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8e35pq | Why are steering wheels placed on the left or right side of a car instead of the middle? | Engineering | explainlikeimfive | {
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"A couple of reasons. First of all, if the steering wheel were in the middle, you could only comfortably fit one person (the driver) in the front of the car. That's not something that most people would be happy with in a car. But more importantly, it's much safer to have the driver on the side of the car that faces oncoming traffic. It's easier for them to judge the distance to other cars when they're on that side.",
"A brief history: In ancient times people would keep left while traveling on foot or horseback, because if the person coming toward you was hostile then you'd want them on your weapon-hand side (which for most people would be the right hand). Horse-drawn carriages followed this guideline with the drivers usually sitting in the middle to hold the reins and whip. In the late eighteenth century, though, a new kind of carriage entered into popular use in the United States and France. It had no usable drivers seat, so the driver typically sat on one of the rear horses. Again, being right-handed, he wanted to sit on the left horse so he could whip in the middle of the team. Since the drivers were now on the left, they wanted to pass oncoming traffic on the left as well, so they could better gauge a safe distance and not get wheels stuck or run people over. So they started driving on the right side, and since they had become commonplace, the rest of the traffic followed suit. France applied this rule to all their colonies, and Napoleon enforced it in every land he conquered. A century later Germany finished off the rest of Europe and by the golden age of automobiles the only places that still drove on the right were part of the British Empire. The Brits held firm to the old ways, and when cars took over they knew they would have to put the drivers on the right side so they could watch oncoming traffic themselves."
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8e3sta | If you choose a 100% renewable / green energy provider, how do they ensure YOUR source is any different? Isn't the grid just the grid? | Engineering | explainlikeimfive | {
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"First and foremost, the electricity producers are tasked with keeping the grid stable. Sometimes that means outputting at full power, sometimes that means not outputting even if they could. They all work in the grid When you select a green provider, your electrical distribution company will charge you for the electricity, and then pay the green provider at the agreed upon rate. If they generated enough power to cover all the people who signed up with them Great! If they didn't, they they need to \"buy\" power from other producers to cover their bill, most other producers can make power cheaper than renewables so they still get to keep some which will help them grow their operation] Your boss is right, when you choose a green provider, your electrons don't come from that wind farm on the hill, they come from whatever is powering the grid at that time; however, your $$$ does go to the operator of that wind farm on the hill and that's the most important part.",
"It doesn’t work like that. You get your power from a mix of sources since they all feed into the same grid. Your electricity company just ensures that they put in the amount you use over the course of maybe a year or so from renewable sources, even if they also produce non-renewable energy. So, if you use 1000kWh over a certain period of time, they promise to put at least 1000kWh of electricity from renewable sources into the grid. You are supporting green energy, just not as directly as they make it seem in their marketing material."
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8e7lxv | Is there an upper limit to how tall a building can be? | Assume a 50,000 square feet cuboid building on a plain, firm land, in a landlocked area and not on any major earthquake prone areas. How tall can it be theoretically? | Engineering | explainlikeimfive | {
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"The limit is based on the weight the construction can carry. If the construction cannot carry that amount anymore, then it will crush under its own weight. For example, the World Trade Centre towers, you might have heard a little bit about it. They were 400 meters high. The metal and concrete construction were properly specced to hold its heights for the first 35 years: Nothing wrong with it. Now due to an external incident, the metal construction is affected and lost its strength. It then collapsed under the weight of the construction on top of it and it folded nicely on top of itself. The space elevator principle is based on a string of 36 thousand kilometers. Currently there is no material which can carry that weight without breaking.",
"The building you describe would probably be limited in size by the wind before its self weight. The higher above ground level you go, the stronger wind forces become. The beams, columns, braces or walls that keep the building from turning over, are larger than what would be required for the weight of the building alone. A building with a single flat side is also the worst shape for dealing with wind forces. When you look at Burj Khalifa, which iirc is the current tallest building, it has a lot of irregular and streamlined shapes as you go up the building. That is to avoid making a big flat wall that the wind can exert a lot of pressure on. Even with all this, you could probably make an exceedingly tall concrete box. The problem becomes that architects don't want to design them, owners don't want to build them, and tenants don't want to live in them. Current skyscraper design is probably limited more by client desires, expense, and other factors than it is by how tall a cube we could pour out of concrete."
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8ehs1z | How do skyscrapers drain rain water? | Engineering | explainlikeimfive | {
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"The roofs on top of large buildings aren't flat. They're mostly flat, but they're not perfectly flat. Typically, each section of the roof forms a shallow bowl, and at the center of each \"bowl\" is a drain. The rain flows down these drains, which pass through the building (often with very large pipes) and into the urban storm drain system at or below the ground.",
"Through pipes. The rain that falls on the top of the building is collected by the roof, and dumped into drain pipes. There are rules about how far it can fall before there is an elbow in the pipe, to protect the pipe. Back and forth it goes until it gets to the ground floor, where it dumps into a storm drain."
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8ekkwl | Why do sedans typically seat 5 people nowadays when they used to seat 6? | Engineering | explainlikeimfive | {
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"sedans use to put a bench seat in front. That is now known to be wildly unsafe, and it was not typically utilized anyway, so they went to bucket seats up front with a console, losing the middle seat.",
"Man I used to get stuck in that damn middle seat during family road trips skiing and am thankful that it no longer exists so no one has to suffer through that"
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8es0lr | How does a seatbelt know when the strap is coming out too fast and needs to lock up (in an accident)? | Engineering | explainlikeimfive | {
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"Centrifugal brakes. The webbing is wound onto a drum that has a clock spring inside to retract the belt. It also houses radial pins which are retained by weak springs within the drum at low rotational speeds, but are forced to protrude by centrifugal force at high rotational speeds - the non-rotating part has a ratcheted surface that the pins catch on when they protrude. In addition I believe they have inertial brakes. A ball bearing in a sloped slot, perhaps? When the car is accelerating or at a steady speed the ball stays at the bottom of the slot. When the car is decelerating quickly enough the ball will roll up the slope and engage in a toothed or ratcheted surface on the drum, stopping it from rotating. What gets really exciting is seat belt pretensioners! I've never heard of them being linked to the brake pedal, and if you think about it that would be crazily annoying, particularly for those weirdos who drive auto with both feet!"
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8esw54 | What makes bolt action rifles more suitable than semi autos/full autos to fire more powerful ammo? | Why bolties more common for shooting large calibers? | Engineering | explainlikeimfive | {
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"It is simpler to make a bolt action system with high strength, ruggedness, reliability and accuracy compared to other system and the weight will be lower and they are cheaper to produce. The disadvantage is rate of fire. But for many application like hunting, target shooting and precision shooting that is not a a huge disadvantage compare to the advantages. Large powerful calibrates are primary used at long range where accuracy it one of the most important factor. For man portable military systems weight is important both for the weapon and for the ammunition. So you either have sniper rifles that can be move quite easy that are light and often bolt action to increase accuracy and reduce weight and ammunition use. Or you have heavy machinguns that are hard to transport and use a lot of ammunition so you like to use them on vehicles where you even have heavier cannon system both smaller full automatic and larger that you load manually or some semi automatic.",
"The main reason is that, by design, the bolt in a semi-auto firearm must be able to move backward using some of the impulse created by firing. The bolt is then slowed by a spring and eventually hits a solid surface (called different things in different types of semi-auto actions). When you have very powerful rifle cartridges like the kind used for big game or long distance target shooting, there is simply too much impulse created from the cartridge going off that a system to slow and stop the bolt would be very expensive, bulky, and heavy. This is why the Barret M82 is 30 pounds and $10,000. On the other hand, a bolt action remains stationary until unlocked by rotating the bolt handle, which means it can be made stronger relatively cheaper.",
"The gas/power used to manipulate the action of a semi automatic, reduces the muzzle velocity (a little bit). This isn’t an issue with bolt action rifles. I’m not an expert.",
"It's because gasses in a semi auto are used to manipulate the bolt, so you have different actions going on ( gas piston, bolt and bolt carrier)besides just the recoil.",
"Because large calibers have quite the kick and when using them you usually aren't targeting someone that is only 30 feet in front of you so you want accuracy, range, and stability. Shooting a full auto 50 cal isn't really plausible unless you are on something like a turret mount because the recoil would be uncontrollable, but for bolt action it's one shot then you reset and take aim for the next."
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8etkwf | how is a turbocharger different from a supercharger? | Engineering | explainlikeimfive | {
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"A turbo charger is connected to the exhaust. The exhaust spins a set of turbines extremely similar to a jet engine. The air gets compressed in the turbines and is then forced out with greater force and is then rammed into the engine. The higher compression in the cylinders makes more power. A supercharger is connected to the engine itself and uses that power to force more air into the engine at higher pressure making more power. Basically the name of the game is more air at higher than normal pressure. The only difference between the two is how they generate power to force air into the engine.",
"Basically, more air in the engine means more fuel can be exploded which means more power. Superchargers and turbo chargers both do this just in different ways. A Turbo charger is just a fan that pushes air into the engine. But how do we spin this fan? Using the exhaust coming out of your engine already! Because it uses exhaust gas to run, turbo chargers are actually quite fuel effiecient. But that fan has to spin up, hence when you take off in a turbo car, the turbo \"spools\" and gradually gets stronger as the exhaust gas spins it. Luckily this is an exponential process - you take off, some exhaust gas spins the fan, which shoves some air into the engine, making it run harder, producing more exhaust gas, spinning the fan even more, etc etc. Basically a turbo charger is just a smart way of recycling the usually wasted exhaust gas to spin a fan and push more O2 into engine for more power. Super chargers are exactly the same thing in concept. They force air through the engine to create more explosions! But they don't have to \"wind up\" or \"spool\"... Why? Because they are powered by the engine itself! This means you get instant air into the engine, no need to wait for a fan to spin up from exhaust gas. A downside to this is the engine is now running itself, and the supercharger, and then even more fuel for when more air is pumped into it. They are very bad for fuel economy, but in terms of speed they are faster. I'm more of an expert on turbochargers (jdm life) so my supercharger knowledge might be off so feel free to correct me!",
"Turbo uses exhaust, supercharger is connected to the belt system, both force air into the engine. With a turbo you’ll see more high rpm power, with a supercharger you’ll find more low end power. There is also a power charger, which does some sort of mix of both. Super chargers are mounted on the top of engines (not sure if it replaces or sits on the carb.) Turbos are connected to the exhaust pipe and the fumes spin the fan that forces are into the engine.",
"There's a lot of answers here that are in the ballpark, but missing details. This is decidedly not a ELI5 type of answer, but the question is broad. Superchargers and turbochargers increase the volumetric efficiency of an engine by increasing the manifold pressure beyond atmospheric pressure. The goal is to put more oxygen into the cylinder so that more fuel can be burned, which means more power can be generated. When you push the accelerator in a naturally aspirated engine, you have only the pressure of the atmosphere pushing air into the cylinders. That means a 1 litre engine can, at most, contain 1 litre of air/fuel mixture (very clever engineering gets some engines above this slightly). Forcing the air into the engine allows you to cram more than 1 litre of air into a 1 litre engine, often as much as 200-300% more. How they differ is in how they generate that pressure. Superchargers are just the compressor which is driven directly by the engine, either with a gear set or belt, typically. Just like any other accessory, driving the compressor takes some of the power from the engine. This isn't a problem, as the supercharger always adds more than it requires. There are a bunch of different types of compressors used, and they all have different strengths and weaknesses. Turbochargers have both a compressor and an impeller. Instead of the compressor being driven directly by the engine, it's driven by the expansion of exhaust gasses. Because the exhaust is considered waste, turbos are more efficient than superchargers, essentially reusing the waste to produce power. But, because they aren't driven directly by the engine, they are less responsive than superchargers.",
"Both try to achieve the same goal: air compression. A more compressed air charge means it can be combined with more fuel. More fuel + more air equals a bigger bang when the spark plug ignites the fuel air mix, and you get more power. Both use a turbine mechanism to achieve this. The difference is how the turbine is powered. In a turbo charger, exhaust gases power the turbine. In a supercharger, the engine itself turns the turbine.",
"Some good answers, but (in typical ELI5 fashion) they're not really ELI*5*. Both superchargers and compressors help your car make more power. If you want to make a lot of purple paint, and all you have is red paint and blue paint, adding lots more red paint doesn't give you more purple, it just makes the paint red. Your car is the same way. You need air and fuel to burn together to make power. If you just add a lot more fuel, you don't get a faster car, you just get a lot of unburned fuel. You need to add more air as well. Turbochargers and superchargers are both basically big fans that force more air into your engine, so you can add more fuel too, and make your car faster. The only difference is that turbochargers are powered by something called a \"turbine\" in your exhaust flow--just like a windmill gets power from the wind. A supercharger is a fan that's powered by a belt connected directly to your motor.",
"That’s what I love about Reddit. From my experience there’s very little condescension if you ask a reasonably intelligent question.",
"A turbocharger IS a supercharger, just like a square is a rectangle. It's a specific type of supercharger that is powered by a turbine spun by exhaust gas flow. Hence the full, correct name - \"Turbo-supercharger\" In common usage though, when somebody says \"supercharger\" they generally mean a compressor (either centrifugal or positive displacement) powered by a mechanical connection to the crankshaft. There's also such a thing as a \"power recovery turbine\" which is like a turbocharger without the compressor section, that feeds power into the crank from an exhaust driven turbine via a fluid coupling.",
"Technically both belt driven chargers and turbine driven chargers can be called [superchargers.]( URL_0 ) These days when most people say supercharger they mean not the exhaust turbine driven type: *Common usage restricts the term supercharger to mechanically driven units; when power is instead provided by a turbine powered by exhaust gas, a supercharger is known as a turbocharger or just a turbo - or in the past a turbosupercharger.* In other words, all turbos are superchargers but not all superchargers are turbos."
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8evmln | How does a diesel engine work? | Engineering | explainlikeimfive | {
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"In a gas engine, fuel is added before the piston squeezes the air (actually, air/fuel mixture). The ignition happens at the right time because it's ignited by a spark plug, which produces a spark at the correct time. Diesel engines compress *only air* and just because of how much it's squeezed, it becomes hot enough that fuel would burn if present. To avoid the fuel burning prematurely, fuel isn't *added* until the time when it's supposed to burn. The fundamental advantage is higher squeezing/compression of the air allows higher efficiency. Compression ignition (due to heat of the air, not a spark plug) goes along with that. Skipping the spark plug is at most a minor convenience, and the method of ignition *itself* isn't the only advantage of the design. The practical disadvantage is because fuel is added *while it's supposed to be burning* the burn is not as fast or sudden, the fuel may not be mixed as well with the air, and the burn may not be as complete or clean. This is the main advantage of newer diesel technology compared to older diesels (besides the addition of turbochargers, though that happened quite a while ago)."
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8ewm6d | Why are there only 3 blades on wind turbines? Wouldn't they be able to capture more wind energy if there more blades on each turbine? | Engineering | explainlikeimfive | {
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"Adding more mass will make it harder for the wind to get the turbine up to speed (think small car vs large truck). Also, adding more surface area increases wind resistance.",
"Wind turbines are very large to be able to capture more wind and have a greater mechanical advantage. With the long lever arms, it is easier for the wind to turn the giant wheel (like a bolt is easier to turn with a wrench when pulling from the end of the lever arm). The objective is to turn the wheel to capture wind energy, not necessarily capture the most amount of wind possible. Adding more blades would indeed catch more wind but it would also add more weight making the turbine more resistant to spinning from its inertia. The balance between size, number of blades, angle of the blades, material of the blades, available channels for the wind to travel along the blades, and many other variables are all calculated during the engineering process to determine the most efficient way to capture the correct amount of wind in order to make the turbine rotate at a maximum.",
"Yo ho ho! Yer not alone in askin', and kind strangers have explained: 1. [ELI5:Why do Wind turbines have three blades? ]( URL_1 ) ^(_57 comments_) 1. [ELI5: Why do wind turbines generally have 3 blades? ]( URL_4 ) ^(_32 comments_) 1. [ELI5: why do wind turbines only have three small blades? ]( URL_3 ) ^(_2 comments_) 1. [ELI5: Why Do Wind Turbines Always Have 3 Blades? ]( URL_0 ) ^(_9 comments_) 1. [Why were windmill blades made with so much open space (almost like they were lattice)? Wouldn't they have been more efficient as a solid blade? ]( URL_2 ) ^(_13 comments_)",
"Wrong. More blades would increase torque, but also reduce blade speed. It's the speed that turbines want. Technically one blade would be the fastest, but it's unstable. 3 blades is the least amount of blades that offers a stable configuration"
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8f5sh5 | Why is "ground" in electrical seemingly optional? | I've always wondered why some devices require a third prong and others do not. What determines whether something needs a connection to ground along with the positive and negative connection? | Engineering | explainlikeimfive | {
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"A ground is used for safety. Let's say you have a dryer with a metal exterior. A dryer takes a lot of power, and so there is a lot of electricity flowing through the wires. But it can happen that one of the wires comes loose, or loses some of its shielding. If it then touches the metal case, and you touch the metal case as well, Bat Stuff can happen. But by hooking the case up to the ground, the electricity is diverted to the Earth via that wire. And since stuff in nature tends to prefer the path of least resistance, you won't get shocked when you touch the case. You have a higher resistance than the wire.",
"Some devices are designated [\"Class 2\" or \"double insulated\"]( URL_0 ). They have a second layer of protection. Often that means a plastic body with no metal parts which can become energized. They don't need grounding."
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8f833q | How do you not suffocate on an airplane during a long flight? | I know that an aircraft cabin is pressurized while at low elevation, but how is there enough air for a really long flight? Ex. Dubai to San Francisco on an A380 with over 500 passengers. | Engineering | explainlikeimfive | {
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"It isn't sealed air tight, far from it. Air is constantly pumped into the cabin through the climate control system. The jet engines take some of the air that passes through their compressors (bleed air) and sent it to the plane's HVAC system for the passengers.",
"They get a constant supply of fresh air from outside. It gets pressurized and conditioned and fed inside. Its not a sealed can.",
"The plane isn't well sealed The engines are taking in air, compressing it, pumping it into the cabin of the plane, and it leaks out through various spots in the cabin. This leaky cabin is why we only pressurize planes to about 8000 feet, to get higher pressure than that requires either better seals or more air from the engines, and will cause more stress on the body of the plane"
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8fbhmd | Why are roof shingles usually textured like sandpaper? | Had a bit of an issue this week where some shingles from my apartment building fell, and one landed on my car, grit side down on the windshield, so now I'm wondering *why* roof shingles have this texture. In my mind it makes more sense to have something smooth up there, that won't catch snow or other debris (leaves, etc.), but roofing shingles seem to be universally textured, and I can't figure a reason. | Engineering | explainlikeimfive | {
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"The granules protect the shingle from UV light, provide color, reflect infrared, and help water to evaporate faster. Edit: also they make working on the roof safer and keep the shingles from sticking during manufacturing.",
"It increases the surface area of the shingle so that water evaporates more quickly, and in being textured, it slows the water down as it runs off. Otherwise you'd get these fast moving torrents of water and that would be bad for the roof (wear out quicker) and everything around it."
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8fc5y1 | Nautical navigation in the 1800s? | I understand a sextant is a great way to determine latitude at night (using Polaris) and during the day given that you know local apparent noon (LAN) and have a nautical almanac (I'll get back to this). Longitude though seems like a Catch 22 because you need to know LAN but the most realistic modern way for determining LAN is by knowing your longitude... I know you can do cool tricks like using a small piece of cylindrical metal to measure when the sun's shadow is the smallest but does that really work on a ship rolling in the sea? I mentioned earlier the LAN problem for the sun during the day too. In my current understanding, early explorers could easily determine their latitude at night. Latitude during the day and longitude seem to be contingent on obtaining a value (LAN) that is almost impossible. Any explanation are appreciated! | Engineering | explainlikeimfive | {
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"This is a well known problem, British government established the Board of Longitude in 1714 to solve it. The French were also working on the problem, probably even before 1700. Galileo solved the problem using the moons of Jupiter, but you needed a powerful telescope and excellent pointing. Thus it only works on land, to find the location of the harbor once you're in it. John Harrison finally solved the problem in 1773 by making a clock that kept accurate time at sea. Then you can use sunrise and sunset times."
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8fijal | Ok, what does that pedestrian crossing button really do ? | Engineering | explainlikeimfive | {
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"It tells the PLC that controls the traffic lights in the intersection that there is someone at the crossing that wants to get across. Cars are identified with magnetic sensors. I.e, when enough metal is on the road at the same time, the PLC knows that there is a car in a lane. In it's most simple form, there is a detection coil right next to the stop line. So that cars that have stopped are properly identified. In more advanced setups, there are coils that identity cars from a distance so that they don't necessarily have to stop. Or speed down. Or to better be able to count cars. Establish that an off ramp is getting full and need to be cleared. And there can be coils that identify trucks from afar so that the intersection is not forcing them to pull the brakes in a manner that they are not necessarily capable of anyway. Or coils that enforce the bus lane by not even reacting to cars. So, what does that have to do with the pedestrian button? Well. The detectors makes the intersection adapt to traffic. A lane won't have a green unless there is actually a car to let through. And if there is a green, it can be a short one intended to let just the one car out. But. Pedestrians are not nearly as fast as cars. The button is there not only to tell the PLC that there is a pedestrian. But to *make sure that the pedestrian has plenty of time to pass safely.* In other words, it creates a safe window in the program that is far too long for cars, so that it never makes sense to keep it going all the time, and shuts it once the timer runs out. Some intersections have bike crossings too. Those buttons trigger their own timer too. Often shorter than the pedestrian button, because the bicycle crosses faster.",
"In some cities they don't do anything. There are always enough people around that the computers are set to always show a pedestrian signal. The buttons basically get taken out as part of maintenance; they are left over from when they used to do something. In less busy cities, there are not pedestrians waiting much of the time, so the pedestrian signal is only shown if someone pushes the button. Sometimes pedestrian signals are shown with green lights, so if there is a car waiting the signal will show anyway. But at night there may not be a car so the button serves as a backup to make sure the signal gets shown.",
"It tells the traffic light controller that a pedestrian wants to cross, of course. Depending on the intersection, it may initiate a pedestrian crossing phase in the very next part of the light cycle, in a *particular* part of the *next* light cycle, or nothing at all. It's entirely up to the whim of whoever decides on the programming of the intersection.",
"It depends, some buttons don't actually do anything and are just there to give the impression of doing something, others will cause the traffic lights to change if they haven't recently changed."
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8fq80a | Difference between Multilink and MacPherson suspensions? | Engineering | explainlikeimfive | {
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"In a Macpherson strut configuration, the strut (which incorporates the spring/shock absorber) is an integral part of providing wheel/hub location, and acts as a suspension link in its own right. With a multi-link suspension, the spring and shock absorber are relegated to just providing the springing and shock absorption, and are not responsible for locating the wheel. Macpherson strut suspension takes up less space, reduces unsprung weight and is simpler in configuration, but it allows for less adjustment/fine tuning of wheel/hub location and can really only be implemented on vehicles with unitary construction. Multi-link suspension is essentially the opposite: increased unsprung weight which usually leads to the use of alloys and specialised materials, more precise engineering and more components which can drive up price, but more precise adjustments are possible. In many cases each parameter (camber, caster, toe, etc) can be adjusted without affecting others."
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8fqmb0 | Why do bathroom stalls not extend all the way to the floor, leaving that creepy exposed area? | Engineering | explainlikeimfive | {
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"Installation Cost: save money by buying pre made walls instead of custom walls Smells: Airflow helps remove gases Cleaning: easier to clean floors Security: people less likely to do drugs, sex, etc. Speed: increased throughput by people seeing which stalls are empty instead of knocking",
"Safety and sanitary reasons. It's easier to clean stalls than enclosed stalls. Plus, you would not want people too comfortable in public bathroom. Edit: a word.",
"How would someone hand you any toilet paper if you went in, “went”, then realized the paper was out as you reached for it?",
"Here in the UK (and most places I have visited), bathroom stalls DO extend all the way to the floor. It's quite uncommon to find those open style \"half door\" cubicles outside of the USA. A small gap at the bottom (20cm or so) is reasonably common. [example cubicles]( URL_0 )"
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8ftqzf | Crowd control | Engineering | explainlikeimfive | {
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"The thing is, people walk at different speeds. Not like electricity or water where it all moves at the same speed. So, over a long path, people naturally will spread out according to the speed in which they walk. Fast at the front, slow at the back and everyone else in between. On a short path everyone reaches the gate at the same time because there isn't much time for the speeds to differentiate. However, in a long path there is time for the speeds to differentiate and so not everyone reaches the gate at the same time."
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8fugbn | - How induction motors work and how inrush current decreases as the motor starts spinning | Hi all! As the title says I was wondering if anyone can explain how an induction motor works and how the inrush current decreases as the motor starts spinning. I read some articles and websites online, but would like to see what people say. Thank you! | Engineering | explainlikeimfive | {
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"An induction motor has two groups of windings that carry current. One is on the outer part and doesn't spin, it is connected to power and generally built into the housing of the motor, this is called the stator. There is a second set of windings wrapped around the center shaft which spins, this is called the rotor and has no wires connecting it to the stator or outside power in an induction motor, all current that flows through this winding is *induced* by the magnetic field of the stator. When the motor is stopped and you apply power, the only thing limiting the current is the resistance of the wire that makes up the windings in the stator, this is going to be pretty low and with the full input voltage applied across it you get a large input current (Inrush = Voltage applied/Winding Resistance) Once current begins to flow through the windings of the stator, they create a changing magnetic field. This magnetic field induces current in the windings of the rotor and pushes it so it begins to spin. Once the rotor is spinning, it now creates a changing magnetic field which induces current in the windings of the stator *in the opposite direction* which creates a voltage in the other direction which is called \"back EMF\". This back EMF eats up some or most of the voltage applied across the motor such that the current while spinning = (Voltage Applied - Back EMF)/Winding Resistance If you have a motor with 2 ohm windings and apply 100V across it, you will get a 50V inrush. If when its running at its rated speed it creates 95V of back EMF then you'll only have 2.5A of current while running."
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8gcgwh | How do flying wing-type aircraft, such as the B-2 Spirit, fly without a horizontal stabiliser? | Engineering | explainlikeimfive | {
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"There's nothing in principle necessary about a horizontal stabilizer. It is useful because it reduces the amount of force necessary on the control surfaces to provide the same torque that makes the airplane pitch up and down. If you can provide the same amount of torque (e.g. through bigger elevons - combination elevators/ailerons) through wing devices, there's no problem. That said, you do lose the passive stability generated by a separate horizontal tail. The real tough part about losing a tail is losing *vertical* stabilizers - which is why you see a lot of delta-wing planes with no horizontal stabilizers, but not a lot of planes with no vertical stabilizer (example: the F-117A Nighthawk has elevons and no horizontal stabilizer, but two vertical tails). The vertical stablizer has tremendous value in preventing flat spins (the aircraft just spins like a top) which have historically been a major way people crash airplanes, especially maneuverable ones. The B-2 has split ailerons - one that can go up, the other goes down - to provide yaw control. They act as speed brakes, causing one side to slow down. Deploy the split ailerons on the left side, the plane will yaw left. As /u/lithuim mentioned, the control requirements for a total flying wing are pretty complicated. There are many planes that have been flown without automated computer control that don't have horizontal stabilizers. There are much fewer that have flown without vertical stabilizers, and they have all had control problems associated with the difficulty in performing yaw control manually without a vertical stabilizer. It's also worth mentioning that even early flying wings in the '50s and '60s had autopilots that helped maintain stability, though not to the same extent as later computer-controlled autopilots."
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8gj4ga | Suppose a new kind of product is created and someone wants to create a factory to mass produce it. How do that company manages to create/buy the machines that the factory needs? Is there a kind of professional who creates that or a company who creates factories? | I am thinking about complex stuff like producing glass sheets, that require pouring the melted glass over a pool of melted tin, that is a rather complex stuff to think about, when creating the factory. That is not a stuff one comes out of thin air and demands some serious thinking. Also imagining a factory that wants to produce something that was never produced before, at least not on the way the factory wants. Who does that? | Engineering | explainlikeimfive | {
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"This is called tooling, and there it is an area of engineering. Tooling engineers build the systems used in factories to manufacture goods. There are sub specialties based on the type of goods being made, and some argue this is a huge advantage China has over the US \\- their programs for training tooling engineers are highly advanced."
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8gmxqs | why do cars get the same gas mileage as they did 20 years ago? | have a Camry from the 90s and i still get 28 miles per gallon, why should i get a new car that gets 32 mpg? | Engineering | explainlikeimfive | {
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"The Second Law of Thermodynamics. Internal combustion engines are heat engines. The greater the heat differential, the greater the power they produce. The efficiency of burning fuel and getting useful energy out of it is limited by physics, and engines have between a 32-40% efficiency in doing this. How many MPG (miles per gallon) a car gets is a combination of engine, transmission, and overall vehicle weight relative to the size of the engine, as well as the size of the engine itself in terms of power and torque.",
"Cars are bigger and more powerful. That new Camry gets around the same mileage as your 90s Camry? Now look at the size and weight difference and compare horsepower and 0-60 times. Get back to me with the results.",
"There are obviously cars that get far better MPG. People in general don't want them. Hybrids and electric-battery cars exist, and they have a strong market share. Good gas mileage is a trade off, smaller vehicle and less powerful engine at the expense of things consumer appear to want, such as a larger vehicle or a more powerful engine.",
"My current car does 50 mpg without blinking. If id gone for the smaller engine option it would be doing 70. And that's just plain old 2L vs 1.6 L mid range cars. If you want to get massive engines then there's only so much you can do. However claiming that fuel efficiency hasn't increased is a massive misrepresentation. You're simply choosing inefficient models.",
"I’m no expert but car’s fuel efficiency hasn’t gone up drastically in years, we are still at best ~40% efficiency and typically hovering around 30%. One of the reason why electric cars are on the rise, they are already at more than ~90%."
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8godid | If silencers on guns actually work to muffle the sound of a gunshot, why aren’t they more commonly found in all the recent shootings across the US? | Engineering | explainlikeimfive | {
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"They just reduce the sound from “jet plane” level to “louder than a heavy metal concert” level; so not that effective at muffling gunshots.",
"Suppressors only reduce the decibels to a point where less damage is done to the ear. They are nowhere near as effective as movies would have you believe for most guns. Best case scenario, it becomes a bit more difficult to locate the shots. This combined with certain drawbacks to suppressors and a more difficult obtainment process, makes them pretty much useless for a mass shooting, especially when you consider that people are gonna figure out there's a shooting going on from all the screaming and commotion anyway even if a gun could be made Hollywood silent.",
"Good quality suppressors are very difficult to come by. They’re regulated as heavily as full-Auto machine guns and assault rifles. Building one may be beyond the capabilities of these shooters. Ordering a non-regulated functional suppressor or components could lead to an investigation which could screw up their plans. Most shootings are done with pistols. Not every pistol can accommodate a suppressor without changing some parts around. Putting a suppressor on a pistol makes it much more difficult to conceal, and also makes aiming more difficult. May cause reliability issues. As far as shootings using rifles, again not every rifle can accommodate a suppressor easily. An AR15 could but will most likely run into reliability issues going off of the AR15s recovered from shootings. The Vegas shooter is the only one that comes to mind who had rifles and presumably the know-how to get his rifles to run reliably with a suppressor. But considering his plan involved multiple rifles and a high volume of fire, a suppressor would have been a hinderance. Also not advantageous to his plan. But just slapping a suppressor on an AR15 increases pressure and will most likely lead to malfunctions. Final note would be that they probably don’t care. Or maybe they want to be as loud and violent as possible. Or it wasn’t pre-meditated to any significant degree. But I’d say for the overwhelming majority of gun homicides, the perpetrator wants to be able to easily conceal the firearm and either escape with it or throw it somewhere. So they use pistols, and a suppressor on a pistol could make conceal-ability more difficult.",
"A suppressor really only muffles the sound of a gunshot by 20% at most. It is still a definitively loud and distinct sound. The added weight at the end of a barrel can help with stability by shifting the center of mass words the front and causing less recoil. As to why they aren't used in these shootings; a suppressor can be very expensive and several states have restrictions/bans on the sale of them.",
"Short Answer: Silencers don't work as well as they do in the movies (especially on full power rifles), and most shooters aren't willing to invest the time/effort to get a worthwhile one. Long Answer: Because A) Buying a good can needs a background check stricter than what it takes to buy a rifle off the shelf, B) building a good can yourself requires technical skill that the average mass shooter lacks and time they are often unwilling to invest, and C) a bad homemade can won't take enough off the muzzle blast of a rifle to make it worth the extra dicking around it takes to build one. Most shootings are taking place with entry-level 5.56mm semi-auto rifles, involving people who don't have a great amount of experience with firearms (with exceptions like the Vegas shooter), who either bought the cheapest AR-15 pattern rifle they could find, engaged in theft, or got it handed back to them in violation of the law after originally having them confiscated (thanks Waffle House shooter's dad!). Buying one of these rifles requires a clean background check (and many of these shooters are either first time offenders or got overlooked by the mental health system), plus whatever state/local specific restrictions apply. Buying a silencer under American laws takes a lot more hassle, since they're federally regulated to a much heavier degree, assuming the buyer's individual state doesn't just ban them by name. Building your own can is doable, but must home made cans are shit. A shit can can work fine for pistols, and especially well for small caliber rounds, even if only for a limited number of shots, but a shit can on a full power rifle is less than worthless. A \"good\" can on a 5.56mm rifle is only really expected to take the report from \"instant hearing damage\" to \"hearing damage if you do it a bunch of times\". A shit can takes it from \"instant hearing damage\" to \"perhaps a handful of shots of non-instant damage, followed by everything else sounding as bad as normal\". And that's assuming that the shooter didn't commit some kind of truly gross error (and remember, most of these people aren't 'gun people', in a sense of bothering to acquire any real skill) like misaligning the baffles so the bullet strikes them on the way out, or using materials that physically can't withstand the heat/blast of the escaping gas from the gunshot, in which case the silencer might disable the gun and/or injure the shooter."
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8griny | I recently read an article about China positioning itself to sell solar energy to the world. Can someone explain how a country can sell energy to others countries? Am thinking really long cables here... | Engineering | explainlikeimfive | {
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"Maybe they mean solar *panels*? China has really gotten the per-panel price down so low that they have pretty much cornered the global solar panel market.",
"The marketplace for energy is like a big basket. You pay if you take something out, you get paid if you put something in. Energy providers can sell energy directly - I own the generation and the distribution so I hook you up and sell you power. That's the simplest way and is limited by distance. I could also just generate power and add it to the grid. You take out of the grid, I put into the grid. I agree to receive as much payment as you're willing to pay for your power. The grid operator takes a cut, I get the rest. I could also sell you generating capability in the case of solar. My product could be solar panels and storage devices. That's fairly particular to wind and solar. Edit: a bit of 'real world'... In North America you can buy power on the market but you could be buying it from someone at the other end of the continent. The power you use isn't the power they generate, but whatever is on the grid. Electric companies have very strict rules about what their sales people can know and even who they can talk to. No inside knowledge can be shared that might affect price.",
"\"Sell solar energy\" ≠ \"provide solar energy\". China wants to sell, in the marketing sense meaning convince, all countries on the notion of solar power by eliminating the primary barrier = Cost. Then they want to sell, in the normal sales sense, panels that allow countries to get their energy from solar rather than fossil fuels. This allows them to redirect money currently going to the middle east and Russia to, you guessed it, China."
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8grjrx | why did toilets have the tanks much higher initially, and what changed that allowed them to be lowered? | Engineering | explainlikeimfive | {
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"Better siphon geometries and more complicated castings allow water to suck the contents down the drain rather than try to push it down. This lead to higher volume lower speed flows, eliminating the need for the pressure/speed benefit of a tall tank.",
"Another part of the answer is that the flush pull was much harder. Old cisterns had a cast metal, inverted bell that lifted the water by suction up to the syphon pipe for the flush flow to start. That requires a heavy action, easier by pulling down on a chain. Nowadays, plastics allow a lighter construction with a plastic piston moving to lift the water up to the syphon lip. That can be operated with a little handle and a twist of the wrist. Cutaway picture and description of cast iron cistern mechanism - URL_0"
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8guznj | Why aren't all bottles twist off? | Engineering | explainlikeimfive | {
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"The bottles for crown-caps are cheaper & more durable than twist-offs. The machinery for putting them on is cheaper and less complicated. The seals are better & preserve the beer better. ...and many consumers associate twist-off bottles with cheap beer so they won't even buy a craft beer with one.",
"I own a liquor/wine retail store. From what I understand from my suppliers is that there isn’t really a reason for having corks on bottles anymore unless they are higher end wines. Specifically reds and whites that are able to be aged well. 5+ years. If you notice that a lot more wines these days are screw caps instead of corks. It’s cheaper. A lot of inexpensive wines are now screw caps (wines under $15) A few brands keep the corks because it’s more traditional and they are probably much larger suppliers that don’t mind spending the extra few cents to keep them corked. (Makes the brand look higher end) Also keep in mind that the screw off’s that are made these days are much better quality then what they used to be. I have a bunch of moscato di asti’s (slightly bubbly) and other sparkling wines that are now screw offs. This is recent... probably within the last 5-10 years. Another big thing is something called “corked” wines. I’m not going to go into big detail.. but I believe that a vast majority of corks come from Portugal and there has been a problem in the past where a percentage of wines have been corked... this isn’t the particles that float in wine. It’s a specific taste that usually only the avid wine drinker would notice. (Or someone who has an excellence Sense of taste) To most it almost makes the wine taste like paper? At least this is one of the characteristics that I have heard. I personally don’t believe I have ever tasted a corked bottle. Maybe my pallet isn’t to that level yet to distinguish the difference. Back to the point... Corked wines obviously affects the quality of wines produced and from a business standpoint. If I where a supplier and a percentage of your wine is being affected by it then it makes a lot more sense to just use screw caps... Moral of this rant... there are plenty of different reasons why specific bottles are corked or not.. MANY wines are switching to screw caps. Only ones to really tend to stick to the corks are the higher end. I wouldn’t be surprised if I’m 10 years everything below the 30 dollar price point was screw off. I even have some wines over 100 that are screw caps"
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8gxph3 | When you turn on a free-standing fan, why is the first notch seemingly always 'high' and not 'low'? | a) Maybe this is just a US thing, I don't have extensive experience living in other countries. b) It's entirely possible this is just the fans I've seen, but it's been every one over a period of 30-odd years so there's at least a little bit of a pattern. | Engineering | explainlikeimfive | {
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"Because at it's lowest setting the motor isn't strong enough to move the blade from a stop. It needs to be moving just a little bit. Unplug it set it to 1 and plug it in, you'll see."
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8h4dng | How do police trace bullets back to the gun they were shot from? | Or is this something you only see on tv? | Engineering | explainlikeimfive | {
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"You're getting some good explanations as to the theory, but keep one thing in mind; it doesn't always work as well as the theory suggests. Wear and tear from normal use of a firearm changes the patterns left on cartridge pieces that might be used to identify the firearm. The firing pin could leave different marks after another 1000 rounds have been fired, likewise with marks on the bullet from the barrel. Also, markings on casings or bullets aren't like DNA. There is no actual proof that they are always 100% unique. It comes down to degrees of probability. Two models of the exact same pistol for instance, only one number apart in their serial number, meaning they were manufactured one after another, could very well have extremely similar imperfections on the moving surfaces. If they are purchased around the same time, have roughly the same number of rounds through them, and both use the most popular brands of ammunition, there is a good chance the markings on the barrel or firing pin will be pretty similar. Is it similar enough to rule out a different gun? How likely is it that the person who bought the very next gun off the assemble line committed the crime you're accused of with the same gun you own? Well that's for the attorneys to argue about and for the jury to decide. But it's not a perfect science, and it's far from absolute proof.",
"Every gun barrel leaves unique marks on the bullets it fires. Police can fire a test bullet into water or ballistic gel and then compare the marks to a bullet used in a crime."
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8h4mni | How do helicopters with large guns on them not spin when they are being fired? | Engineering | explainlikeimfive | {
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"In addition to the previous correct answers: the more powerful helicopter-mounted guns are typically mounted near the center axis (so they don't create a large turning force) or twinned (so the two sides fire simultaneously, counterbalancing each other).",
"As far as I know, large guns aren’t mounted on helicopters. The usual caliber is 20-30mm. (The amazing 30mm gun of the A-10 warthog is not installed on any helicopter). The common guns are 20mm mini gun (Gatling style - 3 barrel only) on the Cobra or 30mm chain gun on the Apache. Smaller helicopters have .5 or 7.72mm machine gun installed. Anyway, it fires forward, so why would the helicopter spin? That said, vibrations are felt through the whole fuselage when firing.",
"I don't know about helicopters, but I know the F-16 has the Vulcan 20mm cannon be an input to the electronic flight control system when it fires since it is mounted off-axis so the flight controls can compensate. I imagine it would be a similar situation with the helicopter flight controls.",
"They do, but the pilot reflexively compensates with the anti-torque rudder pedals. In the early 1970s in Vietnam, they mounted M61s (six-barrel 20mm externally-driven Gatling-style guns) on AH-1G Cobras. They called that variant of the M61 the XM195, and it was mounted to one of the stubby wings on the side where rocket pods were usually mounted. Unfortunately, they had to mount it to the left side and the muzzle blast would unlatch and pull open the front-seater's canopy, so they had to hold on for dear life to keep the canopy closed any time the gun was firing. In addition to inducing yaw, the fact that ammo was disappearing at 3,000 round per minute caused changes in center-of-gravity in both the pitch and roll axes. But again, the pilot reflexively compensates without even thinking about it."
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8hdzdt | Assuming normal temperature water is coming out of a tap at largest possible stream, why does lowering the stream cause the water to get really hot? | Shouldn't it stay the same temperature? | Engineering | explainlikeimfive | {
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"A combi boiler heats on demand, so if the water is moving slower through it then it spends more time in the heating department, and so comes out hotter.",
"It should, but you are also lowering the stream at the heater end, so it heats up more with the same power, and also water stays more in contact with your skin, heating it up more"
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8hf6z7 | Why no mobile company made the Ideal Phone which consists of best camera, battery, display, processor, design and etc..? | Engineering | explainlikeimfive | {
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"Phones are already expensive. Using the absolute best parts available at the time to create some überdevice would cause the price to skyrocket and only a select few people would be able to afford it. It would probably cost the company more money to put something like this together than they could earn by selling it to the handful of people who would actually be willing to pay for it. Also, \"design\" is highly subjective - the \"ideal design\" for one person might be utterly non-functional for another.",
"It would be possible to build a phone from the most cutting edge technology. That would be really expensive but doable. But then you get to other things, like design and OS. Let's take size for example: I have large hands, so I enjoy a 5.5\" screen. Is it the best screen ? For me, yes, for a 10 years old or someone 5' high, it's not. And it goes the same for all the operating system design choices..."
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8hn4cp | How does Motion Controllers know where you are | I am curious, is it a feature of Bluetooth? How can Motion Controllers and VR headsets synchronize perfectly with each other in a space? | Engineering | explainlikeimfive | {
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"Most motion controllers rely on some form of external reference point that stays relatively still. The controllers have a sensor that can \"see\" the reference point and will use it to calculate their own position. Examples: The Nintendo Wii had a \"sensor bar\" you would place below or above your TV. In reality the bar had no sensors of its own - it was just a pair of infrared lights which the controllers could see and use as a reference. The HTC Vive uses a similar setup - you place \"lighthouse\" modules around the play area which sweep the room with infrared lights. The headset/controllers can sense those lights and use them to compute their positions. The Oculus Rift has sort of an opposite set up where the stationary reference points (a collection of cameras around the play area) are the ones doing the sensing and the controllers/headset are the ones emitting the infrared light. A few companies are working on \"inside-out tracking\", which is a new technology that doesn't require you set up an explicit reference device like a lighthouse. Instead, the headset (and sometimes the controllers too) have an array of cameras pointing outwards which can map the room. The computer then picks out its own reference points from this map and keeps track of how they \"move\" in order to calculate the headset's position."
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8hnedw | How my showers draining system works. | So this is how i percieve my showers sewer system to look like, from the side. The snot green is a plastic "cover" which I am guessing works like a water lock? Poorly drawn image: URL_0 The problem is that this green cover a few times a day shoots up and hits the metal lid. The shower drian lid if you will. Which makes a loud metal sound, and sometimes it even pops the lid off completely. Why is this? Also, Is this something I should do on my part, or should I contact my landlord? | Engineering | explainlikeimfive | {
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"text": [
"To make a slight correction to your diagram the water level should be the same inside the cap as outside it. Most likely the phenomenon you describe is due to the vent to your sewer system being in such a place that it allows the wind to push air into the system. This would pressurize the pipe leading away from your shower drain and push that plastic cap up. This vent on the roof, commonly called a \"stack\", can be adjusted by adding an extension to hopefully move the opening away from whatever air currents are pushing air into it. This is something your landlord should handle because it is a modification to the structure."
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8i1tyc | How a ships path can be visible miles behind | Not talking about a normal wake (V shaped waves trailing behind). If you've ever been on a really big ship you can sometimes see a pattern of calm water which matches up exactly with the path of the ship, even though the obvious bubbles and waves have long gone. [This is the best example I can find online]( URL_0 ) | Engineering | explainlikeimfive | {
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"As they move, Large ships displace hundreds, if not (tens or hundreds of) thousands of tons of water. The water's inertia keeps it moving around after the ship passes. The current eddies created by the propeller and hull recycle the surface water for up to an hour after a large ship passes. The water remains smooth as it moves across the surface and down again because the wind doesn't have enough time to allow ripples and wavelets to form."
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11
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8i65uq | Why does a country downstream from a dam suffer a hit to its water supply? Doesn't the net water flow stay the same? | Take Egypt and Ehtiopia. Wouldn't the water volume coming out of the renaissance dam be the same as the water volume going in and the net flow to Egypt remain constant? | Engineering | explainlikeimfive | {
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"One big factor is evaporation. A reservoir generally creates a greater water temperature in the upper layer and a larger surface area, which increases evaporation and reduces downstream water availability. Also infiltration can be a problem, with water becoming groundwater due to the fact that it's sitting in the reservoir for so long.",
"Not necessarily. If you dam up a river, the backup of water above the dam may cause excess water to flow down different rivers instead, as it is easier for water to go down that river. Think of it like your commute at the end of the working day. Most people normally take the highway, and a few people take side roads. But then an accident happens on the highway. Sure, as soon as you're through the accident site it'll open up and you can get just as many cars through, but people are going to be diverting off onto side roads from the highway as well.",
"> Doesn't the net water flow stay the same? Nope. At the very least, the dam is going to form a reservoir, resulting in more evaporation and seepage loss. But the real impact is irrigation, as dams are often used to retain water and divert it to farmlands.",
"It will take up to 15 years to fill the reservoir, during that time flow will be reduced. Also, some of the water may be used for irrigation, that will not be returned to the river.",
"By definition it does not. Dams slow the flow of water downstream by storing it in their resivuoir. This higher water elevation is usually used to divert water to other places like irrigation and city plumbing. The river downstream will flow more consistently, but on a total water per year basis deliver less water. So no floods or droughts, but you also don't get the extra water to store yourself if the guys that own the dam feel like hogging it for themselves."
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8i7dfm | Is the steam that comes out of nuclear power plants irradiated? | Engineering | explainlikeimfive | {
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"> But then that means the water comes into contact with the uranium(?) in order to be heated? No. There is a fluid that passes through the reactor to pick up the heat and then passes through a heat exchanger which transfers the heat to the actual working fluid that turns the turbines. The steam going through the turbines and ultimately being exhausted was never in contact with the nuclear material, a safety precaution in order to stop the potential of contamination of the waste steam.",
"Have a look on [this schema]( URL_0 ), it´s in czech but you will get it."
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|
8i7e6z | Why do pen caps have a small hole? | Engineering | explainlikeimfive | {
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"text": [
"I’ve heard that it prevents children from choking to death on it. If it gets lodged in their throat they’ll still be able to breathe through the hole."
],
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18
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8iol57 | How does Google Maps have real-time, block-by-block traffic information? | Engineering | explainlikeimfive | {
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"Since practically everyone has a smart phone nowadays, they monitor the movement of android devices to get an idea of how fast traffic is flowing. That along with predictive models such as using past data, and analyzing proximity traffic.",
"Every Android phone, every iPhone with Google Maps installed are sending data back to Google to provide location & speed. Collectively, that data can determine traffic density.",
"According to this [Google Blog Post]( URL_0 ) they get data from many sources, one of which is phones that are following Google Maps directions. All those iPhones, and probably their Android buddies when running similar applications, send updates to Google, and that lets Google know how fast the cars are moving.",
"Google owns Waze, which relies heavily on speed data collected by users navigating via the Waze app. The Google maps and Waze maps share all sorts of data. Google also collects and analyzes traffic patterns, which is how it can suggest how much time it would take to get across Los Angeles at 3:30pm on a Thursday afternoon.",
"Also, Google uses the publicly\\-available data from DOTs and similar public agencies that are monitoring traffic. If your city has signs that give travel times, that's based on DOT data and Google can also use that."
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8iqs1f | Why does it take YEARS to do road work on a highway? | Engineering | explainlikeimfive | {
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"text": [
"Some people will say \"it's health and safety gone mad\", and there's an element of truth to that. But it's not the whole picture . Highways need to be *incredibly* flat and smooth. Even a highway you think is a bit shit, is still smoother than a single person is going to achieve on their own. Consider how far a car can travel in a given frame. Let's keep it simple and say 60kph. At that speed, a car will travel a kilometre every minute. That's ~16m per second. But you don't plan highways in 16m blocks. All variations in height need to play out over many seconds to reduce the chance of accidents (and wear on the road and the cars). For your example, it's even harder - because you need to match the grade of the road perfectly .",
"It could be done in under a few months if they could close all lanes and not have to deal with the physical interference, noise and psychological pressure of keeping traffic moving. (Traffic which cares little (or negatively) about the safety of the road workers.) But everyone \"just wants to drive to work\" and they don't care about those other externalities, so the \"SLOW CONSTRUCTION ZONE\" signs are considered annoying, instead of a promise of better roads to come. It could be done faster still if they had the budget to afford 3 or 4 shifts of workers so they could work around the clock. This however requires more planning, and people like good project managers and clever experiencesd engineers are expensive. Mistakes, delays or accidents can also cost more time and money. If they happen and it eats up the budget, work may have to pause until next financial season. Another hangup is if a different project suddenly has a higher priority, and the city doesn't have the resources to do both at the same time. To say nothing of unions or weather."
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8j2ui4 | How does water from taps,showers etc change temperature so quickly even if its just a slight change? | Engineering | explainlikeimfive | {
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"text": [
"The water coming to the tap is extremely hot and cold at the same time. You are merely mixing it at the tap. When you turn the knob the \"new\" temperature only has to travel a few inches (sink) to hit your skin."
],
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8j3jqt | Why is it so hard to build a bipedal robot? | Engineering | explainlikeimfive | {
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"text": [
"The biomechanics for bipedalism are very complex. Humans have numerous tiny muscles that exist solely to provide balance and leverage during walking, running and balancing. Replicating the mechanics of a knee joint can be pretty easy. But if it can’t be replicated, say the extensor mechanism that interacts with the arches in your foot to provide balance and complex foot movements necessary for bipedalism, then it has to be replaced with another mechanism (built by humans) that doesn’t work as well. Replicating human biomechanics is tough. I’m a kinesiology major, not an engineer tho."
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8j3ogr | Why are things like jewelry and precise measuring instruments kept in velvet lined cases? Is it just because it’s fancier, or does the velvet serve a purpose? | Engineering | explainlikeimfive | {
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"text": [
"It is soft. It prevents scratching and movement during transit. Thus it avoids damaging the valuable and often fragile thing inside."
],
"score": [
12
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8j4mb0 | Why are concrete driveways and sidewalks made in squares instead of one giant patch like roads and asphalt driveways? | Engineering | explainlikeimfive | {
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"text": [
"The squares are there to help stop cracks from spreading across the whole slab. If you're talking about the stamped grid marks. Or they are heat expansion joints if you're talking about the individual blocks that are sometimes poured with thin strips of wood in between."
],
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6
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8j7r22 | what exactly is vibrational and chassis tuning when automakers are testing vehicles and concepts? | Engineering | explainlikeimfive | {
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"text": [
"Everything has a resonant frequency, the pitch where the whole structure will vibrate and generally cause problems for people in or near the thing. Bridges sometimes collapse (for example) because the wind or the people walking across it match the resonant frequency and amplify it until the structure fails. In cars, there are certain predictable speeds and engine settings and yadda yadda yadda corresponding to city, highway, neighborhood driving conditions. If those conditions produce vibration in the chassis, it will be heard (and disliked) by the people in the car, and in truly bad cases, it could cause bolts or other pieces to shake loose, creating a hazard on the road."
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8jgc7j | What is the purpose of rubber bands for braces? | Engineering | explainlikeimfive | {
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"I had a variety of orthodontic devices that all served a specific purpose at different stages of correction. After having braces on for a while they added rubber bands from top to bottom to correct an over bite and a little side to side correction. The orthodontist can play with how they're setup to apply the right amount of tension and in the right direction. Over time this moves your lower jaw to line up with the upper half. URL_0",
"Additional question, how do braces like “Invisalign” accomplish the same task as the rubber bands? Or do they just not do that?",
"There’s a couple different types of brackets of braces; traditional brackets and Damon brackets. On traditional brackets, the colored rubber bands are looped in a figure 8 around little prongs on each bracket to hold the wire in place. On Damon brackets, they can be added as a colored decoration if you want, but they don’t serve any real purpose. Unless you’re talking about the rubber bands that realign the over- or under-bite, those are different. But the colored ones are just for looks or they hold the wire to the bracket itself",
"The rubber bands apply a pulling force that slowly moves the teeth to a new position/alignment over a period of months."
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8jtp9f | Why do massive updates to games give barely any extra space to the game when people download the base game (after the update is released)? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"Because most of the update is actually overwriting files that were already there. So instead of adding to the size of the game it stays roughly the same due to files just being replaced with new ones that are similar in size."
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8jwlce | When my turn signal bulb is about to die it starts blinking faster. How does the car know? | Engineering | explainlikeimfive | {
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"text": [
"The resistance in a bulb change when it start to die that also reflect the amount of light is emits. So if you measure the current trough the bulb at a constant voltage you will notice a change when is is close to failure.",
"Usually this happens because your car uses a mechanical heat activated relay to control the blinking (that is when the light is on, current through a bimetallic strip causes the strip to get warm and bend, this disconnect it until the strip cools off). If you pass less current through the strip it doesn't get as warm and can blink faster. So if one bulb is out it will blink faster. This is also the device that makes the clicking noise when your blinkers are one. Newer cars use all solid state stuff, if they do this (I don't know if any do), it's because they have a power meter hooked up to the circuit and emulate the \"feature\" electronically.",
"It doesn't \"know\". The fact that the clicker is going faster means that the bulb is already dead: The dead bulb is an open circuit rather than a closed circuit with some resistance. The change in resistance causes a change in the timing of the electrical circuit that makes the blink/click."
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8jzeu5 | How does a mobile phone connection work | How does my cell phone tell the stream of information that's meant for it apart from all the other streams? Does it work like a radio, where each station has a different frequency? If so, I have a follow up question. There are many more cell phones than radio stations, how do you fit all these different frequencies in a limited band? (unrelated: how do you post a question to ELI5? I had to click the button like fifty times until it decided to work. Yes, I performed a search first, but then what?) | Engineering | explainlikeimfive | {
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"There are some different frequencies, but not enough to all mobile phones. The data is digitalised and encoded. Your phone sends out a connection request. The mobile station picks it up and assigns your phone a digital 'slot'. The details are all very mathematical, but knowing your digital address and slot allows you to ignore all those digital packages not meant for you. With this technology you can connent many phone over the same frequncy and why the more phones connect to a mobile station the lower the connection speed is. There are so many slots, your packages arrive less often."
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8k0k7o | What happens to my plastic bottle after I throw it in the recycling bin? | How long does it take before it is another plastic object for consumer use? | Engineering | explainlikeimfive | {
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"TL;DR: Your plastic bottle will almost always wind up in a bale of sorted \"mixed plastic\" in a few days, maybe a week or so. But it's hard to say how long it will be until that bale is processed into recycled plastic granules/pellets and then used to make consumer goods. Let's start with you putting your plastic bottle in a recycling bin. What happens next? First, the bin gets collected by a waste management company. This may or may not be the same company that collects non-recyclable garbage in any given area, but it's almost always done with trucks dedicated to recycling. The recycling trucks dump their loads at a recovery facility that may or may not be owned by the same company that owns the trucks. These drop offs occur pretty much all day during business hours, but any given recycling bin is typically only collected once a week. Second, the recovery facility sorts incoming \"mixed material\" into different categories using a number of different processes, some automated, some manual. Paper is sorted with an \"air knife\" that blows it onto a different belt while leaving heavier materials behind. Metals are separated with a magnet (yes, even aluminum, though not in the same way that steel cans are). Rotating drums sort out lighter materials (like plastic) from heavier ones (like glass). And \"pickers\" remove things like plastic bags, contaminated cardboard, wet/shredded paper, etc., that tend to jam up the machines. Your plastic bottle will most likely end up in a gigantic container containing almost all plastic, sometimes even sorted by plastic type (depends on the facility). The recovery facility will then compact these plastics (mixed or otherwise) into bales, usually of a size appropriate for moving around with a skid loader. This sorting/baling process can potentially run 24/7, depending on how many different waste management companies are dropping off \"mixed material\" at a particular recovery facility. So, how long does all of the above take? Well, it could be anywhere from a few minutes to an entire week after you put your bottle in the bin before it gets picked up. But once that happens, it'll probably make its way into a sorted \"mixed plastics\" bale in less than 24 hours, as recovery facilities try to process material more-or-less as fast as it comes in. Otherwise they'd wind up with an ever-expanding amount of un-sorted material to store! Recovery facilities sell these bales to plastic recycling companies. Plastic recycling companies take in these bales, sort them (far more carefully than the recovery facility does, and sometimes dividing different kinds of plastic into different production lines), wash them (recovery facility doesn't do that), shred them, and then melt them down to be molded into granules/pellets suitable for use in manufacturing processes. How long does it take for mixed plastic bales to get recycled? Hard to say. Recovered \"mixed plastic\" bales are an industrial commodity. Recovery facilities obviously like to move their inventory as fast as possible, but it could be anywhere from a few days to a few weeks before any particular bale gets sold, for any number of reasons not important here. Once a recycler buys a bale, it will typically process it almost right away. But then there's another indefinite period between producing the recycled plastic granules/pellets and selling those on to a manufacturer. Once a manufacturer buys recycled plastic, it's typically used to make consumer products almost right away. But those two delay periods--between compacting a bale and selling it, and between producing recycled plastic granules/pellets and selling those--make it hard to provide a precise estimate. Again, anywhere from a few days to a few weeks. A few *months* even, if the market in recycled plastic is slow for whatever reason. Shipping takes time too, so add in a day or two for each step.",
"To piggyback on this. I've always wondered if it all goes through some type of decontamination process of some sort....?",
"\" We've all been told that we should recycle plastic bottles and containers. But what actually happens to the plastic if we just throw it away? Emma Bryce traces the life cycles of three different plastic bottles, shedding light on the dangers these disposables present to our world \" [ URL_1 ]( URL_0 )",
"I once met a guy whose dad made a fortune spinning bottles into thread (unsure of the method). The thread ended up in clothing like polar fleeces and such. This was a few years ago so market conditions may have changed."
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8k4lrq | Why is it that even with two different size holes in a pencil sharpener, I still come out with too large of a graphite tip that consistently breaks off? | Engineering | explainlikeimfive | {
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"If your pencil lead breaks constantly no matter how you sharpen it, that probably means the lead is already splintered inside the pencil. This is caused by dropping your pencils or other similar impacts, and the only real fix is to buy another pencil.",
"The two holes are for different diameter pencils. They are supposed to be rather tight to keep the pencil in line with the sharpening bit. Only use the big hole if you can’t fit in the small one.",
"The holes are usually just to hold the pencil centered in the sharpener. The length of the point is determined by the angle of the sharpener, which for most sharpeners is completely unaffected by the holes.",
"Rotating the sharpener around the pencil causes less breakage than turning the pencil. It’s something they teach you to help prevent breaking colored pencils, as they are softer.",
"I haven't got a good explanation but I was taught to turn the sharpener instead of the pencil.",
"The angle of the blade is very much the factor. The two holes likely have the same angle and are simply designed for two different sizes of pencil. You could consider using an X-acto knife to cut your pencils as desired. I would personally just gently dull the end of the pencil on a blank piece of paper so that you wear down the sharp narrow tip before you begin writing, because if the tip breaks off, youre going to have a jagged piece that you may want to use more force on, breaking it further. I would also just recommend a pen."
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8k51nm | Why is coolant (antifreeze) added to a car’s radiator? | Engineering | explainlikeimfive | {
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"text": [
"It's not just anti-freeze, even though that's what we call it. The additives also act as an anti-corrosive, and can make the mixture more thermally conductive than just water. Modern engines are built using a variety of different metals and alloys, and they don't always get along that well when they become electrically conductive with each other."
],
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11
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8k7gge | Why does the air being sucked in behind a fan not feel as strong as the air being blown out from the front? | Engineering | explainlikeimfive | {
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"text": [
"Because the air is being sucked in from all directions around the fan and pushed together into the same direction.",
"Fans blow air in a particular direction, but suck air from everywhere given the chance. Because the air being sucked in is more spread out, it doesn't move as fast and so feels weaker. We can get fans to \"suck strongly\" by restricting the amount of air being sucked in - this is what a vacuum cleaner does. If you find the air outlet on a vacuum cleaner, it will feel much softer than putting your hand over the suction."
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8k9lx7 | How to predict using Machine Learning techniques if someone is depressive by just its twitter posts? | Engineering | explainlikeimfive | {
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"Machine learning, such as deep learning, usually takes a lot of samples and is told \"These are X, these are Y, and these are Z.\" Theses may consist of thousands of samples and the algorithm will try and match certain patterns and give a percentage of how much the piece of data matches X, Y or Z. You know that toddler's game where you put the shapes in a block? The program will try to put the block into a hole and see how closely it fits. Let's say you have a triangle (but you don't know it's a triangle) and you try to put it into a circle. It may go a bit into the hole, depending on the hole's size, but not fit very well, so there is an unlikely chance that is a circle. You then put it into a square. Again, that goes in a little, maybe even goes all the way in, but doesn't fit perfectly. Then you put it into a triangular hole, and it fits snuggly, so you conclude it's most likely a triangle."
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8kgmch | Why is the movement of animatronics so much more fluid and realistic than current humanoid robots? | Examples of realistic animatronic human/humanoid dolls: [Animatronic Head (Greg Townley) - The World's End]( URL_0 ) [First Look at The Na’vi River Journey Shaman | Disney's Animal Kingdom]( URL_2 ) Yet, "realistic" humanoid robots don't seem to be even close to passing the uncanny valley: [Sophia]( URL_1 ) | Engineering | explainlikeimfive | {
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"It comes down to the focus of the project. With your first example, it's a simple animatronic head meant for specific movements which can be focused on (theyve also spent a lot more time on the facial details). While the focus on Sophia is going to be on the artificial intelligence first, and her facial moves and human like actions second. I think that Sophia just isn't to the point in design that theyve pushed for those movements yet."
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8knp8y | If all of Manhattan is basically hollowed out underground for the subway systems, what prevents all the buildings from collapsing in on its self? | Engineering | explainlikeimfive | {
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"Tunnels in cross section are basically arches with the brick or concrete lining acting as the pillars. That leads the stress on the top down to the ground beneath. They will design the tunnels and the routes they take to support the weight above and would avoid hollowing out beneath a building without carefully calculating the stresses they need to account for. Often tunnels will take routes that don't have high loads above, like following the line of a road.",
"It does not collapse because most of it is not tunnels but solid ground. If you look at at map there is no more the 7 lines parallel on Manhattan.The wide train are 3 m wide. lets say that the tunnels are 4 track wide and add 50% for distance between them and the walls. The us 3*4*1.5= 18 m or 126 m for 7 lines. Manhattan is 3km wide at central park. 126m is 4.2% of 300m. That is not basically hollows out. The with estimation of the tunnels are likely a huge overestimation and half that is likely more common. Station are wider but not all over the place.",
"The tunnels (as with all tunnels) have been built so they support at least the exact same (but usually more) weight than the filled in land would have. The Chunnel (i.e. the UK-France Channel Tunnel) is a good example how there can be huge weight over a tunnel and yet not collapse. URL_0",
"The lower floors of the buildings are also hollow. What prevents the upper floors from crushing them?",
"Buildings aren't necessarily just resting on the ground; large buildings are built on top of piles, sunk deeply into bedrock. If you removed everything under the building except the piling, it'd probably be fine."
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8kptv8 | How does this (mechanical) hotel keycard work? All cards have the same size and shape | Engineering | explainlikeimfive | {
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"From the name on it, and the overall shape of the keycard, I think that lock is what's described [in this patent]( URL_0 ). Now I hate trying to read patents to understand their technology, it's like a language all its own, but it seems like there's an electromagnetic mechanism in the lock, and permanent magnets in the card, arranged in a barcode-like way that will move pins in the lock (or not) if the pattern lines up."
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8ksc43 | What does being Turing complete means? | I've seen it's a machine that can solve any computational problem, but can you ELI5 please? | Engineering | explainlikeimfive | {
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"Turing described a minimal, hypothetical computer which he used to mathematically prove results, known as a Turing Machine. It wasn't intended as a practical device, but rather to be as simple as possible to make proofs easier. A computing device that is capable of doing everything that a Turing Machine can is Turing Complete. One way to show that a computer or programming environment is Turing Complete is to implement a Turing Machine emulator.",
"In colloquial usage, the terms \"Turing complete\" or \"Turing equivalent\" are used to mean that any real-world general-purpose computer or computer language can approximately simulate the computational aspects of any other real-world general-purpose computer or computer language.",
"Computer engineer here, A computational device is said to be Turing complete if it can simulate any other Turing machine. In practical terms, Turing completeness specifies a minimum set of instructions that an instruction set architecture must implement, including conditional flow control, arithmetic (addition, multiplication, division), logic (predicate and boolean), and a reasonable (technically arbitrary, but this is not realistic) amount of memory. Many programmable processors, such as DMA processors used to transfer memory between peripheral devices and main memory, are not Turing complete because they do not need to be. They have a specific set of instructions tailored to the task for which they are designed; they can be simulated by any machine that is Turing complete but they cannot perform Turing complete simulation on their own.",
"A Turing machine consists of only a few things. A paper tape, a head that can either read, erase, or write a symbol on that tape, and advance/reverse the tape by one step. It's an impractical computer design, but very easy to reason about and prove properties. It turns out that this extremely limited machine can compute anything that is computable - the mathematician Turing proved this, and so it is a very important mathematical result. Now, your PC is not designed like a Turing machine. For example, its memory is random access - you can read any memory location you want, you don't have to advance a paper tape 10,000,000 positions. However, it is \"turing complete\" - we can prove that it can do everything that the Turing machine can do, which therefore means it can compute anything that is computable. This makes the proof easy. If you show a device can do the few simple things the Turing machine does, it must be a general purpose computing machine. It can be very difficult to proof that it is general purpose otherwise, if you try to take all its capabilities into account. But you don't have to perform that difficult action since this much easier proof is available to you. In summary, it is a way of saying 'we know this machine can compute anything that is computable', and we know from Turing's work what is computable and what isn't. ~~~~"
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8kua6f | What exactly would happen if I opened an Emergency Exit door of an airplane while flying at 30,000 feet? | Engineering | explainlikeimfive | {
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"You wouldnt be able to, because they way its set up on commercial airlines is that you have to pull the door in before you can open it outwards, and the pressure difference from inside the cabin makes it to where the door will not swing inwards with any amount of force one person could use."
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8kvall | How is the container ship cargo weight limit calculated? | Supposedly if they loaded on too much cargo the ship would sink. | Engineering | explainlikeimfive | {
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"It can get very complicated. It boils down to displacement. If I have a boat which weighs 10 tonnes, then it needs to be large enough to take the space of 10 tonnes of water. If that same boat weighs 9 tonnes, it will float a little higher. If it weighs 11 tonnes, it will sink. Now, the bit that gets complicated, is that different water weighs different amounts - sea water has a higher *Specific Gravity* than fresh water, which essentially means it is heavier. So the same size boat is now displacing more tonnes of water, and you can load more tonnes on it."
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8l40wb | if 70 mph is the maximum speed limit in most states, why are car manufacturers allowed to sell cars that can go more than twice that speed? | Engineering | explainlikeimfive | {
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"Cars would be required to be more powerful than this. For example if they need to go up steep hills or carry heavy loads. As well as this, they need to have good acceleration in some occasons. Most people will. Expect to go 0-60 in under 8 seconds. To achieve this you would need an amount of power that would easily get your vehicle going over 90 mph on a flat road.",
"This is a decision you must make as a voter. If you think the tradeoff between safety and freedom is worth it, you lobby your politicians to put this regulation on the books. The direct answer to \"why are car manufacturers allowed to sell cars that can go more than twice that speed?\" is that there's no law that says they can't."
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8l6n71 | Why Don’t Adobe Houses Melt in the Rain? | OK, I’m visiting New Mexico and it occurs to me that the Taos Pueblo, over 1000 years old, is made of mud and straw. And I see lots of adobe houses in Santa Fe. And we all know mud washes away when it rains...so why are these structures still here?! | Engineering | explainlikeimfive | {
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"Live in an adobe house in New Mexico myself and have long studied sustainable building out of personal need. Traditional adobe resists weathering but it does require a smooth coating (rendering) to aid shedding of water and a routine schedule of restoration. The chief thing making adobe bricks resilient is a certain portion of clay (~15%) and sand (50-75%) in the soil. You can't make it from just any topsoil as it typically lacks the clay and has too much organic matter. A small amount of organic material --straw or animal dung-- serves a kind of fiber reinforcement akin to the fiber reinforcement of plastic composites, resisting cracking. The Native American adobe (there are variations in just about every part of the globe) has an additive unique to the continent; the gelatinous juice of the prickly-pear cactus that works as an additional water-resisting stabilizer. This is used in higher concentration for the surface rendering, though generally block and rendering need similar hardness to minimize delamination caused by temperature changes. ('high desert' sees broad temperature swings) The more recent forms of adobe used in the southwest have used asphalt and Portland cement mixed with the soil. Other very different materials are also used, such as formed soil-cement (a mixture of earth and cement poured into forms like conventional concrete) and pumice block or pumice-crete. (a mix of granular pumice and cement again poured into forms like concrete) In the 1950s mechanically Compressed Earth Block produced in machines called 'Cinva Rams' emerged as a popular building method for the developing world. It has sometimes been used as an alternative to traditional adobe blocks here, but is not as well known in the US as in the rest of the world. Straw bale construction is another common alternative and, because of the use of the same adobe surface rendering, can look much like the traditional adobe block housing, but usually with much thicker walls. Sadly, a great deal of the 'pueblo style' architecture you see in the southwest is not really adobe at all but conventional stick-frame construction using sculpted foam to simulate the smooth adobe forms and covered in a dyed plaster. It's often called 'Santa Fake Style' here.",
"How often do you figure it rains in the desert? Adobe is very hard, but it does erode a little in the rain and need to be repaired every so often.",
"Mostly because the roofs are made of a more waterproof material, and they overhang beyond the edges of the walls. So the adobe material doesn't get a lot of direct rain impact. Modern adobe often includes a bit of cement, which increases its durability when wet."
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8l7fhi | How do we know how much weight a crane can lift? | Engineering | explainlikeimfive | {
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"Math and physics. You can calculate the weight of all the pieces, the torque applied, the breaking strength of all the pieces, and all the other variables, and come up with a number. It should be noted that most engineering things are either done with a 5:1 or 10:1 safety factor, so a crane can probably actually lift 5-10 times its rated weight. That extra gap is to account for uncalculated items, mistakes in calculations and measurements, changing conditions, operator error, etc.",
"With cranes this tends to have less to do with the maximum force that would cause the boom to buckle or the cable to fail catastrophically. It tends to have more to do with the load that will cause it to **tip over** at a given radius from the crane's center of gravity. It is critical in most crane operations to both know the weight of the payload, and the lifting radius. Calculating the maximum safe load for a crane at a given radius and size of counterweight, is a matter of first year college physics. The boom itself is always deliberately overengineered with a *safety factor* of 3x-10x. Generally, the crane's winch system is designed to reach it's limit and stall out well before the boom would be in danger of collapse. Most modern cranes have several safety systems and warning indicators to prevent tip-overs, but most cranes are also capable of easily hauling themselves over if such systems did not exist, or were ignored by the operator.",
"I know I'm late to your question, but I want to give it a shot from a different perspective. The other answers seem to have interpreted the question from a design perspective, while I interpreted your question more along the lines of, \"How do we know a 200 ton crane can really pick up 200 tons?\" First, a load chart for a 200 ton crane looks something like [this]( URL_0 ). The only point at which it can actually lift 200 tons is with the shortest boom and at the smallest radius, meaning the boom is raised up as straight up as the crane will allow. Now if you look at the chart you will notice that there are asterisks by the numbers at the top. Those asterisks note that the capacity for those radii and boom lengths are based upon the strength of the crane. This is where if you overload the crane, something is very likely to break. These limits are based on calculations of how much stress the materials can handle. The components of a crane are not designed to be able to lift 5 times what the crane will be used for. Under no circumstances will a 200 ton crane's boom be able to lift 1,000,000 tons. It would be far too expensive for the manufacturer and would weigh so much that the rest of the crane would have to be overbuilt to match. That would be impractical. Then there are the numbers lower on the chart that do not have asterisks. These capacities are based on the leverage the crane has against the load. Overloading the crane at these points will likely tip the crane over, rather than just breaking it. These capacities are calculated by looking at things such as how much the crane weighs, including how the counterweights are configured. Also, different cranes have different types of stabilizers, so you have to look at how those affect leverage. All of those things are controlled and tested by the manufacturer of the crane. The numbers without asterisks on the chart I showed earlier reflect 75% of the tested maximum tipping load the crane can handle. The capacities on that chart are never to be exceeded unless a capacity test has to be performed on the crane, but inevitably some people do try to use the safety factor to their advantage. Once the crane arrives at the job, environmental factors play a role in the lift, the most important of which is ground compaction. This answer has gone on long enough, so... **TL;DR - Crane capacities are determined by strength of materials as well as the leverage they use to lift their loads. There is room for error, but not as much as some would think. Even when lifting loads that are within the limits of the load chart, environmental factors that are neglected can turn a lift into a disaster.** Edit: Clarity and bad wording"
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8lez76 | the Classes of ships and subs | So I've seen things like Ohio class destroyer. So Like is there a real scheme to these names? | Engineering | explainlikeimfive | {
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"The classes are real things, yes. The class names generally come about in one of two fashions, either matching the class leader (usually the first ship of that class produced, for example in the case of the Pennsylvania-class battleships and the USS Pennsylvania), or they are named based on the theme the ships in that class are named after (for example the Royal Navy's Tribal-class frigates that are named after the various tribes of the world).",
"If there's a ship with a new enough design from others of its type (carriers, destroyers, submarines & c) then it's generally considered to belong to a new class. The modern US Navy has *Nimitz*-class aircraft carriers and *Virginia*-class submarines, for instance, and for a fictional example the *Enterprise* on the later *Star Trek* series is part of the *Galaxy* class, and the ship from the original series was a *Constitution*-class ship.",
"Just to expand a bit. The other other explain it for western ships. For Soviet, Russian, Chinese and likely some other countries [NATO reporting name]( URL_0 ) They exist because the original names was not known when they are first observed and names in the local languages are hard to use if the common language is English. They are chooses so they are clear and distinct and the initial letter often designate the type like S for surface-to-surface weapons, F is a fighter aircraft For some reason that is not followed for ships and submarines. So a soviet Alfa-class submarine was project 705 known as Лира or Lyre in english. The Russian names are seldom used so everyone in the west think of it as a Alfa-class All types don't have a name som are just the project number a bit like the Primary US army firearm is the M4 Carbine and that name is not a name but mean that is was the 4th carbine accepted into service since that system started to be used.",
"> So Like is there a real scheme to these names? There is no real rhyme or reason to the naming of ship classes if that is what you are asking. Each country has its own method for naming ships and conventions may change. However, the first ship of a class typically carries the name of the class. For example, the USS Nimitz is the first ship of the Nimitz class aircraft carriers, and the USS Ohio is the first ship of the Ohio class submarines. It is extremely ineffective to conceptualise, plan, design, construct, and maintain a single bespoke ship. Economies of scale come into heavy play in ship building, so it's most cost effective to build multiple ships all based on the same design and the same core components, while incorporating revisions, learning opportunities, and new technologies into newer ships of the same class. All of the Nimitz class aircraft carriers have minor differences that a result of their being built at a rate of one every 5 or so years with a scheduled mid-life overhaul. There are 8 or 9 different designs for the Arleigh-Burke class destroyer organised into 3 major revisions. The first keel of the first revision was laid down in late 1988, while the first keel of the third revision has not yet been laid down at all."
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8lkkje | why do steps have plateau after every 1 to 12 steps?. Eg in train stations. | It is a common architectural design seen in most of the big buildings. Is to reduce leg fatigue? | Engineering | explainlikeimfive | {
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"Giving climbers a place to rest can certainly be one of the reasons (and is part of the reason why they are called \"landings\"). They also are required to create most stairs that change directions, which lets builders fit staircases into smaller areas. That said the key reason you see them commonly is to work as something to catch you if you fall down the stairs, so you only fall to the next landing rather than all of the way down. For this reason most building codes limit how long staircases can go before they are required to have a landing.",
"A few different reasons. Muscle fatigue is one reason. Especially in buildings that may host young kids or elderly that may have more difficultly with a continuous set of steps. Safety can be another, as things/people that for whatever reason fall while climbing will likely stop upon reaching the longer platform. Finally, in some cases, the incline is not too steep, so they design in the longer platforms to better match the terrain.",
"Because of design standards: URL_0 Note that the full title of \"Approval Document K\" is \"Approved document K protection from falling, collision and impact\" which gives us a clue. :-)",
"back when i injured both ankles in a fall, i lived for those plateaus when i was basically relearning to walk.(stairs are terrifying when you haven't walked in a few months). i'd imagine its for people with disabilities that make it hard for them to go up and down stairs.",
"Legally, it's building code. You have to have landings every so often depending on how many vertical feet the stair is covering. This applies to ramps as well. But the reason for the code has been described by others - it's a place for someone to safely stop and rest between flights. You can picture how this would be a great help to the elderly (you'd also appreciate it if you've ever had to walk with crutches). If you're on a ramp, it's even more important to have a place to stop a wheelchair safely without rolling downhill."
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8lnosy | Why are there so many sink holes in Florida? | I'm seeing on the news more and more about growing concerns about sink holes in Florida. I rarely hear about them in other areas. So the question is why? | Engineering | explainlikeimfive | {
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"Much of Florida is built on limestone. Add in warm temperatures and heavy rainfall, and a complex underground network of caves and rivers forms over the years. Water falls as rain, sinks into this system, and after an average of about 20,000 years it comes bubbling up again in a spring, like [this one]( URL_0 ). Some people enjoy scuba diving into the underground caves. They see some spectacular views, but it has about the highest fatality rate of any hobby. A sinkhole is just the opposite of a spring - it's where the water leaves the surface and enters the underground caves. Of course, they seem to appear suddenly, but anywhere built on this kind of geology might have a water-filled cave just below the surface, that has been slowly growing for millions of years.",
"They do happen in many other areas. But as has been mentioned previously, acidic water dissolves limestone and creates sinkholes. You may be hearing about it more because of increased development and population growth in Florida. If there are no roads and buildings affected who really cares? It's kind of like the increased damage hurricanes cause now vs. 50 years ago. Well 50 years ago, the population and development in Florida wasn't anything near what it is today. 10,000 years ago hurricanes were still hitting Florida. It's the ill planned development and population density that make the storms more destructive."
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8loqyf | How do hypodermic needles penetrate your skin without punching out a "core" of flesh? | maybe this is really a physics question in disguise, but I wanna know. Having a core punched out of your skin and subsequently injected into your blood stream sounds problematic. | Engineering | explainlikeimfive | {
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"They are cut on a slant, so they slice a slit instead of cutting a core.",
"You make a tiny hole, and then spread it open -- you don't make one big circular \"core\" all at once. How?: The needle tip is tapered and comes to a sharp point. So the tip (which is solid) is what makes the initial puncture, and the hole is simply expanded as you push the rest of the needle tip in. Edit: A picture would probably help -- Google \"bevel\"...",
"Well, for one, because the edges of the hole at the end of the needle are not sharpened. Imagine a 2\" thick pipe with 1/2\" thick walls that someone neatly sawed through at a 60 degree angle - that's (roughly speaking) what the end of a needle looks like. The edges are flat, they're not great for biting into things. The end of a needle is a pointy wedge with a hole in its side, it pushes tissue out of the way. What you're describing is basically a tissue punch, and they have to be specially designed for the job - they're usually very short, so they don't bend, and treated so tissue doesn't stick to them. And they're very expensive, compared to needles. Like, a disposable punch might be several dollars, while a pack of 100 needles might be 10 bucks."
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8lp7au | How do traffic engineers determine the minimum angle a road can turn at safely? | e.g. a 30 mph residential street is going to have much sharper corners to turn on than an 80 mph highway. How is the angle of that turn decided? - or perhaps angle isn't the right word here, I'm not sure - how is the *sharpness* of that turn decided? I assume there's probably an equation of some sort and they don't just keep running cars down it until they don't crash. But then I guess it's also affected by things like the presence of a median? | Engineering | explainlikeimfive | {
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"There is the American Association of State Highway and Transportation Officials than publishes a book, \"A Policy on Geometric Design of Highways and Streets.\" This book, often referred to as 'The Green Book\" is adopted by most localities when specifying construction of roads. The book specifies the exact geometries of common roadway scenarios.",
"You can find the centripital force acting on an object going a certain speed on a curve of a specific radial length (F=mv^2 /r). If the friction of the tires of a car against a road can't match that force, it'll start to slide away from the center of the curve. Edit: Also, there are also formulas that involve moments to find the tipping points of trucks. If the centripetal force is too strong, then it'll tip the truck."
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8lqd6c | - What is the concept behind car manufacturers not standardizing which side the fuel cap is located? | Engineering | explainlikeimfive | {
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"Car manufacturers have various reasons for where they are placed. Some to do with safety (away from road in case you need to pour gas can into an empty tank), some to do with engineering (it's easier to put fuel cap near the release lever by driver). Different countries drive on different sides of the road, so the auto manufacturer will make the decision based on their biggest market. If you are annoyed by not being able to remember where it is, most cars have an arrow next to the fuel pump icon (on the fuel guage) that points to the side with the fuel cap."
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8ly7cr | How does turning the volume up or down on speakers actually work? | Engineering | explainlikeimfive | {
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"Actual engineer who worked on audio circuits here. Audio (sounds) is caused by differences in air pressure, we call that sound waves (imagine waves in a pond). When a speaker receives voltage, the diafragm of the speaker moves according to that voltage. The amplitude of the speaker movement is controlled by the voltage it receives, the higher the voltage the more it moves. When you turn up the volume, your device increases the voltage it uses for the audio signal thus making the speaker louder. The actual circuit used to increase or decrease the voltage the speaker receives is called amplifier and it's function is literally to multiply the input signal by a factor. Your device determines that factor using the volume controls. Doesn't matter if it is digital control or analog control the result is the same, more voltage to the speaker. Digital gain is similar but instead of having an analog amplifier the multiplication is done on the digital domain where each sample is scaled by the gain factor. Edit: Autocorrect gore",
"It depends on how complex the circuit is, but in a simple setup the knob is attached to an electrical device called a [potentiometer]( URL_0 ) which is basically a resistor that changes resistance as you turn it. When there is more resistance in the circuit, the volume of the speakers is lower."
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8lygfn | How do investigators figure out the root cause of an accident (plane crash, fire, structural failure) by examining the wreckage? | If the cause of an accident isn't picked up by sensor logs, black box, etc, how is the cause determined? I've seen images of all the little bits of a wreckage being collected and laid out for examination - but how is the root cause determined? For example a plane crashes because an engine fails. In the crash, the engine is mangled, along with the rest of the plane. How does one determine the difference in damage caused by the initial failure vs the damage caused by the crash? Am I just underestimating the meticulousness that goes into collecting and examining stuff like this? | Engineering | explainlikeimfive | {
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"This question took me a masters degree to start understanding. The investigators are a team of specialists, many with decades of experience, training and education. In my case I am a welder, aircraft mechanic, avionics technician, mechanical engineer and hold a masters in systems safety, with heavy emphasis and specialization in accident investigation and reconstruction. In my area of expertise, airframe structures, we look for anything out of place. As metals fatigue and fail certain telltale signs show up: cracking of paint in specific patterns; discoloration and distortion of bare metal; the shape, condition and color of a bolt that broke clean from hard impact is different than that of a bolt that failed due to repeated bending (fracture vs yeald). Burn marks and soot patterns from fire vary based on the speed and altitude, fuel feeding the fire (oil, gas, rubber, cloth, electrical wire insulation, etc), oxidizer present and dozens of other factors. There is no easy way to explain it. The key is really knowing the aircraft, how it works, what the components look like new, after normal wear, and understanding the potential failure modes of all of them. Knowing flight and ground procedures, rules and regulations for flight crews, passengers and cargo. A lot of research, testing and consulting with people much smarter than myself goes into narrowing it down to the most likely cause, which, more often than not, is human error at some point leading up to an incident.",
"> Am I just underestimating the meticulousness that goes into collecting and examining stuff like this? yes > In the crash, the engine is mangled, along with the rest of the plane. How does one determine the difference in damage caused by the initial failure vs the damage caused by the crash? For this specific example various failure modes have been tested, and when the plane crashes into the ground some parts are harder and more sturdy that others. If a part that is super hard but has a low melting point and normally gets somewhat bent during a crash gets super warped out of shape, its probable that the part was melted or weaked by heat , indicating an engine fire Basically while yes parts get mangled , different parts get managled in different ways from different causes so they can differentate between damage from ground impact vs fire vs bird strike in the air , etc Its also worth noting that even if sensors fail *which* sensors fail in *what order* can also tell a story of its own, helping tracking the origin point and how the issue spread, and its extremely rare for all sensors to fail at the same time so tis data is always quite useful"
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8lypke | As Hawaii get's more and more covered with molten rock how does cleanup even start? | Engineering | explainlikeimfive | {
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"The media and reddit is blowing this way out of proportion. [Here's a map]( URL_0 ). Cleanup isn't really a big problem. The lava flow is not nearly as widespread as media outlets would have you believe."
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8m3iat | What decides the maximum altitude a plane can fly? | Engineering | explainlikeimfive | {
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"Depends on the plane. If the cabin is unpressurized, you can't effectively fly higher than ~40,000' before your pilot is guaranteed unconscious. Also, as the air gets thinner, the stall speed (the speed at which the wings no longer provide lift) gets higher. At the same time, the reduced oxygen means the maximum thrust the engines can develop drops. Roughly, maximum altitude is where those two curves intersect. Where that altitude is depends entirely on the specific plane in question: body size and shape, wing geometry, engine layout, gross weight, etc.",
"It is a combination of lift (there needs to be enough air density to keep the aircraft up), speed (moving faster through less dense air can provide more lift), and structural limits of the engine. Moving very fast in order to provide sufficient lift from the wings and enough air for the engine to function means the air must be compressed very much in order to feed the engine. That compression heats the air which only gets hotter during combustion of the fuel, potentially melting the engine components. So the limit for a particular aircraft might be defined by any of several factors. For example an aircraft might lack wing surface or speed depending on how you look at it, as more of either would allow it to go higher."
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8m6314 | Why do rockets not use the advantage of aerodynamic lift until they reach a certain altitude? | When looking at rockets I always wonder why they immediately start straight upwards, instead of launching like a plane until they reach an altitude that doesn't allow aerodynamic lift anymore. It always seemed more sensible to me to build the lower stage of a rocket with big wings and only the upper stage is a "classic" rocket. Is it a weight issue? | Engineering | explainlikeimfive | {
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"Up isn't the problem, very little of the rocket's energy is wasted going up. Getting to space isn't the hard part, its staying in space that's the challenge and that requires going *FAST*. A typical rocket will climb out of the atmosphere(~100 km) in under 2 minutes, in that time it will have lost ~1.1 km/s due to gravity. In order to achieve orbit it needs to hit 8 km/s so only about 12% of the total delta V is lost to gravity and getting it out of the atmosphere, and if you gave it wings it may have to carry those beyond their useful limit which would significantly reduce performance due to their added weight Rockets are actually really good at what they do. A space plane that can flip from a jet engine to a rocket engine and use wings to get it up high is the dream, but if you're stuck with just rocket engines then you want to get up and out of the atmosphere as fast as possible",
"Some rockets **are** made like that. Look up Virgin Galactic. Additionally, some (very small) satellites get to orbit on board a modified air-to-air missile fired from a fighter jet.",
"Wings add weight and drag. Both of these are bad when you're attempting to move so fast that air will literally disintegrate you."
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8m84pc | How do "Heavy Doors" work? | How do "Heavy Doors" close on their own? | Engineering | explainlikeimfive | {
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"If you're talking fire doors in hotels and stuff, those big doors that split a corridor into shorter lengths that are always open? If so, they normally are sprung; they want to close, they're just held open by an magnetic or electromechanical catch, so if the fire alarm is tripped or there's a power cut, the doors will be released and they'll all close automatically."
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8malfq | How do buildings under construction not get all warped and structurally unsound during heavy rain? | Engineering | explainlikeimfive | {
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"As a carpenter, wood is actually much studier than you'd think. Obviously you don't want to expose the raw materials to the elements for a long period of time (over a week, give or take) but wood is fairly resistant. As for the heavy winds, we use \"wall braces\" to keep the walls in place. Additionally, walls are designed to support each other. Ideally you would plan to have at least two walls perpendicular to each other by the end of the day. As the final step,you have plywood which \"ties\" the wall together. It is next to inmpossible to knock over a wall that has been properly built and sheathed (had plywood put on) The main components of a house you would really want to shelter is the insulation (not just the actual \"insulation\" but anything inside the walls, including drywall\" but by this point you would likely have roof rafters up and some plywood to protect the inside.",
"Sensitive parts of construction are done quickly and with planning around weather. A house for example could be easily framed, wrapped, and have a roof on it in under a week. The time consuming part of home construction is getting approvals and the interior work done.",
"The rest of these guys are clueless. Wood framing is built with what is called \"green\" wood. That means that it has a very high moisture content, sometimes it is sopping wet. Go to a lumber yard their framing lumber is sitting outside. What causes most warping is differential drying, wood shrinks when it dries if one side is exposed to the air and dries while the other is against a surface and not nailed down it will shrink at different rates and bow, twist and warp. Plywood is so filled with glue that it does not absorb much water. When a framed building is in place most pieces of wood are nailed in place and are subject to similar amounts of drying on all sides of the lumber.",
"I did some work in a condo building under construction, and it rained heavily for a week before the roof was finished. Nothing warped, but there was a lot of interior flooding, and the chimney stack (which gets drywalled from the inside for fire code) had to be rebuilt, and lots had to be repaired from water damage and mould."
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8mc9tf | How does a metal detector not detect itself? | Engineering | explainlikeimfive | {
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"The \"sensor\" part of a metal detector is made of coils of wire, usually in a plastic of epoxy shell. Metal detectors don't detect metal directly. They detect changes is magnetic permeability. They send out a magnetic signal and measure how the surroundings effect it. Sure, many parts like the arm are metal, but they don't move relative to the sensor. You use the detector by sweeping it from side to side. If there is a metal object, the magnetic permeability is different and when you sweep the sensor through the space around the metal object, it beeps. Only change in metal-i-ness around the sensor makes it beep, and the metal arm doesn't change.",
"Metal detectors don't detect metal so much as they detect *change* in the amount of metal around them. All alone, the metal in the detector serves as a baseline that goes up as it gets near other metal sources.",
"A metal detector puts out a magnetic field and looks for disturbances in it caused by the presence of other metals. It doesn't detect itself because it is constructed to recognize its own field.",
"A metal detector doesn't detect itself in the same way a scale doesn't weigh itself. It *does*, really, but it's calibrated in a way that makes that measurement the zero point.",
"The detector coil is part of a tuned circuit that can induce an electrical current into any metal around the coil assembly. The amount of current induced is determined by the conductivity of the metal and it affects the tuning of the coil detector circuit. That change of frequency is converted into an audible signal such that the tone heard is changed by different metals, so an experienced detectorist can tell the difference between rusty iron and gold, for example."
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8mjz9t | Why do piano notes keep playing when you hold the key down? | Surely if the sound just comes from the hammers hitting the strings and causing them to vibrate, the note would last the same amount of time if it was held down or not? | Engineering | explainlikeimfive | {
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"There are stoppers that mute the string and stop the vibrations when you lift the key. There is a ~~petal~~ pedal on pianos that remove all the stoppers which will allow for the string to keep vibrating even when you lift off the key. They will continue until you release the ~~petal~~ pedal OR the string stops on its own.",
"Because a tiny hammer covered in felt hits a string when you press the key. It's not too different from what happens when you thwap a guitar string with your thumb quickly. The string keeps vibrating and making sound while you hold down the piano key. When you stop pressing the key, the little hammer and some other felt pads sit back down on the string and stop it from vibrating. This way the string does not make any sound. There is even a way to let the strings keep making sound after you stop pressing the pano keys. There are usually two or three foot pedals on a piano. When you push one of the pedals down with your foot, it lifts up all of the little felt pads that were keeping the strings quiet. Since pushing down the pedal lifts up all the hammers and pads, the strings inside the piano still ring out, even after you stop pressing the piano keys. I recommend looking up photos of the inside of pianos to get a better idea of how they are put together. (Edited for clarity)"
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8mp4kl | Why are highways made using small blocks of concrete one next to eachother, and why aren't all roads made like this? | Engineering | explainlikeimfive | {
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"Concrete roads always have little gaps in them. That is necessary because the road will contract and expand depending on how hot it is, and a solid concrete surface would crack due to this. Asphalt roads don't need that, since the material is a little more flexible. Concrete isn't used for all roads because the kind of concrete that lasts long is expensive. That's why it's usually used on roads which have a lot of traffic, where using a long lasting material is worth the investment."
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8ms5td | how does a PIT maneuver work, and why is it an effective way of stopping a vehicle? | Engineering | explainlikeimfive | {
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"There are two important aspects to a PIT maneuver 1. A car tire that is rolling has significantly more traction than one that has started sliding. 2. The traction from the rear wheels keep the car going in a straight line The goal of the PIT maneuver is to push the back end of the target vehicle to the side and start the rear wheels sliding, once they start sliding they lose their traction and don't keep the car going in a straight line anymore and hopefully cause the car to spin out."
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8msfvg | Why does a car steer with its front wheels, while an aeroplane steer with its tail rudder? | Engineering | explainlikeimfive | {
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"Cars turn in two dimensions. The road 100% controls the up/down dimension (unless you're driving a rally car or otherwise aren't constantly in contact with pavement). Planes turn in three dimensions. If a car points to the side, unless it's on ice, it will start traveling the new direction. If a plane points to the side it won't immediately start moving that way. Much like a car on ice, a plane trying to turn \"flat\" will slide sideways. Planes turn by banking. It's more like turning a bike, except a bike still has the road to control up and down. If you bank a bike you'll turn that way. If you simply bank a plane you'll sort of turn that way and sort of slide downward that way. The rudder turns the half turn half slide into a controlled banked turn.",
"Cars steer with the front wheels, because the engine is usually in the front, and its weight makes for better traction. Also, it is much easier to turn the front and have the rear wheels follow. An airplane doesn't really steer with its rudder, most of the turning is done with the ailerons on the wings. The rudder is more about keeping the airplane aligned with the direction of travel.",
"Tails create a lot of drag and if you where to position a control surface Infront of the aircraft engine you would end up with an incredibly unstable plane. It'd be like oversteer on steroids. Imagine trying to keep a pencil oriented vertically, you try two methods. One is you tie string around the tip and tie the string around your finger, it works well and the pencil is very easy to keep vertical like that. Next you try balancing the pencil vertically on top of your finger, it's incredibly hard to keep it there. It's essentially the same as that for aircraft, but your finger represents the planes engine. Cars can do it because they have stabilisation at the back as well. If you look at something like a hovercraft and how hard they are to control you get an idea of what the issue is."
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8mtxes | Why do cars sometimes ‘snap’ ‘crackle’ and ‘pop’ after they’ve been driven? | Engineering | explainlikeimfive | {
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"Metal expands as it heats and contracts as it cools. You're hearing the engine contract, cool, and go back to its cold shape.",
"If you referring to that like tinging noise after you turn the engine off then it just the metal parts mainly exhaust starting to cool down causing the metal to contract and as it contracts it makes those pining noises."
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8mvo44 | If noise-canceling headphones emit sound waves to counter the outside noise, why do the emitted sound waves not themselves harm your hearing? | If the sound waves must be loud enough to match the outside, regardless of that they appear to cancel the noise, why are these sounds not considered harmful? | Engineering | explainlikeimfive | {
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"For the same reason that they cancel the sound. The way noise-cancelling headphones work is that they cancel the sound *before* it reaches your eardrum. So your eardrum doesn't get vibrated, and you don't hear a sound. Since there's no vibration of your eardrum, there's no harm. Of course, in reality, noise cancelling headphones don't work quite that perfectly, which is one of the reasons that you don't use them to protect your hearing. The other reason is that they really only work well for repetitive sounds in certain frequency ranges. They are best for ambient noise like the sounds of an airplane.",
"There seems to be misconception about active noise cancellation. Noise transmission is viewed in source-path-receiver model. If you're in a room and a sound wave at 400 Hz is directed at you at 0 degree, and 80 dBA, you will hear it. But if another source is emitting sound at 400 Hz that reaches your ear at exactly 180 degrees and 80 dBA, then they will cancel each other. It's called destructive interference. The magnitude at 400 Hz will be negligible at your ear entrance. Your ears will not get damaged in any way. This principle is applied in head phones, the error mics capture the magnitude and phase of external noise and the speakers emit sound which is 180 deg out of phase for given range of frequencies. The net effect is negligible noise and they are actually safer for your ears. Why they aren't used at construction sites or other work sites 1. Expensive 2. They can't cancel impulsive noises Noise cancellation works only for steady state noise (aeroplane engine at constant speed) The cancellation algorithm takes time to adapt to the external noise and hence they can't cancel anything that changes in an instant for eg. Speech. This where passive sound absorption is used. PS. I'm an acoustic engineer",
"Noise-canceling isn't about drowning out the external noise with louder sounds - it's about generated a phase-shifted version of that noise that precisely cancels it out.",
"They are harmful. Active noise cancelling headphones are not safety gear. They are for convenience. Hearing protection uses passive noise blocking. Electronic ear-pro has external microphones that allow only low-volume sounds through."
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8n0k2o | How do scopes work? (firearms) | Might should like a stupid question but I guess that’s partly what this subreddit is for. How do scopes accurately portray where the gun is targeting despite being situated above the barrel? Edit : Thank you for all your responses! | Engineering | explainlikeimfive | {
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"The scopes reticle/crosshairs arnt parrell with the barrel, it aim downwards slightly and meet the travel of the bullet at certain distances. So if you zero your scope for 200 meters. The bullet will travel lower than the point your aiming at untill it hits 200 meters, then it will travel higher untill it drops back below. Here's an image of this. URL_0",
"[Copied from another post about this]( URL_1 ) The scope is set to only be precise at a set distance. You have to adjust it for different distances or expect the misalignment. [Another, but basically the same picture ]( URL_0 )",
"The focus point can be adjusted on a scope depending upon the distance that you are attempting to shoot, so that the line from the scope and the line from the barrel crosses at the target.",
"Short answer: They don't. Slightly longer answer: Scopes are \"zeroed\" to particular ranges. This means that the scope is angled slightly down so that when the reticule (The image inside the scope) is pointed at a target a particular range the firearm will be angled upwards just enough to arc the bullet the correct range. This is why in some scopes you see multiple horizontal lines, each of those lines is zeroed to a particular range so a shooter just needs to align the appropriate line with the appropriate range and the shot should be angled correctly for that range. Have a look at zeroing on bow sights, because of the lower projectile speed the sight needs to be more dramatically angled so it is a bit easier to understand, with guns the adjustments are quite small so it's hard to see.",
"The sights are not parallels to the barrel because the bullet don't fly in a straight line but drops because of gravity. So the barrel point a bit up. It looks a bit like [this]( URL_0 ) So you have to zero the sights at a distance. So rotate the optics or move the reticle so it intersects with the path of the bullet at that distance. So for longer distances you have to set the range and change the sights so the barrel point more and more up to hit the target. If you fire at a targer closer then the set distance the bullet will be below in the beginning and above the sight for most of the distance. But for a rifle at distances of 100-200 yards the difference is not that high so you will hit the target if it is closer to you. a 5.56 round have a path that is 1 inch from flat and 150 yards. So a sight set to 100 or 200 yards will result in a hit at closer distance At longer distances you might start to miss it and have to set the sight as a the same bullet at 500 m start to drop closer to 30 inches at that distance. So the offset of the sight is not a problem. That is except for the just after the barrel. If you lay on the ground it is possible that there is a rock in front of the barrel that is not visible in the sights but it is easy to see if you look outside the optics. It is more of a problem if the distance between the sight and barrel is large like in a tank. If you have a hull down position and hide behind a hill and expose as little as possible you have to make sure that the barrel is above the hill to. Early US M4 tanks in WWII only had a sight on the roof so it was hard to determine if they was blocked by the ground so a sight behind the barrel was added in part for the reson to make sure the barrel is clear from the ground."
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8n2nnm | Why is it my phone is cold while my laptop have to use all its fans cooling while doing the same simple tasks, browsing reddit, streaming music etc.? | Engineering | explainlikeimfive | {
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"Phones use less powerful components that are significantly smaller, generating less heat. Phones will also get hot overtime though just like computers, you just can’t hear the cooling systems in the phone like you can with the computer."
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8n32eo | What's the difference between oil types (5w20, 10w40, etc and also synthetic) for everyday vehicles? | I say everyday vehicles because I assume front loaders, cranes, so on to be on a totally different type of oil altogether.. or am I wrong? Thanks | Engineering | explainlikeimfive | {
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"Those numbers refer to the viscosity grade of the oil. Like many fluids, oil flows differently at different temperatures The first number refers to the viscosity of the oil when cold (the W stands for \"winter\") and the second number is the viscosity at 100 degrees C. I do happen to know that although it sounds like WD-40 refers to something similar, that is just a trade name referring to the 40th formulation of the \"water displacement\" oil.",
"The first number is how thick the oil is when it's cold. The second number is for the thickness of the oil when hot. Some engines require a certain oil because of stress tolerances of the engine. Synthetic oil is engineered to resist breaking down and making sludge, but is expensive. Conventional oil breaks down faster, but is cheaper. Also, you'd be right in assuming different engines take different oil. The biggest differences being how it is used."
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8n3bda | How do they get the model ship inside the bottle? | Engineering | explainlikeimfive | {
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"The individual pieces that make up the ship all fit through the mouth of the bottle. The builders use specialized tools to build the ship inside the bottle. It's a technical challenge that those with the hobby enjoy. You can probably search to find videos of people assembling them."
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8n53ua | What would have happened if the Deepwater Horizon/ Macondo Well blowout could not have been stopped? | I have been reading a lot about the Macondo Well blowout and the Deepwater Horizon tragedy. In many of the articles one of the major risks in capping the well was the possibility of a “subsurface blowout” that would fracture the earth and allow the entire 110 million gallons of crude in the formation to contaminate the ocean. My question here is what would the environmental impact of such a catastrophe be on the gulf and the rest of the world? | Engineering | explainlikeimfive | {
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"Fishing in the Gulf and the Caribbean would be stopped for years as the US and other countries worked full force to clean this shit up. Maybe we would even see the development of some kind of new crude oil eating bacteria or something as a necessity to get it cleaned up as fast as possible. The economies of many places, from carribean islands, to coastal towns would be devastated from the impact on commercial fishing and tourism. Maybe these people would find temporary work helping to clean up until everything got back to normal. There would be a lot of health issues. People might die or certainly get cancer and stuff like that, these could last for many years to come as people eat contaminated seafood. On the positive side, we would hopefully see nations and people come together to help clean the disaster, we might see cool new oil cleaning tech that we don't have in our current timeline, and we might also see new discussion on the ethics of drilling for oil in the ocean, or new safer methods of doing it might come out of such a disaster"
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8n5bif | The ground wire in audio cables. | I've read several articles and posts now on the inside of audio cables, and I learned a lot, but I still can't wrap my mind around the ground wire. I'm not trying to understand ground loops just yet – just what the ground wire really is. I know some simple explanations for what it does, sort of, but I'm still left confused. What I know: * Unbalanced cable = one signal wire \+ ground wire * Balanced cable = hot signal wire \+ cold signal wire \+ ground wire * signal is sent twice \(one flipped\) over the signal wires, flipped again to cancel out the noise. This is just using the hot and cold wires, correct? it's the gear on each end that does the polarity inversions, not the ground wire? * \(I don't really understand why it's called "hot" and cold". I know the waveforms are opposite polarities on each wire\) * The ground wire somehow **reduces the amount of noise** entering the cable's signal from radio frequencies in the air. \(not sure how that happens though\) * The ground wire somehow provides **safety** from electrical shock in electrical wires \(audio wires too?\) * The ground wire somehow provides some kind of **"reference"** to 0V – I get it, but I really don't get it. Some aspects I know I don't understand: * Is this the same thing as the ground in **electrical wires**? Does it work the same? \(I know analog audio cables are transferring electricity too, but clearly there is some difference\) * Why is it **called "ground"** or "earth"? I read that with electrical wires, there's a metal rod that sticks into the earth somewhere...? Without totally understanding the details behind that, intuition tells me that's not the case with audio wires – they just go from one piece of audio equipment/instrument to another. Correct? * I was looking at [these diagrams]( URL_0 ) to understand how things are actually connected inside cables \(and what happens when you just wire up one type of connection to another\). Is the green line they call **"Shield"** the same thing as the ground wire? Why is it drawn into the side of the cable wrapping? People say balanced cables have 3 wires, but is the "ground wire" actually more of **a wrappi**ng rather than a wire? * I can't wrap my mind around **electrical circuits** – is that a pre\-requisite to understanding the ground wire \(and eventually ground loops\)? Sorry for the long post. I've seen the posts for electrical ground – I tried to explain how my questions here are different and are asking for a simpler, new angle on understanding this as it pertains to analog audio cables. \(And whether or not it is the same thing.\) Many thanks to whoever has the patience to help me get this. | Engineering | explainlikeimfive | {
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"> The ground wire somehow reduces the amount of noise entering the cable's signal from radio frequencies in the air. (not sure how that happens though) These signals actually **induce** a current into a wire. The radio signals enter the ground wire, instead of the cable that's transmitting the information. > The ground wire somehow provides safety from electrical shock in electrical wires (audio wires too?) If the wire is carrying enough power, yes. > Is this the same thing as the ground in electrical wires? Does it work the same? (I know analog audio cables are transferring electricity too, but clearly there is some difference) Yes, and there really isn't a difference. > Why is it called \"ground\" or \"earth\"? I read that with electrical wires, there's some metal rod that sticks into the earth somewhere...? Without totally understanding the details behind that, intuition tells me that's not the case with audio wires – they just go from one piece of audio equipment/instrument to another. Right? It will inevitably go to actual ground. That equipment is plugged in, isn't it? > is the \"ground wire\" actually more of a wrapping rather than a wire? Grounded wires **can** also be wrapped around to create an actual shield. And when your signal is carrying information, it can help a lot. I don't know if they do that for audio cables, it can get pretty expensive. > I can't wrap my mind around electrical circuits – is that a pre-requisite to understanding the ground wire (and eventually ground loops)? Honestly, yeah."
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8ndop6 | Why does the position of an object on a scale affect its weight? | Engineering | explainlikeimfive | {
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"This is because of what is known as moment. Pick up an axe by its head and it’s rather easy to lift, try and lift the axe head up from the far side of the handle and it’s much more difficult. The axe head exerts more of a moment the farther away your hands are (ie the fulcrum). The same thing happens with flat scales. The further away the mass is from the fulcrum (the base) the more of a moment it will exert, making it pull down more."
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8ne1kn | How electric cars like tesla work | Isn't it in over-simplified terms a servo motor with the amount of electricity controlled via pedals acting as potentiometers... don't get me wrong I'm pretty sure there is more to this than just this... | Engineering | explainlikeimfive | {
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"no not really, thats about it. The trick has always been in designing a battery that can hold enough charge to make them economical. Tesla is special for their batteries, otherwise what you said is how they work.",
"Basically, yes. Lots of small battery cells assembled into a large battery pack connected to very powerful electric motors power the drive train. Interestingly, the individual battery cells that make up the battery packs are not much bigger than a standard household AA battery. They use 2170 cells which means they are 21mm in diameter and 70mm long. How the signal gets from the pedal to the motors is bit more complex than a simple potentiometer. Essentially an on-board computers knows the position of the throttle pedal. The throttle tells the computer \"I want to go 50 mph.\" That information gets fed into an on-board computer that is monitoring the wheel speed or how fast the wheels are spinning. The wheel speed computer tells the throttle computer \"the wheels are currently spinning at 30 mph\". Another computer then calculates how much power is needed from the batteries to get the wheels turning as fast as the throttle's position is asking for.",
"Yes, but more sophisticated. A servo motor usually uses very cheap, yet inefficient kind of brushed DC motors, which uses the Lorentz force on a conductive wire to turn. Electric cars use Brush-less PM machines or Induction machines which uses synchronization of rotating magnetic fields using Permanent Magnets and Electromagnets, which are a lot more efficient, but a lot more expensive and harder to produce. Instead of potential meters they use hall effect sensors, because this is more precise and durable. But yeah, a Tesla is just a scaled up and overengineered version of your RC car when you were younger. The real question is, why did it took them so long to start selling electric cars, as they are so simple? _Welcome to the world of car manufacturer conspirancy theories_"
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8nkqqx | Why is it that every hospital or commercial property I've been to is bug free, yet most residences tend to have issues with them getting in? | Engineering | explainlikeimfive | {
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"There are plenty of bugs & critters in commercial buildings & hospitals. They just take extra precautions so the customers/ patients never see them. You'd think in a major hospital/ research facility it'd be pretty sterile, but we have giant roaches, silverfish & even bats in older halls that only workers go down on a fairly regular basis.",
"Public buildings like stores, hospitals, etc, have two sets of doors along with a wall of moving air. This \"airlock\" keeps the majority of bugs out, and the blowing air above the second door layer is yet another blanket of bug repelling science. It physically blows the bugs before they can get in.",
"Commercial buildings definitely have issues with bugs, and owners work very hard to reduce or eliminate them. They have professional level cleaners maintaining the facilities a regular basis -- even as often as daily or hourly in scenarios where extreme cleanliness is a quality-of-service level concern (food production, health care, contamination-sensitive manufacturing, etc.). At some point, professional level exterminators manage infestations with the best methods and technology that are currently available. It's just an inevitable cost of doing business and you budget for it accordingly and carry on. But this same process is expensive and intensive effort for homeowners. It's a frustrating and disgusting complication to daily life. So, while it's managed in stride and (usually) well managed in professional situations, private homes are usually playing a little bit of catch-up in the anti-bug war. There's just too much other stuff in our lives to keep our homes at a sterile level of production."
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8nn3v0 | Why do public toilet seats have a U shape but home toilets have circular seats? | Engineering | explainlikeimfive | {
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"I think because in public you dont want to accidentaly touch the fron of the toilet with your penis so they just removed the front",
"The U shape is for the people who clean them. Urine has a tendency to splash up and accumulate on the bottom front of a circular toilet seat. There's no reason you can't use the U shaped seat at home, my mother in law certainly did. I think people don't out of tradition mostly.",
"Also longevity. The open shape allows the material to form and reshape if someone sits down on it so it won’t break if there’s a lot of work it has to do. At least that’s what I’ve read the last time this question was around.",
"Home toilet? I mean, I’ve got a urinal at home. You can have whatever you want at home. There’s no such standard of circular or u shaped toilet seats. U shaped is actually better. Easier to clean, more hygienic, and easier to wipe oneself etc.",
"I always thought it was for helping the seat to stay clean when a man who doesn’t raise the seat but then dribbles."
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8nnx5s | How does the check engine light in car knows when to light up? | Engineering | explainlikeimfive | {
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"Cars have lots of sensors that their entire job and function is to notice when something is funky. Each one sets off its own code that can be checked to easily find why your car is trying to kill you"
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8nps8t | the difference between the Hong Kong and New York agreements? | ELI5 the difference between the Hong Kong and New York agreements? Are there any high quality journalistic sources that give a good, neutral overview? All I know is that they're both some kind of Segwit compromise. | Engineering | explainlikeimfive | {
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"Are you talking about Bitcoin, or something else?"
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8nx03n | How do GPS satellites not get overwhelmed by the number of phones and devices that use them? | Engineering | explainlikeimfive | {
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"they broadcast a signal down, the GPS receiver receives it from multiple satellites and uses the timing of the signal to calculate position. There is no uplink from phones or devices.",
"It's like a lighthouse; any number of boats can SEE the lighthouse, the lighthouse shoots out a beam of light, the light house doesn't need to send out a beam for each boat. Each GPS sat is just like a lighthouse; it is sending a signal ANYONE can just look at and see. Your phone is looking for multiple of them, and your phone does the math to figure out where you are based on the satellites and the time of the signal."
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8o1qf3 | Whats preventing Tesla from producing more cars? I know Tesla is increasing the number of cars produced per quarter/month/week but it seems they undershoot their goal each time. Also,why don’t they lower their goals a bit to make it more realistic? Wouldn’t that make their investors happier? | Engineering | explainlikeimfive | {
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"It takes time and money to increase production capacity. They simply can't put enough parts together quickly enough. If they spent a billion dollars on more manufacturing lines, it would still be several months before they were operational. You can throw as much money at it as you want, there are some things with fixed time that can't be accelerated(9 women cannot make 1 baby in a month) They already have more capacity coming on line, if they build too much capacity too quickly then they can end up with more capacity than they need which is a waste of money. Going super deep in the red to get into the black a tiny bit quicker isn't always a good plan.",
"Production issues. Elon Musk admitted they went all in on automation before it was ready. They have shut down unexpectedly at least twice since starting the model 3, mainly to correct bottlenecks in their manufacturing process. As far as the lofty goals, well if you don't set the bar high you'll never achieve great success. I also don't think they anticipated the demand for the model 3"
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8o97dv | Why can you replace only the glass in some phones and you have to replace the full display on others? | And what are the best phones right now that, in case of the screen breaking, you could replace only the glass? | Engineering | explainlikeimfive | {
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"On some phones, the screen is bonded (glued) to the glass. This was done supposedly to achieve a clearer image, having less material and an air gap between the top surface and the display. So rather than use a heat gun to carefully separate the screen from the glass (and put it back together with the same quality as manufacturing), I usually just opted to go the easiest route and replace the whole unit. It’s usually not that much more expensive anyway."
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8o9x9w | What is it about (steam or otherwise) turbines that makes them so ubiquitous for power generation? Do we really not have anything more efficient after more than a century? | It's almost mind-blowing to think that large-scale electricity production is just a handful of different ways to spin a turbine (nuclear, wind, hydro, coal; all basically the same core principle). Is there really no better commercial technology available for generation? | Engineering | explainlikeimfive | {
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"> Is there really no better commercial technology available for generation? Not really. It is important to note that time is not an assurance that improvement will arrive, that is just an expectation from our advances in other areas. It turns out that steam turbines are just really good at producing electricity from a heat source. That isn't to say there have not been advances, but it is more in the form of better steam turbines rather than a fundamental shift in techniques.",
"Almost all power sources are just to turn a shaft. It’s all about torque. Nuclear powered aircraft carrier sounds like it should have some Star Trek looking engine. Nope. Radiation boils water. Water turns to steam. Steam spins turbine. Turbine turns shaft. Crazy that we can do all the amazing technological crap that we come up with, but haven’t found a better way to make something spin.",
"If there was a better way, we'd be using it. Most navies are getting away from steam generation and switching to gas turbines (in the US, the same General Electric engine that's used on the Boeing 767). Steam plants can take *hours* to get up to the minimum operating temps (running the main engines on cold oil is a really bad idea), but gas turbines can be online within 10-15 minutes. Combine the gas turbine with a variable-pitch screw and you have a 20K-ton ship that can stop in its own length. Can't do that with the old steam turbines connected to geared drives. ***************** Per deleted post: I referenced a 20Kt ship, not an 90Kt carrier. It's also obvious that you can't use a gas turbine engine on a submarine. Cherry-picking a few ships to counter my point makes him nothing more than a troll, and **my point is correct**. Furthermore, carriers likely **would** use gas turbines except for the fuel requirements. The less space needed for fuel oil can be used for flight ops. The **hours** required to bring a carrier's nuke plants online and steam turbines up to operating temp is a serious vulnerability. A ship using gas turbines can go from a cold plant to underway in less than 30 minutes. The US Navy got rid of the Boiler Technician rating back in 1996 because it was a dead-end profession; no new ships were being built using steam boilers and turbines. The Gas Turbine System Technician was added because new ships use the GE CF6 LM2500 series gas turbine to power the shaft. Add a variable-pitch screw to the shaft and a Spruance-class destroyer can stop in its own length. The Spruance class has been around since 1971, so the switch from steam to gas turbines has been going on for 47 years.",
"It maybe the energy source we are tapping into for electricity generation. If you think of traditional energy sources, hydro and wind are kinetic energy sources (water and air are not reactive in generating their own electron). Natural gas and fossil fuel have a lot of chemical energy but they excel in combustion. Heat generally doesn't change chemistry but make them more energetic so we are still trying to harness kinetic energy. But then look at solar and fuel cells, light is a electromagnetic wave and are not kinetic energy, so we can tap into that directly. Fuel cells work through electron exchange, so we can also ignore kinetic energy. I think the way we harness energy is more source dependant than it is technology dependant.",
"mostly that they're cheap and safe. while there are fluids with better thermal properties, they're much more expensive and tend to be fairly dangerous in one way or another. methods besides turbines tend to be restricted to low power applications because of practical constraints.",
"There have been a lot of advances in PV cells. But it is much more convenient to get our energy from something where we know that we will reliably have x for power production and that it will not be impacted by weather, seasons, etc Spinning magnets to either create a torque or electrical power ( depending on which side is your input) is a pretty effective method. But don't be fooled over time there have been a lot of advances in this technology. Is there/could there be something better? Quite possibly, but just because we want there to be advances in a particular field doesn't mean we will ever see them, or be able to anticipate when those will be seen. Look at almost any field of science and you'll see that many times we get bottle necked on some level of tech for decades, centuries, Millenia before something new comes out.",
"If you can find a more simple, elegant solution to \"inclined plane\" to do all they work they do, then you will have indeed found a better alternative to turbines, and will be the richest person on the planet.",
"The most efficient machine for converting energy known to man is a turbine. It is known as F1FO ATPase and is the thing that creates energy for our cells in the mitochondria. It uses protons flowing like water in a dam to turn a turbine that pinches together two molecules into one which, when broken apart again, releases energy. ADP + phosphate turns into ATP using that turbine. This doesn’t answer your question directly, but what it does do is tell you that so far we don’t have a more efficient way of converting motion into electricity because this method is already incredibly efficient.",
"I’ve been studying fluids and thermodynamics engineering, and steam turbines are actually surprisingly efficient. The main most common form of power generation is to take heat energy and convert it into mechanical energy. Because of thermodynamic laws, you need to take energy from a hot source (burning coal, nuclear power, etc), make it go through a process that uses up some of the energy, then dump the rest of the energy into a cold source (the air or some body of water usually). Water is good at absorbing energy and turning into steam, which can be pressurized to turn a turbine. This process works really well and has little losses, essentially being as good as you can get thermodynamically. Any other fluid could be used too, but water is well studied and is readily available.",
"As someone who operates large water boilers and steam turbines, I can tell you that it is by far the most efficient method of generating electricity by burning fuels. The big advantage of using water boilers is the huge latent heat that water can absorb and release during a phase change, as well as the huge volume change associated with the phase change. \\(1 gallon of water becomes 1700 gallons of steam\\) This change in volume allows the heat sink \\(condenser\\) to operate at significant vacuum, lowering the condensation temperature of the water.",
"Do we want a power plant for ants?! No, we do not want a power plant for ants, so let's talk about what's useful at a human scale and in human lifetimes. Can some future whiz-bang technology double the efficiency we get today? I think not, and here's why. Efficiency in todays power plants runs roughly from one third to two thirds of thermal output. Lets split the difference and call it 50% efficiency. If you want more, you've got to run hotter. (We would existing turbines hotter if we could, but we can't, because the steel would melt.) Let's say the end of mankind as we know it is 10 000 years from now, when Skynet has taken over the world and built a Dyson sphere around the sun, sucking up close to 100% of its power. That's as good as it gets. No matter what, you run up against the laws of thermodynamics. So, don't expect any miracles. Even with cold fusion, powerful Terminator-style alloys, or alien technology, you can only double today's efficiency.",
"Can you think of a more efficient way to move a coil through a magnetic field?",
"Water expands 1600 times it’s original volume converted to steam at a mere 213 degrees. What liquid do you want to use?",
"Water is incredibly powerful when it is heated and changes from its liquid to gaseous state. It expands something like ~200 times in volume. Capturing that expansion is key.",
"All this talk about thermodynamics and efficiency. How about the fact that a properly designed steam turbine has only one moving part and will run for decades if fed clean oil and dry steam?",
"Water is an amazing compound. By the way the actual power generation has nothing to do with temperature other than just keeping it steam throughout the process. The only reason it has to heat up so much is that you don’t want the steam condensing back to water on a turbine blade. Coal, natural gas, and nuclear, all do the same thing which is heat up water until it turns into steam. The steam passes from a higher pressure point to a lower pressure point and it’s path it through the turbine which in turn turns the turbine and creates power. Harnessing higher pressure that is going to lower pressure has been used for thousands of years (think of windmills or a water mill)."
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8o9xv4 | Why is it that a car journey feels less bumpy if you are the driver? | Engineering | explainlikeimfive | {
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"For the same reason that you cannot tickle yourself: *anticipation of effect dampens perception of effect*. When you are driving a car you control the speed, direction, and changes in velocity while also viewing road conditions that generate bumps. Using all of this information, you can usually correctly anticipate movement (bumps) and anticipated movements are accounted for in the brain and somewhat canceled out in our perception. Passengers do not control any of these factors and generally do not pay attention to the road in the same way and thus do not dampen perceived movement. Neuroscientist Daniel Wolpert brilliantly explains these effects in his talk, [The real reason for brains]( URL_0 ;).",
"I also think it’s because as the driver you know what bumps and curves are coming up and are making adjustments with your body to minimize how much they jostle you. And I’m not just talking about leaning into a turn. I think while you watch the car in front of you hit a bump or you see a dip in the road ahead your own body (even subconsciously sometimes) makes adjustments by shifting its weight or flexing certain muscles or keeping other body parts loose. I don’t know if this is true but I’ve thought about it a lot when my head is bouncing all over while riding shotgun in my wife’s car.",
"For regular 5 seat cars, the location of the driver seat right behind the front wheel helps because the suspension is there for you. My parents have a 8 seater, and sitting in the last row feels like bumps would throw you in the air. I bumped my head in the roof on too many occasions in that car Edit: fixed several typos. Autocorrect attack"
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8od7yj | What is the space in between sidewalk squares for? | Engineering | explainlikeimfive | {
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"Those spaces are called Contraction Joints. Concrete is poured wet, as it dries, it shrinks, just like a wet sponge shrinks as it dries. When concrete shrinks, there's a risk of cracks forming. So builders put those contraction joints into sidewalks to reduce the risks of cracks forming, and to provide a place for crack to form (you'll often see cracks in contraction joints).",
"If the space wasn't there, natural expansions th contraction over time would make it crack much easier. Plus its easier to just lay it in chunks instead of laying it all at once. Makes it far easier to repair in the future as a bonus, too.",
"All of these answers are wrong. The cracks exist to break your mothers back. They work using the quantum principles of super symmetry.",
"When the temperature changes, objects change is size slightly. Normally when objects get warmer, they expand, and colder, they contract. This is for metals and most things. Water does the opposite but that's a different story. Either way the squares need that space so when they expand they have some room. If they did not have it, cracks will form from the pressure when they push against the adjacent squares."
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