so you need rockets (Curiosity) or airbags (Pathfinder).",
"Mars has an *annoying* atmosphere. As in, Earth's atmosphere is nice, because it is thick enough to slow you down, and things like parachutes work well. You need to deal with a lot of heat, so you have to carry a heatshield, but on the other hand, you get a huge slowdown for free just from the atmosphere. The Moon has none at all, which means it won't slow you down, but it won't damage your spacecraft either, and it doesn't require you to bring a big heavy heatshield, so that's also fine. Mars has a thin atmosphere which is nearly useless for slowing you down, and while large parachutes can slow you down a little bit, it's nowhere near enough to actually let you land. On the other hand, the atmosphere is still thick enough that you need a heat shield to survive entering the atmosphere. So you have to carry the extra weight of a heatshield, and you have to withstand a lot stress entering the atmosphere, but it doesn't help you slow down so much that you can land. It's the worst of both worlds."
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6s22l7 | What is the difference between a degree in computer science and a degree in software engineering? Will getting one over the other limit job opportunities? | Engineering | explainlikeimfive | {
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"Think of Computer Science as a science, and Software Engineering as engineering. Computer Science generally deals with algorithms; the high level implementation, optimizations, and proofs. It's usually more academically inclined. Software Engineering generally deals with the practical implementation; how to get the product to the customer by next Wednesday. It's usually more commercially inclined. That said, different schools will label their programs differently. One school's CS degree could have very similar course-work to another school's SE degree. Check out the actual offerings from the school's you're looking at, and see what kind of jobs graduates from those degrees get.",
"You're getting some mixed answers here, in part because degree programs are different at different schools. Computer science degrees teach you how computers work. Architecture, programming, operating systems. Heavy on math and theory, light on business tools. Software engineering degrees teach you how to use computers to produce complex software. Project management, business applications, etc. Heavier on actual business tools and application of them in a professional setting. There is some blending between the two; most software engineering degrees will teach you to program and most computer science degrees will teach you how to put together relatively simple software products.",
"CS = work on solving new problems SE = making existing solutions more efficient Jobs = okay for both, most commercial gigs will not tax you compared to the theoretical work of the degrees. If you are worried about employment, SE is probably the safer choice. Either way, be good at solving problems and hungry at finding new opportunities. Very competitive fields. Source: CS and EE bachelor degrees, grad 2006",
"Computer Science is a very broad, and usually entry level degree that will lead to later career choices or a broad spectrum job where general knowledge is needed. Software engineering, is a very specific degree focusing on the critical development of software, and limits the teaching and the degree's scope to the more narrow strokes, avoiding other things like networking and IT security or other aspects like hardware design and development. Like all career fields, there are broad definitions and very narrow slices within that field."
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6s2mlx | Why are there so many people against windmills? Is there any sufficient evidence saying that they are "dangerous" and "not clean"? | Engineering | explainlikeimfive | {
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"Having worked in the industry, it is largely because people are skeptical to change and the \"not in my back yard\" mentality. When you do a project you often see great resistance when it is first proposed, then as you engage with the community the attitude softens. Once the project is completed you see the vast majority very much in favor of the project.",
"There's a very incorrect, yet oddly successful, marketing campaign by the energy industries that utilize coal, nuclear and other non-renewable sources of energy that labels and works against renewable energies. Images like this iconic image from [Japan Times]( URL_0 ) highlight the irony of an \"ugly windmill\" in contrast to large nuclear cooling stacks in the imediate background. Unfortunately, energy co-ops and energy producers hold all the cards, and until either despiration hits, or we run out of all non-renewable resources, it is going to be an argument that won't be solved until we are long dead, and our children are also long gone from this earth if we ever survive.",
"1. A lot of people think they're an eyesore. There are tons of yuppies and limousine-liberal types who are all for green energy so long as it doesn't ruin their million dollar view. Personally, I think they look cool as heck and I love sitting on the porch and watching them in the distance. 2. Wind power is, generally speaking, much more expensive per MW than conventional sources are currently. Although the price has been steadily falling for years now. 3. Wind power is intermittent in most places. That means that you need to have quick-reaction producers online and ready to start generating if the wind drops off. This adds to the price of wind power, because you have a bunch of mostly idle plants sitting around just in case. As an aside, and in response to u/Uchihakengura42 , nuclear energy is more or less a renewable power source. Uranium is incredibly common in the Earth's crust and can be \"mined\" from seawater indefinitely."
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6s2zhz | With all the advances in technology, why have we not even come close to replicating exoskeletons like the Iron man suit? | I know exoskeletons do exist out there, but most of them are impractical or really limiting in one regard or the other. What is limiting our ability to make something as efficient? | Engineering | explainlikeimfive | {
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"A power source with a great enough power-to-weight ratio is one thing. The best we got is gasoline, but gas and combustion engines are heavy unless you're on the scale of a weed-wacker. Next after that is power transmission: taking energy from the motor and putting it in your arms and hands. Either this is done with relatively slow electric actuators, or very heavy but highly efficient and fast hydraulics. You could do it with electric motors and transmissions, but that would require far more electricity than what's portable for even an exoskeleton. Iron Man's suit appears to function with the power and accuracy of encoded induction motors comparable to CNC machine tools, but small enough that they have a form factor like a wrist watch or kneepad. This is far out of the reach of modern technology and may be that way for some time. Problems like these aren't matters of making things smaller like in computers. (So Obadiah Stane was wrong on at least one front.) Mechanical issues like these are matters of increasing pure material properties like energy density in batteries, conductance in wires, and compressive strength in structures.",
"Power Iron man's suit only works because he has the arc reactor. We don't have one of those yet so modern exoskeletons that are doing heavy long duration work need a generator. They can either be tethered or carry a little Honda generator on their back, but batteries just aren't good enough for completely remote long duration operation",
"Another issue I don't see mentioned here that applies to the Iron Man suit in particular is the concept of impulse, which in physics is the change of momentum in a period of time. Tony may have an iron casing, but he's still flesh and bone underneath. If he flies into a brick wall, his suit hits the wall, and then his body hits the inside of his suit. Now, with a suit he'd have the forces of impact more evenly distributed across his entire body, but that's not going to be enough to keep him from being crushed to bits when a tank's cannon knocks him to the ground at several hundred mph. Cars have airbags, crumple zones, and all sorts of technology designed to help reduce the impulse experienced by the human body in a car accident, but none of those things are mentioned (or even practical) in a suit like Stark's. Furthermore, the relatively low mass of the Suit compared to an aircraft or other heavy object mean his momentum will change much more than a car hitting another car, more like a motorcycle getting hit with a tractor trailer. It's the same reason why football players get concussions, only much much worse. So even if existing technology did make it possible to fly around in a suit like that, you probably wouldn't want to.",
"I actually had to research exoskeleton solutions for improving ergonomics at my internship (Aerospace Precision Component Manufacturing Plant). The amount of energy required to power such a suit is enormous, however, we actually are making progressing in this field. Here are some cool exoskeletons: [Lockheed Martin Exoskeleton Technologies - FORTIS]( URL_4 ) [EquiPois - URL_1 ]( URL_2 ) ( However no one there seems to pick up the phone or reply to emails. [Micromo - Chairless Chair Exoskeleton System]( URL_5 ) [Ekso Bionics - EksoGT]( URL_0 ) And this is sort of related: [Open Bionics - Augmented Arm Prosthetic]( URL_3 )",
"DARPA (the government organization that makes all the USA''s new war tech) is actually working on this. Look up the DARPA next Gen soldier. They plan to have our infantry outfitted with exosuits (not like iron man but like a mesh outfit) that will amplify their strength and carrying capacity. Also, they are gonna have integrated combat systems and a whole bunch more cool shit"
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6s42am | Why do aeroplanes retract their wheels immediately after take off but put them back down so far in advance? | Engineering | explainlikeimfive | {
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"As an aircraft is taking off you want to gain speed. Air is particles, and atoms which create drag and slow any object down. They retract their landing gears because you make the aircraft more streamline and allow it to come to speed faster, due to the less drag. It also saves fuel. If something is more aerodynamic it will consume less fuel to propel it. When an aircraft is coming in for a landing, fuel consumption isn't a main concern, so thats out. You are trying to slow an aircraft down. By creating a larger surface area for the drag to act against you are slowing down the aircraft which will cause less stress on the brakes and tires. Basically you retract them to gain airspeed faster and put them down to create more drag to slow the aircraft down.",
"They need to keep as much of that speed as they can when taking off, and having the landing gear down messes up the aerodynamics of the plane. When landing, they can put them down well in advance because they're trying to slow down, and it probably helps create more drag than if they waited till the last second, plus it would give them more time to make sure it's out properly, and if not, try and fix it before it becomes a problem."
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6s80zh | how did medieval peoples make anvils? | Engineering | explainlikeimfive | {
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"Medieval European anvils did not look like the anvils we're used to seeing today. The classic anvil most people think of (probably from cartoons) looks like [this]( URL_1 ). The large flat surface is known as the face and is where you strike with the hammer. The pointed part coming out of one end is called the horn and is used for bending and shaping metal. The round hole is called a pritchel hole and is mostly used for punching through metal. The square hole is called a hardie hole and is for special tools to fit through for things like punching and bending. This style of anvil is called the London pattern anvil and dates from around the late 1700s. Medieval smiths used much simpler anvils. Many times the anvils they used were simple metal blocks. They also sometimes made it so the anvil could be lodged in a tree stump or wooden block for extra stability. Here's an [illustration]( URL_0 ) based on Anglo-Saxon England (England before 1066). As you can see, there's nothing fancy about it like holes for punching or a horn for shaping, but it gets the job done for hammering. I don't know all the specifics of medieval ironworking, but a solid block like that wouldn't have been too hard to make. It's also worth noting that these anvils would be smaller than modern anvils, as shown in the illustration, which would have made them easier to make and potentially shape. Medieval smiths also did use horns for shaping, but they were made separately in the same manner as other equipment and not part of the anvil. I'm not sure when smiths started attaching horns to the anvil or how they first did that.",
"The kind you're probably thinking of were forge welded (heated up and hammered together) from wrought iron billets. That kept the correct slag composition in the wrought iron to keep it strong. Simply casting them from molten iron in stone or sand resulted in brittle cast iron anvils that didn't hold up. That process was occasionally used for other things, but it's no good for smithing. Once steel technology developed, steel-clad wrought iron anvils became more popular. Those were produced in the same manner, but with a steel top layer for durability and strength."
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6sdf0w | Please explain the difference between getting my oil changed and buying oil and adding it in? | So for example I have to check if my oil is low in my car, and add oil if it is. So since that oil is new, why would I have to get my oil changed? | Engineering | explainlikeimfive | {
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"> So since that oil is new, why would I have to get my oil changed? Oil can be burned off but it also wears out. Old oil with bits of metal dust, burt oil leavings, etc. can build up within the oil still within your car. That needs to be cleaned out and the way to do that is drain out all the old oil then add new stuff in. Imagine if you took a bath every day, but only ever replaced the water you lost every time you got in and out. That water would get really nasty fairly quickly wouldn't it?",
"Oil isnt consumned like gasoline it will lose its ability to lubricate well and get all sticky and sludgy... Adding more fresh oil will help lubricate the system but you still have all of that gunk that needs to be removed",
"Oil loses it's viscosity after exposure to heat and pressure due to combustion. Eventually, oil will become thin and not lubricate sufficiently. The oil filter also needs to be replaced at regular intervals to insure particles suspended in oil are removed and don't collect."
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6sdylu | Why is weatherstripping for doors/windows hollow? | Engineering | explainlikeimfive | {
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"dlbyoao"
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"text": [
"Combination of reasons, it uses less product and therefore cheaper. A large hollow foam tube can be squished when the door closes and make a good seal. A smaller solid tube will leave gaps and won't squish as easily."
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6se8ps | Why does the "Auto" mode in car climate control change the zone to "feet only" and then slowly change to "feet + torso". | I'm not sure if this standard on all cars but this is how it works on my Subaru BRZ. | Engineering | explainlikeimfive | {
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"text": [
"Normally, hot air comes out of the footwells, and cold air comes out of the face vents. That's because hot air rises, so by pumping it out of the footwells it will rise into the rest of the car. If it came out higher up, it would be difficult for it to heat the lower parts of the cabin. And vice versa for cold air. > Why does the \"Auto\" mode in car climate control change the zone to \"feet only\" and then slowly change to \"feet + torso\". I would expect this to be true if the car is cold to start with, because it's pumping out hot air at first. As the car warms up, the air it pumps out is no longer hotter than the air already in the car, so there's no need to pump it all out near your feet any more. But if you get in a hot car then the air conditioned air should come out the face vents right from the start."
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6sflyt | Vacuum Ballon | If a helium ballon floats because helium is lighter than air then what would happen if you made a ballon filled with a vacuum what would happen? Would the ballon float because nothing is less than air, or would it fall because there is nothing in the ballon to keep it up, or would something completely different happen? | Engineering | explainlikeimfive | {
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"A zeppelin would be a better analogy. If you had a zeppelin with hydrogen in the gas bags, it can lift a certain amount because the hydrogen weighs less than the air it displaces. A vacuum zeppelin could lift more, **except** that it would be difficult to hold a vacuum without changing the film the bag is made of. If you have a thin bag with hydrogen on one side and air on the other, at the same pressure, the hydrogen leaks out very slowly and the bag stays \"full\". If you put vacuum on the inside, the bag will collapse because it's not strong enough to hold its shape against the pressure of the atmosphere. If you make the bag out of something sturdy enough to stand up to air pressure, it will weigh a lot more."
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6sh5vm | In programming, what is an N + 1 and why is it a threat to performance? | Engineering | explainlikeimfive | {
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"I suspect you're talking about the N+1 **query** problem? It's an issue where, instead of issuing a query to get all your data in one go, you query once for some index value (\"1\") then for each returned value issue a new query for the data (\"N\"). for each id in : select id from purchases where user = \"longhairedgirl\"; select URL_0 from products where URL_1 = id; This is not performance friendly. Each \"select\" has overhead, especially if the table is being mutated while you try to query it. Each \"where\" clause has a cost, especially if there's no index, but even the index has a cost. A join of some kind is often more performance friendly, as it requires only a single pass to the database. Within the database the join can also more efficiently execute the where clause."
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6sjpwd | Why do we cover railway tracks with rocks ? Does it improve the stability when a train goes by ? (due to the vibrations) | Engineering | explainlikeimfive | {
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"Although it doesn't look it, the ballast is pretty solid. It distributes the load from the passing trains, keeps down unwanted vegetation (there aren't many plants than can grow on rocks) and enables water to drain away. This last one is very important. Where I live, there was some flooding back in May, and parts of the railway line were submerged. The water, having just washed down from the hillsides, was very muddy, and so the ballast is caked in mud. And so right now the line is closed for a couple of weeks while they clean the ballast. This is because the mud is preventing the water from draining away properly. In the winter, the wet mud would likely freeze, and water expands when it freezes: and so it might actually lift the tracks slightly. If we have a winter where the ballast freezes, unfreezes and freezes again a few times, that could loosen the tracks and make them dangerous.",
"To my knowledge. Its actually to help deal with water. the rocks make it really hard for water to pool around the tracks and help to prevent moisture damage and in some environment ice build up.",
"The rail ties sit on the rocks to distribute the weight of the train. It goes like this. Train wheels distribute to the rail, which spreads the weight to several ties, which distribute the weight to a larger ground area. The rocks help disperse the weight to a greater area rather than just the footprint of the tie."
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6sm5pk | How are huge bridges constructed across a large body of water? | It never made sense to me especially when the water can be 10s to 100s of metres deep. How do you construct support beams in the water? How do you stop the force of the water from breaking down the bridge? I'm not talking about smaller bodies of water such as rivers where support is usually on either side of the water. I'm sorry if this is a dumb question but it never made sense to me. Thank you | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"Yer not alone in askin', and kind strangers have explained: 1. [ELI5: how do they build bridges over large bodies of water? ]( URL_4 ) 1. [ELI5: How did they build Medieval bridges in deep water? ]( URL_2 ) 1. [ELI5:How do bridges get built over bodies of water like lakes and rivers? ]( URL_3 ) 1. [ELI5: How are bridge piers built ]( URL_0 ) 1. [ELI5: How are bridges and highways built across water and marshes? ]( URL_1 )",
"Basically, they drop a big ass tube (called a cassion) into the water, then they very slowly fill that tube with the various materials needed to construct the support beam. This allows the tube to sink further down into the ground. You also need to have divers going down to the bottom to excavate the edges around the tubes. This is incredibly dangerous work. Think about sticking a straw into a glass of water, and blowing air down into it. Just enough for the straw to lift up a little bit and force water out. Then you keep doing it, over and over, until all the water is out of the tube and it's just filled with air. With little divers at the bottom helping out. Once all the water is gone, you fill it with your cement."
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"https://www.reddit.com/r/explainlikeimfive/comments/4749i2/eli5_how_did_they_build_medieval_bridges_in_deep/",
"https://www.reddit.com/r/explainlikeimfive/comments/4om9ae/eli5how_do_bridges_get_built_over_bodies_of_water/",
"https://www.reddit.com/r/explainlikeimfive/comments/19g968/eli5_how_do_they_build_bridges_over_large_bodies/"
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6smds6 | How much technology dependent are modern vehicles? | I constantly read 'the board computer does this, the sensor tells the board computer that'. So how much technology is there exactly in a today's car and what is it responsible for? | Engineering | explainlikeimfive | {
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"There are dozens of board computers and way more sensors and actuators controlled by them. For example: A sensor measures the mix of gasoline to air. If that mixture is not good enough for optima performance of your car the computer tells the injectors to put more or less gasoline in the system. Another good example is drive by wire: There is no more physical connection between your gas pedal and the engine. The pedal sends a signal to a computer which then controls the speed."
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6snhbn | How do superchargers work? | Engineering | explainlikeimfive | {
"a_id": [
"dle2s6f"
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"text": [
"There is a chain/belt/driveshaft coming off the engine that turns an air compressor that pumps high pressure air into the engine's intake manifold."
],
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6swaw8 | What is double-clutching and are there any advantages to it? | Engineering | explainlikeimfive | {
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"text": [
"\"normal - clutching\" : you press the clutch in, move the shift lever to neutral, then into the next gear you want, and release the clutch. Double clutching : you press the clutch in, move the shift lever to neutral, release the clutch. You then press the clutch in again, move the shift lever into the next gear you want, then release the clutch. You're pressing the clutch in twice per gear change rather than once, hence the name. In the past, this was required for non-synchronized transmissions. With modern synchronized transmissions there isn't really any need for it. That said, I'll occasionally double-clutch with my car in the winter when it's very cold and it's not fully warmed up yet, as it makes going into the next gear just a bit easier.",
"Double clutching is releasing the clutch midway through a shift (during the time when the gear shifter is in neutral). It's an advantage when one is driving a car without synchronizes (which most modern transmissions do have). It's usually an introductory step to learning heel and toeing (using the heel of the braking foot to engage the throttle so the engine is operating at the same RPM as the clutch during a braking downshift)."
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6t04so | Why do we not need to test our missiles frequently to make sure they work? | Engineering | explainlikeimfive | {
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"text": [
"Part of it is not make sure they work, but to make sure others remember that they work. Otherwise you need tests if you change things or just to make sure that everything still works after years and that the people involved know what to do."
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6t167y | Why do Asian (Korean, Japanese) flight routes go way up North of Russia? | I have noticed that on a sunny day I can see multiple planes, flying over the city. I looked it up today and was surprised that all of the ones I had seen were from Tokyo or Seoul to Europe (Germany, Denmark, France, etc). Their routes seem weird to me. Here's some examples: URL_0 Tokyo one puzzles me (DLH715). I don't understand why would it go that far to the North. It is shorter? But is it safe? Why is there dotted line over some distance? | Engineering | explainlikeimfive | {
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"Instead of looking on a map, try looking at that flight path on a globe and you will see that it is the straightest and shortest route. 2D maps do not show the Earth in it's proper shape - especially far north and far south. For example, say you wanted to fly from Alaska to Norway ( see [this map]( URL_0 ) ). You wouldn't fly East or West to get there. You would fly North over the North pole, then South. If you tried to show that on regular world map, that route would appear weirdly curved.",
"Because the earth is round and not flat like a map, that is the shortest route. If you have a globe, it shows this much more clearly. Edit: It can also be because countries charge transit fees to fly through their airspace. Sometimes it is cheaper to fly around a country rather than over it",
"What your asking about is the [ETOPS]( URL_0 ) requirements on the flight route. The first answer is if they meet various safety requirements (mainly, ability to fly with an engine out), they don't have to be near an airport all the time, because it's very unlikely to be a problem. So if your plane is certified to fly over the pacific, it's likely certified to fly over the poles, and it won't be a problem (legally) and they can fly the shorter, cheaper route that crosses the pole. Boeing has [this document]( URL_1 ) that includes ETOPS maps for 60, 120, and 180 min ETOPS certifications (pg 8-10), and you can see that if you have 120min certification (basically the ability to fly with an engine out for two hours) then you can follow a great circle through northern Russia (but not to the pole). If you have the lower certification (60min), then you probably can't fly across Russia or Mainland China on those routes, which would be a huge detour.",
"So this is something called a Great Circle Route. Most flat maps are actually kind of deceiving in that a straight line is *not* the shortest distance between two points, due to the curvature of the earth. It will look like a curved line (like in your screenshot). You can use a great website called the [Great Circle Mapper]( URL_0 ) to see what the shortest route is. I've done Haneda-Munich to show you what I mean. Now, that's not to say that there's not individual concessions that need to be made for any given flight path. The Munich-Japan one looks like it's very deliberately staying away from North Korean air space, but otherwise looks normal."
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6t2h2u | What is a spacecraft hull made off and why? | I always thought it was aluminum, but I've been searching for information and I can't find it. Aluminum has a melting point of 659 °C though, which doesn't seem high enough to be able to come back into the earth atmosphere. | Engineering | explainlikeimfive | {
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"text": [
"The hull and structure is indeed made of aluminium. But all of this is coated with various kinds of high temperature insulation tiles which are the characteristic white body and black underbelly. There are a few different kinds on the skin of a space shuttle designed to withstand various thresholds of heat (because not all parts reach the same temperature on re-entry). It was the failure of one of these tiles on the leading edge of the left wing of Columbia which allowed hot gases to enter the wing cavity during re-entry. Acting like a blowtorch the superhot jet of air cut through the internal aluminium structure causing the wing to collapse and dooming the shuttle."
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6t3842 | How does the differential work on a truck equipped with Duallies. (4 wheels in rear of truck) | Engineering | explainlikeimfive | {
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"text": [
"The two wheels are linked together, their purpose is to let you spread the load between two normal tires rather than having to use a super wide tire on each side If you treat each double tire as a single unit(they effectively are) then you see the differential works exactly the same as usual. The inner and outer wheels on a side don't move at significantly different speeds"
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6t4mr4 | How do solar inverters work? | Engineering | explainlikeimfive | {
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"text": [
"Solar power is produced as direct current, most residential applications are AC. The inverter converts the DC current to AC. > The inverter takes the direct current and, in simplified terms, runs it through a transformer. It is almost as though the inverter is tricking the transformer into thinking it is getting AC by forcing the DC to act in a way similar to AC – the inverter runs the DC through two or more transistors that are rapidly turned on and off and feeding two different sides of the transformer. URL_0"
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6t5xt3 | If a nuclear missile were to be launched right now at the west coast of the United States, would we be able to stop it? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"Depend on the missile speed. If it is like the most recent Russian test. You won't have time to react. If it is a slow one. Well, let's say the military spent quite a bit of money on a nuclear missile defense in the past 35 years.",
"As people have said, it kind of depends. If its a non-hypersonic missile, once launched you would have somewhere in the realms of 25-60% chance of knocking it out before it arrived. I'm not sure the evidence supports much higher % odds than these. This doesn't take into account the possibility for actions such as pre/during launch cyber attacks (which seem to have caused a lot of recent N.korean missile launch failures), or the possibiilty of sabotage to missile components pre launch / during manufacture (again assuming N.korean, this seems to be the likely target of your question). If you factor these in, and bear in mind the possibility of missile failure (they're not easy things to get right), then again assuming Korean launch I'd say the odds of taking it out look somewhere in the realms of 30-90%, depending how effective US assets can actually be on the ground or in cyber actions against n.korea. TL;DR - coin toss odds"
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6t7bda | Difference between algorithm and logic? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"An algorithm is a way of doing something (for example, a way to sort a list of names into alphabetical order) Logic is the study of making decisions Logic is very important for algorithms because they have to make decisions as they go (should that name go before or after this one?)"
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6t8d3w | How did people get fountains to work in medieval times? | Engineering | explainlikeimfive | {
"a_id": [
"dlio7i7",
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"text": [
"Gravity, water and air pressure, and an understanding of fluid mechanics (especially siphoning). If you have a body of water (a little pool, for example), and a very narrow tube leading from the bottom of the body of water to a position that is higher than it, the pressure from the pool will force the water up the tube (so long as there is no air in the tube; this is called siphoning). So you begin the siphoning process manually (sucking the air out of the tube), then install the tube into the fountain. So long as the pool of water never goes empty, you never have to restart the siphoning. It'll just keep going forever.",
"Rome used aqueducts (artificial rivers) which filled large cisterns. These were then connected to a series of underground pipes, which were provided pressure through gravity. This pressure would be a lot lower than what we expect today, but enough to fill baths and run fountains. We still use a very similar system today in the form of water towers. The weight of the water helps maintain pressure. That way some of the load is taken off the pumping engines."
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6tchwz | Riding a bike with 20+ gears | I feel like I'm 5 asking advice on how to ride a bike but seriously. I haven't been on a bike since I was a kid and roads a bike I bought from Walmart that didn't require changing gears. Now that I'm 22 I inherited a really nice bike and I have no idea when to change gears. | Engineering | explainlikeimfive | {
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"text": [
"You change gears when you need more rotation from less distance of foot travel. Basically, like gears in a car. For instance, at first gear you can pedal up anything because there's almost no resistance from the gears. You will move slowly, but the bike will feel almost weightless to your muscles. As you need more speed you up the gears and each turn of the pedals will let you put more energy into the back wheels. This let's you move faster, but requires more work from your muscles. The easiest way to describe when to shift is when you no longer feel resistance from the bike. When the wheels \"catch up\" to your feet, you can easily up the gears and get more speed. Another way to tell when to shift is when you're pumping too fast. A good rider uses long, slower than expected strokes to keep speed. If you're pumping like you're trying to get the bike to climax, you're doing it wrong. One last thing, have your seat up high enough that your knees are nearly straight at the bottom of your downstream. And so your knees never go past 90 degrees at the peak. Your muscles will appreciate the lesser strain, and you'll have a better ride in general.",
"if I can echo the OP here, I have a bike with 27 gears. That's 9 on the right hand and 3 on the left hand. I generally can get by just shifting on the 9 on my right while keeping the left gear in 2. If I were to go through all 27 in theory, how would I do it? Do I keep the left on 1 and go to 9 on my right switch and then switch my left gear to 2 and then try to get my right to 1 again???",
"One thing to watch put for is don't use extremes in the gear ratios. For example do not use out most gear on the front and the inner most on the rear. Doing so puts a lot of stress on the chain and prematurely wears it out. It might have 20 plus ratios but you should probably only use a put 16 of them or so for maximum life and use. Optimal cadence for pedaling distance is about 90 to 100 rpm. It's hard to get used to at first. Slower cadence feel more natural but they actually fatigue you faster. It's more cardio that way hut it gives you much smoother power. Also, don't think of the downstroke as the only power stroke. Especially if you have clips instead of standard petals. If you don't have clips, get them, it opens up a whole new world of distance biking. With clips you have the down stroke, then at the bottom you pull your foot backwards like you're scraping dog shit off the bottom. Then as the foot comes up you can pull up using the clip for a more consistent power around the entire stroke of the pedal. If you do have clips and have never used them. You will fall over. It happens. It happens to everyone that gets them almost. Anticipate gear changes before you go up hills. Trying to change in the middle of a high power stroke makes it more likely to jump the sprocket or break the chain. Neither are fun. You might have to increase to a faster cadence at the bottom of the hill to downshift, but you'll be less likely to need to shift mid way up. Edit. The real advantage of years is you can switch to a gear that allows you to maintain cadence independent of terrain."
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6tcm8l | Why does self-assembly furniture always use Allen bolts? Why not other kinds of screws? | Sometimes it's both. My desk needed the included Allen key *and* a Philips screwdriver (not included). | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"Allen heads are more resistant to stripping than Phillips heads(good for more than 1 assembly), you can get more torque on them(good for less wobble), you can use them in tighter spaces, and they come in a wide variety of sizes and head styles For your desk, i'm betting that all the bolts that took the allen key were exactly the same and the screws were different size/length. This would let them poke yoke the assembly process and makes you less likely to put the wrong screw in the wrong hole",
"I am actually very passionate on the subject of screw/bolt heads. Phillips heads only advantages are it's ubiquity and ease of manufacturing (which go hand in hand) , all Phillips can be made security screws, they suck as transferring torque (stripped heads) and the sizes are easily misinterpreted causing you to strip them more easily, best application is putting them on threads that self tap (wood screws, plastic, sheet metal, drywall...because you are already pushing pretty hard). Torx are the best because the driver either fits or it doesn't, there is no in between(it's almost impossible to strip the head), can transmit the most torque by a long shot (used to hold engine blocks together or very small parts), but they are expensive to make and no one has torx drivers at home (by far my favorite head, mostly due to application ). Allen is in the middle of these two being somewhat resistant to stripping, kinda easy to make,most people have a driver set at home, these are probably the overall best head type. Flat heads are basic af but the lack of self centering prevents power tools form being used. Hex heads are like Allen but only require one driver for all sizes (medium hard to strip, easy to make, ubiquity), they also do not require as much clearance to turn them. JIS (Japanese Philip) improved version of Philip, can transmit more load, can be distinguished by hash marks between each groove, still not suitable to hold any kind of load, their greatest weakness is being stripped by Phillips drivers because who the fuck has Japanese screw drivers (other than me). I spent 3 months of my education learning about bolts and use them every day."
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6tg5sj | Why are there so many fire hydrants in America compared to European countries? | Engineering | explainlikeimfive | {
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"It's not that there are less, but that they do not have the same shape. In the U.K., for example, they typically look like little boxes (typically yellow or grey), marked with a big \"H.\" In Germany, our fire hydrants can have a lot of different appearances. It's probably not that we have less, but that you don't notice them. :)",
"In some places,the fire truck carries the URL_0 the scene they open a plate in sidewalk,then screw in the hydrant.",
"I think there aren't actually more hydrants in the US than Europe. You maybe see fewer, but that doesn't mean they aren't there. In Germany for example hydrants commonly are either integrated into the facade of buildings or underground. [Here is a picture from Wikipedia of a standard German underground hydrant.]( URL_0 ) Stand alone hydrants like the common red pillar types familiar from the US are comparatively rare. They probably would get in the way."
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6thoxb | How does Mazda new HCCI gasoline engine work ? | They claim that it is the best of both worlds between diesel and gasoline engine, how is it possible ? | Engineering | explainlikeimfive | {
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"It's a gasoline engine, but it uses a supercharger to attain very high pressures in the cylinder, causing spontaneous ignition without the use of a spark plug. Together with a uniform blend of fuel and air, the result is more efficient combustion than the hot, explosive combustion caused by a spark. Detailed video on [this page.]( URL_0 )"
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6tibj6 | How does a city perform maintenance on a water or sewer main without shutting off service to houses or businesses? | The street I live on is having the water main replaced, but I haven't noticed a stop in service. I was curious how this works. | Engineering | explainlikeimfive | {
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"If that was the main where your home draws water, they'd have to shut it off. You're apparently getting your water from somewhere other than that main."
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6tkl3s | Why do/did automatic gearboxes use planetary gears? | I know that today there are many different types of automatic gearboxes (good ol' classic automatic, all kinds of semi-automatic robotic thingys, cvt:s etc), but back in the day pretty much all automatic transmissions in cars used planetary gears instead of "normal" gears consisting of different cogs. My question is: *Why were planetary gears preferred for automatic instead of "normal" gears?* I know how planetary gears work and I know how normal gears work. I also know how torque converters work and I know that classic automatic transmissions used hydraulics in some way to determine which gear should be engaged. But why use planetary gears? Do they work better with the hydraulic system? I also know this has been asked here at least once before, about a year ago but I didn't find any satisfying answers there. | Engineering | explainlikeimfive | {
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"In a manual transmission you have to synchronize the gears, get it up to speed, then release the clutch to engage the new gear The synchronization was difficult for a vacuum controlled system. With a planetary gearset your gears are always locked together and synchronized, you just need to use a clutch to lock the right component. No difficult alignment or synchronization necessary"
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6tlnzs | Modern day battery capacity and mAh. I have a DJI Mavic Drone that takes 3830mAh batteries. My Galaxy S8+ has a 3500mAh battery. The Drone battery weighs 2x as much as my S8, and is 6x thicker. What accounts for this? | **DJI Mavic flight time:** 27mins with the 3830mAh battery. Battery Weight: 240grams **Galaxy S8+**: 3500mAh battery. Total device weight: 173grams The galaxy S8 also has a screen, processor, RAM, waterproofing, and everything else besides the battery. Actual battery Weight must be 100grams tops. The total mAh here is almost the same. Yet one device weighs much more than the other, and is *much* larger, and is JUST a battery. What accounts for this? Why could DJI not make a smaller battery with double the mAh? What physical constraints are happening here? | Engineering | explainlikeimfive | {
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"The phone's battery, I'm pretty sure, is 5 volts; what's the voltage of the drone's battery? The energy stored by a battery is the product of amp-hours and voltage, not just the amp-hours by itself.",
"The issue is two fold, but first two basics: Current, measured in \"Ampere\" basically means \"How many charge-particles, electrons, flow per second?\" while Voltage, measured in \"Volt\" means \"How much energy carries each of those charge-particles?\" Now for the two aspects: --- First, the possibly less important thing of the two but still valid: mAh, or \"ampere hours\" means that you can pull one ampere for the given time, so if it says 3.8 Ah (the same as 3800 mAh) you can pull 3.8 ampere for an hour, or 7.6 A (double 3.8) for half an hour, 1.9 A for two hours or 0.38 A for ten hours. The massive difference between a drone battery and a phone battery is that the drone battery needs the energy in a much shorter time, if you fly half an hour you need to pull the 7.6 ampere from it to fly your drone. Fully draining it in 20 minutes - the phone could burst into flames or at least get damaged. Therefore the drone battery needs to be heavier due to more powerful components and wireing. --- And second, the more important thing: Ah (or mAh) is telling you nothing about the transferred *energy*. For that, you need to take a look at the voltage at which the one (or 3.8 or whatever) is pulled. You calculate Wh, which is \"watthours\", which basically means of much Watt (= energy) a system gives or takes per hour. You calulate this by Ah * voltage. So if you have two batteries both with 3.5 Ah and your mobile phone works at some 5V you have 3.5 Ah * 5 V = 17.5 Wh. But the drone might require 12 V or even 18 V and as such you'd get some (let us just use the mean between 12 and 18 for this example) 3.5 Ah * 15 V = 52 Wh. This means both have the same Ah, but one can deliver that 3.5 Ampere-hour at 5 V and the other at 15 V, meaning the drone battery can store and deliver 3 times the energy of the phone battery.",
"The drone battery is 3.8 Ah at *11.4 volts*, and the galaxy battery is 3.5 Ah at only 3.8 volts. The drone battery holds a lot more energy. Also, because size is not as much of a constraint, the drone battery is probably not packed in the same way. I'm guessing inside that battery are cylindrical cells rather than teeny little flat rectangular ones.",
"I wish they wouldn't use mAh as a measure of battery capacity: it doesn't tell the whole story. It's OK as a *relative* measure for comparing batteries for the same voltage, but *only* at the same voltage. To get a measure of the energy capacity of a battery, which is what really matters, you need to multiply by the voltage to get a mWh figure. For example, if you have a battery marked 2,000mAh, its energy capacity at 5V will be 10,000 mWh (10 Wh), but at 12V it would be 24,000 mWh (24 Wh), and probably more than twice the size!",
"On top of everything else mentioned, the safe discharge rate for a drone battery is gonna need to be a *lot* higher than the discharge rate for a phone battery. Your drone might draw as much as 10 amps constant current, whereas your phone might only draws 100 milliamps. Using those ballpark numbers, the drone battery needs to safely discharge 100x more current than the phone battery on average which means; - Cells that can tolerate said discharge rates need to be physically bigger, - The packing of the cells can't be as dense to allow heat dissipation, - The size of the wiring between cells and the circuitry will be bigger to accommodate the high amperage."
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6tlrmo | How does a elevator algorithm work? | Considering there are multiple elevators going to the same floors. | Engineering | explainlikeimfive | {
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"The elevator algorithm is simple: 1. If there is a called floor in the direction the elevator is going, go there 2. Otherwise, if there is a called floor in the opposite direction, change directions and go there 3. Otherwise, go nowhere (or sometimes go back to a specific floor) Generally with multiple elevators there is no coordination beyond not servicing a floor that another elevator is already headed to. There *are* more advanced algorithms. The Marriot Marquis in NYC was known for having wait times of up to 30 minutes, and about a decade ago installed a [destination dispatch]( URL_0 ) elevator system. Users punch in their desired floor in advance, and the system tells them which shaft to use. This allows a computer to optimize the elevator use more efficiently.",
"The elevator algorithm is actually pretty important in computer science. It is pretty simple to understand though. When called, the elevator will prioritize the calls that are in his way, then it goes the other way. If the elevator is going up, it will continue to go up until there's no one left calling it in the up direction or until it reaches maximum floor. Then it goes down with the same logic.",
"The real answer is: It is *very* complex. In the most simple approach you click a button and a software determines which elevator goes there and then blocks all others from answering that call. In reality... it is vastly, vastly complex and engineers and computer scientists work on it. For example, modern algorithms try to base where which elevator does what based on the time of the day, for example: * in the morning a lot of people want to go from the \"bottom\" to all kinds of floors further up with a massive peak demand * during the day most people want to drive around in the house but the demand is spread more evenly, the algorithm might attempt to reserve elevators for certain parts of the house, one might jump a request if it already has many buttons pressed from the inside, and another is scheduled for that outside-request * unless it is around noon, when all kinds of people want to go down for lunch * and in the evening a lot of people might want to go down to the bottom again. * many, many, many more cases depening on time, the type of house (only offices in there? A mix of office and businesses? Entertainment venues (a club on the roof? A gym in the middle? Shopping center? What if there is also residence in it?). All that said: There are probably entire careers in CS/engineering for \"elevator optimisation\" and every manufacturer has his own, probably very well guarded algorithms with custom ones for special houses (you can bet that the Burj Khalifa does not steer its elevators with Siemens-Standard-Algorithm#2 they might roll out for some 5-story residence). **tl;dr There is no general answer and the issue is vastly complex.**",
"A little bit offtopic, but if you know a little bit of programming [this]( URL_0 ) game may be interesting. You control a bunch of elevators to move people in the most optimal way.",
"Omg omg omg!! Ive been waiting for this to be a question for soooo long! Now I'm too excited to remember what I wanted to ask about!! I feel like a kid who meets their favorite celebrity and can't muster a word!! I have considered elevator algorithms for far more time than I want to admit. Now I can't even get it out. Ok so.. in a medium apartment complex, 10 floors, say.. and two elevators. Which one comes first? A or B? Probably A because of programming. If elevator A is on the 10th floor and opening while someone on the 8th floor calls I find that elevator B will come up. But wouldn't it make more sense for the algorithm to be 'pause', 8 is calling. I know that if I call the elevator *after* A on 10 is closed it will come to 8, but not if 10 is still open and it's next stop is undefined. I guess it's all about programming, but can a 50yr old system be reprogrammed that way? Also, what gets elevators stuck between floors? Like, why does that happen? If elevator A is on floor 5 and elevator B is on ground floor, who gets the call for floor 3? More complexly, if elevator A is on floor 10, and B is on 3 and both are programmed to return to 1, which goes to 5 going down if someone on 6 wants to go up at the exact same time?",
"They aren't standardized. so... any algorithm you can come up with that makes sense is and is relatively simple is probably being used by an elevator somewhere."
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6tn5qh | Why/how distinct gear upshift sound with CVT? | I rode in a 2017 Honda Civic with a CVT. While it was accelerated hard to 60, I heard and felt what certainly seemed to be distinct gear upshifts. But it has a CVT. So, it supposedly has a Continuously Variable Transmission and does not have distinct gears. So, ELI5, what was I hearing and feeling? *edit: Thanks for the replies. It seems that Honda added in simulated gear shifts. Which is dumb. Seems like there should be a software hack to remove them! | Engineering | explainlikeimfive | {
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"What you're hearing and feeling is a manufactured sensation of gear changes. CVT doesn't have traditional gears and doesn't \"shift\" however, manufacturers found that drivers did not enjoy the constant torque (and sound) of a CVT transmission and added in simulated gear shifts. IIRC it's mentioned in the Engineering Explained video on CVT: URL_0",
"Was it in sport mode? I owned an accord CVT which had simulated gearshifting when it was in sport mode but in regular eco-mode drive, it was a constant RPM. Edit: I didn't think people would get worked up over transmissions this much",
"I’ve driven a Nissan Maxima that did the same thing. The CVT is programmed to make a few sudden changes to the overall ratio as opposed to sliding across the range, simulating shift points you’d feel with a traditional transmission. If you want to feel a CVT without the shift points, I believe the Fusion Hybrid I drove for a week operated in this manner.",
"On a side note, I would really love to own a car with a manually controllable CVT. It'd be so nice to be able to have the responsiveness of manually changing gear ratios, but still have the advantages of a CVT. Does anyone make one of those? Edit: to clarify, I don't mean a CVT that can act like an x-speed manual transmission. I mean a CVT with a lever that can access any of the CVT's theoretically infinite gear ratios. I'm sure there's probably some reason this is a bad idea, but I have no what that might be.",
"When you press the accelerator hard, CVT changes the usual ratios to quasi-sport-mode to give you a boost of torque and passing power. That is what you feel. Source: Honda manual.",
"Use to be a tech for acura and honda. Also former service advisor. People complained they didnt like the way it felt, manufactuer decided to implent a replicated gear shift.",
"I had an '03 Murano SL. That was allegedly their first use of CVT in North America. It felt like it had one gear from 0 to the limiter at 118. It would stall out at 105 until I fluttered the throttle (lifted it and matted it in less than a second). Then the tach would drop from 4000 to 2000 and the speedo would resume the climb to 118. I called it \"supercruise\" after the same-named feature of the F-22 Raptor. It was like an easter egg. tl;dr Never felt any gear shifts. Just one long puuuuuuulllll...",
"The Subaru Impreza CVT doesn't have this. It doesn't pretend to have discrete gears. Kinda cool to start up a hill, the engine revs to optimal and just hangs there as you accelerate through the transmission.",
"It really bothers me that some CVTs mimic the shifting sensation, for the same reason some electric/hybrids play fake engine noises... because people are accustomed to old, less efficient, louder technology. Personally, I love the way my CVT pins me back in the seat and doesn't let up. It's the closest I'll ever get to a rocket. And I love how silent my hybrid is (in electric mode). The new tech is better; I say embrace it.",
"I own a 2016 Civic Turbo with CVT and have never heard of felt any kind of shifting in it.",
"I noticed this with my Nissan Sentra, I told my dad I could feel the shifts and he told me I shouldn't be able too. I was confused because I know for a fact you can feel it but also know my dad is very knowledgeable about cars, now I got something to tell him.",
"This has nothing to do with the \"simulated shift\" topic, but for those reading... Stay away from CVTs if at all possible. Their efficiency is far outweighed by the cost of repair/replacement. I own and operate a transmission shop. 90% of all available CVTs on the market are non serviceable and require replacement when problems arise. Some at the tune of 5k or more for JUST the unit, no labor, expensive special fluid or taxes included. I have witnessed customers literally scrap their 4 year old car due to the cost of unit replacement. The repair often times costs more than the vehicle is worth. Until they start making serviceable units, I would keep with conventional automatic transmissions. With technology like dual clutch drums, and the usage of sprags and other types of one-way clutches, we are now able to make 7,8,9 and even 10+ speed automatic transmissions, allowing for more efficient gear changes and keeping the vehicle in it's optimal performance range. Just my thoughts on the matter. I hope this information helps someone on their next purchase.",
"It is used to mimic a geared transmission so that drivers don't get confused thinking they have a slipping trans or that there is a problem. Too radical a shift (pun intended) from geared to completely gearless. Our '16 Impreza has the CVT and does the 'gear shift' thing. I don't get it - if it's a CVT then be a CVT - don't fake out like you're a slushbox just so the cavemen don't get confused.",
"There's actually a standard gearbox hidden in your car, a little gnome is doing the shifting for you.",
"basically people would come into dealerships saying their car was broken because it wasn't shifting and the word of mouth made bad press for big companies, things were changed to have more distinct gears so the cars would sell better but it also defeats the purpose of a CVT",
"Can someone ELI5 what this question is asking?",
"How bout buy a manual gearshift for real gear changing euphoria ? You know...actual shifting",
"My wife's Forester does that. It drives me crazy. Apparently they changed it for the model year she got to be more pronounced.",
"People expect to feel gears. So they program cvts to \"shift\". As in, they dont change gear ratio until certain speed, load, and rpm parameters are met. Making them effectively pointless.",
"Yeah, I honestly hate the fake gear shifting. Ideally, I would like to manually control the CVT's RPMs with a lever like you do in a plane. That way I can floor it at 2K if I wanted to, instead of the daft thing hesitating winding up and then jerking me and my passengers like we're in a cartoon. To simplify the concept, I kill for something that is like what they do with a lot of today's automatics and have a \"gear mode\" that branches off of \"D\" With Low, Eco, and High RPM settings. Low RPM for down hills/engine braking (if you are into that). Eco for best MPG at all times. High RPM for passing people quickly and waking up your whole neighborhood (ideally). \"D\" would vary between the 3 modes \"intelligently\" for people less interested."
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6tp4qm | Why are drivers, specifically GPU drivers, updated so often and regularly? What does a new driver contain? | Engineering | explainlikeimfive | {
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"They write their drivers to work in any circumstances they can think of, and then somebody tries something they didn't think of. They get a report of \"If I run Game X at this resolution and then Windows sends me a pop-up low battery warning, it crashes the computer.\" So they try to fix it. It can also be for security, like somebody figured out a way to install malware by using some bug in their driver or in a popular game, so they have to fix that, too. It's also possible that they figured out a better way to make the card do something, and they add that to their driver. Another issue is the old programmer's rule that if you have eight bugs in your code and you fix one, that leaves eleven.",
"Bug fixes. Most of these companies get large numbers of bug reports, both in an automated sense (when your PC crashes or the driver crashes) and also from users via their support. They regularly fix the most common bugs. They also fix security issues, as well as enable the driver to properly respond to new conditions (like a new OS version).",
"A lot of graphics cards drivers in particular contain patches for [broken programs on your computer]( URL_0 ). Basically to avoid you buying a game that doesn't work, the video drivers can actually contain the code patches to fix bugs in how various games are mis-using the graphics hardware."
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6tptv1 | Why do the metal prongs on most electrical plugs have holes in them? | Engineering | explainlikeimfive | {
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"The outlet themselves are designed to grip the plug. The outlet itself has some bumps in the contacts that you plug into. Those plugs fit into holes on the plug, and grip it a bit better than it would if there wasn't the holes in the plug."
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6txwdl | Why can't we see satellites from the ISS? | I was watching the live feed from the ISS and people are asking why can't we see satellites ? | Engineering | explainlikeimfive | {
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"The orbital path of satellites is a sphere significantly larger than *the entire Earth*. While there are thousands up there, most of them are the size of a car basically. You aren't going to just randomly see *anything* in space. It's SO huge, if you're not trying *really really hard* to find something, you never will. It's like if you're in the Sahara ATVing. You might wonder \"I can't be the only one ATVing in the Sahara, where is everyone\". Well, the Sahara is 3.5 million square miles. It's like finding a needle in a haystack, when the haystack is the size of 7 countries. It doesn't matter if there's 2000 needles, or 5 million. You probably won't see another one even if you search for the rest of your life.",
"Most satellites are in what's known as \"Geo-Stationary Orbit\" which is about 22,236 miles up. You're not gonna see something the size of a car from that distance. Space is BIG."
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6tzc02 | Why can't we put a high quality camera on a satellite orbiting another planet? | Engineering | explainlikeimfive | {
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"There's a couple reasons, [I think this page has a pretty good rundown of several of them for the Cassini camera in particular.]( URL_0 ) In a nutshell: it takes a long time to develop a probe from design all the way through launch, so its electronics will always be a bit out of date by the time they fly. The camera has to survive some really extreme conditions in space, for many years, with no maintenance. Deep space probes to distant planets tend to have low bandwidth because the radio signal is so weak at that range, so they need to be very efficient with data. Consumer-grade cameras actually have designed-in \"cheats\" to easily take good looking pictures for the human eye under Earth lighting conditions, but taking scientific pictures in deep space means designing for different lighting, filters, and sensitivities."
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6u5lm0 | why are most top fuel dragsters V8 why not use V10 and V12 to go faster?. | Engineering | explainlikeimfive | {
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"The NHRA regulations constrain the engine used to power a Top Fuel drag racing car to the basic layout found in the second generation Chrysler 426 Hemi made from 1964-71. Although the Top Fuel engine is built exclusively of specialist parts, it retains the basic configuration with two valves per cylinder activated by pushrods from a centrally-placed camshaft. The engine has hemispherical combustion chambers, a 90 degree valve stem angle; 120 millimetres (4.8 in) bore pitch. The configuration is identical to the overhead valve, single camshaft-in-block \"Hemi\" V-8 engine which became available for sale to the public in selected Chrysler automotive products in model year 1951. The engine displacement is limited to 8,190 cubic centimeters (500 cu in). A 106-millimetre (4.1875 in) bore with a 114-millimetre (4.5 in) stroke are customary dimensions. Larger bores have been shown to weaken the cylinder block. Compression ratio is about 6.5:1 .",
"V8, v10, v12 is designation for how many cylinders It does not say anything about how big the cylinders are. 10 and 12 have more components. More valves, more bearings, more variables. Top fuel dragsters are ballpark 8 liter supercharged making 10,000hp. Just the supercharger requires more power than most consumer engines can produce.",
"Number of cylinders doesn't necessarily equate to more power. The the volume (or the size) of the cylinders is what matter."
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6u7ex8 | Why do most Airplane engines have a swirl on the front of them? | The swirl on the engine looks like its on the middle bearing, so it swirls in a hypnotic way while the engines are on. Can some explain why these exist? | Engineering | explainlikeimfive | {
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"It provides a visual indicator as to whether the engine is turning or not. Very useful for ground crews and maintenance people."
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6ua6zf | Why do we use bronze on buildings and statues if they're only going to turn green? Do the creators know the work will turn green or is it a sign of a lack of upkeep? | For example, in Philly there's a building with green roofing because the roof is bronze. Did the architect know his work would change color or what? Thanks! EDIT: Copper. Not Bronze. | Engineering | explainlikeimfive | {
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"It is a choice made in the design of a building... Originally it had to do with longevity and reliability of the materials - if you look at steel, the rust that forms when it is left exposed is detrimental - as it rusts the material degrades and gets thinner and weaker until it fails. So steel needs maintenance and protection to last long term (such as bridges being painted, or having steel encased in something like concrete to protect it). The green of bronze however is actually helpful - initially it is nice and shiny, then degrades to the green colour we commonly see, at which point that green layer has created a barrier around the metal, so it doesn't degrade any further. So once it reaches that point it needs relatively little maintenance and upkeep - great for a feature dome at the top of a roof you don't really want to have to access regularly. It is also worth remembering that most of these structures are traditional ones, built when technology and materials were far more limited, so the green roof was a small price to pay to be able to actually build the structure that may not have been possible with other materials. Nowadays it mainly comes down to architectural reasons - it will be used to give a certain look and design to a structure that the architect desires.",
"Yes, architects know their roofs are going to turn green. It makes buildings look classy and old. Sometimes they'll even chemically treat a new copper roof to get that verdigris color (it even has a cool name) without having to wait fifty years for it to change naturally.",
"Side note, many colorblind people are completely unaware that the Statue of Liberty is green.",
"Copper -- and by extension, bronze -- has two oxidation modes: brown and green. Newly installed copper roofs, for example, are chemically treated to promote the brown oxidation, since it will protect the underlying copper from weathering. The green oxidation, on the other hand, will continue to chalk and flake off, eventually eating all of the copper away. There was a case many years ago when a group of \"good samaritans\" cleaned some oxidized bronze statues in their city to restore their luster. But they were taken to task for it, since the brown patina that they removed served to protect the bronze from the other mode of oxidation. Statues that are already green should probably be scrubbed and retreated to promote the less corrosive oxidation.",
"On sculptures this is called a patina and is intentional. You can treat the metal to prevent oxidation sometimes with coatings or slow it or speed up oxidation with different textures.",
"they have known it turns green for thousands of years. its even called verdigris, which came from verte de gris ( from verte de grece), meaning green of greece. i believe it was used as far back as 300-400 bc"
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6uek5m | How are large/giant sinkholes fixed? | An example of size being URL_0 , and how do they ensure they stay "fixed"? | Engineering | explainlikeimfive | {
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"text": [
"fill it with big rocks. then smaller rocks. then sand. then pour cement on top. then layer dirt on top. the reason why sinkholes become sinkholes is because water washed apart dirt. replace dirt with rocks that water can't wash away, your problem is solved."
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6uhibw | What happens to building under construction during an earthquake? Are there precautions taken to mitigate damage? | Engineering | explainlikeimfive | {
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"Most of the measures taken to make a building resistant to earthquakes are installed in the early part of construction(tie-downs, shear paneling ect...). It's not really to mitigate damage during construction, as much as it's the easiest order to build, but it does mitigate the risk quite a bit."
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6uhv29 | How does a boat motor work, and how does a small propeller move such a large object? | Engineering | explainlikeimfive | {
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"text": [
"The small propeller moves a small stream of water, just as wide as the propeller, at a speed that's much faster than the boat moves. This stream of water has more mass than you'd think, because water is quite heavy, and it has a momentum (m • v). All this momentum goes into the boat, because momentum is conserved (the old physics rule \"Every action has an equal and opposite reaction\") so since the boat has a larger mass it gets a smaller increase in velocity. It's surprisingly effective because well designed boats experience very little water friction, that's how you can paddle a canoe that floats your weight, plus the canoe, plus your stuff.",
"A boat motor works similar to a car motor except they're usually direct drive or have a simple single speed forward gearing, as well as a reverse gear. You have a shaft attached directly to the engine rather than a complex gearbox. The shaft runs through what's called a stuffing box, which is a grease filled tube that keeps water out. Very large boats (cruise ships, container ships) use azimuth thrusters. Large rotating pods which contain an electric motor that is powered by a diesel generator in the ship's engine room. So no need for a stuffing As for the propeller, they work the same way an airplane propeller does, or a fan, or a swimmer doing the front crawl. They scoop water and push it back. Since every action has an equal and opposite reaction, this causes the boat to move forward. Thrust. You can change how much water they scoop by adjusting the angle of the blades, the pitch. The sharper the angle, the more water they can scoop at a lower rotational speed. Allowing them to work more efficiently. Since water is more viscous (thicker) than air, you don't need as much surface area or rotational speed as you would with say a plane. Nor will you need to go nearly as fast. So you can make the prop smaller. The downside with adding more pitch is it requires more torque, rotational power. Which is why boats are often powered by high torque engines like diesel, steam, and electric as opposed to gas. With a gas engine, the prop has to turn faster, so it needs a shallower pitch to get the same amount of speed. But it's doing less work per rotation. So when buying a prop, you really have to keep in mind the ones that are designed to work with your engine. There's another issue that crops up too if you spin a prop too fast: cavitation. The spinning prop creates a low pressure zone which causes water to boil at below 100c. Bubbles of steam appear and pop, which cause shock waves that can actually damage the prop."
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6uimc7 | Why does a smart phone touch screen work after the glass has been broken? | Engineering | explainlikeimfive | {
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"text": [
"It cuts your finger because it's literally broken glass. Nothing special there. The glass in a modern touch screen phone is actually there to protect the underparts, it takes no part in making a touch screen work it's just that the real touch sensitive parts are fragile and need that protective layer that's transparent, so glass is used. The kinds of touch screens used in modern phones are known as capacitive. Unlike a resistive touch screen (the older fashioned kind) it does not actually sense the pressure of your touch but actually the electric charge of organic matter. It's basically all based on static electricity. So there are 3 basic parts to a touch screen phone. Ther's the glass layer for protection. The capacitive layer to sense the touch and the screen layer to display the information. These 3 layers are all sandwiched together. In an LCD display, the \"display\" portion is also layered. There are the crystals and the light. The crystals themselves don't emit light, they just change color when different electrical charges are passed through them, and are semi transparent. The back light shines through the crystals to make the display show. Some modern types, such as Samsung's AMOLED screens actually integrate the display and the sensor making the screen appear much closer to the glass. AMOLED screens don't use crystals, they use diodes that emit light, so they also eliminate that backlight issue. That's why they can make better blacks.",
"The glass is separate from the digitizer piece. Digitizer is it's own clear plastic panel under the glass."
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6ukpd0 | How does a manual transmission in a car work? | Engineering | explainlikeimfive | {
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"text": [
"Yer not alone in askin', and kind strangers have explained: 1. [ELI5: How to drive a manual transmission. ]( URL_6 ) 1. [ELI5: How does a manual transmission work? ]( URL_2 ) 1. [ELI5: How does manual transmission work and what's it's purpose? ]( URL_7 ) 1. [ELI5 A manual transmission/stick shift ]( URL_5 ) 1. [ELI5: How a manual transmission (and clutch) work ]( URL_4 ) 1. [ELI5: How in the fuck does manual driving work, and why is it considered better than automatic? ]( URL_1 ) 1. [ELI5: What makes a stick shift different than an automatic other than manually shifting the gears? Is there anything better/worse about it? ]( URL_3 ) 1. [ELI5: How do manual car engines work as opposed to automatics and how do you drive a stick shift? ]( URL_0 )",
"A manual or [standard transmission]( URL_0 ) transfers power from the engine to the differential using a clutch and pairs of gears that are manually selected using the gear selector and locked to the output shaft. Most cars today with manual transmissions have five or six different forward gear ratios, however manual transmissions with anywhere from three to seven gears are common, depending on the year the vehicle was made. The clutch is positioned between the engine’s flywheel and the transmission. Normally the clutch is engaged, and locks the transmissions input shaft to the flywheel. When the clutch pedal is depressed, the clutch is disengaged by a thrust bearing, and no power is transferred. With the clutch disengaged it is possible to select gears. To start the vehicle moving, the clutch is slowly released, and slipped slightly. It is important not to ride the clutch once you are moving (by resting your foot on it for example) because this can lead to clutch slippage (which over time can cause clutch failure) or premature thrust bearing problems. Gears are normally selected using a shift lever, often mounted on the floor, but sometimes mounted on the dash or steering column. The gears are usually layed out in an H pattern, with neutral being the space between gears. Early manual transmissions (and those found in some trucks, heavy machinery, and racecars) use an unsynchronized design. In an unsynchronized transmission the gears are selected by sliding them on shafts until they have engaged the dog clutch. In order for the gears to engage properly they need to be spinning at the same speed as the output shaft, otherwise the gears (actually the dog clutches) will grind. To do this you can either double clutch by essentially using the clutch to shift into neutral, and then again to shift into the desired gear, or you can rev match by matching the engine’s RPM with the RPM the engine will be at for the road speed in the desired gear. Unsynchronized transmissions are generally tougher than synchronized ones, as the synchros are usually made out of soft brass that wear much more quickly than the steel gears. Because of this added strength, unsynchronized transmissions can usually be shifted quickly without using the clutch, especially when designed with fewer teeth on the dog clutches, as in some race cars. Most transmissions found in modern cars are synchronized. The synchronizer is attached to the dog clutch, and consists of a cone clutch and a baulk ring. When you try to shift gears and the parts aren’t at the same speed, the cone clutch contacts first, bringing the gears and output shaft to the same speed using friction. The baulk rings prevents the dog clutch from engaging until everything is synchronized. The synchros are usually made out of a soft metal such as brass, which makes it important to use the clutch when shifting, because imperfect rev matching will be masked by the synchros. Continuous abuse will lead to synchronizer failure, and grinding. Rev matching while using the clutch can reduce wear on the synchros as they will need to work less hard to match the speeds of the gears and output shaft. In a sequential gear box (usually found in race cars) there is no neutral between gears, and the gears are selected in sequential order instead of in an H pattern, often without using the clutch. In a semi-automatic manual gearbox the actual rev matching, gear shifting and clutch control is handled by a computer. In these transmissions the clutch is usually only used to get the vehicle moving. When the driver presses a button or pulls a lever the computer takes the car out of the current gear, blips the throttle or waits for the engine RPM to drop to the appropriate level, and then changes into the requested gear. Manual transmissions are generally more fuel efficient than automatic transmissions because there is no fluid coupling or fluid pump like in a traditional automatic transmission, and no belt to slip like in a continuously variable transmission. A standard transmission also allows the driver more control over which gear they are in, potentially allowing for a lower engine RPM."
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6umd3n | Why do some large trucks have tires that are set 6"-12" off the ground? | The second and third tires in this [pic]( URL_0 ) don't appear to have any functional use. They don't make contact with the ground. WHat are they for? | Engineering | explainlikeimfive | {
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"text": [
"These are extras that can be put to use if the load is super heavy. When the load is lighter they are retracted to reduce friction and save a little fuel.",
"They are lift axles and can be raised or lowered depending on how heavy the load is. The axles have air chambers that are filled with air from the trucks air supply to lower them. The more axles you have the more weight you can legally haul. When the truck is empty it is best to leave them up, the truck steers much easier with them up."
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6uom8o | Why do buses and ambulances have chains hanging from the rear axles? | I've always seen the, hanging below the trucks and no one has ever been able to tell me why. I've heard static discharge and possibly warning the driver that the airbags are under-inflated but no one was sure PS I just stumbled upon this sub and I love it! | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"They are for snow chains. (I'm assuming you are seeing these up north. There is basically a round hub that you press a button and it pushes the hub against the wheel. Then as the wheel spins, the chains spin, and the chains go under the wheel, to increase traction."
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6uppe5 | Waterwheels, the old timey styled ones. | Firstly, I'm not a complete dope, I have a pretty general idea of the waterwheel concept. They build the waterwheel structure, and the attach a log or something in the center to turn when they attach it to the water. For the life of me I can't imagine the much use they could have gotten out of it before electricity. Maybe attach some gears and use it for a bellows? Or I think they used it for flour? Anyway, clearly I don't know what I'm talking about, but hopefully the leading expert of old world waterwheels sees this. | Engineering | explainlikeimfive | {
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"text": [
"What do most industrial machines use electricity for? To drive a motor. What does a motor do? It spins a shaft. What does a water wheel do? It spins a shaft! Look at [a picture from inside an old mill]( URL_0 ), there are shafts that run along the ceiling, these are connected to the water wheels outside. They power looms and weavers and spinning wheels that need rotational energy. You can use a water wheel to power anything that needs an input of rotational energy like grain mills, or milling machines, or lathes, or generators Today we use electricity as a nice way to transport energy from one place to another, but almost all electric circuits have a mechanical analog and while they're a lot bigger you can get the same results with gears, springs, dashpots, and weights, they're equivalent to transformers, capacitors, resistors, and inductors"
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6uww2t | How does greater locking surface in a firearm increase its accuracy? | Heard it multiple times in videos of Forgotten Weapons youtube channel. As I understand this is primarily about semiautomatic weapons and follow-up shots? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"Say you have a hammer and nail. If the hammer head is bigger than the nail, then the force of the hammer is distributed evenly against the head of the nail. The nail will go straight into the wood. Now say you have a hammer and a nail, but the nail head is bigger than the hammer. When you hit the hammer against the nail head, it's likely to be slightly off center. This will cause more force on one side, and the nail will go into the wood at a slight angle. So the bigger the surface area of the hammer head in relation to the nail head, the more likely the nail is to go into the wood straight. This isn't as big a deal if you hit the nail dead center each time, but it's hard to do that. People tend to be slightly off with their aim when swinging a hammer, and even reliable machines have tolerances where they are slightly off. Now just substitute the hammer for the locking mechanism in a gun, and replace nail with bullet, and you'll get why increased surface area on the locking mechanism increases accuracy."
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6uyk6n | In most old cars, the transmissions shift was placed on the steering column. Why did automakers start placing it between the front seats? | EDIT: I watched some videos of manual cars that have shift levers at the steering column. The whole time I was thinking... FUCK THAT! The modern day H-pattern is MUCH more straight forward. | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"faster shifting due to a direct and shorter mechanical linkage to the gearbox. it also co-incided with the move from bench seating to bucket seats."
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6v2j5o | why does spectral leaks occur when doing fft? | Engineering | explainlikeimfive | {
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"text": [
"The sampled signal does not fully represent the real world underlying signal. Critically, in this case, the sampled signal is recorded for a finite time. Let's imagine a single sine wave at a given frequency. Its analytic Fourier transform will contain a delta spike at that frequency and nowhere else. But when you take an FFT of a finite domain signal, you apply a windowing function. If you choose no windowing function, you get a rectangular window by default. You can say that the measured signal is your original perfect (infinite) sine wave multiplied by a function that equals 1 during the sampling window and 0 everywhere else. That's the rectangular window. Since this multiplication occurs in the time domain, there must be a convolution in the frequency domain. This convolution is a good way to picture the leakage. Take the Fourier of the sine wave, and you get a spike. Take the Fourier of the rectangular function, and you get a sinc wave, which is periodic and peaks at the center and decays with distance. Now convolve these two transformed functions in the frequency domain by sliding the sinc function over the spike. This blurs out the spike, and you get your leaky FFT."
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6v2m25 | Why we can't just put removable batteries in electric cars. | I mean, if a household inverter battery can be plugged in and out, a car battery shouldn't be much of an issue Imagine "petrol stations" just being these huge battery warehouses where you pull up, take out your battery, put another fully charged battery in, and drive off. The old battery is then charged up in the warehouse Wouldn't this work if we're able to standardise the kind of car batteries being used? | Engineering | explainlikeimfive | {
"a_id": [
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"Elon Musk: We have, basically, the LA-to-San Francisco pack swap capability in place, and I believe all Model S owners in the California area have been invited at this point to try it out. And what we're seeing is a very low take rate for the pack swap station. So we did an initial round of invitations, where we did basically, like, 200 invitations, and I think there were a total of four or five people that wanted to do that, and they all did it just once. So, like, okay, clearly it's not very popular. And then we said, okay, let's expand that invitation to all customers, but I would expect that all customers behave roughly like that initial sample group. It's just, people don't care about pack swap. The Superchargers are fast enough that if you're driving from LA to San Francisco, and you start a trip at 9 AM, by the time you get to, say, noon, you want to stop, and you want to stretch your legs, hit the restroom, grab a bite to eat, grab a coffee, and be on your way, and by that time, the car is charged and ready to go, and it's free. So, it's like, why would you do the pack swap? It doesn't make much sense. We built the pack swap into the car because we weren't sure if people would want to choose the pack swap or not. We thought people would prefer Supercharging, but we weren't sure, so that's why we built the pack swap capability in. And, you know, based on what we're seeing here, it's unlikely to be something that's worth expanding in the future, unless something changes. URL_0",
"This is something that's been discussed. The problem is that the battery costs ~1/2 of the price of the car. They take up a bunch of space, and it would take time to switch them. The \"gas station\" would be a very expensive business to set up, you'd need several batteries for each type of car. If changing them takes 20 minutes, like an oil change, it's going to have to cost $50 for the technician time. When a supercharger can put 50% charge in the car in the same time for $5, it's hard to see this becoming popular.",
"A commodity is something that is interchangeable with something else. One ton of Grade B wheat is the exact same as any other ton of Grade B wheat in the world. I can trade you my ton for your ton and never give it a second thought. A battery isn't like that. A battery has a different lifespan depending on how it has been treated in the past (both how many charges it has been through, and how quickly it has been charged/drained). You wouldn't want to go to a \"gas station\" and swap your brand new Tesla battery for a three year old gas station battery. A gallon of gas is a commodity. You buy a gallon anywhere and its the same thing. Same with electricity. That's why its a lot easier to charge the car instead of replacing the battery. That said... If you don't own the car, say it was an Uber, the battery could be swapped out at a station and be charged outside of the car.",
"There have been attempts... there was a company called Better Place that attempted pilot runs in some locations a few years back but folded. Tesla has also explored the idea. The issues are that the batteries are huge, incredibly heavy, and unique size/shape for each model. The batteries weigh over 1000 pounds and are the size of a queen mattress. Not exactly easy to swap without a giant crane and storage for waiting batteries and actively charging ones. And a Tesla Model S battery is different from a Leaf, or BMW i3, etc. so do you have to keep a dozen different types of batteries on hand? Have brand/model-specific battery swap centers? Could you imagine having BMW or Ford specific gas stations? Where would we fit all the different stations? The other option would be to get all electric car makers to create a standard... but given all the R & D spent already and timeframe for designing vehicles and getting them to production, that could take a decade or more."
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6v6hog | Why are chairs slightly incurved instead of the other way around ? | It's well known that the right posture for humans is to bring at a reasonnable degree your shoulders back and extend your thoracic cage. Then why do all chairs litteraly put you in a position where you shoulders are leaning in and your chest is sinking due to their incurved shape ? People suffer from chronic pain from this whole "desk/chair posture". Wouldn't a slight outercurve in chairs correct this problem and help improve our postures ? | Engineering | explainlikeimfive | {
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"text": [
"Some busses and trains in sweden actually have chairs that make you sit with a straight back, but i think It's cause people find those types of chairs more comfortable"
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6v7y8t | What's the actual, non-conspiracy-theory explanation for why World Trade Center 7 fell? | Googling this leads me down so many rabbit holes. | Engineering | explainlikeimfive | {
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"It had a huge gash torn out of it by one of the other towers. That included some support structures. Further damage from fire led to the tower being too heavy for the remaining supports to hold up. And the gouged portion basically \"tugged\" the standing portion down when a crucial pillar (79) couldn't hold up further. Edit: had a wrong piece of info. Added the exact failure point for the major collapse.",
"A very, very tall building next to it collapsed, with a lot of debris hitting 7 WTC. This debris started fires that eventually led to the structural collapse of the building.",
"According to both the FEMA and NIST reports, the building collapsed due to uncontrolled fires causing some beams to expand and push a main girder off it's seat, causing the center of the building to collapse. Apparently there was no water pressure at the WTC so firefighters couldn't actually do anything.",
"My source (Wikipedia) says the nearby North tower of the WTC destroyed it. Debris and Fire struck the WTC 7 building and eventually caused it to collapse",
"URL_0 There is a gash starting at the top of the building going at least 10 stories down from a tower falling into it. The entire south face of the building is smoking and fires are raging on at least few floors. URL_1",
"I highly reccomend taking the guided tour at the Memorial if you get to NY NY. Ask to go with Eduardo if you can."
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6v96rx | Power button on a smart phone | Engineering | explainlikeimfive | {
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"text": [
"I suggest that you read a few button tutorials for embedded systems like arduino or raspberry pi. The processor can read the state of the buttons at any time. So they do this in a loop. When they notice the button is pressed they start a timer and wait for the button to be released. By measuring the time it takes for you to release the button they can do different functions. To save on battery they might not do it in a loop but rather have an interrupt logic in place that will signal the CPU whenever the state of a button changes and make it run the button checking logic. There may also be additional logic on the circuit board that detects if the power button have been held down for some time without the CPU reacting and do certain functions like toggling the power. This can be useful if the CPU have crashed or if the device is out of power.",
"Software. If the button wasn't pressed on the previous time, but is now, save the time. If the button was released and press time was < 1 second, turn off the screen. If the button is still being pressed and more than 1 second elapsed so far, show shutdown screen. If the button is still being pressed after 5 seconds, perform a shutdown.",
"It is worth noting that a smartphone power button is not a physical on/off switch like a typical light switch, but instead something more akin to the key of a keyboard - when you press the button a piece of software in the phone registers this and then performs the appropriate task, which can be programmed to be different depending on how long the software determines the button is pressed."
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6vbahv | why do bombs and missiles explode in the air above their target? Instead of hitting the ground/target and exploding? | Edit: thanks for all your informative responses! I have a lot to read through | Engineering | explainlikeimfive | {
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"In an impact detonation (bomb that explodes when it hits something), that \"something\" absorbs most of the force of the impact. If it is the ground, a majority of that force is transferred into the ground and some of it is reflected back up into the air (think of a ball bouncing off the ground, it bounces back in the air at a very similar angle that it impacted the ground). For harder targets where you want to concentrate the force in a smaller area, this is the technique you want to use. In an air burst bomb (using either a timed fuse or a radar proximity fuse) the shockwave travels toward the ground, is reflected by the ground, similarly to the ball theory and meets with more shockwave. The meeting of this ground reflected shockwave and the Air shockwave change the direction of travel of both and causes them to run parallel with the ground. This amplifies and shockwave and pushes it outward, away from the central point of detonation with more force. Fragmentation damage is an effect in both cases. In impact detonation the frag is reflected upward and out from the ground (since it impacted the ground first) and is slowed somewhat by the contact with the ground. This is the desired effect for harder targets. In an air burst the frag pattern is directed toward the ground with more force (initially) and then pushed outward by the outward directed shockwave. This is more desired for softer targets (vehicles, radars, personnel). Alternatively in a delay burst, the weapon impacts first and then a set time delay is executed prior to fuse actuation. This allows the velocity of the weapon to penetrate harder targets so that it may detonate inside the target. This is useful for concrete, buildings (sometimes depending on construction), armored targets (depending on the weapon), tunnels or bunkers. There are caveats to each of these situations.",
"With a ground burst, a lot of the explosion energy is used up by the ground/material very close to the explosion center. An air burst can cause damage over a larger area, but causes less destruction at the explosion site than a ground burst. It becomes a question of strategy. Does the attacker desire to damage an entire city or level a single block?",
"In WWI the combatants lobbed enormous volumes of artillery shells at each other, to not much effect (on a per-shell basis, at least): it's estimated that over a hundred shells were fired for each combat fatality during the war. The ineffectiveness was because the shells were mostly impact-fused, meaning there's a little gadget in the nose of the shell that is deformed on impact, setting off the explosive charge in the shell. This detonation, while quite fast, still took a little bit of time, during which the shell would usually bury itself into the earth. When it finally exploded, the surrounding dirt absorbed most of the force and shrapnel. Artillerists experimented during the war with timed fuses instead of impact fuses, set before the shell was fired. The idea was to calculate how long the shell would take in its path through the air, and set the fuse to burst the shell while it was still a few meters in the air. When it worked, this was extremely effective and greatly multiplied the destructive power of the shell. The problem was that it was very difficult to get right and rarely worked, resulting in the shells usually exploding high or in the ground anyway. In WWII, the British invented a device called the cavity magnetron, capable of generating high-energy radar waves. The secret of the technology was given to scientists in the United States, who embarked on a project of miniaturization and mass-production. The end result was a device that cost $18 and could be screwed into the nose of an artillery shell and used to trigger the detonation. This device (called the \"VT fuse\" misleadingly) could be set for a variety of ranges. The original purpose of the VT fuse was for anti-aircraft work; hitting a moving target directly with a shell was essentially impossible, while detonating nearby required precise estimation of altitude and proper setting of a timed fuse. With a VT fuse, it was only necessary for the shell to come with 60 feet or so of the plane and it would detonate. But it also occurred to the developers that it could be used to air-burst shells, with the radar reflection from the ground detonating the shells at extremely precise preset altitudes, causing massive destruction to whatever/whoever was underneath. While the VT-fused shells were used by naval vessels from 1943 onwards, they didn't appear on the battlefield until their first use in late 1944 during the Battle of the Bulge (and *very* handy they were then, an under-appreciated reason for eventual American success in the battle). The navy got them earlier because of security concerns - the US didn't want the Germans or Japanese to get their hands on a fuse that had failed to fire, but over water this would have been virtually impossible. By late 1944 it was clear that Germany was finished, so even if they had acquired one of these fuses it would have taken too long for them to reverse-engineer it and put it into production. Bonus factito: the cheap miniaturized cavity magnetrons eventually made an appearance in consumer goods - inside the microwave ovens we all have now.",
"USAF Ammo Troop here (we build all of the ordinance that's dropped by aircraft) Depending on the fuze, guidance unit, mission requirements, payload, purpose of the munition, etc. , you're able to set the fuse to when it should detonate, HOB (height of burst) for how far above the target it should detonate, or even set the fuze to wait after impact to detonate, and build the bomb accordingly to allow penetration before detonation. (Big ammo troop pun) search BLU-109 on YouTube. Reason I mentioned guidance units, is depending on whether or not it can be controlled from the pilot or ALO on the ground, they're able to guide the munition and detonate it as they see fit. To fully answer your question as to \"Why\", it totally depends on your target. If it's a soft target, say personnel or just normal trucks, it's easier to use an air burst with small metal projectiles (brass normally I think) that will essentially shower the target with shrapnel. This can even be applicable with heavier targets, such as tanks, because there's certain types of shrapnel that can pierce straight through armor or even engine blocks. A good example would be to search a flechette. They're like small little steel darts that are used as airburst shrapnel for personnel targets. We used this early on in the war, and back then the size wasn't regulated by NATO. Meaning each size of the flechette differed, some much larger than others. There were reports of targets being literally crucified to trees - so NATO demanded the regulated size of them to now be a 1/4\" long. I may be off very slightly on some things, but that should answer your question.",
"Before fancy fuses, everything hit the ground and exploded. The problem with this is that when something hits the ground it takes a little tiny bit of time before the fuse can tell the explosive to go bang. By that time, especially in soft ground, the explosive is now slightly in the dirt. When it goes bang, the energy wants to go where it is easiest to go. So a lot of it goes up and out of the dirt. This makes a cone shape extending upwards from the explosive. The problem is that people and thing at ground level aren't in that cone. So most of your explosive is wasted unless you got really close. Then people figured out how to make fuses explode pretty much whenever they wanted them to. So now we can use that explosive as well as all the sharp bits of metal along for the ride (shrapnel) to be more useful. Anti-aircraft and missile defence applications explode in a cloud of shrapnel when their guidance tells them they are close enough to the target. This increases likelihood of hits to control surfaces, fuel, engines and electronics. A lot of anti-personel applications also allow for airburst usually on a timer or range. This allows the warhead to send shrapnel downwards and means people hiding behind walls or in trenches aren't really safe. Most anti-tank missiles use a shape charge warhead which requires a precise distance from the armour in order to be most effective. Most will have a long pointy bit on the end to trigger the warhead at this distance. It may therefore appear like it explodes before hitting the target. However some specialized munitions have various range finders that cause detonation if they are close enough.",
"Exploding in the air means that the shrapnel hits a larger area and goes into trenches that a ground detonation would miss. The devices used to achieve this are called proximity fuzes.",
"if a bomb explodes on the ground, it does a lot of damage in a small area. That might not be the best solution. If it's detonated a little above the ground, the force is less, but the area is larger. If you want to destroy buildings, you can turn more of them into rubble, rather than just turning one corner of one into dust. It all depends on the objective you're trying to accomplish.",
"It's called the \"mock stem effect\", first seen in the Halifax explosion in WWI. In an airburst, the downward-going shockwave will reflect off the ground and join up with the shockwave radiating out parallel to the ground, thus intensifying it. When Hiroshima and Nagasaki were smoked in 1945 those bombs were both airbursts, the lesson being learned in Halifax.",
"If a bomb or missile explodes in the air, it's using a proximity fuse. This is usually a tiny radar in the fuse that detects when it approaches the ground (or an aircraft for anti-aircraft missiles), and triggers the warhead to go off. The fuse would be set to go off at the distance that the warhead will be most effective. You'd see proximity fuses on weapons designed to kill through the blast wave of the explosive going off or through shrapnel created by the metal casing around the warhead splitting up into pieces. The best targets for these types of weapons are troops in the open, or in uncovered trenches, unarmored vehicles, or aircraft. By exploding the weapon above the ground, all the blast and shrapnel spreads out as much as possible and isn't wasted moving dirt around. It's an old concept that dates back to even before the invention of radar, but back then they used a fuse that was timed to explode after so many seconds and then fire it at targets that were just slightly more than that many seconds away. It's where the \"bombs bursting in air\" from the US National Anthem comes from. Timed fuse bombs. And as a side note, when the radar fusing technology was invented, it was called \"variable timed\" fuses to hide the fact it was actually being triggered by radar fuse. There's also a proximity fused weapon used against tanks. This is a missile that is designed to fly over the top of the tank and explode when it gets directly above the tank. When it does, it creates a molten metal slug that is fired by the explosion directly down into the tank.",
"The reason for an airburst is for troops in the open or light skin vehicles. When bombs go off at point detonating there for armored targets and generally the most available fuzes carried by assets. Then there's delay, which is for hard targets such as bunkers, bridges, airstrips, buildings, and punching through a thick canopy forest. Reason you would use delay on a building is because there is 2-3 feet of plywood roofing material rafters that would absorb most of the shrapnel if it was air burst or point detonating. This is for 250, 500, 1000, and 2000lb bombs as well as small rockets and missles as well as artillery.",
"There are a lot of great comments in here. For some things - like surface to air and air to air missiles you have a much better chance of shooting down the aircraft if the missile explodes in the area of the aircraft rather than just hitting it. Because it's actually pretty hard to hit a plane in midflight as compared with getting near it and then blowing something up. Likewise other targets are more affectively damaged or destroyed by point or area detonation depending on what they are. Troops out in the open you probably want to use arborists – but those same soldiers in a bunker you want to hit directly."
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6vct49 | How/Why do vehicle engines just suddenly catch fire? | Like when you see a car on the side of the highway literally on fire (presumably the person was just driving it), how does that happen? Shouldn't this be something that doesn't happen to passenger vehicles? | Engineering | explainlikeimfive | {
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"There are rubber hoses filled with flammable fluids, either fuel or oil. Given wear or damage, they can burst spewing flammable fluids onto hot surfaces. Hot surfaces like exhaust piping, which can ignite the fluid."
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6vcuyu | How do car parks know how many spaces are available? | Engineering | explainlikeimfive | {
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"Is there a barrier on the entrance and exit? Barrier on the entrance goes up, -1 parking space. Barrier goes up at the exit, +1 parking spaces. Start with a known quantity, cos we can count the number of bays, deduct 1% to allow for dicks parking in two or more bays, and away you go. When the number reaches Zero, the car park is full. Illuminate the neon sign that says Car Park Full.",
"There's two popular methods you'll see in most garages these days. They either work on a per-garage or per-space basis. **Per-garage:** Generally done with induction sensors. These are the same sensors used to trigger traffic lights. If you ever notice [these sorts of shapes]( URL_3 ) in the road as you're coming up to a light, you've noticed an induction loop sensor. It's a common misconception that these are weight sensors; they're not and weight sensors (they look like big metal plates) never really reached wide usage because induction loops are easier to implement and less intrusive. Essentially, your car disrupts the magnetic field being created by the induction loop buried beneath the concrete, which triggers a response in the system. In a parking garage there will usually be two sensors: one at the entrance and one at the exit. That way, they can simply count number of cars in, number of cars out, and subtract from the total number of spaces to give an idea of how many spots are available. The major problem with this is with false positives; multiple cars passing over a sensor in rapid succession may get flagged as a single car. That's why the count is often off. The counting option for ticketed garages is simple: take the total number of spots, subtract the number of tickets which have been taken and add the number of tickets which have been returned to get a count of available spots. Same goes for garages with barriers/gates. **Per-space:** These are the systems you see where there's a board that has a count per floor of the garage. There are a few technologies for this but ultimately it boils down to putting a sensor above or below every parking spot in the garage to detect if there's a car on it. Sensors can be ultrasonic, magnetic, or infrared, or a combination (the type depends on the environment). Each sensor is connected to a centralized system (either wired or wirelessly) and reports whenever a space is empty or full. These systems are much more accurate, with > 99% accuracy in most cases. Sources: URL_0 URL_2 URL_1"
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6vdstk | How do traffic lights work? How are they coordinated with each other and why do some lights stay on for longer when there are more cars? | Engineering | explainlikeimfive | {
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"Most lights operate on a simple timer. They know to change every X seconds. The 4 lights at an intersection are controlled by 1 control computer (and wired below ground) so there are never any accidents of green in both directions. The timing of this computer is set by a traffic engineer who works for your local city. If you live in a really small town, this job is likely done by someone else but in larger cities, they hire people whose only job is to maximise the flow of traffic. To that end, some lights have more features. Such as the ability to change the timing of the light based on the time of day. For example, give a longer green to people leaving the city at the end of the working day, but in the middle of the day keep it 50/50. Or a light that has a dedicated left turn only green that can be activated or deactivated depending on rush hour timing. Even more advanced are lights that are connected to sensors under the roadway. These lights can actually decent when there's traffic backing up and change the timing of their greens. Again the parameters of these lights are programmed by traffic control people. Lastly, and most advanced are lights that all connect to a network and feed live data back to a control center. This is super uncommon but might happen in a place like New York where traffic i almost always trouble and lights need constant monitoring depending on conditions. This kind of system is very uncommon and expensive to operate but useful for those narrow use cases where it makes economic sense."
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6ve28e | Difference between torque & horsepower | Engineering | explainlikeimfive | {
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"Imagine turning a heavy hand crank. Think of torque as how hard you're turning the crank at that exact moment. Think of horsepower as how much crank turning you do over time (how hard did you turn the crank over one minute). Torque is like work, and horsepower is work over time."
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6vfr4l | How do construction workers deal with bad weather? Aside from slowing progress, doesn't it have a chance to majorly destroy the work they've done? | Engineering | explainlikeimfive | {
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"I work in the residential construction industry in NC. There are many factors but planning and timing play a big part. What I see is lots of building in the fall, a good bit in spring. Some in summer and just a little in winter. Each season has its advantages and disadvantages. Winter is cold with hard ground, spring has a lot of rain. Summer gets hot. Fall is great. But you still have to watch the weather to see what's going on. If the slab is supposed to be poured and it has been a hurricane in the past week, you are going to be delayed. Working with professionals, they can get the house up quick. Once the house is dryed in it's mostly fine. Weather can still mess with the schedule but not as bad as it can in the beginning.",
"If the workers are suspecting rainfall, they cover susceptible work with waterproof tarps. A lot of times, good workers will cover the susceptible places with tarps regardless of whether or not rain is expected.",
"I work IT for a road paving company. From what I know of their work schedule and the required weather conditions, it has to be over a certain temperature and decently dry. Can't make asphalt in the rain apparently."
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6vgvd0 | Why do Diesel cars have more torque but less Horsepower compared to their Petrol counterparts? | So I was doing a little research and learned that Diesel generally has higher energy content than Petrol. This is what generally gives it higher torque (in combination with a higher compression ratio). I was wondering if Diesel has higher joules of energy stored in it than petrol, why are there no diesel sports cars? Even compact diesel sedans have lower HP than their petrol counterparts (take Jetta TDI and Cruze TD vs the Mazda 3 Skyactiv Petrol). Shouldn't they have more horsepower because they store more energy? | Engineering | explainlikeimfive | {
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"Horsepower is a function of torque (HP = torque x RPM / 5252). Most diesel engines that the average consumer can buy rarely rev past 5000 rpm. So doing the math, a petrol engine that can rev to 7000 rpm is going to make a lot more horsepower that a diesel at 5000 rpm can do.",
"> So I was doing a little research and learned that Diesel generally has higher energy content than Petrol. This is what generally gives it higher torque (in combination with a higher compression ratio). This is actually a common misconception; the difference is almost *entirely* due to the higher compression ratio of diesel engines. You can't directly compare the energy contents of the two fuels because the two cycle types are so radically different, and once you factor in the fuel delivery processes the energy content differences are all essentially washed out anyway. Source; two degrees in M.E., finishing up a doctorate, primarily focused on diesel engine thermodynamics."
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6vjj01 | Is braking with your front wheels as fast as braking with your back wheels? | Condsidering you're using the same energy to brake. (car, motorcycle) | Engineering | explainlikeimfive | {
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"Front wheel braking is more powerful. As you brake, more weight is thrown onto the front wheels, so they can actually brake harder and harder. The opposite is true for the rear wheels: as they become more lightly loaded, the less they can contribute to the overall braking force. Every car has bigger, more powerful brakes on the front because of this phenomenon, and it applies to bikes just the same.",
"When a car brakes, the front noses down and the back lifts up. This puts pressure on the front wheels. So, if the front wheels are doing the braking, that added weight on them increases traction, hence better braking. Back wheels brake too, but as the vehicle weight shifts forward, they lose traction. They are much more likely to start skidding, which reduces traction even more and lessens braking power. However, if we talk about a bicycle/motorcycle, while the same principle applies, the front wheel can brake so hard the back of the bike can lift off the ground, and flip over.",
"The braking force you can get from a wheel depends mostly on the amount of weight it carries. If the center of gravity is right in the center of the car, all wheels will more or less brake with the same force. However, once you start braking, the load of the vehicle shifts towards the front, and therefore your front wheels will have more traction than the rear wheels. That is very noticeable on a bicycle: If you engage the rear brake, the wheel will easily start skidding without slowing you down much. Meanwhile, pulling the front brake too tight will give you so much braking force that you might flip over. The same thing happens in a car (except the part where you flip over), which is why the front brakes need to be stronger than the rear brakes."
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6vkwnx | How did astronauts get "air conditioning" in their space suit to be comfortable on the moon? | How can you controllably eject waste heat, CO2, farts, water, etc.? | Engineering | explainlikeimfive | {
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"Each space suit is its own little self contained environment. You can radiate waste heat into space. The space suits contain chilled water circulation systems to maintain the astronauts body temperature and comfort. CO2 is absorbed by cycling the air through CO2 scrubber canisters and a little bit of oxygen is added back to the mix. Urine is collected and disposed of after the EVA. Only thing that might be an issue is if you suddenly have to take a dump, but astronaut meal times and meal contents are closely controlled as to avoid unexpected bowel movements. Almost all EVAs are scheduled well ahead of time, so you have time to adjust your diet accordingly."
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6vo8kf | Why is engine power measured in horses? | Engineering | explainlikeimfive | {
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"URL_0 Inventor James Watt had an improved steam engine. To market this engine, he came up with a calculation to measure how much power was outputted by a horse turning a millstone. He ended up with the the figure of 33,000 foot-pounds per minute. That is, a horse could lift 33,000 pounds a distance of one foot, in one minute of work. He then used this figure to advertise his new engine. The benefits would be that it was able to constantly output as much or more energy than a horse, while doing it constantly, without unhitching and rehitching as you would several horses as they tire. The engine also used cheap coal rather than wheat and oats, among other benefits.",
"Because back when Watt was developing the idea, for thinking about how well or poorly a steam engine worked, the alternative to steam engines (used mainly for pumping water out of mines) was to have a literal horse turning a capstan."
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6vq5ml | Explain the following to someone with no knowledge of physics: force, momentum, power, work. | This is actually an interview question for engineering at Cambridge University. I didn't manage to get a decent answer from my physics teacher and this question is really bugging me. I know all of the aforementioned "things" in the context of equations or physics problems, I just can't explain them in layman's terms. | Engineering | explainlikeimfive | {
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"You push a rock and it moves, that's force, you stop pushing but it keeps moving, that's momentum, the rock is in new place now, that's work done, if you pushed harder it would be there faster, that's power.",
"Think of the force as the capacity to change an object's speed. This means if you apply a force to an object that doesn't move, it will accelerate in the direction of the force (only if nothing prevents it, like a table, or air friction, etc.). If the object was moving and you apply force against its direction of movement, it will slow down,and eventually accelerate the other way if you continue to apply force. Momentum is the product of mass and speed. This quantity is the image behind the fact that a light object moving fast and a heavy object moving slow can have the same impact on a surface for exemple. (not exactly right, but gives the idea I think) Work is the energy you spend to move something. It is the product of the amount of force and the distance on wich you apply the force on the object. Let's say you are pulling an object lying on sand. The object will experience some friction (=force) from the sand. If the force applied by the sand is constant and, say, equals to 10 Newton, and you pull the object on a 10 m distance, then the work is 100 Joules, meaning you spent 100 Joules to move the object in those conditions. Power is the product of speed and force. In the same exemple as above (object pulled on the sand), the power (in Watt) is your capacity to pull the object with more friction from the sand. The more powerfull you are, the more fast you can pull the object, or at the same speed but on a more \"frictionnal\" sand. Hope this helps."
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6vu7uc | Why are calculators still clunky, unintuitive, and slow, relatively speaking? | Engineering | explainlikeimfive | {
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"People think that technology just gets cheaper and better automatically, but it doesn't. It gets cheaper and better because researchers are coming up with better designs and inventing new technologies. So your calculator is just as expensive and just as good as a calculator from 1995 because it *is* a calculator from 1995. There are a couple reasons why no one has made a cheaper, better calculator since 1995. 1) All the R & D money is going into computers. Your computer can do all of the things your graphing calculator can do, and so much more, but you can't use a computer instead of a calculator because 2) A new calculator would have to be approved for the SAT/ACT/GRE etc. in order to be approved. And any new technologies on it-- especially internet, might mean it doesn't get approved."
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6w3n1c | Why do launched missiles often fail? | I am just wondering what particular factors make it so difficult for a weapons program to successfully launch a missile. What typically goes wrong, and why is it so difficult to get right? Related question: Does a failure mean the engineers still can't get it right, or are they testing different variables each time to gain more information? | Engineering | explainlikeimfive | {
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"Try making something weighing several tons travel thousands of miles at hundreds of miles an hour and attempt to make sure it doesn't stray anymore than a dozen feet from its target. If you have little experience in physics, programming, and rocketry it's not going to be easy. Hell a large part of much of modern missile tech was from research done in Germany in WWII. If you don't have access to those resources you're already decades behind."
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6w6h7b | How did /r/BlueShed withstand hurricane-force winds, while everything around it disintegrated? | Engineering | explainlikeimfive | {
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"1) It didn't get hit by any major debris 2) it has a very small sail area/wind loading area compared to the strength of the structure. A house would be framed very similar to blue shed (16 inch on center studs, etc) but have a much larger sail area so a much larger wind loading/force on the structure 3)the quality of the construction varies. this structure may have had superior quality of construction. edit: to add to this i remember when hurricane hugo i think it was tore through town and a certain construction company ended up getting sued out of existence because the hurricane exposed their slap shod building practices. (found a news article from this happening with another hurricane: URL_0 )"
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6w7iiz | Why do AC units have to be re-filled/maintained when fridges and freezers can stay cool for years just ruuning on electricity? | This occured to me when I drove my car to my bi-annual service and had to spill out my money on this. Edit: Thanks a lot to all the people that answered. My AC was barely cooling but I will be more aware of what's going on the next maintenence. | Engineering | explainlikeimfive | {
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"Automotive air conditioning has much more difficult set of situations to work with. Many of the components are sharing the same space as the engine, exposed to the temperature and environmental extremes the engine is, which makes them degrade over time. The compressor is bolted to the engine and gets its power from it. This means the compressor much accept power from an external source, usually through a pulley attached to a shaft with a seal, which will wear and leak eventually. This also means the compressor moves with the engine, and the system accommodates this motion with rubber hoses which is also a major source of leaks. Parts are made to be replaceable, so some type of seal or gasket is necessary between them, which can, yes, leak or fail. Contrast this with a stationary application like a refrigerator. The entire system is usually plumbed with metal tubing with only small amounts of flexing, so no dynamic seals or rubber hoses, or gaskets. Even the compressor is encased in a metal shell, the only seals are for the electrical connections, but since there is no motion, these seals can be made to never wear out and almost never fail."
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6w8av0 | When planes fly low, why is it harder to detect them? | The obvious answer to me is because they "blend with the ground" or the bounceback on radars dont go through mountains/whatever ... but what about over oceans etc. | Engineering | explainlikeimfive | {
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"The Earth is round. So the land and even the ocean curves away from you. This is obvious if you are out at sea and looking at another ship. If it is far away you can not see its waterline. And if it is really far away you can only see the masts as the rest of the boat is bellow the horizon. This also applies to radars which is why the radar on a ship is placed high up so that it can \"see\" further over the horizon. If you fly a plane lower then you will also be bellow the horizon to anyone looking for you."
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6waik8 | How do cable companies lay cables underwater, especially those that go very deep? | Engineering | explainlikeimfive | {
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"Fun fact there are plans in place to cut underwater cabling. It would severely fuck certain countries over in case of war. Can't fix the cable so all new cable would have to be laid.",
"Large boats tow what looks like a plow along the bottom, essentially tilling the cable under the sea floor. This mechanism can be pulled by long cables and directed remotely."
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6wbvee | Why aren't there seatbelts in public busses? | Engineering | explainlikeimfive | {
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"The chances of you needing a seatbelt in a bus are far less than in a VW bug. The weight and size of a bus makes it inherently safer to drive in than a smaller vehicle. The only problem here is if the bus flips. Then you're fucked.",
"I was once in a bus accident on a non-divided highway back in the 1990s. We were driving along in a coach bus at 100km/h. A van driving in front of us slowed down, signaling a right turn at a side road. Our bus pulled into the oncoming traffic lane to pass them on the left. The van turned left, and got T-boned by the bus at 100km/h. As a passenger near the back, it felt like we hit a speedbump. I just felt light on my seat for a second, then everything was normal again. We were then confused as to why the bus was pulling over. The van had its entire side destroyed. You could see the steel beam that runs down the side of the van in relief, every body panel and window on the left side of the van was crushed. Fortunately there were no serious injuries in the van (but probably lots of whiplash). In summary: momentum is awesome. As far as I can recall, this bus had no seatbelts. (Ironic part: we were watching a movie on the bus. It was \"Alive\", and our bus accident happened shortly after the airplane crash scene was shown.)",
"Town/ city buses drive at about 20-30 mph. They have handles often on sides or the seat in front. They are not legally required to have seat belts atleast here in the uk. And the inconvenience of having to take it on and off every time someone wants off. On the other hand, coaches or intercity buses travel on highways often faster and are required by law to have them and in proper operating condition. And people don't get on and off as often. These are checked on every inspection and yearly MOT(government vehicle check) Ps- city buses can drive fast but only when no passengers on board",
"Physics! The bus weighs so much that an accident will not be nearly as dangerous to passengers as a similar accident when in a smaller lighter vehicle. Most accidents would be with another vehicle, not a brick wall. So the bus would not instantly decelerate.",
"FYI buses in the U.K. Won't emergency stop either. This is because they don't have seat belts so hitting the brakes hard would increase the risk of injury more than hitting the object and slowing down using this (slow speeds only) One of the things a friend got told when learning to ride a motorbike was don't mess with buses they won't stop for you",
"The rationale for a school bus is that in the case of young children, if there is an accident most of them are probably going to have trouble undoing their seat belts on their own, and in the case of something like the bus catching fire, it would be more hazardous to have them than not",
"Here in the UK there are some, but they tend to be on the longer distance, newer coaches rather than local or town-town bus routes. There definitely doesn't *seem* (feel free to correct me anyone...wait, this is Reddit, of course you will :D) to be any legal requirement or enforcement on people belting-up on public transport, even when belts are provided. Often wondered why meself.. I'd've thought that *not* having em would play merry hell with public liability insurance costs..",
"Here any buss made after 2004 has to have seatbelt for all passengers. Unless its made for urban traffic only. And its illegal to not use seatbelt if one is available."
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6wcc7k | why can't the United States create a pipe network to pipe water from flooded areas to places that need it? | So like flooding and excess rain in Texas could be piped to California or where ever it was needed. | Engineering | explainlikeimfive | {
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"Money. Water is a very low cost material. What's actually rare in the desert southwest or California is low-cost fresh water. California has a coastline on the Pacific Ocean, but the cost to remove the salt is too high. If you're going to spend money on pumps in New Orleans, where there is much need and not enough money, you're only going to buy pipes long enough to reach open ocean, not 2000 miles to California.",
"Step 1. Cost Finding a way to make money off of relocating water. Thats the only way that would ever happen. A cheap product (water) wont make money being transported thousands of miles because at the end of the route they would charge by gallon. Step 2. Geography Find a cost effective route. One that doesnt go through protected land. Elevation plays highly into this, it takes a series of pumps to go horizantal, more so to bring water against gravity. Step 3. Environmental When rain is dropped it helps support wildlife in that area. Step 4. Politics After implemented, It would come to the point that local governments would fight over who has the \"most need\" of the water.",
"The problem is the relatively short amount of time the water arrives in. When that much rain falls all at once, the established infrastructure is overwhelmed, which leads to flooding. You'd have to completely redo all the underground drainage. A very expensive proposition. After that's taken care of, you need a huge pipeline that crisscrossed the country since, you never know where the next storm will hit. You now have to buy the property from everyone who owns the land the pipeline sits on. That's another huge expense, even if everyone sells willingly. Those who resist and force/fight eminent domain, will drag the process out for years and the legal costs could easily double the construction costs. Then, there's the issue of operating costs. Electricity for the pumps, maintenance of the pipeline, worker's pay and benefits. That would likely make the water coming out of the pipeline ungodly expensive. I haven't even mentioned the environmental considerations. When a storm like Harvey destroys a town, most of that water would be contaminated. Fuel from damaged storage tanks, chemicals from the paint store that was blown away, the saltwater mixed with rainwater caused by storm surge, etc, etc, etc.... That would all have to be tested for and mitigated, before it went into the pipeline. If you do it afterwards, you might need to decontaminate the entire pipeline if something really harmful were introduced to it.",
"You're talking about moving millions of gallons water thousands of miles. It just isn't feasible. Moving the water takes energy. Conveniently, the Sun provides a huge amount of energy for moving water around the entire planet. That energy drives our weather, which moves [37.5 million *billion*]( URL_5 ) gallons of water all around the planet. It also conveniently moves the water thousands of feet into the air, where it can land thousands of feet above sea level, where the Earth conveniently uses gravitational potential energy to move the water around through streams and rivers. Also, conveniently, the Earth has been geologically active for a few billion years so there's water trapped underground in convenient locations. It's relatively simple for us to move water a short distance from where the Earth and Sun have maneuvered the water to where we need it - we've been doing it for [thousands of years]( URL_4 ). Still, historically humans have settled in areas that already have lots of fresh water that flows relatively slowly - ie. in big, wide rivers like the Nile. But water is [heavy]( URL_3 ), so getting it uphill is very difficult. So the first problem is moving the water *out* of flooded areas. That's already tricky, and the reason areas flood in the first place. Flood plains are wide, flat areas that collect water and down allow it to flow easily out. Places that experience chronic flooding, like coastal cities, often spend millions of dollars building drainage systems that provide an outlet for the floods. That often means constructing [levees]( URL_2 ) that channel water around flood plains so it collects where we want it instead of spreading out all over where we don't want it (like our streets and buildings). Those are powered by gravity, though. We don't pump it out, we just let gravity do the work. Some places *do* use pumps, but that's only moving water a few hundred feet at most to put it back inside the levees and rivers to let gravity do its thing again, not thousands of miles into another state. We do, of course, move water around that we use from natural sources of water to somewhere else. Controlling our water is often a [massive undertaking]( URL_0 ). Even locally, we [prefer to use gravity]( URL_6 ) to move our water around. We use *small* (relatively) pumps to move a little bit of water into the towers where gravity will deliver it to our homes. And that's not to say anything about the massive cost of laying the pipes and pumps. There's a lot of stuff in the way: rocks, rivers, mountains. Pipes are subject to damage, so you have to build them big and bury them and protect them. Just replacing the pipes in Flint could cost [billions]( URL_1 ). That's just replacing the water pipes in one city. Granted, that also means digging up old pipes and the pipe has to be safe to deliver clean, drinkable water. Nonetheless, you're talking about transporting millions of gallons of water thousands of miles away, through mountains, across different elevations. It's just absolutely not possible. It's also mostly unnecessary."
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6wef3r | Is there any research showing that STEM toys actually have a positive impact on children's ability in real-life STEM? | Engineering | explainlikeimfive | {
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"Lots of research has been done with STEM toys, but most of it asks the question of whether or not the toys help children learn STEM topics. Even less research is longitudinal (meaning the researchers observe subjects for a long time, usually years). One study that almost answers your question found that children who developed early interest in STEM topics had more access to opportunities to engage in STEM activities later in life. URL_0 B.S. Physics Teaching, M.S. (candidate) Instructional Psychology",
"I don't have any studies available for you, but I started programming as a hobby after my parents gave me a Lego Mindstorms set, and eventually it led to my enjoyment of mathematics in high-school. You can't know if you enjoy something unless you try it."
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6wejr0 | what determines when water is too deep to drive through? | I live in Houston, and the news coverage is of course all about the flooding and weather due to Harvey. There are dozens of stories daily about people needing to be rescued from cars when they drove into flooded sections of road. My question - in situations like this, what exactly determines when it's "too deep" to drive through? Is it a mechanical/electrical problem, where exposure to water prevents the car's engine from running? Or is it a physics issue, where at a certain depth you can't maintain enough traction to push the mass of the car through the mass of the water? Or something else completely? | Engineering | explainlikeimfive | {
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"Physics issue, vehicle becomes buoyant and drag from the water exceeds friction from the tires, usually when the water gets several inches above the floorboards.",
"I'm from Arizona, where rain is uncommon, but flash floods are when it does rain. To my understanding, there's a point you reach where the car loses traction / becomes buoyant and can be easily washed away by the current. Lots of people die this way. Don't drive through anything flooded if you don't know for certain that it's only a few inches, and even that can be too much, depending on what type of car you have.",
"Mostly, it's the level of your air intake and the weight of your car. Serious off-roaders (and people that want to look like serious off-roaders) will install a snorkel that effectively puts their intake somewhere around the level of the roofline. Hard to figure exactly how long you would be able to hold your breath while trying to pilot a car under water, but maybe... you also have to think about buoyancy, friction, and current. With the snorkel, the engine may still run. If the tires don't have traction, and the current is noticeable, does any of this matter?",
"Mostly it's electromechanical & air related. If air can't get into or out of your car because the intake / exhaust is under water you will stop pretty quickly. Dirty water can cause a short so electrical systems are a problem also. Many cars don't have waterproof seals, so driving through water that goes above the bottom of the door / above chassis level is just a messy idea. Unless you know for sure how deep the water is, and it's no more than a splash, don't risk it - there can be ground damage and deep potholes after a storm. Stay safe."
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6wi5x1 | In hurricane prone US areas, why are houses still made primarily from wooden structures? | Engineering | explainlikeimfive | {
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"Wood is not a problem, that's why. It's completely possible to build a structure out of wood that can withstand hurricane force winds. Source: structural engineer who has designed a handful of wood structures.",
"Wood frame houses tend to be the least expensive route of building a house. With modern hurricane code, they can actually withstand a pretty big bearing. Check out the Mythbusters episode.",
"Lets talk about a flood proof house. First of all, regardless of what you make it from, all the shit inside the house is toast if there is a flood. But at least the house is ok right? Well lets talk about that. Because if you have carpeting, that would have to be ripped out. If you have drywall for the internal walls all that has to be ripped out. All your thermal insulation has to be ripped out. Hot water tanks, furnace, cabinets, counter tops, etc. all have to be ripped out. But at least you have the frame? Well what did that frame cost to build? Was it twice as expensive as a wooden frame? If so you would (thanks to the time value of money), be better off just rebuilding after the flood. What happens if that frame gets damaged? You flood a wooden frame house and then throw a tree through the wall and that house was just as screwed before the tree as it was after. But some kind of flood resistant house? You throw a tree through that wall and you might need to demolish the house. Regardless of what happens a \"flood proof\" house would be looking at a major insurance claim AND months of renovation work (at a time when contractors are hard to find), after a flood before it is livable again. Frankly you are probably better off bringing in contractors to re-develop entire neighborhoods at a time after a flood just like you would for new housing construction.",
"Pretty much anything you build it out is going to take some serious damage in a big hurricane. Timber frame houses are cheaper to rebuild afterwards."
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6wifd1 | why do the front wheel studs in a semi or meat truck stick out? | To better explain. I see all the front tires in heavy duty vehicle have the studs stick outward and the tire itself closer to the cars body. But if you look at the rest of the vehicle the studs are in the middle or closer to the cars body. | Engineering | explainlikeimfive | {
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"It's because all of the wheels and rims on that type of truck are made the same. The wheel itself (metal part in the middle) is made to bulge out so you can take one and turn it around so it's touching the other one to make the double wheel that is in the back. Having the front tire be just a single tire is better for turning the vehicle, and having all tires made the same allows for a spare tire that can go anywhere on the truck. So what you are seeing on the back tires facing you is actually the inside of that front tire."
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6wlw1v | What effect does opening a dam have on areas that are under heavy flooding? Ex: Houston | Engineering | explainlikeimfive | {
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"There are different kinds of dams, but generally speaking dams have a spillway, and they usually have floodgates as well. Spillways and floodgates are used to manage the water behind the dam in emergency situations, because you don't want the dam to fill up so much that water comes over the crest of the dam. So the dams near Houston are very full because of the hurricane, and the Army Corps will be opening floodgates on two of the dams. The impact on the downstream area will be that there will be more water than is already present, because more water is being released. But the alternative is if you don't release the water, maybe the dam overtops, maybe the dam fails - and that's a catastrophe that would kill people and destroy tons of property. With a controlled release they can warn people."
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6wmvts | How come when a plane stalls or it's engines die and it's gliding, it's almost a miracle if it lands safely. But gliders glide with no engines at all and land safely 99% of the time? Is the weight/wing ratio so much off between the two? | Engineering | explainlikeimfive | {
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"It's not a miracle when an airliner glides to a safe landing -- they're designed to be able to do it, within reason. But yes, the reason is exactly as you say. Gliders are build *extremely* light with *very* long wings. The U-2 is a powered airplane designed much like a glider, and it so wants to fly that it's hard to land -- it keeps bouncing back up into the air!",
"A plane stalling is not the same or even similar to an engine dying. Stalling is when the plane is no longer generating enough lift from its wings to keep flying. This can happen in a perfectly-functioning plane if it pulls up too steeply, but more often happens when the plane is going too slowly to generate the needed lift. Recovering from a stall is difficult, and if you're not high enough you simply can't do it before crashing into the ground. A plane with no engine running can continue to glide for a long time, slowly losing altitude and speed as it does. It can dive to regain speed at the cost of altitude (you'll want to do this *before* stalling). This is how gliders are *designed* to work, though powered planes can do it just fine.",
"You also have to take into account with the idea the glider knows it will be gliding all the way down with a predetermined landing strip. If a plane engine goes out, it's an emergency and the pilot has to scramble to find a large enough area to land as he's losing altitude. It's never in the playbook for the airplane to glide to a random airstrip.",
"If you are in a glider, you will never far away from your runway. You are always close enough to land. And a glider will glide very far compared to the altitude it loses. Meanwhile a plane does not glide as well. It will lose altitude faster than a glider. And you may not always be very close to a runway. If you are at 40,000 feet and you loose an engine, you have a lot of time, and it will be easy to find a runway (assuming you are over land), and you should have no problem landing safely. If you just took off and you are at 2,000 feet, you are in some serious trouble. Also, I imagine panic is a factor as well. A glider has no engine, so it can't lose an engine. If you don't fly often, and you loose an engine, its possible to panic, and not know what to do. This is less of a factor with professional pilots, but I imagine it can affect some recreational pilots quite a bit. I would also argue its not a miracle when a plane lands safely after loosing its engines. A plane loosing one engine happens more often than you would think and it doesn't even make the news. A plane loosing both engines (at altitude) should be expected to land safely, and if it does, it might get one quick mention and you don't hear about it. Of course if it crashes, its in the news for weeks.",
"The ratio you are asking for is [Glide ratio]( URL_0 ) The value is the number of units you can travel horizontally for each unit vertical. A glider has a value of 30 so for each meter it descend it can travel 30 meter. And they can have a ratio of 60 The value for a small single engine prop plane Cessna 182 is only 9.3 but a large commercial jet like a Boeing 747-200B have measure values of 15 But that dont explain it all. There are other significant differences. A glider without pilot may way below 500 kg and have a stall speed(minimum flight speed) of 75 km/h so the required landing area is small and it can land safely in a field with growing crops. The Cessna is not that different with a weight of 900 kg (excluding fuel) and a stall speed of 91 km/h but I guess that the air brakes on the glider gives it a shorter stopping distance then you would guess from the numbers. The value for a Boeing 747-200 is landing speed in the region of 270 km/h so stall a bit below that and the empty weight is 170,000 kg empty so it is not hard to understans that is cant not land except on a prepared runway. A landing on a softer and uneven surface will break it apart. But back to the comparison of the Cessna and the glider. A powered aircraft will only land as a glider if there is a problem. If the engine dies shorty after start, before landing or any other time you are flying low you have only a short time to react and limited or no possible places to land the plane safely. Remember you only have 9 time your elevation in flight distance and the fist step is to try to restart the engine and you vill loose altitude For a glider a gliding landing is the norm so you are never in a situation where you fly near the ground except when you are going to land. You \"engine\" is rising air produced by thermals or hills so you know that you might not find something that give you lift so you plan your flight for it. The safety rule is that never fly out of gliding distance to a landing spot. It look like when you are at 500m/1500feet above ground you should land instead of trying to gain altitud. That givs a potential landing distance of 15km away. So a glider should never be in a situation where glide landing is impossible but a powered aircraft often is. So the primary difference is how/where and in what condition you fly the different aircraft and the training of the pilot for small aircrafts. For larger aircraft the problem is that a large aircraft can't land on a unprepared surface safely even if you make the prefect approach the landing is likely catastrophic."
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6wnn4m | how we know that the golden record on voyager 1 won't get destroyed by space rocks during its billion year lifespan | Engineering | explainlikeimfive | {
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"We don't know that, but it's pretty unlikely. Space is called *space* for a reason, it's almost incomprehensibly empty. As the probes drift farther from the sun they're entering a region of space even emptier than the already empty void between planets. When you point a telescope at a distant star, you're seeing light that has traveled a hundred trillion miles without ever crossing paths with anything of substance. Eventually the probe will be destroyed by a collision or the slow erosion of cosmic radiation. We have no real estimate of how long that may be.",
"The density of matter in interstellar space is *extremely* close to zero -- about 1 atom per cubic centimeter. Statistically speaking, Voyager 1 is *unlikely* to every hit anything large enough to cause major damage."
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6wsy60 | What is Turing completeness? | My electrical engineering friend tells me this is incredibly complex. But it appears to be a selling point for a lot of the cryptocurrencies, so it seems important. What is Turing completeness and what does it mean for development to be Turing complete? | Engineering | explainlikeimfive | {
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"Ahoy, matey! Yer not alone in askin', and kind strangers have explained: 1. [ELI5: Turing completeness ]( URL_0 ) 1. [ELI5: What does Turing Complete mean? ]( URL_6 ) 1. [ELI5: Turing complete programming language ]( URL_7 ) 1. [ELI5:What is Turing Complete and why is it so important to Digital Currencies (such as Bitcoin and Ethereum) ]( URL_5 ) 1. [ELI5: What is \"Turing completeness?\" ]( URL_1 ) 1. [ELI5, \"Turing complete\" ]( URL_4 ) 1. [ELI5: Turning completeness ]( URL_2 ) 1. [ELI5: What does saying something is 'Turing complete' mean? ]( URL_3 )"
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6wszd6 | Why do power lines emit a loud buzzing/static sound when it rains? | [video for clarification]( URL_0 ) | Engineering | explainlikeimfive | {
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"It depends on the power line, there are a couple high voltage (220Kv I believe) above my house, and they buzz all the time",
"Power in the US is alternating the direction of the current 60 times a second (50 in the EU). So a wire may have a tiny 120Hz oscillation. Considering you have a three phase current that gives an overtone of 360Hz, etc. This is what gives you that transformer buzz sound. Designers are trying to eliminate the sound but when conditions change it can make things out of tune. So rain can cause things to change and sound differently."
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6wtquw | why are radiators places under windows? Doesn't the heat just get wasted combating the cold through the glass? | Engineering | explainlikeimfive | {
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"Actually it is specifically because \"cold come in the window\". By putting the radiator there you create an air pocket that keeps the colder air from the window getting very far into the room if it can come in at all. This is why stores will have small AC or heater units over doors to make an air barrier, and why grocery stores are able to display produce without it being behind glass."
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6wyqau | how does a single house far in the mountain region or forest get electricity or sewer management | how do some single house which is far away in some forest or mountain region, how do they get electricity and what about their sewer management system | Engineering | explainlikeimfive | {
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"They don't mostly. Depends how rural they are, in the US they mostly do run electricity out to those rural houses. Not always though, many people who actually live that far our just use generators. As for sewers and water, they have wells and a septic system. My old house had those and my Mom's does too, it's not particularly rare and you don't even need to be all that rural to need them. Lots of towns have no sewers, or don't provide it for most people.",
"I grew up pretty rural, we had power though. For sewer we built our own septic system. House was on a hill, we buried pipes down the hill and dug a septic pond with proper treatment traps. That was 40 years ago and its still going strong today. For potable water we were part of a rural coop. No my grandmother lived on a hill and a shack until she passed in the early 2000's in Alabama. No power, nothing. Gravity plumbing from an outhouse (no joke) to a Chemical trap at the bottom of the hill ( a service came once a month and cleaned it out) Higher up the hill, they had a 200 gallon water tank and a big propane tank. When I say \"shack\" you get the wrong idea. You know those prebuilt barn kits you can buy at major home improvement stores? She bought one of those and the family built it into a two room efficiency for her. It was what she wanted. It was honestly super pretty, it was more of a \"tiny house\" before the craze began. For heat (its Alabama LOL) they would use an extra blanket or rarely turn on a burner on the stove. She lived happily like this for all of my adult life. She had chickens and goats for meat and eggs. And her 2nd husband had a small barn at the very bottom of the hill (maybe 600 yards or so) where he did body work for cash and (I suspect) modified vehicles to hide things for people. LOL They had a quiet life in the hills. And honestly, with some slight differences, I think that wouldn't he a bad way to spend my golden years if I ever found myself to be alone and unmarried.",
"The sewer is not a problem. There are many, many modern homes that are not on a sewer system, even in seemingly developed areas. It is called a septic system. Septic systems usually work very well and are relatively easy to maintain. Electricity is a different matter, but still not usually an issue for the rural home owner. You see, living in an isolated area is a lifestyle choice that people make knowing full well the implications. Life is tailored around the fact that you are not connected to the power grid and many rural residents want it that way. Even so- just because a home is not connected to the grid does not mean they have no electricity. There are many options for power that do not involve the grid. One is a private generator that is fired up whenever electricity is truly needed. Some homes have solar cells for minimal electricity. Or even a wind/water turbine. Many rural homes have a large cylindrical tank that is filled with LP gas. This tank of gas provides for many of the energy needs that others use electricity for."
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6x32my | Why do semi trucks in the United States have front wheels where the lug nuts protrude past the edge of the tire while each subsequent wheel has recessed lug nuts? | Currently on a road trip from southern to Northern California and all the trucks we've passed so far have this pattern. Is this an industry standard? Or does it relate to safety in some manner? | Engineering | explainlikeimfive | {
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"It's because the front wheels are single (like on your car), while all other wheels are dual. Because of this, the centers of the wheels extend past the edge of the rim. This means you can put two wheels face to face, and bolt them together to the hub. When you look at a set of dual wheels mounted, you see the concave side of the outer wheel, so you see the lugs as being recessed. Since the front wheels are single, they mount them the same way they do the inner wheels of each dual pair, meaning with the convex side facing out.",
"Hey guys, first time commenting here on ELI5. Truck mechanic here and I wanted to take a chance to clear some misconceptions up that some people have as well as confirm others. The front (Steer) wheels are singular and are turned essentially the same way that any car or truck wheel faces with its concave facing outwards. The inner rear axle dual also faces this same direction if you look closely at a set. The second rear wheel on each hub is placed there because the axle is rated to hold a higher weight than that of single rear wheels. If a vehicle has what is called a \"super single\" (A single overly large rear axle tire) it has the same weight capabilities as a axle with duals. The benefit to this is fuel economy which tires play a factor in. While at one time they were very expensive in there early years they have drastically come down in price to actually be cheaper than replace 2 tires in some areas of Canada and the US. I was reading some of the comments about interchangeability of the Steer tires vs the rear drive tires. To say that people are wrong and you cannot interchange wheels and tires would be incorrect. But to be clear though, most Highway tractors have different steer tires vs drives. Again fuel economy is a factor. The steer tires are generally smooth grooved with more of a streamlined pattern. At the end of the day they are there for just that. Steering. there is no driving force being applied to them so they are designed to provide minimum rolling resistance with optimal steering capabilities. The drives usually have a more aggressive tread pattern for a balance of grip, wear control and you guessed it fuel economy. Also side note here, retreaded tires are illegal as steer tires here in the great white north. Not sure but it is likely the same in the US. Lastly as far as wheel torque goes this is something of a myths/legends vs science question. A lot of mechanics I knew of coming up in the trade believed that torquing a lug nut on a car was as easy as listening for a the right amount of bangs coming from their impact gun when tightening a set. Some believed in the \"hit em hard with you gun and then torque em after\" approach while others swore if they did not use a torque wrench on every lug nut a wheel was sure to come off. The correct answer is the last one but with a procedure of assuring each lug actually moves when torquing. If they do not and your torque still clicks you have torqued nothing. The lug nut is already over tight which can be as dangerous as under tightening. Over tightening stretches the wheel studs which the illustrious engineers worked so hard to design. This one of the big causes to wheel coming off on trucks and cars as the studs can shear or time allowing the wheel come loose. TL:DR Amounts of tires and sizes are all put into the design of the vehicle when ordering it for maximum efficiency. In some cases the customer can go against recommendations due to cost or applications. As far as maintenance goes. If your the one working on it. Torque it. The cost and damage of a 150+ pound tire and wheel into someones windshield costs far more than the time it takes to use your torque wrench. As far as everyone else goes. choose a reputable dealer/shop for your tire needs. p.s. Those shiny pointy things some drivers put on lugs are just for show. For show and for me to slice my leg on when servicing a truck... but they do look cool ;)",
"all the wheels are the same wheel. URL_0 the rears just have an extra set 'turned around'",
"This has already been answered a few times but I'll give it a go. The rims on a semi truck, all ten of them, are the exact same wheel. Most standard is 22.5\" overall diameter and 8.25\" wide. These wheels can be run on any position on the trucks, even the trailers. That means, that the rear wheels on a truck (known as the drive wheels) are mounted face to face. If you take the outer most wheel on a drive axle of a semi truck, and then turn it around, you could install it to the front (or steer) axle. The same goes for 1 ton pickup trucks, cargo vans, anything with \"duallie\" applications.",
"Why do some of them put spikes on those tires?",
"The front has only one tire. The rear has two together. All the rims and tires are the same. In the back the way they have two is they have the outside rim flipped around. Doing this causes the two rims to touch. They have to have all of them the same because of flats or rotating tires. So in the front they stick out because the rim needs to be able to be put in the back so the plate of the rim sticks out past the lip. So if they have to put them together (make them kiss). The tires wont touch.",
"I didn't see anyone else mention this, but maybe I just missed it. The centers stick out so much because the hubs behind the wheels are HUGE. They are massive because they are heavy duty and built to run hundreds of thousands of miles. Much like pickup trucks have different style hubs - a smaller truck, using Ford terminology - say an F-150, has smaller hubs and therefore can use car-like wheels. Go all the way up to a Heavy-Duty F-350 or more correctly a Super Duty F-450, like a Tow Truck or something - you are going to see a wheel that starts to look a little more like a full size Tractor wheel, and the hub behind it is similar. These are simply massive, with much larger, beefier bearings inside. The wheel is shaped like a \"cone\" to both provide clearance for the huge hubs, and to locate the actual tire in the right place so the steering works properly. If the wheel were flat instead of cone-shaped, the tire would be much too far outboard of the steering system, and instead of turning in place and pivoting in the center of the contact patch, it would turn in an arc sort of around the hub. Hard to explain, I guess.",
"There's a lot of true info here in these comments but they are missing the salient point. That is, the centre of the tyre tread must be in the same vertical plane as the centre of the wheel bearings assembly. This is to ensure correct loading. Trucks must have enormous heavy-duty hubs, brakes, and bearings, so the wheel needs to provide room to accommodate these. It's the same reason your car wheels are offset with the majority of the rim being inboard from the plane of the wheel stud holes. The hub, brakes, and bearings have to sit inside the wheel and centred. That's why the studs and nuts are on the outside. The rear wheels on trucks are dual, so the outer wheel appears to the eye to be dished or concave because the hub sits inside the inner wheel.",
"What if you turned the front wheels around and mounted them backwards. Would it affect performance?",
"I have sort of a related question. Why do some Semi trucks have spikes on their lugnuts?",
"In europe the semi's front wheel lug bolts are required to have a protective guard ring over them. In the US we add spikes to make them stick out more",
"The rear wheels are mounted in pairs. Bolts that extend as far as both tires would be too weak (and get it the way when working on the wheels. The front wheels are mounted as singles and don't need a hub that looks different from those on your car.",
"In a regular car, you have a spare. That spare can be used on any of your four tires. Semi's, and duelly pickup trucks have the front hub stick out farther, while keeping the tire under the vehicle for this same reason. To be able to use one spare, in any position. It is also so they only have to carry one type of spare, which could save on weight. Which on semis is a huge deal. That's 200lbs (est) less cargo they can carry What happens if you blow a front (steering axle) tire, but only have a rear axle/trailer axle wheel? You're hooped.",
"the easiest explanation is that the tires that appear concave, are actually a dually axle, its 2 tires. The outer tire is concave for mounting purposes. The inner tire would look similar to the front tires. URL_0 The reason the inner tire, or the front tires stick out, is because they cover the braking system, similar to a passenger car",
"Diesel mechanic here. The front and the back wheels are exactly the same. The front wheel has the concaved part of the wheel facing inside so it goes over the hub. The back has duel wheels, so the inside tire mounts just like the front and the outside tire is flipped around so the concaved part of the wheel is facing out. I hope this makes since. This is the first time I've tried to explain this.",
"Am I the only one that drives by trucks and think one of those lugnuts are going to magically pop up and propel into my brain while I'm driving. It's a weird anxiety...",
"Nothing here has been dumbed down enough so I'll give it a try. On an axle with two tires on each side there are two different rims that are bolted to the same hub so the rims have an offset. The rim that you can't see on the inside is protruding out. It touches the side of the outside rim that is protruding in.",
"The correct term is offset. The fronts use negative offset (like fast and furious cars) which brings the rim further inside the fender and puts it directly atop the a-arm which affects how tightly it will steer. The rears use both positive and negative offsets. They position the inner tire closer to the axle (using negative offset) which puts the lugs further out alowing the outer rim to bolt on safely and more securely. Positive offsets are what give big 4x4 trucks a wide stance. They extend the rim out well beyond the lugs to widen it, giving a lifted truck better stability.",
"TL/DR: Front axles have a single wheel and the wheel fits around the axle hub. On the rest of the axles, the first wheel looks just like the front with the single wheel, but an additional wheel is flipped 180 degrees and placed against the single wheel, giving it an opposite dished appearance. The \"]\" and \"[\" are the wheels, the \"o\" are the hubs, and the \"---\" are the axles. Front Axle [o---o] Rear Axles ][o---o][ This allows greater weight carrying capacity on the truck where it is needed, and single wheels on the front where steering is needed but where less weight exists. An \"18 wheeler\" will have 2 wheels on the front axle, and 4 wheels on the other 4 axles, equaling 18 wheels. __"
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6x4cbx | Why most ammunition's dimensions have decimal number instead of round number? | Such as 7,62x49 instead of 7,60x49, and 5,56x49 instead of 5,50x49. | Engineering | explainlikeimfive | {
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"A reason for some strange values is that they were original designed in inches. The 7,62 are developed from the .30-06 Springfield cartridge where a diameter of .3 inches was used. 5,56 is derived from a .222 inch cartridge where the value make a bit more sene"
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6x6s1g | How does the Goodyear sphere wheel work? | Engineering | explainlikeimfive | {
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"Google tells me that there's a bunch of magnets inside the rubber sphere and electromagnets in the wheel casing. So kind of like how a maglev train levitates.",
"It doesn't work. It's just a concept tire, and doesn't actually exist yet. ."
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6x6z1m | Why are trailer tractors so boxy and non-aerodynamic? | So, first of all, there's a shape difference between tractors in Europe and the ones in the US. EU ones are pretty much a flat surface slamming against the wind at 100+km/h. US tractors have a front mounted engine, which gives them a seemingly more aerodynamic shape. However, when brought together together in the same plane, the engine and the windshield have roughly about the same surface area, with both surfaces being mostly flat. I don't know if the US trucks are more aerodynamic or not, so some input on this would be nice, as well. The overall question, however, remains: how come aerodynamics doesn't play a role in truck development? Considering the fuel costs, the large front surface area and the speed they cruise at, you'd think aerodynamics would be something truck manufacturers should be all over. | Engineering | explainlikeimfive | {
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"EU tractors generally have their passenger cabins mounted on top of the engine instead of having the engine be in front of the cab. This makes the tractor look much boxier than American styles but it also makes them shorter. Being shorter is much more important in Europe because many cities have small roads with tight corners that aren't as common as in America.",
"I'm an aerospace engineering grad so can provide some insight into this. The default, most aerodynamic shape is a tear drop (fat end into the air). A lot of people think a knife edge is more aerodynamic because that's what they see on fighter jets. But take a look at this fighter from 1945: URL_0 It has a rounded nose, just like a modern commercial airliner. Sharper noses only came in when people wanted to go super-sonic where aerodynamic rules get really different. Anyways. If a teardrop is the best shape, how far off that shape is the front end of a truck and how could we improve it? It turns out, when you start to consider the stuff you need in the front of the truck (and where it has to be places so that you are good with local highway axel loading rules), you just don't have much wiggle room. You could improve things a bit, but not that much. The interesting thing is that the BIG aerodynamic savings are to be had at the back of the truck. That flat surface deviates significantly from the tapered end of a teardrop and creates all sorts of excess drag. The problem is, that;s where stuff gets loaded in and out of the truck, and a giant spike sticking out the back of a truck isn't a safe idea.",
"A reson for the difference in max length. In continental Europe the max length is 18.57 m, 40 ton and 24-25.25m, 60 ton in Scandinavia (the reason is that the extra length make it possible to have 3 instead of 2 cut to length stack of timber because the forest industry is important in Scandinavia). It is economical to have a cab over engine design to maximize the amount of cargo space I the US there are no federal max length but there are weight limitations of 36 ton so there is no incentive to make it compact the same way With those limitation is it not strange that the designs are different. I is liley the case that increasing aerodynamics would reduce capacity and be non economic. A flat front is not that bad for aerodynamics. A flat back is worse Look at how [aircraft wheel covers looks like]( URL_0 [5]-1784-p.jpg). They are quite bulky at the front and pointier at the back. That design are that way for a reason",
"As much as it looks like things aren't aerodynamic these things tend to be heavily engineered towards being as fluid as possible while considering other handling characteristics. Many others have pointed out about the size restrictions so I'll leave that unsaid as other commnets seem to be more than enough for that. What I will say is take for example the Volvo FH16 truck. You look at it and you think it's very poor for aerodynamics but when you start looking into it you'll notice subtle things about it's design aimed at improving the CD (coefficient of drag) rating of the vehicle. Take a look at this image; URL_0 Looks like a massive flat wall with a slight curve at the very edge of the cab doesn't it. Now look at the plan overview of the truck and what do you notice. URL_1 I bet you should be able to see that there's a very slight curve to the entire front of the vehicle. The figures I've found is that most Cab over Engine trucks in Europe have C^D around 0.55-0.70, Most mainstream modern cars are around a value of 0.20-0.35. Now when you consider how much larger of a profile a truck has, then you soon can see that the figures are pretty impressive as an FH12 is from the figures I can find about 3 times the size of most cars in terms of cross sectional area (I've looked up a few common European models). It's also worth adding that a lot of the parts on a truck (or other vehicle) are designed to be for aerodynamics not just in terms of the coefficient of drag but also to aid grip for the tyres, improve road holding and other performance metrics beyond being a fluid shape. Making a vehicle perfectly aerodynamic is possibly a great idea if you plan to use it in a straight line on perfectly dry roads with no wind or weather influences. For real world use you have to deal with pushing them into the road surface to press through rain to combat aquaplaning, to work against strong winds to prevent roll overs and many other things."
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6xfiie | A wheel is bound to the car by lug nuts, but how does an inflatable rubber tire stay bound to a wheel, especially under tremendous forces? | Engineering | explainlikeimfive | {
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"The hole in the tire is reinforced with a steel cable. With air out of the tire, the tire store can slip these reinforced holes over the wheel. Then when the tire is filled with air the tire can't get off because the hole is smaller than the flange of the wheel.",
"The shape of the wheel prevents the tire from falling off. Most car tires are inflated to a level around 30-35 pounds per square inch. That's a lot of force pushing against the lip on the wheel rim. That would have to be over-come on a significant portion of the rim to force the tire off."
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6xfyry | If I'm using headphones on a PC, and I mute the PC volume, is there still technically a signal being transmitted to the headphones? | Engineering | explainlikeimfive | {
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"No, there isn't, on wired headphones. If you attached a meter to the wire you'd see zero voltage.",
"Kinda related. Some headsets have a dedicated mute button. But a lot of them only set the volume to ultra low. So if you enable it's physical mute feature, record yourself, and amp the volume of the recording to maximum using audacity: you may be able to hear yourself. Sadly, it means you can spy on someone \"muted\" with this kind of headset, as long as you're using a voice chat software that continuously transmits audio (Skype for instance). Teamspeak, Discord, etc... anything with sound-level voice detection should keep you safe.",
"I think you are assuming the headphones do the muting? The signal is sound is stopped at the pc, not the headphones.",
"A lot of people are saying 'no' but that's probably not entirely true. The DAC in the sound card won't be outputting a signal but unless you have a motherboard that has well designed audio front-end as a feature or the audio front-end is shut off entirely when the channel is muted you'll still be able to observe a signal at your earphones either by listening or looking at the voltage with an o-smell-o-scope.",
"On wireless headphones there would still be some minimal signal. Just enough to tell the headphones they're connected and there's nothing to play.",
"A sound card is basically a digital-analog converter (DAC) that sends electrical signals to vibrate drivers (magnets) that make the actual sound. When the computer is muted, no signal is sent to the drivers. The computer can tell the headphones are connected because the ground is still connected. EDIT. I described analog headphones. USB headphones still acknowledge (or \"ack\") a response to the computer to let them know they're still plugged in.",
"Yes there is still a signal on the amplifier and headphone side. If you think of your brain as the computer, and your throat and vocal chords as an amplifier and speaker, then hitting mute on a PC would be like if you stopped talking or singing. You wouldn't be completely silent as there are other noises your throat makes like breathing, and your vocal chords still have some movement even when not engaged. It would just be very quiet, so as almost not to be heard. The headphones are the \"load\" in an amplifier circuit which does \"work\". When there is no input signal that circuit still exists, but with little work to do there is some noise and there is still the electrical circuit which is sending power through the headphone. Think of the \"click\" you hear when you plug headphones in, even when nothing is playing. With a PC and other digital sources there are 2 halves of the system. The digital side and the analog side. When you mute your PC, pause an MP3 track on your phone, or stop playing a track, you are stopping a digital signal or zeroing it out. That signal is converted by a DAC chip (digital to analog converter) before going into a pre-amplifier or operational amp (op-amp), and from there to the headphone amplifier. Even if there is no signal coming from the DAC chip, the headphone and various amplifiers form a circuit and there is a signal there in the form of a noise floor. The noise floor, or point at which noise is audible, can be very low on a good amplifier, but it's always there."
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