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dcb642 | How do engines with forced induction create vacuum? | I’m specifically focused on how engines with centrifugal supercharger create vacuum at the manifold when the engine in high RPM with low load. Let’s say engine is at 4K RPM with 20% throttle, how can the manifold have a vacuum? | Engineering | explainlikeimfive | {
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"Centrifugal superchargers create pressure or boost by spinning an impeller at very high speeds, relative to the RPMs of the engine driving it (often they spin upwards of 8:1) They increase the air density in this way. Since they are inline with the intake of air, they can only really add to the air dynamics of the engine. The vacuum effect is just the displacement caused by varying degrees of air pressure. Obviously in a low load situation the impeller spins slower and creates little, if any, boost. But it wouldn’t harm the natural flow of air the engine creates by running."
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dcqyye | Why do they lay down grooved pavement when working on roads? | Engineering | explainlikeimfive | {
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"This is to promote adhesion between the layers and also to stop sliding of the asphalt off a concrete or hard base.",
"Same reason you sand before you paint. So the new layer has something to \"grab on\" to. Otherwise it would risk flaking.",
"Flat surfaces do not bond well. Adding texture to the underlayment gives something for the surface material to grip. Where the surface itself is grooved, it's to help reduce the amount of water or ice that sits on the surface making it easier for tires to get traction"
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dczt77 | How are rings resized? | Engineering | explainlikeimfive | {
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"Making one larger is easy; it gets hammered on a mandrel, a tapered piece of steel. This stretches it out. Making a ring smaller is harder; it has to be cut and material removed."
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dd2b4p | Why don’t cars utilize conditioned air (A/C) into the engine of a car? | If cold air is more dense, more fuel is injected to compensate resulting in more power? Why not take the cold air from the A/C and reroute into the intake manifold? | Engineering | explainlikeimfive | {
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"Because running that air conditioner would take power from the engine, and chilling the incoming air wouldn't provide much benefit. And chilling the amount of air a car engine uses would require a really big refrigeration system. One thing that is done, on some super- and turbo-charged engines, is use an intercooler, which passes the very hot air that comes out of the turbo through a bunch of narrow passages cooled by air, to cool it back down near the outside temperature. This doesn't take any power from the motor.",
"It takes a lot of energy to make the cold air via A/C so it would be a net loss to make extra cold air just to feed into the engine. Instead, a turbocharger achieves the same effect much more efficiently.",
"Some people have experimented on drag cars with running the intake into the car's interior, running the AC on max to cool the cabin as much as possible. Then when running down the drag strip the engine basically has a pool of cold air to draw from. From what I remember one guy that did it thought it maybe made a little difference, if it was a really hot day but might have been wishful thinking and there are far better ways. Like ice, water, methanol etc to cool the intercooler."
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dd97wx | Why traffic jams never decrease even when new roads are built | Engineering | explainlikeimfive | {
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"Decreasing a bottleneck in one area simply allows traffic to flow faster through there to the next bottleneck down the road. If it gets there faster it backs up quicker. Plus bad drivers causing accidents and incidents and it may never be perfect until we teleport. Then there will be queues at the teleport terminal.",
"Paradoxically, adding roads can actually increase traffic jams. & #x200B; Imagine there are two roads from one end of the city to the other, each road is split in half. We'll call these Roads A and Roads B. The first half of Road A is narrow and the traffic depends on how many cars are traveling down it. Let's say your travel time is equal to The # of travelers (T) divided by 25 (T/25). The second half of Road A is very wide. Wide enough to accommodate all travelers at equal speeds. We'll say the travel time is just a flat 50. So if you go down road A, your total travel time is (T/25) + 50. Now look at Road B. Road B is the same as A, just in reverse. The first half is the wide half and the second half is the narrow half. The total travel time is 50 + (T/25). With the same travel time, people will split their commute evenly across both roads. Let's say you have 1,000 travelers. They will split evenly, 500 travelers down each road for a travel time of 500/25 + 50 = 70 minutes. Now, let's add a road at the halfway point between roads A and B (the point where each switches from narrow to wide and vice versa). And we'll make this a short, wide road so it has a flat commute of 5 minutes. So rather than having to stick to road A or road B, you can travel the first half on road A and second on road B, or the other way around. You now have 4 options: A- > A (T/25 + 50) A- > B (T/25 + T/25 + 5) B- > A (50 + 5 + 50 = 105) B- > B (50 + T/25) If there are no travelers, it's clearly better to do A- > B, as this is 0 + 0 + 5 = 5 minutes. This is better than all A/B (50 minutes) and also clearly better than B- > A (flat 105 minutes). In fact, as long as T/25 + 5 < 50, it's better to do A- > B and that happens at 1,125 travelers, which is more travelers than we have. That means *everyone* will want to do A- > B. But if you do that, the commute time increases to 85 minutes. You've increased the average commute time by 15 minutes. What's even more paradoxical is that if adding roads can increase traffic, then removing certain roads might actually *reduce* traffic!"
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ddb9of | Why don’t we have any mobile/cellular signal inside of elevators? | Engineering | explainlikeimfive | {
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"The closed metal enclosure of the elevator acts as a shield that blocks the radio signals from getting in or out of the elevator. Rather than getting in or out the signals produce currents in the metal enclosure and don’t travel further. It’s called a Faraday cage.",
"Metal is almost perfectly impenetrable to radio waves. Most buildings actually contain very little metal, and are riddled with holes like windows. This allows radio waves to pass through and around the walls and enter the building relatively unabated. Elevators are nearly perfectly sealed (by the standards of radio waves - any holes less than about a half inch across may as well not exist) metal boxes, and so radio waves have almost no chance of entering them."
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ddgq3v | How do pool tables know the difference between the white ball and the colors when returning a sunken white ball? | Engineering | explainlikeimfive | {
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"It’s not by color. It’s either by weight/diameter, as the cue ball is different than all the others balls. Or, it’s by a small magnetic detector that just basically puts the cue ball down a different path in the table"
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ddipyx | how does wireless phone charging work? | Engineering | explainlikeimfive | {
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"When the magnetic field near a wire changes, it generates electricity in a wire. Conversely, electricity in a wire generates a magnetic field. This means that with two wires next to each other, suddenly running electricity through one will generate a brief pulse of electricity through the other. If you wrap both these wires into coils, the effect becomes much stronger. Now, quickly switch electricity on and off in one coil- you will get pulses of electricity in the other coil and thereby transfer power.",
"There are two wireless charging that I'm aware of: Inductive charging, and Optical charging. Inductive chargers essentially created a magnetic field. When a matched device enters that field the magnetic energy is converted back into electrical energy. These are not very efficient (compared to a wire), require close proximity, and are susceptible to electrical and magnetic noise Optical chargers essentially points a lazer at a solar panel. Light hitting the solar panel causes electrical energy to flow. We can tune the panel and the laser for specific light instead of solar panels for all available light too! Optical charging delivers less power as well and is suspectible to air quality and also needs line of sight, obviously"
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ddmmy7 | Do diesel car engines get blocked with particulates when not being used? | I was advised to drive my diesel car a minimum of 10,000 miles a year to stop the particulate filter blocking. This is now impossible for me to do but selling the car is not yet an option. What is the best way to protect the engine? I use the car mainly for local journeys. I've been told I need to give the car a 'good run' at a decent speed to burn off the particles, I'm wondering if I am better off not using the car for short trips if this is what creates particles to build up. Am I right? | Engineering | explainlikeimfive | {
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"What is the year make and model of your vehicle? Some newer vehicles monitor the condition of the filter and will run a cleaning cycle periodically. Besides that you can remove the filter and have it taken care of. Post this over on r/mechanicaladvice identifying your vehicle, you’ll get real, relevant answers there."
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ddo95h | How its possible to friction weld wood? | Recently saw a post on Instagram where it showed wood being friction welded together. I understand how friction welding something like titanium would work. But wood? Thanks in advance. Edit: Link : [ URL_0 ]( URL_0 ) | Engineering | explainlikeimfive | {
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"It's a surprising and novel achievement. Basically you need to heat the wood so fast, and with so little oxygen supply, that it doesn't have the opportunity to combust."
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de084w | how a refrigerator works. | Engineering | explainlikeimfive | {
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"Okay, so in a refrigerator there is a compressor, and that compresses a liquid, Freon, and what happens when you compress something? It gets hot. That hot freon then gets circulated through a big windy tube, similar to a radiator in car, to dissipate the heat and cool it down. Then that compressed freon gets uncompressed, or it condenses. And what happens when you de pressurize something? It gets cold. Hence when you use up a can of aerosol and it gets colder. That cooled freon is then ran past a fan that blows air over, cooling the air and that air gets put in the refrigerator. And when you blow warm air past something cold, something called thermo equilibrium happens, and as the air cools down, the freon warms up. Then the cycle repeats.",
"To add to the previous answers, the cooling system in a fridge lasts so long without a refill because it’s a sealed system: there’s nowhere for the coolant to go, unless something breaks."
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de0pfs | How power plants and grids store electric if not in demand by consumer ? | Engineering | explainlikeimfive | {
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"Good question! They don't actually store electricity. When they say they have extra, what they mean is they have the generators to make more than they need. If the generators are at a dam using water to make electricity, it's hard to \"save\" the extra capacity because if the water doesn't flow through the dam there will be a flood upstream. So they have to let the water flow downstream without using it to make electricity. Some places, they actually use the extra water to make electricity, then use that electricity to pump water uphill into a storage reservoir, where they can use it later. That tends to happen at night, when everyone's asleep and not using electricity. In fact power companies typically charge less for electricity at night to encourage factories to use it when people are asleep and not using it. A power company needs enough generators to supply power at the time of highest demand. Then some of those generators sit and do nothing when demand is lower, and a machine doing nothing is a waste of money, so power companies look for ways to keep the generators running all the time so they are making money instead of sitting doing nothing.",
"One way is to use electricity to pump water up into a reservoir, then later use gravity and water-powered generators to reclaim the energy",
"Frankly, the power is not stored. The electrical grid is, however, interconnected and managed so that excess capacity in one area can be sold to other areas (to a certain extent).",
"Electricity can’t currently be ‘stored’ in the quantities required to be useful for national-scale grids. There are industrial scale batteries but these aren’t really - to the best of my knowledge - suitable for the scale needed. So what do we do? We have excess electricity and we need to do something with it. How about we convert it to something else? Electricity is, after all, energy. Energy can’t be destroyed but it can be converted. Examples of such energy storage mechanisms are ‘Electric Mountain’ in Wales ( URL_0 ), which takes electricity from the grid at periods of excess and pumps water up a mountain, then during times of peak demand releases that water through turbines to generate electricity. There are all kinds of possible ways to do this. Some are more efficient than others, and they all have their own problems and suitable applications.",
"They're not storing much. For a few seconds you have energy stored in the rotating mass of the generating equipment. You can also use battery storage to give you enough time to bring more power generation online. For long term storage, you can pump water uphill into a reservoir, and then turn it back into electricity as needed. Though usually cheaper to skip the uphill pumping and rely on rain. The vast majority of the power generated is used in less than a second. Often they'll have spare generators already spun up and ready so they can increase power generation more quickly. They'll also predict load based on past usage. I remember a story about power plants starting up more generation at the start of a commercial break, anticipating the increased load from people making tea.",
"Before we begin, a lot of context. The actual answer is at the bottom, if you want to skip the context. ------- **In Theory** To give an overly detailed example from someone who works in the field, all generators have an operating band, where there is a low and a high \"sustainable\" (operating) limit. Within that, there can be several stages of operation with different levels of marginal costs. Let's say I've got a 500MW steam generator, but if it's online, it must be running at at least 200MW. I can run it above 200, but no higher than 500. Below 200MW, I can get vibrations in the machine and that's really bad. In order to generate more power, I need to move the water and steam through my unit faster, which means running my circulating pumps harder. Eventually, say at 350MW, I may need to bring on another pump, in addition to adding another set of fuel burners. This makes my unit less efficient, because i'm now burning approximately double the fuel to go from 350MW to 500MW, and I have to consume more of that power i'm generating in order to run my extra pumps. When i'm at 350MW, I may be consuming 5 MW of that to run my pumps and other auxiliary equipment, so i'm pushing a net 345 MW to the grid. When i'm at 500MW, I'm probably consuming closer to 10 or more, so i'm only pushing 490MW to the grid, despite burning fuel to produce 500MW. Less power to the grid means less money in my pocket. --------- **The Money** Units summarize and communicate this economic behavior to their grid operators in the form of \"marginal cost curves\"; for each level of output, getting 1 more MW of power will cost an additional $X per MW. This affects not only the cost of power that I *will* provide, but the cost of all the other power i'm already providing. So if my marginal cost at 300MW is $20/MW, i'm making about $6000/hr from energy sales at that level. If my marginal cost at 400MW is $80/MW, then my grid operator will tend to not operate me at that level, because it will cost them $32,000/hr for just that extra little bit of power (400 vs 300). If someone else is willing to provide that last 100MW of power at $50/MW, I will probably only get to generate 300MW. Conventional generators tend to get more expensive at higher output levels due to lower conversion efficiency and higher losses. There would be no reason for your marginal cost to go down as your output goes up, because that means you have lower operating costs as you produce more, which is impossible. There is no economy of scale for thermodynamics. Even nuclear and renewable generators have \"flat\" marginal costs, where their operational costs are the same at all times, so they simply want to produce as much power as possible when they can at least make their operational costs back. ----- **Now that we know why I might not be always generating all that I can all the time, the actual OP question.** If another grid that's connected to mine needs energy and is willing to pay $100/MW for it, then my operator will happily sell my $80/MW energy to them, because i'm willing to sell it and they're willing to buy it. This is usually scheduled in advance with look-ahead markets. I as a generator probably don't know that my energy is being sold to another grid. All I know is that i'm generating into the grid and someone else is expected to be taking energy out at the same rate. If it's a completely separate grid, this transmission will usually be done with HVDC (high voltage direct current) lines, but the same process can occur within a grid between two control areas across AC lines. This selling of energy or the capability to sell energy to someone outside your normal area is often called a \"surplus\" in the news, but it isn't really seen that way in the industry, because there energy doesn't get bought from \"here\" and sold to \"there.\" Power grids are pools of energy, so the energy you buy comes from everywhere. When we say you're buying from me, we're really implying that i'll put in as much as you're going to take out.",
"Grid connected battery technology is developing and now grids can store energy. It’s still new technology but quite on the rise. Tesla did a really big battery project which is a 100MW / 129MWh lithium-ion battery, Edit - Tesla battery project is the Hornsdale Battery in Australia.",
"It is important to separate power plants from the grids. They are cooperating, but have different goals. Power plants want to maximize their profits. Those are private companies. There are two main income sources for them. Either generate energy to sell, or get a contract with the grid operator to reserve some of their electricity generation potential for them (grid operator can command them to generate more, or less energy). This means that even though the power plant will only generate for example 80% of their potential electricity generation to sell, while losing the potential profit from the remaining 20%. For them to be willing to take this potential loss, the grids has to pay them better. Grids on the other hand are state companies and profit is not the main goal. The security of the electricity grid is. Because electricity cannot be stored efficiently on national level, grids main job is to ensure that at any given moment the production of electricity is equal to the consumption. To do that, they need to be able to start generating electricity at the moments notice. But because Grids cannot generate electricity on their own, they contract power plants to leave some of their electricity generation potential unused until the grid commands them to use it. As others already said, there are ways to convert electricity to other forms of energy, store it and in time of need, convert it back to electricity. Our current methods are very inefficient though. Which is why the question of \"how to efficiently store electric energy\" is the current big obstacle. **TL;DR:** We cant store electricity, which is why its Grid's job to ensure that electricity production is equal to its consumption."
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de8eym | Why are american bathroom stalls built with giant gaps? | Engineering | explainlikeimfive | {
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"Because it is significantly cheaper to do it that way, and it's normal in America so nobody really complains about it. Really the answer to like half of the \"Why is X so weird in America?\" questions is \"Because that's the cheapest, lowest effort thing companies can possibly do.\""
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dej0gw | How do anti:lock brakes work? | Specifically, what's the difference between these and just normal brakes that apply friction to brake pads on the wheel? I should state that I know nothing about cars at all | Engineering | explainlikeimfive | {
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"Car brakes work by applying a frictional force between the brake pads and the brake disk (or drum in some cars). This attempts to slow the rotation of the wheel. The car slows down because this force is transmitted to the ground through friction between the tire and the road. So brakes slow the wheel, wheel slows the car. In most cases, the slowing force from the brakes, when applied very hard, is higher than that of friction between the tire and the road. When this happens, the wheels \"lock\" and the tire starts to slide on the road rather than roll. This results in 2 bad things. First, maximum rolling friction is higher than sliding friction, this means your car actually slows down less when the wheel is locked rather than rolling. Second, when the car is sliding and the wheels are locked, there is no longer any steering force. If the wheels are still rolling, then there is still some steering available to the car. If the car is sliding, then basically it just goes in a straight line. This is bad for obvious reasons. Anti-lock brake system is simply that. A system that detects when the wheels are \"locked\" and reduces braking pressure to allow them to start rolling again. Some (most) systems use a system of pulses - the explanation of this is probably beyond an ELI5. A car with no antilock braking basically relies on the driver to modulate braking pressure according the driving surface and situation to keep control of the car. In the hands (feet?) of a skilled driver, a normal brake system can perform slightly better than some (of the less advanced) anti-lock brake systems. However for most drivers, in an emergency situation, a modern ABS is a much superior option to keep control.",
"Skidding is not the fastest way to slow a car to a stop. It is fastest to brake just a little less than what it takes to skid. It is hard for a human to hit the brakes just hard enough to slow the car, but not hard enough to skid. Anti-lock brakes have a computer that takes over when the driver slams on the brakes."
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dek34g | How do foundries actually dope silicon when chipmaking? | So i get that transistors are basically 2 diodes sandwiched together and photo-etched onto silicon but how do you make those negative and positively doped zones that a transistor needs when we're talking about nanometers in terms of dimensions? | Engineering | explainlikeimfive | {
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"The entire ingot can be doped by including certain impurities when making it (or by exposing it to a powerful neutron flux from a nuclear reactor). But the tiny individual doping operations are done photographically. A compound is placed on the wafer, and a mask is used to expose certain areas of it to light to harden it. The parts that did not get hardened can be washed away with chemicals, leaving only specific areas of silicon exposed to be doped. Then, the hardened areas are removed with other chemicals. Rinse and repeat."
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deshlc | Why they tear up random rectangular strips of the highway just to fill them back in again with lesser materials. | Engineering | explainlikeimfive | {
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"They needed to get to something under the road. I guarantee you they aren't just digging up the road for the fun of it, they need to go through it for a reason. Replacing the material is necessary but using somewhat less durable material is much easier.",
"Probably isn't actually weaker asphalt, but a weaker base. When they backfill for a utilities trench there are numerous reasons why they might not get the gravel as packed as the original road was, all to do with trying to get heavy compaction equipment into a narrow trench. They'll cover it with top tier asphalt, but it'll sink more than the rest of the road."
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df0b9k | How do CNC machines measure so accurately? | It seems like magic, lets be honest. Big, vibrating, metal-chewing machines that turn out parts that are flat to the 5th and 6th decimal places? How is there no slip? how does it compensate for vibrations and the super fast movements? | Engineering | explainlikeimfive | {
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"CNC machines need to be **very** rigid. They have big, thick braces and sturdy arms, and the entire structure is bolted firmly into concrete floors. You can move an arm pretty quickly when it is made up of several inches thick of hardened steel. Vibrations can be kept to a minimum with dampening and simply balancing everything very carefully.",
"First, lots of very rigid metal. If you look into buying machining equipment you quickly find out that even the toy sized stuff is really heavy. A mini lathe that fits on a desk is about 60 kg/130 lb of metal, and that's a small, very light thing. Second, very precisely machined surfaces to start with. The entire construction is made to be rigid and tight fitting. Third, [screws]( URL_3 ). You turn a screw once, things move by a millimeter. You can easily turn a wheel by a tiny amount, and a computer can do such things even better. You could make a finer screw that moves things less per turn if you need more precision. Fourth, [rotary encoders]( URL_0 ). While consumer level machines (and 3D printers) blindly follow instructions like \"turn this motor by 10 degrees\", and have no clue how much it actually turned and if it even turned at all, the better machines precisely measure motor rotation so they know at all times exactly what the position is, and if something is slipping. Fifth, [anti-backlash devices]( URL_1 ) and [ball screws]( URL_2 )"
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df4wxg | How did bridge builders of old ensure both ends of the bridge would perfect meet in the middle before laser measuring was around? | Engineering | explainlikeimfive | {
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"They had things like levels and surveyors tools. With a surveyor's kit you could get quite accurate - like to within an inch or two or better. Read up on how they built cathedrals. Iron right angles, bubble levels, chalk lines, plumb bobs... the tools of stone masonry and carpentry really haven't changed much in over a thousand years. Furthermore, they couldn't quite build out from the bridge pylons the way they do now with cantilevere'd tower supports etc.. They'd build a pier out in the middle of the river and build it up to a certain level with stone (using my level and surveyors scope from the shoreline to measure the right height). Let it set and remove the wooden caisson they used to dig down to the river bed. Then they'd construct a wodden trestle/framework that spanned from the pier to one shore, while the foundation on that shore is being built. Make both sides of the trestle level is easy enough, then the trestle framework supports the stone until the mortar sets. Rinse/ repeat on the other side, or until you reach the other bank.",
"I'd ask the same question about *tunnels*. You can't even see where you're headed. IIRC there's some tunnel connecting Manhattan with either New Jersey under the Hudson River or Brooklyn under the East River (I don't remember what side) that was built in the late 1800s with two excavation teams from both sides that met in the middle with an error of less than 1 inch. I can't get through my skull how this could be done at the time.",
"Bridges aren't as precisely sized as you think even today, physics won't let them be The longest span on the new Tappan Zee is 370 meters long and can change length by 20 cm between a hot summer day and a cold winter day. Bridges are built with expansion joints which let them expand/contract as temperatures change and also take up some slack during construction With old stone bridges a lot of stones were adjusted for their spots. Have a smaller gap than expected when you get to the middle? Smack the stone with a hammer until it fits! Then add some mortar to take up all the slack. Voila! A bridge that \"meets perfectly\" in the middle Our precision measurements let us better understand the load capabilities of structures and make parts in advance so we can build them faster, but every engineering design supports some slop in the final measurements",
"There's a lot of talk about protractors and string here ... that's only partly right. Traditional survey instruments (same principles often used today) measured angles, horizontally and vertically, extremely precisely. Quick precise distance measurement came later, but one carefully measured baseline and a series of intersecting angles can coordinate points better than you'd expect. GPS isn't as useful as you'd think for surveying in closed-in or covered areas, so we still use similar instruments almost every day. Behind all the software, modern terrestrial survey instruments still just measure horizontal angles, vertical angles, and slope distances. Leveling is even easier! In fact, if you want to be really basic, you can even do it with water. Source: professional land surveyor.",
"The original Huey P Long bridge in New Orleans has a 1.5' \"jog\" in the roadway about 1/3 the way up the bridge. I suspect it was due to measurement errors. The lanes were super narrow, at 9' wide, with no shoulders and most driver had to grip the steering wheel tightly while traveling the bridge, Especially if an 18 wheeler was next to you. URL_0",
"They did pretty much what I did when I built a fence around my house: * Put a stick at either end. * Have someone at one of the sticks, looking so that both sticks line up. * Have someone else put remaining sticks in a line between them, lining up the top of the sticks with that sight line. Accurate to within a millimeter or so, if done carefully. Sure, they probably didn't line up sticks, but the principle remains. With two set points, one at each end, it's very easy to accurately put stuff on a straight line between them.",
"They used other measuring techniques. Things like string/rope and anything to measure an angle could be used to draw two parallel lines, then you just stay within those lines.",
"Even more impressive is the transcontinental railroad, one of the tunnels they dug out from both sides and met up in the middle. They were within an inch I think",
"String. They lined up string measured by levels and squares. Also stone masons back then were absolutely ace when it came to construction. Roman masonry work still functions in some places after 2020 years.",
"laser measuring? I can build you a bridge with 100 feet of rope, a hatchet, and a few* pitchers of lemonade",
"To blow your mind, the middle of this bridge washed out and they rebuilt it. It has a bit of a kink now. Steel beam suspension bridge. URL_1 . URL_0",
"Modern measuring tools are more practical and resistant to human error, but precision isn't necessarily a modern thing. The [theodolite]( URL_0 ) has been around since 1600 and along with other precise distance measuring tools it allowed for incredible precision in building. Antique topography instruments were much more mechanical and human dependent, but topography isn't a new science and the old instruments weren't necessarily much less precise than modern instruments.",
"2500 years ago, a tunnel was dug under Jerusalem to allow water to be available from a spring. It’s called Hezikiah’s tunnel.",
"There's a misconception that ancient and even recent past is super primitive compared to our modern (as of 2019) knowledge and tech, but people have been super innovative and resourceful for several thousand years. Just the last 10-20 years have probably doubled our knowledge of the total so far...",
"They would sometimes make mistakes & #x200B; [ URL_0 ]( URL_0 )",
"Today we seem puzzled that people could have built anything without digital technology and other gizmos. But, we tend to forget that great builders, like the Romans for example, constructed roads, sewers, aqueducts, bridges, structures, without lasers and computers. Their works have stood for centuries. And, imagine doing math with Roman numerals! They even had rudimentary cranes and understood gearing. They could do a lot of things we do now, but just didn't have power machinery to do it. For a great look at ancient building techniques, read the Roman author and architect, Vitruvius, who wrote [De architectura]( URL_0 )."
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df7ofm | How do they make eye glasses for infants? | Engineering | explainlikeimfive | {
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"At just a few weeks old, a baby with healthy eyes will track you, moving their head and eyes to follow you. If your baby isn't doing this significantly later than most babies exhibit this behavior, they may have eye problems. The optometrist may then use a machine to get more information about the eye and determine what kind of prescription is needed. It would likely have to be updated fairly frequently as the baby's eyes develop, but it could lead to allowing a baby born with moderate vision problems to see more clearly.",
"I assume you're asking how the refractive error is measured. The old school way is to use a retinoscope and neutralize the refractive error with lenses. This gives you a decent measure of the prescription without the patient having to do anything. Autorefractors do basically the same thing, but are kind of hard to use with little kids.",
"I saw a post on Reddit today, looked like less than a year old",
"You temporarily paralyse the eye’s ability to change focus with special drops (cycloplegia), then you use an autorefractor to determine any optical imperfection in the eye (basically bounces a beam of light off the retina and back to the machine). This tells you the ‘strength’ of the lens required to correct the optical imperfections of the eye. This can be performed under anaesthesia if required. It is quite accurate."
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dfa56i | Why is a 10mm socket so damn important to Mechanics working on Automobiles. | Engineering | explainlikeimfive | {
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"It’s the most common size between all carmakers. And it’s small in size so it’s easy to lose."
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dfe8a0 | How do laser distance measuring devices work? | Engineering | explainlikeimfive | {
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"A computer measures the time the laser takes to go and come back. Knowing this, you divide by half and then multiply by the speed of light and get an accurate distance measurement."
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dfejgn | How are missiles and weaponized rockets able to fly long distances despite being small in size ? | to fly long distance rockets need fuel . But missiles on jet planes are small and have trackers and sensors and explosives in them and it seems less space is available for flying fuel . How do they manage to fly long distances ? & #x200B; EDIT: thank you all for taking out the time to answer. | Engineering | explainlikeimfive | {
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"They are really small and only meant to fly once. This means that there is WAY less space taken up by nonessential things compared to airplanes. No pilot to worry about, no landing gear, no refueling, no things to swap out, nothing that needs overbuilding to prevent wear and tear, just the explosive payload, fuel and the control system. So compared to their weight, they can carry plenty of fuel for what they need to do."
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dfh4di | Why cockpit of modern day Aircrafts have mechanical control panels rather than digital ones. | Modern day aircraft control panels have hundreds of knobs, buttons and other mechanical components. Is there a functional/engineering limitation that is stopping their digitization? | Engineering | explainlikeimfive | {
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"Actually, modern aircraft are almost all digital fly-by-wire systems. All those knobs, buttons, and mechanical controls are just backups in case the main system goes down for whatever reason. The great thing about having mechanical backups is that they wear down quite gradually over time, so repairs and maintenance can be done on a regular schedule, which means they should always be available and ready to use. Digital systems tend to fail all all at the same time (think of a regular computer crashing, or losing power), so having a digital backup isn't always the best option. Many modern aircraft have three or four digital backup systems, and the one mechanical backup system for when shit hits the fan.",
"Having worked for a charter airline, I can tell you the following: Many modern aircraft *do* have digital instruments in the cockpit--but they will *always* also have mechanical instruments as a backup. The mechanical gauges still work if you lose power, and they're not subject to the random glitches that digital instruments are (like your phone). Thr major watchwords in aircraft design are simplicity, reliability, amd redundancy. If you gotta have it to fly safely, you can net your last dollar there will be two of it.",
"How many times have you had problems with your phone? Have you ever tried to open an app and had nothing happen, or had it lag really long, or had the touch screen get wet and have nothing work? For you, this is annoying. For a pilot, this is life and death. Nobody wants to crash a plane because the touchscreen controls messed up for a dumb reason. Additionally, your phone is useless without power. But with an actual button you can press or a knob you can turn, you can pull wires or whatever that work without power. So a lightning strike might kill the power but the pilot can still land the plane. Finally, button/knob technology is older and we've had more time to figure out how it works. It is a \"mature\" technology, and it's the same reason why the space ships that NASA sends up are still made of 1970s and 1980s technology. It works reliably and we know exactly why and how to fix it when it doesn't.",
"If you look at a modern cockpit of something like am A350 URL_0 you'll see pretty much all the gauges are gone and all the displays are digital. There are a lot less knobs and switches, but there are still a lot. All these knobs and switches are digital. They are connected to the flight computer. The benefit of physical switches is you get to know them by feel and can operate them without looking. If your hand is in the slightly wrong position, the switch will feel wrong, as opposed to a touch screen where you need to watch what you are doing. When flying, you need to be looking out, not looking down."
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dfoyk8 | Why does modded minecraft lag so much? | Engineering | explainlikeimfive | {
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"Its becaise of the way how minecraft is coded. Java is not the ideal platform to play games with",
"Because of how coding works. Minecraft is not a modular system, or atleast, it was not writte with mods in mind, and written in java, which isnt ideal. Then they REWROTE the entire code and database with C++. The lag is NOT from graphic as many would belive, but rather from the actual poor optimisation, or extensive calculation. Take dwarf fortress for example, a game with almost no graphic, but as the simulation gets more complex, the computer requires more and more resource to operation, and thus, generate lag. in Minecraft, while the graphic is simple, the amount of blocks is pretty big, and slap poor modding optimisation in, you get system lag.",
"Backgrounds are really easy to render. You pull a picture from memory, scroll around it a bit if you're moving, ez math. Minecraft has no background. Immovable objects are really easy to render. You use math to describe planes (polygons), then you tell the game that physics-enabled objects can't go through those polygons. Ez math. Minecraft has no immovable objects. Physics-enabled objects are hard to render. Not only do you need to build laws of physics for your objects to follow, you need to decide on a \"resolution\" to run time at, which is your framerate. Computers can't calculate physics continuously the way reality does (fun fact: not even reality does this, reality's framerate is a Planck-second), so each frame is the result of billions of math problems solved based both on the previous frame and on the player's current inputs. The solution to those problems is the frame, and the more math needs to be done to calculate each frame, the slower it gets, and the lower your framerate. *Every single object* in Minecraft is a physics-enabled object because all of them can be removed. There's no ez math based on things in the world which never, ever change, so everything must be calculated on the fly. Of course the game can take shortcuts like not rendering textures on things the player can't see, not rendering objects updating outside a certain chunk radius, and not rendering anything at all outside your loaded chunks, but even with all that optimization regular Minecraft just barely works on decent modern hardware. Then modmakers come along and shovel in *even more math!* More textures to load, more interactions between different kinds of physics objects, more everything! Worse, unlike Mojang, modmakers don't all work together to make sure all the things they're shoveling together mesh well. There are a ton of super-inefficient ways to make sure everything gets done right in general, but they slow things down even more. It's all a mess of code on code on code, all competing for your CPU and GPU's attention, and if it needs to slow down and lose some framerate, it's gonna."
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dfqpja | What’s the significance of weighted keys on a piano? | Engineering | explainlikeimfive | {
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"There is no such thing as weighted keys on a 'piano' . Piano keys are just keys. If you're referring to weighted keyboard keys.. then they are weighted to simulate the resistance of an actual, analog, piano . Why simulate piano key weight? Because if you learn on a keyboard then attempt to play a piano you're going to suffer in music quality and fatigue. This is why people should learn guitar on accustic rather than electric, to build up hand strength and fine motor memory.",
"Piano Keys have weight. That's just how they exist. Keyboards try to simulate that. One of the advantagres of the keys having weight, is that, it makes it quite easy to control the volume your playing at, which is very important in piano.",
"Different pianos, depending on its value and quality, will also have different weights to their keys. For most players, the keyboard on an upright may have enough weight for the player to be able to have some levels of dynamics while keeping it light enough to make a strong attack on the keys where necessary without much effort. However, lighter keyboards generally do not so perform well when it comes to softer dynamics, but that is often overlooked for the average player. In fact, the average player may not be able to fully utilise a heavier keyboard that even smaller baby-grands tend to have compared to an upright - the average player may find it difficult to produce precise and consistent dynamics with a heavier keyboard. For a professional player, having heavier keys allows for a broader range of dynamics, and with the skill required at a professional level, the player can control dynamics precisely and consistently. Lang Lang is a pretty good example of utilising the full dynamic range of concert grands - he is able to produce tiny tinkling notes where he touches the keys just enough for it to make a sound but never so lightly that he misses any notes, and he is able to give full attacks on notes that resonate through the entire hall but never so brutally strong that the notes lose their quality timbre (although this can be contributed to the quality of the piano); then he controls the way the sound comes from the piano and either broadens or dampens the sound using his upper body."
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dfr81c | what advantage does dual exhaust have over single exhaust? | Engineering | explainlikeimfive | {
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"One way to look at a car engine is as if it were a giant air pump. The engine creates an enormous amount of noise and pollution in the form of exhaust, so that exhaust is channeled through emissions gear and a a muffler to reduce pollution and noise. You still want a lot of airflow, too, despite the restrictions. & #x200B; An *honest* dual exhaust might try to improve airflow by running a line from each bank of cylinders into its own pipe, with double catalytic converters and double mufflers. But carmakers know that the vast majority of people don't look under a car to check. So they will prominently display dual exhausts while running both lines through the same single catalytic converter, for example. Or, if you're a Corvette, you have a couple of fake pipes.",
"There is no advantage afaik, only disadvantages for actual dual exhaust. Given backpressure between tailpipes which are split on a V8 engine (left and right manifolds) doesn't do much. Each handles 4 exhaust valves which are on each side of the engine. If you look at the firing order for a V8 you will see it's not symmetric, so sometimes the left and the right pipes would fire causing different exhaust pressure rates as cylinders fire. On a properly tuned single or cross pipe exhaust (fake 2 barrel exhaust with a pipe between them) the pressure going out the tailpipe(s) create kind of a constant vacuum which helps pull exhaust out of the combustion chamber when the exhaust valve opens. When it closes, there is a slight additional vacuum in the cylinder, so when the exhaust valve closes and the intake valve opens, it's already sucking fresh air in before the piston starts goes down to pull in fresh air causing a very slight effect similar to a turbocharger putting more O2 in faster. It creates more horsepower and torque simply by leveraging the existing airflow that's already leaving that causes a vacuum via the exhaust valves. There are a square-billion different exhaust and engine combos. That's what dynos are for. Install and test. Compare to the others. Repeat and select based on what you are doing. (Quick line, top speed, acceleration, etc) there are tradeoffs for everything. Don't get me started on a ELI40 on fluid dynamics and pre-ignition."
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dfvynh | how do hydraulic presses and hydraulics in general work? | I wanna know | Engineering | explainlikeimfive | {
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"Imagine you're in a rectangular room with two movable walls that are on opposite sides of each other. Now imagine you're in this room and it's packed *full* of people. One wall starts moving inwards. What happens is the people closest to the moving wall will push those around them, and they all push each other until the wall on the opposite side moves outwards to make room. This is how hydraulics work. You can try this out with a simple experiment. Get 2 syringes and fill one full of water and fully depress the other (such that it is empty). Now connect their openings together tightly (a rubber tube is best). Depress the syringe full of water and watch what happens. The syringe stoppers are the movable walls. The water are the people inside. The syringe tube are the remaining 2 walls."
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dg4avd | How does combustion happen in a JET ENGINE? | Engineering | explainlikeimfive | {
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"In a normal engine, the combustion is contained within the cylinder; the metal of the cylinder, piston, and seals keep the gases and the fire inside the cylinder. In a jet engine, the combustion is \"contained\" by the huge quantities of air that's sucked in the front and forced to move towards the back. The speed of this air moving through the jet engine is faster than the speed of burning of the jet fuel, so the flames and the expanding gases have no choice but to push out the back of the engine, and not out the front. Think of it as a cylinder that's made of a wall of air, rather than of metal.",
"Perpetually. When the compressed air is halfway through the turbine, it meets many nozzles that are fixed on the shell of the turbine and spray kerosene vapor into this burn chamber. Because everything is so hot, this kerosene combusts almost immediately and produces more heat which exhausts through the rear of the engine"
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dg5w86 | Why are generators (home generators, portable generators, worksite generators, etc.) typically louder than even something like a tractor trailer, car engine, etc.? | Engineering | explainlikeimfive | {
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"Size and cost. A car has a lot more sound dampening, a better/more effective muffler, etc. A generator is made to produce power for cheap. You can most certainly get a quieter generator, but it'll be heavier and cost more. If you're on a worksite, you might not mind a big loud 75 decibel generator. If you're camping and want to sleep next to a generator, you might want a 50 decibel generator."
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dg7p7w | How is it when we launch spacecraft out to orbit other planets or eventually to go to the moon, the craft don't collide with all the space junk up there? | The ISS is under constant threat of being destroyed by debris, so when we (eventually) launch astronauts to go to the moon or when we launched like Juno or New Horizons, how did they not get destroyed by the space junk? | Engineering | explainlikeimfive | {
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"There is very little space junk out there compared to how much space there is, most of the junk is in certain locations notably where we want to put many of the satellites, the probes spend very little time in the danger area - URL_0",
"Sometimes things do collide. We track the orbits of much of the debris satellites that are up there and manage the orbits of new items so as not to collide. This has to be done very carefully as there are many items something could collide with. Last I saw there was around 10,000 debris satellites so it is a lot of work. As I recall the sbirs system was supposed to track all the items in orbit.",
"As others have stated, we do track everything that we can that is orbiting the Earth, but another thing to remember is that the Earth simply is huge compared to what's orbiting it. You could easily put 20,000 pennies evenly spaced in a large parking lot and most of them wouldn't even be close to each other. It's a bit like that, but imagine that the parking lot is much much bigger, and the pennies are much much smaller."
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dg89as | Why are drivers on the left/right of a car, when having the driver dead set in the middle would grant better vision on both sides? | I’m sure it has to do something with direction of traffic and what not, but I think it’s just more natural that way no? | Engineering | explainlikeimfive | {
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"When driving on the right side of the road, turning left is the most dangerous turn. So having the driver's seat on the left gives the driver the best visibility on that side to see into the lanes they need to cross through on their left turn.",
"Sitting on one side allows for more passengers in the front seats. Some early automobiles and trucks did use a center-driver layout, but it isn't an efficient use of space within the cabin if you want more people than just the driver to fit in there. You still occasionally see it on special use vehicles and exotic sports cars.",
"The driveshaft went down the middle of rear wheel cars which used to be more common, so it was easier to sit on the side of it.",
"Sitting in the middle would not give better vision when you are overtaking another vehicle. That is the situation in my experience when there is a significant difference between the driver and passenger position. To have center driver in a regular car the width of the car would need to be increased for 3 person in the fromt or the same width and persons in the from. For racing all trave in the same direction, only the driver is in the car and have a equal center of mass is relevans so then a driver in the center is a good idea. But for regular driving it is not."
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dgd61h | How is it possible that the Flint Water Crisis is still ongoing after 5 years?! | Engineering | explainlikeimfive | {
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"Because the water lines are underground, and specifically under roads. Digging up roads is a slow process, and it always involves giving people advance notice do they can park somewhere else, towing cars that didn't move, etc. Then, many of the lead pipes that cause the problems are actually the property of homeowners. The city wasn't responsible for them, until the state appointed director caused the city to switch to a water source that corroded the lead pipes and damaged the scale that protects the lead from the water. (That corrosive water would have been fine for new plumbing). The replacement of water mains is a single, huge project, but homeowner's pipes are tens of thousands of individual projects. They have to negotiate access to the property, they have to deal with things like getting permission to cut down a tree that has grown near the pipe, etc. They've got state and federal money now, but this work is inherently disruptive to the lives of people who live there. The only fast way to do it would be to kick everyone out of the city for a year, dig up every street and yard, then move them back in.",
"Depends on what you mean by \"still going on\" A lot of US cities have high lead levels in their water. Flint is now back to normal levels URL_0 They are finishing up all of the pipe inspections The more important issue is that citizens have no trust in the city anymore",
"Because, while other cities spent the previous decade and lots of dollars to update their pipes, flint didn't.",
"For one Flint is a perfectly example of what happens whenever a city doesn't manage their piping/sewage system correctly. This whole situation was bound to happen. Now why is it still happening? It's not",
"After Flint in 2014/5 Michigan’s water regulations became the strictest in the country. The water is clean. It’s the late 19th/early 20th century INFRASTRUCTURE that is contaminating the water (with lead, mostly). Michigan has recently (2019) approved the budget to have EVERY municipal pipe/waterline replaced within flint (20,000+). Which is great, but nearly impossible to do in a reasonable time frame."
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dgddwm | How do AWD cars get towed on two wheels without being damaged? | Engineering | explainlikeimfive | {
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"Short answer: they don't. Longer answer: they either lift the car entirely (by pulling it onto a truck) OR, they lift it temporarily in just one end and fiddle in a small wagon-esque set of extra wheels that end up under that set of wheels. And then they move their truck around and lift the car from the other end. Same thing as if they can't get the parking brake disabled; the only thing they care about is not causing permanent damage to the car that makes their job cost them money rather than earning them money.",
"With 4WD cars, they simply disengage 4WD and they can be towed safely. With AWD cars, since all four wheels are drive wheels and it cannot be disengaged, they need to be hauled on flatbeds. They can be towed short distances if shifted into neutral, but this still causes the gears in the transmission to be spun around. Without proper cooling or oil flow, this can cause damage to the transmission overtime. & #x200B; Most 4WD and AWD systems operate in different ways. Always consult the vehicles manual to see how to safely tow the vehicle."
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dgfmsi | Why does fixing a dent in a bumper require the removal and repainting of the entire bumper? | Engineering | explainlikeimfive | {
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"The main reason is that when the paint is damaged it’s not recommended to paint only one area. The paint work will be extremely obvious. Typically when painting anywhere on a car, you need to paint surrounding panels as well. It’s called blending and there is no really good way to do it in just one area. Bumpers however are the exception as they are typically plastic versus the other areas on the car which are painted metal. The bumpers on colored cars are almost always a shade or two different from the rest of the car.",
"It doesn't really but if you don't repaint the entire thing to the nearest seam you will be able to see the repair."
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dgjejf | Why do polarized sunglasses make the rear windows on cars look weird? | Answered. | Engineering | explainlikeimfive | {
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"Car windows are are tempered so that if they break, instead of forming large jagged shards that could stab and kill you, they shatter into many tiny little fragments that probably won't hurt you too much. In the tempering process glass is heated up and then rapidly cooled down again. The surface of the glass cools faster than the center of the glass and contracts, causing compressive stresses, while the center of the glass expands, producing tensile stresses. These stresses are what help the glass shatter into tiny pieces. They are distributed unevenly throughout the glass in a somewhat chaotic pattern. When you look at a tempered glass window through polarized lenses, the rainbow patterns that you see are a result of stressed [birefringence]( URL_0 ). The stresses introduced into the glass by the tempering process cause the glass to warp and refract the polarization of the light as it passes through it. And your eyeglasses then selectively filter out certain polarized wavelengths and let others pass through, which looks like a dappled rainbow pattern, based on the variable pattern of stresses in the glass."
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dglck8 | why do some power lines have orange basketball looking balls attached to them? | Engineering | explainlikeimfive | {
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"They're called marker balls. You'll often find them near mountain passes, in the deep valley areas, near major freeway crossings and around airports. The marker balls are placed on power lines to make the conductor crossings visible to aircraft",
"To make them more visible to planes. If you notice they are in areas with water or near bridges. It's so small aircraft, like float planes (though not just float planes) don't strike them."
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dgnckl | What is the difference in unleaded gas and diesel gas? Also why is it so bad to put diesel gas in a non-diesel engine? | Engineering | explainlikeimfive | {
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"Diesel is a non-vaporizing oil. A Diesel engine sprays atomized diesel into the sparking chamber. Gasoline is a vaporizing oil. It goes to a gaseous state when injected into the sparking chamber. Putting a liquid into an environment that’s expecting a gas has a whole bunch of obvious consequences.",
"Unleaded Petroleum is a mixture of thin petrochemicals that is incredibly flammable and will burn with even the smallest spark. Diesel is a mixture of thick petrochemicals that isn’t very flammable and won’t ignite even if you throw a match onto it. Diesel needs to be pressurised to burn. Petrol engines don’t pressurise the fuel, Diesel engines do. Diesel won’t burn in a petrol engine, it will just flood it and make it not work. I’m sure you can imagine how well a petrol engine would work if you filled it with water. Petrol would burn in a Diesel engine but diesel lubricates Diesel engines. Adding petrol thins it out which means that it’s not being lubricated properly and will damage the engine."
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dgndyt | If hexagons are strong tiled shapes in nature, why are they not used more in human construction? | Engineering | explainlikeimfive | {
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"Because for construction, squares and rectangles are just as good and much easier / simpler to manufacture and work with.",
"Cause rectangles are almost as good, and triangles are way better, so the middle-ground that are hexagons is rarely used"
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dgoi6q | How does electrical "grounding" work? How is it there are different intensities of being grounded(grounding rods more strong? | Is it the gauge of wire, the number of power cords used? How is it done in apartments? | Engineering | explainlikeimfive | {
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"Electricity seeks the path of least resistance. If it is able to return to ground, it will; if not, it will attempt to energize anything it can reach. In the case of appliances, if it can’t use the grounding prong, it may try to go through the metal you touch and out through your feet. Things normally work from “hot to neutral” and the grounding conductor is not used. If something goes wrong, the grounding prong allows things to short out “safely”. The point is to trip the circuit breaker in the panel, so that no one gets hurt and nothing catches fire. There aren’t “intensities” of grounding; there is simply grounded or not grounded. We could discuss impedance to ground, but that’s not ELI5. A house uses a rod and small wire; bigger installations use bigger wire to handle the POTENTIAL damages, but they’re still equally “grounded”."
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dgoz6m | why does two syncing turn signals go out of sync after a few seconds. | Engineering | explainlikeimfive | {
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"The typical speed of turn signals is all in the same narrow window but getting an exact speed isn't really important enough to make high accuracy components. The result is that they all have slightly different (but consistent) speeds. If you're watching them, they'll appear to drift in and out of sync as their cycles line up. [Here's a visual representation]( URL_0 ) - notice how sometimes it's just chaos & other times it makes nice patterns? That's the same thing going on with turn signals. This same effect has lead to the widely held (but incorrect) belief that a group of women working together will find their menstrual cycles going into sync with each other. They're not actually in sync, they just line up every so often & will stay close for a few months.",
"They are only close to sync. The odds of 2 circuits engaging at the same time, with the same frequency is practically impossible."
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dgr8kh | How can a smaller engine (e.g., Hennessey Venom V8) make more horsepower than a larger engine (e.g., Bugatti Chiron W16)? | Just saw a video about the Hennessey Venom F5 engine, which apparently makes 1817BHP, which is significantly more than the Bugatti Chiron Super Sport 300+ and its W16 (1578BHP). I'm guessing it has something to do with capacity and RPM, but if that's the case...why not stick with the simpler V8? | Engineering | explainlikeimfive | {
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"Horsepower isn't a static measurement of the engine's power, and it changes at different RPMs. Every engine has a different HP curve across its rev range. Take a look at the torque for each engine though, and you will see numbers that are more proportional to their displacements.",
"An engines power is closely related to the mass of air per time it pumps through itself. The amount of air can be increased by many factors. The volume of each cylinder can be increased(liters), the number of cylinders(v8/w16), the rate at which each cylinder performs a cycle(rpm), the pressure of air(turbo/supercharger/psi). All these factors add to the total air molecules an engine can pump through itself. Everything in an engine comes down to pumping/sucking this air in, mixing it with the right amount of gasoline(14.7parts air:1part gas), exploding it, and getting new air in. More air=more power. Other factors (such as engine efficiency) play minor roles in the amount of power produced, when compared to the amount of air being pumped through."
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dgxeva | Why do sidelights always have a combination of red and green on left and right sides of airplanes or boats? Why red and green? | Engineering | explainlikeimfive | {
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"Red sidelights will always be on the port(left) side and green always on the starboard (right). I’m not sure about planes, but on ships, the lights will be able to be seen from straight ahead of the vessel to 112.5degrees on its respective side. So if it is dark and you can see a sidelight, and can observe its direction of movement, you can roughly tell what direction that vessel is going and how fast in relation to your own. This is a really vital tool in order to help lookouts and ships masters avoid collisions.",
"So you can tell if a vessel is approaching or heading away. Especially at night, it is very difficult to tell from the silhouette whether it is coming or going - and this is a big deal...",
"They are high contrast colors, easy for the eye to pick out of a field of black(night sky or sea at night). They are always on the right and left so that you can tell the direction of travel depending on how they appear to you. Green=right red =left. If you look at it and see them like this you are behind it or it is traveling away from you. If they appear as if the green is on the left and red on the right it is traveling toward you."
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dgxks8 | Why are some fairly modern commercial aircraft built with propeller engines instead of jets? | Engineering | explainlikeimfive | {
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"Propeller aircraft are usually less expensive for the same size and slightly more fuel efficient, although slower. Makes for a compelling argument for shorter routes without high demand, and some airlines such as Silver run almost exclusively propeller aircraft.",
"One thing to keep in mind is these are still jet engines, and not piston engines. They just don't use the jet stream for propulsion, but propellers connected to the engine via a shaft. The reason is that propellers are more fuel efficient at speeds they operate at. Regular turbo-fan engines really need to be bigger to be efficient (thats why you see companies like Boeing trying to use the biggest engine possible in latest 737 models). They are also cheaper to produce and mantain. One nice \"safety\" feature of using turbo-props (thats how their are named) is that you have very little power-lag in case of aborted approach. Because they regulate thrust by propeller angles, the engines are allways runnig at speeds, so there is no spool-up if you require thrust, you just need to change the angles.",
"Not sure how modern is modern, but the Seattle-Portland route that Alaska flies is frequently serviced by Dash-8s or Q-400s, I believe. It was designed in 84, with the upgraded Q-400s coming out in the 90s. Edit: they are more fuel efficient and require a lot less runway to take off.",
"TLDR: The aircraft are cheaper to operate and more efficient for short-haul flights. Disclaimer: I used to work for a commuter airline that operated only propeller driven aircraft. Modern commuter aircraft use Turbo-props which are effectively small jet engines with a propeller attached to the front via a gearbox. Turboprops are considerably more expensive to build and maintain vs traditional piston engines but they are more powerful and much lighter. For aircraft weight is everything, as each pound you can potentially carry directly translates to profits. But why turboprops instead of Turbofans (jets)? Turboprops use less fuel than Turbofans but consequently they also fly slower. What airlines discovered in the late 80's, early 90's is that passengers on commuter (short) flights were willing to take slightly longer flights if it meant paying less per ticket. Turboprop aircraft are more efficient for short haul flights. They are also less noisy which means they operate under less restrictions than jets. They can also land in shorter distances, and even operate on gravel with fewer modifications making them better suited for operating out of smaller airports. Particularly in the far north where airstrips are mostly chip-stone/gravel. All aircraft are very safe (statistically speaking you are hundreds of times more likely to die in the cab on the way to the airport) but *arguably* turbo-props are safer than jets because they can land in shorter distances, propellers are more responsive (less throttle lag than jets) and can fly at much slower speeds making them easier to handle in the case of an emergency."
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dgyre6 | Why do violins still have traditional wooden tuning pegs instead of modern mechanical gear tuners like guitars? | Having played both violin and guitar, I never understood why violin makers insist on using those primitive wooden pegs that were used for 500 years rather than the more convenient mechanical tuners found in modern guitars, and even on upright basses. Why is that? | Engineering | explainlikeimfive | {
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"There are geared tuning pegs for violins, but the violin remains heavily rooted in centuries old tradition, so it takes a long time for changes to really take hold. Violins used to have strings made from intestines. Now metal strings are the norm. Changes can and do happen, it just takes a long time. Having said that, there is not a huge advantage of geared pegs over traditional. Yes, they're easier to use, but properly fitted standard pegs work fine once you've had some practice, and since massive amounts of discipline and practice are required to play the violin even remotely well, something as small as updating the tuning pegs doesn't seem all that important."
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dh3rrc | how do major cities avoid flooding? tokyo just got hit with a major typhoon and is fine, but the country side is heavily damaged. | Engineering | explainlikeimfive | {
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"Tokyo actually has a massive anti-flooding tunnel system. This helps them from having issues from flooding in a very wet region of the world. URL_0",
"Large-scale design measures are needed to prevent flooding in big areas. Sometimes natural disasters overwhelm the design measures — Houston floods on a pretty regular basis, and Louisiana has issues, despite major engineered systems that are supposed to help. Tokyo has invested a lot in planning for typhoons — likely major pumping systems, drainage planning, and planning of land use. Major engineering for things like that aren’t done as much in rural areas.",
"Tokyo has massive tunnels and silos underground that can contain floodwater and has pumps that can pump floodwater into a local river that were built to avoid flooding from typhoons and during the rain season",
"Parts of New York City flood often when there is a monthly high tide, or basically any storm, so I would say they don't always avoid it well. [ URL_0 ]( URL_0 ) It all comes down to infrastructure. Most cities have some sort of drainage infrastructure in place for regular storm water, and if a region habitually floods, that structure is more likely to be built to accommodate large amounts of water. Most non-city areas don't have the resources to build that infrastructure, and can't always rely on natural water outlets to accommodate excess water in a way that doesn't cause issues.",
"No idea about Tokyo,but I lived in New Orleans for a while,and that city is in a bowl below sea level. To avoid flooding they have huge pumps that can move something like 80,000 gallons a minute of URL_0 least once while I was there, the rain was so heavy that even those pumps could not keep up, and that was not a hurricane, just heavy rain. Urban planners install complex drainage systems to take water off the streets and direct it out to someplace where it will not be a problem. Rural areas mostly just depend on whatever natural drainage there is, and at best the drainage systems are local and rudimentary: ditches along roads and such.",
"Many don't. But those that do avoid it do so by having anti-flood systems. That is massive drainage systems feeding into tunnels or canals that move water away from street and housing areas and into places that can better handle the water. These systems also often involve pumps to move water from low lying areas into the channels or holding tanks for it. Tokyo has invested a lot into such a system.",
"Not only does Tokyo have a better flood drainage system, I imagine other utilities are more well-protected inside the city than out. If your power line is knocked over, someone’s going to come along to fix it much more quickly in a metropolitan area of 13 million than in a small village of 500. The same probably goes for keeping water, gas, Internet, etc. flowing. The roads will be rebuilt first inside the city; the stores will get restocked first."
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dh4r76 | I read today that the SpaceX space suit was tested to double vacuum pressure. If a vacuum is zero, then how can it be doubled? | Engineering | explainlikeimfive | {
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"A single vacuum pressure is when you have a vacuums worth of pressure on one side and an atmospheres worth of pressure on the other. If they double this, this means they had double atmospheric pressure on one side and a vacuum on the other.",
"We typically consider pressure difference, not absolute pressure, in tests. Even if one side is close to absolute zero pressure (a vacuum) you can still alter the pressure on the other side and that will increase or decrease the forces on the structure.",
"It means the pressure difference was 2 atmospheres. If you have a suit pressurized to 1 atm with a vacuum outside, the difference is 1 atm. Pressurize the suit to 2 atm in a vacuum, and now the difference is 2 atm."
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dh6bdk | Why so computers consume more power when doing intensive tasks when the computer efficiency is basically fixed at the CPU speed and even if computers are idle the CPU is still ticking at the same speed? | Engineering | explainlikeimfive | {
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"The transistors in your CPU only use power when they're changing states. The clock(which is what you see the frequency for) helps time the moving of data and instructions from one stage to another, but if your computer isn't adding anything then the transistors in the adder will stay in the same state using no power until it starts adding again. Every time the clock ticks it'll release the data from the adder into the answer register but its the same answer as before so just a few transistors flipped states. If you have lots of stuff going on then the transistors in the adder are changing every cycle, and the floating point unit is multiplying its heart out, the memory controller is pulling things in and out of RAM, while the scheduler and instruction decoder for every core and pulling a fresh instruction each cycle. In the second case the majority of transistors in the core will be changing state every cycle so it creates a lot more heat than when just a few are changing state",
"No computer sold in the last 10+ years has its CPU ticking at a constant speed. & #x200B; This is true of very simple computer systems, but for years and years, computers have automatically changed the speed of the CPU cores depending on workload. Also, the exact details of what your computer is doing will cause different circuits to be turned on and those have different power usages (Especially, for example, if the GPU is used.)."
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dh8kg5 | Can anyone explain to me what are those tubes for? | I dont know how to explain what I mean that's why here's a link to it: URL_0 It's a tube on a cable that's holding shopping mall logo above the entrance- there are several cables all of them connected to the ceiling above Do you guys have an idea what those tubes are? | Engineering | explainlikeimfive | {
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"text": [
"They are used to tighten the cable. It's not one cable, but two and they are linked by that thing that can screw closer and make sure it has the required structural integrity"
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dhkj4r | Why is it that most airliners have the small wings behind and the big wings in front, and very few have the big wings behind and small wings infront? In other words, why is the Canard arrangement so rare? | Engineering | explainlikeimfive | {
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"With a \"regular\" tail, if the plane gets tilted so that the tail is really high or really low, the air pushing on the tail will push it back toward normal, level flight. With a canard, if the plane tilts so that the canard is really high or low, the wind pushing on the canard will push it even farther up or down. So a plane with a canard is harder to control.",
"Stability is problematic in Canard systems. Modern computers make it less of an issue, but what if the computer has a problem?",
"The [Piaggio Avanti]( URL_0 ) is a beautiful plane with great specs. I'm not a pilot so I can't speak to its actual value compared to other planes of similar cost.",
"If you look at what aircraft have canard wing you will fine that they are primary used in supersonic fighter with [delta\\_wing]( URL_0 ) (triangular wings) where is can be useful for improved maneuverability and lower stalling speeds. The airplane trend to be unstable and have increased supersonic drag so max speed is reduces. The design also sensitive to where the center of mass is in the aircraft. The current fighter with canard wing are unstable design that need computer to keep them stable in the air, it increase maneuverability but reduce safety. So it can be a good idea on a fifer where you care about other stuff then you do for passenger aircraft. & #x200B; The fist used in a moderna aircraft in production was Saab 37 Viggen where one main reason was that you can when you land use it as a air break and also push the front wheel down. The result is that is can be used on short runways only 800m long for take off and 500m long for landing in a dispersed base system that is harder to disable then 2000 m runway the previous Saab 35 Draken used. The design have no large advantage and lots of disadvantage for subsonic aircraft when you car about fuel efficiency and safe aircraft. The only use on a commercial passeger aircraft was on the Soviet [Tupolev\\_Tu-144]( URL_1 ) . mall retractable canard to control the airflow. It is was a supersonic passenger aircraft like the Concorde with a delta wing. If was taken out of service after 55 scheduled flights because of poor reliability and crashes in testing. There is also a few small propeller pusher aircraft with the propeller in the back like the [Rutan\\_VariEze]( URL_3 ), [Beechcraft\\_Starship]( URL_2 ) . The design never have become that popular. So the advantages of a canard wing is in in situation that do not apply to commercial airliners. You care about efficiency not maneuverability in a large speed range like fighters do."
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dhpzua | How come sometimes the water gets hotter in the shower when you flush the toilet? | Engineering | explainlikeimfive | {
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"text": [
"Because the cold water goes to fill the toilet and not to your shower. If there's not enough cold water for both your shower gets hotter as it's a higher percentage of hot to cold than previously."
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dhqzqs | Where do astronauts park their rockets when they go to the ISS and how do they enter it? | Engineering | explainlikeimfive | {
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"text": [
"There is a docking port on the ISS, and they link the capsule up to that. Keep in mind that by this point the booster rockets are all gone and have fallen back to Earth, so the only rockets left on the capsule are the maneuvering rockets.",
"The \"rockets\", the boosters that actually get them into space, don't actually go all the way to the space station but rather detach and fall back to earth as they ascend. The capsules they ride in dock with the space station, you can see a video of it here - URL_0 (there are other videos on YouTube as well). Basically, it has a docking port that connects to the docking port on the ISS. A seal is formed between the station and the capsule, and they can then open it up and move between the two freely. It's not like a 7/11 where people are going there constantly, these are planned missions executed by governments and they are spread out over several months."
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dhv06v | How exactly do gears (In cars) work? | Engineering | explainlikeimfive | {
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"When you are dealing with engines, you can use the output to do one of two things - generate torque or generate speed (one at the cost of the other). Torque is good for moving heavy objects, and speed is good for, well, speed. The problem is that you only have _one_ engine in your car, and sometimes you need torque, while other times you need speed. Since you can't have custom engines for each purpose, we solve this problem with gears. Connecting the engine to a large gear and it will rotate the gear slowly, but the gear will rotate even when under heavy load (torque); connecting the engine to a small gear will rotate the gear quickly (speed), but the gear will not rotate when under heavy load. The transmission in the car is responsible for shifting gears in response to what you need from the engine at any point in time. When you are just staring moving, you need torque to get the car going, so you use a low gear to generate that. However, you quickly max out the speed the engine can spin that gear, so you upshift to a smaller gear, which can generate more speed.",
"The engine in most cars can only be spinning at speeds between about 500 and 5000 rpm. If this was directly connected to the wheels, the car's top speed would be ten times the car's slowest possible speed. Gears allow you to change how fast the wheels spin relative to the engine. For example, in first gear, every rotation of the engine could be one rotation of the wheels. In second, every rotation of the engine is two rotations of the wheels. Now, instead of the top speed being limited to ten times the minimum speed, it is limited to double that. Add more gears, and boom, you can go any speed between like 5 and 150 miles per hour. You may notice that for speeds below 5 miles an hour, there's a bit of an issue. This is where the clutch or torque converter (manual vs automatic transmissions) becomes useful - it allows the engine to spin at slightly (or completely in the case of the clutch) different speeds from the gearbox/wheel assembly. Those sort of require a separate explanation, tho.",
"The bits that are extended from the circle of the gear enmesh with eachother, acting like a long series of levers that push against one another. Because having two different lengths of lever act on another can let you exchange torque for speed, and the \"lever length\" of a gear is the distance from the center of the gear to the teeth of the gear, you can exchange torque for speed with gears as well. Imagine an example where you have two gears, one big and one small. The big gear has 25 teeth going around, and the small one has 5. This gives you a 5:1 gearing ratio. If you manually turn the big gear, you will be exchanging torque for speed. It will take more effort for you to spin the big gear (since its bigger and heavier and all), but because of the 5:1 ratio, each complete turn of the big gear will make the small gear spin 5 times. This is useful if you have a big powerful engine in your car that doesn't spin very fast, but you want to drive the wheels quickly so you can drive on the highway. If you manually turn the small gear, you will be exchanging speed for torque. Because the non-driven gear is bigger, it's like pushing on a big lever with a small lever (or pushing on a crowbar with your arms) meaning that you can can get a lot more torque out than you put in. But, because of the 5:1 (technically 1:5 in this case since it's the other way around) ratio, each complete turn of the small gear will only spin the 1/5 of the way around. This is useful for if you have a less-powerful engine that spins quickly, and you need a lot of torque to get up a hill or start moving. Since I'm guessing your question is indirectly about a car's transmission: A car's transmission is just a few sets of gears that you can switch between depending on how you need to make the car behave. Modern cars will usually have 5 or 6 pairs of gears that they can choose from for different speed/torque ratios (plus one more special one for reverse). 1st gear is a high ratio (usually around 3:1) that's only used under 5mph, just to get the car moving from a stop. From there, the ratio slowly decreases to get higher speeds and lower torques. 2nd gear has a ratio of around 1.7:1, 3rd is 1.3:1, and 4th is 1:1. 4th gear isn't really a gear though, since it's just directly connecting the engine to the driveshaft (since a 1:1 ratio means no echange, why have gears?). Since gears 1-3 all have a ratio greater than one, they trade speed for torque. Gears 5 and 6 have ratios less than 1, meaning they trade torque for speed. Because of that, they are not very good for accellerating and are normally used for maitaining high speeds on the highway. If you're a more visual learner, [This 1936 video from Chevrolet]( URL_0 ) is pretty darn great"
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di8d5d | How does electricity create mechanical motion? | Okay, I vaguely understand that by electrons going through a copper wire filament, it gets heated up and that heat creates light or something. So I guess I understand how electricity creates light (bonus explain this better to me too). What I am completely baffled by is how electricity (which is just the movement of electrons down a wire) creates mechanical energy like turning a fan or a motor. How does this happen? Do the electrons push something or pull something? How can they? Is it magnets? Help pls. | Engineering | explainlikeimfive | {
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"An electrical current flowing in a wire creates a magnetic field round it. If that field interacts with another magnetic field, either from a permanent magnet or one from another wire with current flowing, there is a force produced that can produce motion. Usually the wires are coiled together to increase the strength of the fields and thus the forces",
"It's magnets. When electrons move, they create a magnetic field. Wrap wire in a coil, and you get a magnet you can turn on and off by turning the electricity on and off. This is called an electromagnet. In an electric motor, you mount some electromagnets on a wheel so they can rotate, and turn them on and off so that they pull on a matched set of (often) permanent magnets. The \"electric motor\" wikipedia page has a good animated gif showing this."
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di90ug | How were the first astronaut suits designed when our understanding of Space was still super limited? | Engineering | explainlikeimfive | {
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"Our understanding was not so limited. While people had not been in space when they were designed we did have information about temperature, pressure, lack of oxygen etc. the first ones were not perfect but we at least knew what the big problems to solve were.",
"We already knew it's vacuum up there. It was known back in the ancient days when ancient geographers found out that air in the mountains is thinner than air down in the valleys. The logical conclusion was higher up = less air until there is none at all, which is perfectly true.",
"The first astronaut suites is slightly modified high-altitude pressure suit for aircraft use. The US Mercury space suit was a Navy Mark IV suit with minor modification that is designed for use in high altitude aircraft. Pressure suits was first tested in the 1930s and developed a lot in WWII and after that. The knowledge of vacuum, atmospheric pressure drop and that life need air is from the 17th century. The first time a human lost consciousness from high altitude flight is in 1862 with a balloon that reached 11.8 km Lockheed U-2 introduced in in service 1957 have a service ceiling 80,000 ft (24,000 m) where the pressure is 3% pressure at sea level. So for all practical purpose a sute that work there will work in vacuum. The Bell X-1 reached similar altitude in the late 1940s so the problem of a suite that a human can use in vacuum was developed before the human space flight program started.",
"Our understanding of space was not super limited, so the basic designs were straightforward. As with many things, experience drove improvement, as did added data such as radiation levels and variations."
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diihxh | Why is an LED's anode leg longer? | My understanding is that being grounded is important to extend the longevity of the diode, having the anode make contact first seems shortsighted to me. & #x200B; EDIT: There seems to be confusion on what my question is, it's not > why are the legs of a diode different sizes? but rather, > why is the anode leg specifically longer rather than the cathode? | Engineering | explainlikeimfive | {
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"The anode leg is longer so you can tell which leg is which. Once you place it into your breadboard or breakout board the length of the legs doesn't matter as long as its oriented correctly.... The longer leg allows you to orient it correctly. LED's are polarized meaning if you put them in backwards they don't work."
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dil68x | Why are pistons in car engines circular? | If they were a rounded square shape they could be packed more efficiently for the total engine block size. So why aren't they made this way? | Engineering | explainlikeimfive | {
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"Distribution of stresses are more even in circular pistons. But you must also consider the early history of manufacturing process. To make a round hole in a cylinder block, what you need is a \"drill like\" rotary mechanism either to polish it or to make it very even. To make a cylindrical piston, what is needed is a spinning lathe. These are the basic workshop tools and are easy to train and develop mfg processes to make them efficient. Non circular shapes that are of high precision either become hand crafted and/or require CNC machines for repeatability. Until relatively modern times, it would have been impractical to make these parts using this type of equipment. Also while you can pack things in \"tighter\" conceptually, that isn't really the issue for most engines - you need sufficient strength, cooling etc etc all of which requires space/volume anyway. To reduce the size/weight of the engine - mostly it comes through better materials (like not using cast iron for example) and it isn't the shape of the piston or cylinder that makes the most impact.",
"[1992 Honda NR750 Video]( URL_0 ) has this characteristic (rounded square shape). To answer your question I don't think making the pistons oval was too difficult. It was making the piston rings seal well that was tough to do."
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diswdp | Before modern day technology how were such intricate and huge structures like the Houses of Parliament or the St Victus Cathedral in Prague designed and built so perfectly? | Engineering | explainlikeimfive | {
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"All modern day technology does is expedite the math involved in the design and construction of buildings. The math itself already existed - architects were versed in physics, mathematics and geometry so that they could properly measure all the distances/weights/amounts and ensure everything was both designed properly then placed properly during the actual construction. It's why architectural planes and diagrams exist, as well as why architecture utilizes so many models. Once everything was properly diagrammed and modeled all you had to do was multiply measurements to scale and you got the exact coordinates and distances for *everything*. All we've done is switch the diagrams to digital, the models to 3d graphics and the math to calculators. A lot of older tools are still employed to make certain everything is level and properly placed. Outside of that, a lot of hard work is the answer. This is the son-and-grandson-of-architect-but-not-architect explanation. An actual architect can probably explain it better. EDIT: Well I did not expect this to blow up! I've learned a lot, thanks everyone for the discussion!",
"Victorian Era engineers often built things with huge safety margins. (Not sure how strong to build that support? Make a good guess and multiply that by ten.) That's why the Tower Bridge is still cookin' along, not to mention that the Thames embankment sewer structures were only recently outgrown after 150 years of service. Also, you can tour Europe and see examples of Cathedrals that, in their builders' desire to build taller and more elegant churches, exceeded the limits of material physics and had to be reinforced... or taken down, if they didn't outright collapse. Check out Beauvais Cathedral, for example. FWIW, I'm not deliberately beating on the French here (yes, I am). I'm merely citing examples near to mind and one should not take this to mean English engineers are any better than their French counterparts (they are). *dons his trusty Brodie helmet and awaits the inevitable onslaught from across the Channel* 😁",
"Also dont forget they often got it wrong - many buildings and cathedrals collapsed during construction and had to be rebuilt (e.g. Lincoln Cathedral in 1237) . There was a lot of trial and error and rules-of thumb.",
"For anyone who hasn’t seen this series, [Secrets Of The Castle]( URL_0 ) is a fantastic documentary about how things were done at a 13th century castle. I’ve learned so much from this series and constantly go back and watch it.",
"With a semi-circle and a straight-edge. It also took forever, like 20-30 years. Part 1 - Before very recently the math involved was trig and geometry and it had to be done by hand. You can actually design a building with a straight edge, ruler, and fractions. Algebra is a recent invention, the Greeks didn't have it yet their structures have stood the test of time. If you know how long the blocks of stone are that you are cutting (as an example) you can give me a precise measure measurement based on the full size using a fraction. This is specific to the job but it is effective. We see examples of this across the ancient world, from India to the Incas. Part 2 - The time, we can make a building like Freedom tower in about 7-8 years, not an insignificant amount of time but not terrible considering the scale of that project. A lot of that time includes construction not conceived of in ancient times, like AC, plumbing, electrical, windows, radio towers, etc. Considering the added stuff we have to do in modern times, 7-8 years is bloody rapid. In ancient time we were looking at lifetimes of work for one building.",
"The novel Pillars of the Earth goes into a lot about medieval construction techniques and is also just a great read.",
"There was a time before consumerism where the vast majority of people worked until they died on projects such as these, in order to make enough income to put food on their family's table, a roof over their head, and taxes paid to their king. A lot of them may realistically have not had the option to work on anything else. ELI5: Cheap abundant labour, construction lasting decades/centuries, and not many other jobs to do.",
"Go to Rome, and realize we've been doing that for WAY LONGER. And then go to Giza, and see the Pyramids. It's all math. If you do the math right, the structure will hold. Marcus Agrippa was an amazing engineer. His works are all over the empire. You just go with what bears the load best, and then design around that. Take Jenga for example, you instinctively know what blocks may or may not cause structural failure.",
"They were (most of the time) over-designed. One of my favorite quotes is \"Any idiot can make a building stand up, but it takes an engineer to make a building barely stand up.\"",
"The thing modern tech do is mainly speeding up the building time. eg. Its faster to use a machine than having people dig a hole manualy with spades, showels and picks. Or lifting stuff with a crane over using a simple rope´n´pully system. The large old building could take 100s of years from start to finish.",
"Let's just say, for example, that there were twenty buildings erected 500 years, but only two survived to the modern day. It might be argued that those two buildings were \"**designed and built so perfectly**\" yet 90% of the buildings failed. It's a fallacy to look at all such extant buildings and conclude that engineering was so perfect in the past."
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divhtp | How do traffic lights around a city, state, or even country coordinate with each other so they don’t cause constant traffic jams? | Example being that a light has to go off around the same time as the one behind it or 5 blocks behind it so that as soon as you go past a green light the next light doesn’t automatically turn red. | Engineering | explainlikeimfive | {
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"There are many ways this can be accomplished, such as timers and inductive sensors embedded in the roadway that detect the presence of vehicles. In a city I once lived in (Fredericton, NB, CANADA) the lights on Brunswick street were on a timer. Once you caught the first green, the rest would turn green as you approached if you accelerated at a reasonable rate",
"Back in the old days, they would all be on a timer, so the traffic guys would drive a car to see how many seconds it took from light A to light B and set the timers appropriately. Nowadays, they are often all networked together, either with fiberoptics, or even old fashioned cat 5, which enables a central computer to adjust their timing constantly according to traffic levels."
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dizzjs | flat head screws. With Robertson and even Phillips widely available, why in the world are flat head screws still being produced? | Engineering | explainlikeimfive | {
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"flat screws can apply more torq than phillips. and cheaper to produce than phillips. phillips are intentionally designed to cam out when alot of torq is applied. the point of using phillips is to help power tools. with phillips the tool don't slide out from the head. that happens with flat slot. robertsons are superior. but they're even more expensive than phillips. most applications don't need expensive screws",
"Quick note about long term durability. You can easily re-cut a slot to remove a screw, but you can't easily re-cut any other shape. That's one reason marine bolts and screws are often slotted.",
"With almost everything where someone says \"I'll make a new standard and it will be better!\" The new thing gains some ground but doesn't eliminate the old one. Instead we now have multiple standards for lots of things from screws and electrical plugs to audio and video standards. When someone makes a new one the old one still sticks around for at least a while. I learned from [this great video]( URL_0 ) about the history of a lot of screw types. I'd recommend checking it out. Turns out Phillips started out being used by Henry Ford because it's naturally torque limited. Workers on the factory line could just run a tool as hard as possible and when it kicks out of the screw its tight enough. Because they ended up on Fords which were everywhere they started getting used more often. Hex and Torque screws both started out as proprietary security screws in electronics that the manufacturer didn't want you taking apart. People quickly recognized their superiority in resisting stripping and they gained some popularity as well. & #x200B; edit: I might be a little off on Henry Ford's involvement, but watch the video it explains it better than I can.",
"One thing that I didn’t see mentioned in other comments is that it seems flat head (or “slotted”) screws are commonly used in places were they might be painted over, but due to their design, paint can be scraped out of the slot, allowing the screw to still be removed. You might encounter this situation when doing home maintenance, specifically with outlet and switch covers or door hinges that someone negligently painted over. If other screw designs—such as Phillips (plus), Robertson (square), Torx (star), and Allen (hex)—are painted over, it can be near impossible to unscrew them without using some sort of extractor tool.",
"*Slotted* screws may be used where a power tool might apply too much torque, such as on soft plastics. Some are designed to be opened by a coin or other flat bit of material, allowing for easier access. Slotted screws are less likely to trap other materials and easier to wipe/brush clean, such as on food prep stations. And when you're in a pinch, a hacksaw or Dremel disc can cut a slot into almost any screw. (Flat head screws are the ones that can be driven so they're flush with the surface. Common drywall screws are flat-headed, but driven with a Phillips driver.)",
"Slotted screws are also used for visible trim components like light switches and door hardware because they are considered more attractive. Square and cross screws have an association with construction or industry and slotted screws are considered more refined. Most brass screws, for example, which are intended to be visible are sloted. At least that's my impression after a decade in fine carpentry and home building.",
"Slot head screws are (were) often used in machinery or areas that would be painted over because one can use the same tool (screw driver) to both clean out the drive slot and turn it",
"I would prefer flat heads to phillips, honestly. You can apply more torque with less threat of stripping. That said, I do prefer robertson (square) screws to both flathead and phillips.",
"technical reasons aside, a line of exposed screw heads looks much better if they are flat/slotted than anything else. slotted brass screws in natural wood looks nice, phillips or anything else looks awkward and unfinished to me.",
"I hate phillips head screws so much. As somebody who's built an absolutely ludicrous amount of furniture by hand out of obstinate refusal to buy a powerdrill and a bunch of friends and siblings who needed help setting up new homes, The number of screws I'd stripped by one slipped hand or just trying to undo a very tight fit... ugh. Flatheads virtually never strip, while phillips head feel like they were fucking designed to strip at even a slightly non-ideal angle.",
"Slotted screws are common in electrical work due to the versatility of the flat head screwdriver. You can use a flat head to tighten/loosen conduit locknuts, pry out parts/nails/staples, debur freshly drilled holes, adjust the fit of bulb and switch housings, knock out tabs and punches, and sooooo much more. Holding screwdrivers (which hold the screw when you remove it from or insert it into a terminal) also work the best on slotted screws. When you're lifting leads in a cabinet where dropping a screw could have serious repercussions (or at least a temporary inconvenience), a good holding screwdriver is a must-have.",
"Flat heads are used for a couple different reasons. I hate them, but there are reasons. -They’re cheap to manufacture. -They’re aesthetically unobtrusive. For things like light switch covers, your eyes aren’t drawn to them. -They’re the easiest fastener to remove when you don’t have a proper screwdriver available. Again, light switch covers come to mind. Don’t have a screwdriver? Grab a knife or whatever other flat skinny thing you have handy. -They’re easy to clean debris or paint out of. They’re pretty common for fasteners that are meant to be used on floors or the bottom of boats and other marine equipment because you can quickly and easily scrape paint or debris out of the slot if it’s been covered up. This can be difficult or impossible with a phillips/robertson/torx/allen that’s filled with paint or whatever else. -For really small fasteners, like tiny set screws, you can apply more torque with a flat driver in a smaller area. If you’re familiar with jewellers screwdrivers, you’ll know what I mean. The tiny little phillips screws are easy to strip when you’re taking something old or corroded apart. In small applications like this, the debris filling the fastener drive issue comes up again. Flat head is definitely the easiest to work with there. -It’s easy to visually tell how much they’ve turned. For things like carburetors, where you need to incrementally adjust little screws to calibrate something, you can easily see 1/8, 1/4, 1/2 etc turns. That’s all I can think of right now. There’s other reasons they can be a “better” fastener for certain things. We all hate them, but for some things they work pretty well."
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dj0xf9 | Is it better to kill my phone every day or keep it charged a little and top it off everynight? | Engineering | explainlikeimfive | {
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"Don't kill your phone everyday. Technically best not to have it empty or full actually and maintain it around 25-80% URL_0",
"Modern day battery chargers have known limitations set with every type of battery installed on the phone you're using, so the worry about over charging doesn't really exist in any real mainstream phone. The most common batteries implemented in phones in this day and age is Li-Po(Lithium Polymer). These batteries are commonly engineered for max discharge/performance/use, and charging.. The only issue that's every been noticed with these batteries are high temperature venting... which can typically be described as: instead of venting to the point of expanding and eventually exploding, they actually just burst into flames. That can be good or bad. With a typical Lithium ion cell, the temperature constantly goes up until expansion until exploding. That ends up being similar to a frag grenade... which is very uncommon with typical use. The best lithium ion batteries I used a decade ago were capable of over 100 Amps of use (which is a crazy amount of current for any use of any personal battery). Lithium Polymer batteries are actually more capable of delivering that same level, or well beyond a stable waveform(which is a high and flat peak down to 0 current) than Li-ion, of current for a longer period of time. The great and bad things about Li-Po cells(Lithium Polymer batteries) is that they don't swell and fragment into pieces..they just burst into flames when shorted out to ground(typically with a quality cell within a unmonitored system/unqualified user). I've been involved with with electrical testing industry for two decades... the time period for a lead and liquid based battery system will not be replaced before you get a new phone, i.e. don't fucking worry about it. I've seen battery strings go beyond the amount of time I've been alive. Edit- all these battery tests on how to keep battery cells consistent are over decades old. I'm not saying there isn't sound logic to it, but you're more likely to replace a phone over a broken screen and being outdated over a battery... Li-Po cells are as durable as they get for typical electronics. That's why they're in everything. The only issue comes up when you personally replace those cells with something that isn't capable of delivering for your needs.",
"Allegedly it's best to keep it between 40-80%. So you'd want to drain it to around 40% and then only charge it to about 80%. This, of course, is entirely impractical. But I think you're better off not draining it completely every day."
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dj54th | Whatever happened with that radioactive incident in Russia a little while ago? | Engineering | explainlikeimfive | {
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"I'm not 100% certain anybody outside Russia really knows yet. It was nothing, then it was a missle mis-fire, then it was an accident related to the retrieval of a nuclear missile. Basically, I'm just posting to come back for a better answer."
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djc15l | What are the restraints in building the tallest building known to man? | Engineering | explainlikeimfive | {
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"Vertical gravity loads or compression forces, basically the weight of the material used to make the structure secure increases the gravitational load on the building meaning you need more to hold it up. Which is why many tall buildings are now using a kind of tapered pyramid so the top floors have a small surface area and so less mass pushing down on the lower floors which are spread out over a larger area meaning they can support more weight.",
"1. Structural : The higher your build the more weight the lower floors and the foundation have to support. Since we are using the same material you need bigger surface area to support more weight. So you have to make some choice to allow that higher surface area. You can build a larger bottom floor, but that's not always possible, especially since most tall building are in urban area where space is limited by streets and other building. You can make your top floor smaller, but at some point there isn't any place remaining for people to live there. Or you can have a bigger structure inside your building, which limit the amount of usable space inside. 2. Ground : The ground have a specific capacity to support weight by surface area, so if you build anything too high, the weight per surface area will be too big, the ground will not support your building. You then need more complex foundation to increase the capacity of the ground and those cost more money. If you are lucky, you can reach the bedrock, but if you are unlucky it might be impossible for the ground to support some amount of weight, creating a limit on how tall you can build. 3. Mechanical : A building isn't just structure, you need to send water, move the air, have elevator, electricity, air conditioning, etc. The higher you build the more difficult it is to make those things work. For example, elevator have a limit on how high they can go, because they are supported by cables, so for the highest buildings you need the elevator to stop at some floors, people go out and go into another elevator to go higher in the building. So as you build higher it cost more and more. 4. Building material : Usually concrete is made at ground level and them a pump bring it up high, but if you build too high that just doesn't work. You need incredible pressure to bring the concrete as high and the time it take is too long, the concrete might solidify before it reach the top. So you need new method that are rarely use, since such high building are so rare. This add even more cost. It similar with other construction material, you need impressive methods to bring steel so high for the structure. Conclusion : What is important is the price per usable square foot. As you build higher the price per square foot drop, but only to a limit. At some point the increased price for building higher will increase the price per square foot and building two towers of a certain height will cost you less than building 1 tower of double the height. Usually the biggest building are not really efficient, their price per square foot is really high and it's more about the prestige, people are ready to pay more for the same square foot. But there is a limit on how many of people are ready to do that, so you can't build super high building everywhere.",
"I assume you mean constraints? Tall buildings are heavy. To support the weight of the floors above, the beams and columns need to be very large. Large columns are heavy. All of this weight needs to be transmitted into the building's foundation, so the foundation needs to be very robust to withstand all the load. Tall buildings also get lateral loads when the wind blows on them (Because it's such a large surface, it's kind of like a massive sail.) So aside from just the building load and the occupant load, you also have big wind loads to deal with. Cost and zoning are soft constraints - can the neighbourhood support a 200 storey building? Where will all of these people park?"
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djpkz1 | why do companies like AT & T have workers hand dig holes every 20 or so feet to lay fiberoptics/wiring? | Engineering | explainlikeimfive | {
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"That work is likely contracted out, so it's not AT & T, per se. They'll dig holes periodically in areas where they suspect other lines may be. Digging by hand, if you got a pipe or wire, you can either feel it and back off before damage, or if it is a simply cut, the repair is easy. Heavy equipment like a 'ditch witch' or an auger type tool would severely mangle that same obstacle to where repair would be much much longer, not to mention a cable wrapped up in a machine can be a huge problem too. It's just safer than trying to one-shot a tunnel, trench for the fiber.",
"It's not really every 20', it depends on the use, if it's in a residential area these Pull Holes will be closer together but in more rural areas they can be up to about 250' apart or will just be up on power poles instead. The labor is contracted out but they don't just tell these guys to go out there an lay cable. AT & T will have a topographical survey done first, then they go on to design plan & profiles for the cable/FO, the whole design process can take anywhere from a couple months to a couple years depending if any railroads or pipelines are being crossed. Source: I designed AT & T underground cable installations for a year."
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djvuh2 | Why do some supermarket entrances have that ribbed floor? | It's so loud and annoying to push a cart over it but there must be a reason. Thank you all for your answers, I think the majority agree it's to shake water and dirt off the cart if the carts get rained on. | Engineering | explainlikeimfive | {
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"It knocks dirt and mud of shoes and wheels. It lets the place look a little cleaner at the end of the day so they don't have to clean during the day."
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25
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dk17y1 | How do gas stoves turn off the flow if the fire goes out? | Engineering | explainlikeimfive | {
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"I don't think they do? Well, when I had one they all had pilot lights. This kept a slow burn of the gas flow at all times. When the pilot light fails/is blown out the gas flows freely as it's not being burned. House smells like ass, you check the pilot lights etc.",
"My stove doesn’t do that but it will keep sparking if there’s no flame. It has a thermal sensor that tells it to spark if it is below the ignition temperature of gas. I would guess that your stove does the same but is connected to a solenoid valve that cuts off gas instead of the sparker"
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dka7dh | Why do American toilets have so much water in them? | Engineering | explainlikeimfive | {
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"it depends. alot of residential toilets are old. there's no need to replace an old but working toilet. the old standard was 3.5 gallons. since 1994, it was 1.6 newer units use 1.28. but if you walked into a home built in the 1960's or even older, you'll probably still find a 3.5gallon toilet",
"Water contains the smell. Compare this to, say, shelf toilets, squat toilets, etc. - both of which have their own advantages, but one disadvantage is that they let smell propagate more. The more water your poop and pee can be submerged or diluted in, the far weaker the smell."
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dkdlwp | how does a space bar stay level? | Engineering | explainlikeimfive | {
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"Space bars have a leveling bar attached to them, similar to a sway bar in an automobile suspension. Force on one side is transferred in part to the other side to keep it even.",
"There's a metal bar that goes from one side of the spacebar to the other and is anchored to the base of the keyboard. If you pull your spacebar up you can see it. Shift, Enter and Tab sometimes have them too."
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dkehqi | Why do certain four-cylinder engines like the Chevy Colorado get worse mileage than any other four-cylinder, like a Honda Civic's? | Engineering | explainlikeimfive | {
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"A variety of factors play into gas efficiency. One of which is weight of the vehicle. The curb weight of a Colorado is 4,000+ pounds whereas a civic is < 3,000 pounds. The aerodynamics of the vehicle also plays a factor. A sedan is generally more aerodynamic than a pickup truck. And each engine style is made different. An engine can be optimized for fuel efficiency or for power. Both can be four-cylinders, but one optimized for power would get less mpg than one optimized for efficiency"
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dkpzxn | How does a car blinker work? | Engineering | explainlikeimfive | {
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"In the past, car flasher units used a Bimetallic Switch similar to that used in simple analog thermostats. A bimetallic strip is a piece of metal composed of two different metals with different rates of thermal expansion, which have been welded together and then rolled into a thin strip. Heating the strip causes one layer to expand to a greater degree than the other which causes the strip to bend or curl. In a thermostat this bending can be used to close a pair of electrical contacts, thereby controlling either a heating or cooling device. Automotive flashers used a small heating coil which was wired in parallel to, and placed next to the Bimetallic strip. Tuning on the signal switch initially heated the coil which caused the strip to bend, closing the switch and turning the signal on. This created an audible clicking sound that also served as an indication that it was working. But when the switch was closed, most of the current then went through the contacts and not through the heating coil which has much higher electrical resistance, because the two devices were wired in parallel. This caused the heater to cool down, after about half a second the Bimetallic strip would cool enough to bend back to it's original shape, opening the circuit again. While this was a pretty reliable and simple system, fabricating them could be a bit delicate. The heating coil could also fail or fracture due the the material stresses caused by hundreds of thousands of heat-then-cool cycles. Modern cars use a wholly electronic switch that is smaller and more reliable, lacking moving parts. It's also easier to fabricate because high temperature swings aren't a concern. This typically consists of a capacitor and a transistor that is controlled by charging and then discharging the capacitor."
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dkxvc5 | Why are the seats bus drivers sit on bouncy not still? | Engineering | explainlikeimfive | {
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"If the bus driver was sitting on a normal chair, all the vibrations and shocks, both from the motor and the road surface, would be directed straight to their spine. This is OK if you are riding the bus for maybe an hour most days, after 8 hours of it for years, their back would be gone. Foam just doesn't cut it for absorbing this stuff. So the seat is fitted with springs and dampers. This absorbs all the vibration and shocks, preventing injury.",
"I don't drive a bus, I drive a truck. My truck has a fully adjustable seat in it. It has more adjustment than the average car seat. The big thing here is the way a bus or truck rides. The suspension is designed for carrying heavy loads, not soaking up bumps like a car. So they added an air spring to the base of the driver's seat (the brakes run on compressed air, so the seat taps into that system). It's purely for the comfort of the driver. You're sitting in the thing all day long after all. The seat in my truck has a big air spring in the base of it, but it also has an adjustable shock absorber as well. You can set the height of the seat using more or less pressure in the spring, and you can set the amount of absorbtion in the shock absorber. If you want to bounce away like you're sitting on a big beach ball, you can. If you want to have the thing firm so you feel every little zit in the surface of the road, you can do that too. Ideally, you set the height of the seat to suit your legs, then you set the shock absorber to suit your weight, so you're not constantly bouncing away all day long, but you **DO** have a little movement in the seat to make the day more comfortable. The seat in my truck also has two lumbar support sections, as well as adjustable side cushions. The whole seat moves forwards and back like a car. But the base can also be moved forward and back independantly of the backrest. The whole thing can be tilted forward and back too. The backrest is two parts that can be adjusted for angle independantly. You can also let all the air out of the spring in the base to make getting in and out of the cab easier, and when you get back in, you flip a single switch, and it goes back to the setting you had it on previously. It's all about making the driver comfortable, and well supported, for long periods of time. On top of all that, the whole cab on my truck also has it's own springs and shock absorbers, so the whole thing floats a little."
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dla6sd | What makes smooth bore cannons accurate, but the same can't be said for rifles? | Learning about modern main battle tanks, some of them have 120mm main guns that have a smooth bore muzzle as opposed to rifling. Historically, muskets and other muzzle loaded rifles weren't considered accurate at anything resembling long or even moderate range. Rifling of the barrels dramatically increased accuracy. Have main battle tanks ever used rifled bores, and if not, how can the smooth bores be accurate enough to effectively engage targets at ranges over 3,000m? | Engineering | explainlikeimfive | {
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"Rifled bore make the projectile spin, which make it more stable in flight and increase the accuracy. But making the projectile spin isn't the only way to stabilize a projectile. Tanks used rifled gun for a long time, but some specific projectile used by modern tank are better without rifling. One of them is the HEAT round is an explosive that shape a jet of molten metal to pierce the tank. HEAT round are more stable in a rifled barrel, but the spinning decrease their effectiveness on target. So in that particular case you could make a case for both rifled or smoothbore gun. But the main reason why modern tank use smoothbore today is because of APFSDS. Those are Armor-Piercing, fin-stabilized, discarding Sabot. Those are like arrow, long but narrow with allow them to concentrate their force into a small area which make penetrating easier. You see in the name that it's fin-stabilized, so it doesn't need to spin to stabilize itself, in fact making that round spin would create air resistance on the fin which wouldn't be good. The M1 Abrams for exemple used to have a 105mm rifled gun using APDS (Armour Piercing Discarding Sabot). The problem is that APDS had a limit on how narrow you can make the round before it become instable even if you spin it. It's about the ratio of diameter vs lenght, once the projectile is about 6-7 times longer than it's diameter, spinning the projectile just don't do it, so fin were used and for that you need a smoothbore gun.",
"The precision of those smoothbore cannons is _significantly_ better than any musket. They’re also computer-controlled to compensate the vehicle movement (be it ocean wavesor land inconsistencies) and aim. Some tanks use sabot rounds, too, which interact differently with rifled barrels.",
"Muskets in general fire a round ball. The ball have bad aerodynamics when if flight and any unsymmetrical part will have a effect on one direction. The did not fit the barrel perfectly because muskets was in general muzzle loaded so you need to be be able to push it down it to load it, this can result in some bouncing in the barrel that reduce accuracy. Rifled increased accuracy because the spinning make it stable like a gyroscope and any unsymmetrical part will not longer push in one direction but get canceled out because of the rotation. Tank guns are breech loaded so the shell fits perfectly and is of hard precision manufactured stuff not lead that can get deformed when fired . What you fire out of them is not a round ball but [HEAT shell]( URL_1 ), [Armour-piercing fin-stabilized discarding sabot (APFSDS)]( URL_0 ) and some more types that if you look at them all have fins that is used to stabilize there path. So they way that they are stabile in the air is just like a arrow. You can see the difference if you drop a arrow or a ball from some height and the effect of wind on each. Tanks used to have rifled cannon in the past. The first smooth bore tank guns was introduced in the 1960 because the long APFSDS projectile, HEAT shells and anti tank missiles fired from the cannon work a lot better with a smooth barrel. The Soviet T-64B main battle tank was the first tank that used that. Today most tank design uses smooth bore guns because APFSDS and HEAT is the best ammunition to defeat other tanks. The fins on the projectile will keep them stable in flight. Fins that deploy or is a part of the projectile make the ammunition more complex so it is nothing you would like to do on small caliber guns. You add the complexity when it is needed like for HEAT even on man portable launchers. There are experiments with [ URL_2 ]( URL_2 ) There are British Challenger 2 because they what to be able to used High-explosive squash head (HESH) that squashes explosible on the target and detonate. It is good against solid amour and bunkers. You can fire APFSDS and HEAT from them by having rotating drive bands, think of a ball bearing, so they do not rotate when fired from rifle gun."
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dlsx8t | Why do trucks accelerate so slowly even though they have lots of torque? | Engineering | explainlikeimfive | {
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"Most trucks have very low gearing. They are designed to move heavy things, not move fast. So they have a ton of torque, and the gearing means it's moving the wheels slowly, but with an immense amount of power."
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dlvk9e | Why did it take so long to invent the bicycle? | To me a bicycle seems like too simple a machine to have only been invented in the early 19th century. Since wheels and frames already existed before that, what had to happen to invent the bicycle? | Engineering | explainlikeimfive | {
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"Quality of materials. Yes, a lot things could only have invented after the used materials to be used also have been invented. Like for the steam engine. The power of steam has been known to the old greeks, but they haven't had the materials strong enough to build a pressure tank. At the moment engineers know all to build a space lift, but we haven't invented the materials for it.",
"It seems very simple now, but someone had to figure it out first and thats the tricky part. Most invetions seem very easy and obvious once they are invented, but obviously no one thought about them prior to them being invented, so it's a little paradoxial. For reference; Humans have been able to control fire since somewhere between 1.4 million BC and 230.000 BC. URL_1 The earliest known written language is Sumerian which dates back to 3000 BC. URL_0 This means that it took at least 200.000 years from the invention of controlled fire to a written language. Almost all of human history has happened in the last 8000 year, coincidentally, about the same time written language has been around, which shows its great importance. Technology advanced exponatially. The first bicycle was invented in the early 1800s, the car in the late 1800s, first flight with the Wright brothers was in 1903, and The US put a man on the moon in 1969. Now we have thousands of sattelites, we have GPS, cell phones, we can send rovers to Mars etc.",
"You need strong steel for the spokes and rims, and later the chain and gears. Iron was too brittle and prone to rust. Steel really couldn't easily be mass-produced and finely worked until the modern era. Also, bikes aren't much good without rubber tires and good roads, both were often in short supply until the modern era. There is a reason trains existed before cars."
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dlw677 | Why only one color LEDs are affected by a static shock? | I walked by a string of multi colored LED Christmas lights after getting off my couch and a static shock zapped one of the blue lights. Now all of the blue LEDs on the string are no longer working, while all the rest of the colors are working normally. Why might this have happened? | Engineering | explainlikeimfive | {
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"text": [
"Each color of LED gets its own circuit. Only one of the circuits has failed; the circuit for the blue LEDs. Further shocks may break other circuits, or simply continue to run through the already damaged circuit."
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dlzgok | What is the difference between a 2-Stroke Motorcycle and a 4-Stroke Motorcycle. | There are 2-Stroke motorcycles and 4-Stroke motorcycles. I always hear that 2-Strokes have better displacement/power. Why is that? Also, why aren't there more 2-Strokes being made if that is the case? | Engineering | explainlikeimfive | {
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"A four stroke engine operates in much the same way as your mouth does when you eat. You take a bite of food, you chew it, you swallow it, you take another bite of food. A two stroke engine operates like a professional eater where chewing is for chumps. You bite, swallow, bite, swallow, bite, swallow. You eat faster but its much less efficient, makes a terrible mess, and sounds disgusting.",
"A two-stroke engine has a power stroke (the gas burns) every time the piston goes down. A four-stroke has a power stroke only every second time the piston goes down. Two-strokes generally produce more power for their weight (because they have twice as many power strokes per revolution), but require oil to be mixed in with their fuel, and generally burn more gas to produce a specific amount of power.",
"Fuel and exhaust are leaving and entering at the same time with two stroke motors due to leverage in displacement. As a result, there is perceived more power but also there is potential efficiency loss of intake fuel leaving as well. Four stroke motors separate fuel entering and exhaust gasses leaving with a separate stroke for fuel intake and exhaust output. Hence the two and four stroke notations. The better displacement per power may also be perceived as better performing due to the lack of metal mass of the two stroke engine. This is due to the include overhead mass of materials for additional valves and mechanics to account for the exhaust and intake cutoff cycles. Lighter engines are smaller by that regard. Those two stroke engines may not need to worry about the scale and ratio of lost fuel with the exhaust along with the engine size.",
"Two strokes have more torque at higher rpms, whereas four strokes have more power at lower rpms and tend to be more efficient. They also tend to be heavier. This is why two strokes are more popular in sports cars and high end racing vehicles (motorcycles and such) where fuel efficiency is less important, weight ratios are more important, and the rpms will be in the higher ranges more often; rather than standard passenger cars where the goal is higher fuel economy and most of your driving will be cruising at low rpms.",
"A gasoline engine combines fuel (gasoline) and air, with an ignition source (spark) to make an explosion. That explosion pushes a piston down which turns a large crank that you can use to turn other things (like a car's wheels). The engine has to take in the fuel and air before the explosion, and then let out any gases from the explosion. Because it's tough to shove air and fuel around, it's easier just to let the engine do the work sucking it in and blowing it out. We'll start in the middle of a four-stroke engine, since it's easier to understand. Once the bang happens, the piston travel down, then travels back up again. When it's traveling up, it's pushing air up, and if we open a valve at the top of the engine, the piston will push all that air right out. That's the exhaust. Then, the piston is traveling down again because momentum, and if we open another valve, it'll suck the air and fuel mixture in, that's called the intake. Then the piston comes back up and squeezes the air and fuel really tightly, because if we push it together, it makes a better explosion than if we just let it hang out, that's called compression. Then there's another spark, and another explosion, that's called ignition. So, if you start from the beginning of a four-stroke cycle, it's Suck (Intake), Squish (Compression), Bang (Ignition), Blow (Exhaust). See, four steps to the cycle, four stroke. Suck Squish Bang Blow is a common expression, and simply put if you have an engine that won't start, you're missing one of those steps. In a two-stroke engine, the steps are combined so it's a little more complicated. A two-stroke isn't as ELI5 friendly as a four stroke. Intake and exhaust happen at the same time. The Intake valve of most two strokes is made so that the intake gases actually sit underneath the piston, than a smaller reed valve is aligned above the piston that opens and let's the air in when the exhaust is going out on the downward stroke. Then compression and bang. Piston goes down after bang and the intake/exhaust cycle happens. Two stroke exhausts are also designed differently. They use an \"expansion chamber\" which as most of the exhaust escapes it allows the pressure wave to expand then some of it go back up the exhaust towards the port. When timed correctly, that helps seal the intake charge into the cylinder. So, why do two-strokes make higher power to weight than a four stroke. That's easy. For the same number of rotations of the engine, the two-stroke has twice as many explosions. However, because of the specialized nature of the timing of a two-stroke, it makes power in a very narrow RPM window, called it's power band. That power band can be tuned by changing the length and size of the exhaust, the size of reed valve, and adding different exhaust scavenging devices. Also, since the exhaust and intake are both open at the same time, it's very easy to put unburnt fuel down the exhaust. Lowering efficiency and increasing emissions. Also, to simplify the engine design, and because the air charge sits underneath the piston where we need oil anyway, it's okay to have oil mixed in with our fuel source and then use that to lubricate the engine instead of having a separate oil system. So yes, two-strokes make more power, but only in a given band. They're also less efficient, worse for emissions, and have the added trouble of how to properly lubricate them. For those reasons, four-strokes have always been the go to for engines large enough for a car."
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dm0npf | Why do trains go through tunnels when there is nothing above the tunnel? | Engineering | explainlikeimfive | {
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"Trains have steel wheels running on steel tracks. This combination have very low friction which makes the trains very efficient but also makes it very slippery. This means that trans have issues going both up and down hill. It is much simpler to just go through a hill then to build a long ramp to get up the hill at a shallow slope. The UK is perticularly known for their flat tracks due to the low power of early trains which set a standard at 0.05% It is rare to see any railways in the world with over 5% gradient. Roads on the other hand will first start signing a steep incline when it is close to 10% as rubber on asphalt have much higher friction.",
"Same reason cars go through tunnels with nothing above them. There are basically 4 options. 1) go over, but this may be too great of a slope for the trains. 2) cut through, basically cut off part of the mountain so there is no dirt above. The benefits to this depend of the location. Can they just blowup the area? That’s quicker and cheaper than a tunnel but cant be done everywhere. 3) change the route of the train. But this may make thing longer, it may not be possible, etc. 4) build a tunnel."
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dm3e1k | How does the transformer increase or decrease voltages and currents. For example, how does a step down transformer decrease the voltage and simultaneously increase current? | Shouldn't the current supplied by the source be the constant? The structure is obviously not some sort of a compressor that compresses the current and make the voltage high of some kind. The knowledge I have about transformer is very basic and that is that the e.m.f induced in the secondary coil depends upon the number of turns of the coil and works on the principle of mutual induction and I also understand the fact that the power is kept constant but how can a transformer *increase* current for say? | Engineering | explainlikeimfive | {
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"The formula for electric power is P = V * I, where V is voltage and I is current. A transformer works by taking the energy on the input/primary side, and putting it through windings/coil, which create a complementary magnetic field. That field, in turn, links the input coil to the output/secondary coil. The magnetic field from the primary changes constantly, as the alternating current supplied to it changes. Each time the primary field changes, it \"induces\" a current to flow in the secondary coil. In a transformer that had the same number of windings in each coil (not a very useful device AFAIK), the changing field on one side would be mirrored by a changing field on the other side, and the current and voltage would be the same. But the idea of a transformer is to change the voltage, usually. So, if you put 10 times as many windings on the secondary side as you do the primary, the changing magnetic field will still induce current to flow, but at only 1/10th the voltage, as there are ten times as many wires that the primary field has to induce that current in. However, each one of those wires in the secondary coil carries current, and so you have ten times as much current on the secondary side as you do the primary. It's important to note that no electrons or current actually cross from the primary to the secondary; all of the action is induced by the changing magnetic field. So you are NOT limited to the amount of current on the primary side. Conservation of energy does apply though, and although there are small losses in any transformer, in general, P^in = P^out. So, since the voltage is reduced by ten, the current must increase by a factor of 10."
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dmrh74 | A lot of building now seem to have 10-20 foot (3-6 meter) high ceilings, why? | As a person who works in an office, I noticed that the ceiling is a good 15 feet off the floor. A lot of building out here and a lot of newer buildings seem to have really high ceilings. Is there a reason? Is there some rule or law that places with lots of activity or people must have a ceiling over 7 feet? | Engineering | explainlikeimfive | {
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"Probably just the current aesthetic. Realistically, it'd cost more to heat and cool. It's not more efficient, but it does make for a more inviting and comfortable space, which can be good in most public or business settings.",
"Also for many retail/office buildings that are rented instead of owned, having taller ceilings allows for a wider variety of companies to rent that building.",
"Heat rises. Some places are beginning to adapt to temperature increases by making ceilings higher. If you go Dubai most buildings are constructed with this in mind.",
"For single or double story building it is actually cheaper. There are regulations that every employee is entitled to a certain amount of air. I think it's 4 cubic meters but I could be wrong. Anyhow, if you make your offices (and classrooms!) higher you can legally fit more ~~sardines~~ people in the same space."
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dmw2xx | why bus drivers' seats are on springs or something to keep them separate from the rear of the bus. | Whether it is school busses or public ones, I always see the driver bouncing disconnected from the bouncing of the bus. Is it because busses have insufficient suspension or what? | Engineering | explainlikeimfive | {
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"Purely for comfort. You either don't spend very long on a bus as a passenger, or you can move around, change position. A bus driver can't because he or she is driving the bus. That's all.",
"Sit on a rigid seat on bumpy roads with bad suspension for an 8 hour shift and you're going to develop back problems quickly. You can cushion the seat but letting the entire seat move with you is more effective at preventing those issue. Buses are really good at moving lots of people from one place to another. They're built for that purpose rather than a smooth ride so rather than change the whole vehicle just make the seat nice for the person who is in the bus the longest. The passengers are only on for a little while at a time.",
"It's usually hydraulic or air damped. If you sit on it the whole day, it makes a huge difference. Large trucks als ohave them, as well as vans intended for longer travels..."
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dmykau | What is happening when an older house starts making creaky noises/ ‘settling in’? | Engineering | explainlikeimfive | {
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"Newer houses do it as well, just not to the same extent. it's all about heat. The materials used to construct the house will expand and contract with heat and cold. Different materials will expand and contract at different rates and that makes them rub against one another a little bit. It's that rubbing that causes the sounds. Think of the sound that ice makes when you pour liquid over it. That's the same cause, the temperature change is causing the material to expand or contract and therefore make noises.",
"Usually it's made of wood. What happens is that wood, along with moisture on or around it likes to expand and contract depending on the temperature. If it gets hot, it expands rapidly, and if it's cold its contracts rapidly. The noise you hear is the friction and a single piece of wood rubbing up on another piece of wood. Thus happens usually at the joints or intersections of the pieces of wood. If you hear a loud like bang, it means that there was a lot of pressure build up on a piece of wood and when it finally gave out and it moved, even a fraction of an inch or centimeter, it made a noise when it settled again. Rub your hand along a smooth wooden table while dry, it will probably just move over it. Now wet your hand slightly and you will notice the jumps due to friction. Now put some pressure on the table and try to move it again. It will jump further and make a louder noise."
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dmzy6q | How does the dual function of your rearview mirror work? | When you flip the switch on your mirror and it moves slightly then shows everything dimmed. Typically used during the night to dim headlights. Is it a reflection? Or a second piece of mirror in the assembly? | Engineering | explainlikeimfive | {
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"There's a pyramid like piece of glass inside and \"flipping\" the mirror looks at a different angle reflecting more light away from your eyes.",
"A car mirror is slightly wedge shaped. Ordinarily, you see the image reflected off the reflective surface on the back of the mirror. When you change the angle, that image misses your eyes, and instead you get the weaker reflection off the front, which is just like seeing a reflection in a window."
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dn0eju | why are diesel engines louder than gasoline engines? | Engineering | explainlikeimfive | {
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"Fuel is injected into gasoline type engines at 20 to 80psi Fuel is injected into diesel type engines at 10,000 to 30,000 psi Most diesel powered engines are equipped with a turbocharger, which actually helps muffle the exhaust sounds. Even still, due to the density of metal, sound is transmitted much quicker and louder from the engine compartment of diesel powered vehicles."
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dn5j1m | How dangerous can it be if a plug isn’t pushed completely in? | So for a long time I’ve been having to check plugs to make sure they are plugged all the way into the outlet, I have been trying to stop checking so much but I do have a question that hopefully someone here can answer. Is there actual danger if a plug isn’t inserted all the way or am I just worrying for no reason? | Engineering | explainlikeimfive | {
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"If it is in direct contact with the metal receptacles, then it is unlikely to be dangerous. Problems begin when there is a space between the contacts that will allow it to arc--that can create heat, which can cause a fire. If this is a laptop, cell phone, etc., then the danger is reduced--they tend to use very little amperage (the amount of actual electrons passing by), so are less likely to cause problems.",
"Nothing to worry about as long as you don't have little children walking around with forks. :) Some folks recommend that outlets be installed \"upside down\" (with the grounding plug on top) or, if it's sideways, with the neutral conductor on top just to protect against the case where something falls onto an exposed plug. If a plug isn't all the way in there's a small chance that it could heat up if the device is pulling a lot of amps, but you have breakers and other circuit protections to keep it from being much of a safety issue. From the tone of your post I'm getting a bit of an OCDish vibe, like this might be negatively impacting your life. If you're finding that plugs are actually not staying in you can replace the outlets with new high quality ones for a couple bucks each."
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dn7cwq | How does a rope and pulley system reduce amount of strength needed to lift heavy objects? | Engineering | explainlikeimfive | {
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"A single pulley doubles the length of the rope. So you end up doubling the distance you have to pull thereby cutting in half the amount of energy per unit of distance."
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dnddly | does the water I flush down my toilet or bath in ever become drinking water? How does this happen? | Engineering | explainlikeimfive | {
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"Yes and no. Depends. But yes. Most places treat the water so that it is clean enough to drink* and dump it back into the watershed. This is because people tend to get upset about the idea of drinking toilet water, regardless of how much it has been cleaned. A few places where water is becoming scarce, like parts of California, are recycling cleaned water directly back into reservoirs for use. People are not entirely happy about it but hey, drought. Deal with it. Ultimately, though, all water is recycled. That's kind of the whole water cycle thing. Water used upstream gets used again downstream. Water in the ocean evaporates and ends up back upstream. Edit: u/garysai made a good point. I should have said that the water isn't made clean enough to drink, it's made as clean or cleaner than it was when it was pulled from the watershed to be used the first time. So, if you're getting your water from a river, the water going back into the river is at least that clean.",
"Just to clarify, several of the posts say wastewater plants clean up water to drinking water quality. That is usually not the case. Discharge limits are typically set so that the there is minimal, non-detrimental impact to the receiving waters but the discharge would not meet drinking water standards. In some areas where water supplies are extremely limited, there have been wastewater plants set up to produce drinking water, Singapore for example. Source-40 years in water treatment in US.",
"Yes, but typically not in your own home. Used water flows out of your house and into the wild, then into a reservoir of water treatment. The water is first treated by a chemical process that filters out big chunks of waste and causes them to clot together, then the water gets filtered for smaller things. The water then gets disinfected which kills bacteria in it. This water then goes to be stored in towers etc and is reused for other peoples drinking water.",
"A lot of the water you drink at one time passed through the kidneys of a dinosaur. Water has been around.",
"The water, yes. The other crap in the water such as poo, toilet paper, bacteria and toxins, no. The water from your toilet is thoroughly filtered both through several man made techniques as well as by the nature as it is being released into the water stream. Most of it will end up in the ocean before seeing another person again and from the ocean it will get evaporated before becoming rain and ending up in your water supply again. So anything you flush down with your water will be filtered out, sterilized and broken down.",
"I'm going to try and give you a relatively detailed response. We'll see if I succeed. I'm just going to talk about my area. So let's begin with you taking a dump. You flush the toilet and runs into the sewer drain and by gravity runs to the lowest point in that particular run. It just so happens that that particular point is not the wastewater plant, so it collects into a cistern and is then pumped into a pressurized pipe where it is forced to the plant. Now that it's at the plant it meets up with everybody else is wastewater and it passes through a large screen, this is going to catch the bigger debris, like basketballs or whatever. now it's going to pass through a second screen, and this one's going to filter out the majority of the solid things. So whatever wet wipes didn't clog the pumps, huge nests of hair, condom balloons, tampons and pads like crazy, tangles of rags, baseball sized chunks of solidified fat, etcetera. Next stop is a settling tank. Up until now, we've kept the water moving pretty quickly to keep suspended particles suspended. At the settling tank the water is slowed down drastically, and in this particular tank skimmers push across the top to move away floating solids and skimmers push along the bottom to push away sinking solids. After this the water spends awhile coursing a circuitous path through an aeration tank where natural aerobic bacteria eats a lot of the biological components as well as the majority of the dangerous pathogens. Now it's going to take a run through a different style of settling tank, this one is shaped like a cone. And the goal is to skim the foam off the top and the sludge off the bottom, both of which are by products of the organisms. After this we pass it through a large sand filter where any final parts should be strained out. Last but not least, it is heavily chlorinated and then it is shot down a channel to be both aerated and released into a stream. This system is designed to handle around five million gallons of water a day. It can handle 7 for a little while if it's necessary. On days with extreme weather, the system gets 14 million gallons. All of the excess overflows the tanks and is released directly into the river. Now, my county gets their water from groundwater via deep wells. However, the neighboring county gets their water from a water collection point in the middle of a large lake that happens to be not too far away from the outlet of that small stream mentioned earlier. Now I am not as knowledgeable about clean water as wastewater, but it's not really that much different. With wastewater we're collecting human waste and getting it clean enough to dump into nature. With clean water we are collecting water from nature and getting it clean enough to push into humans homes. Our water is well water, so there's the degree to which we depend on the natural filtration of the Earth, but basically water is pumped out of the ground and into large tanks, it gets filtered a little bit, it gets chemicals added, it gets softened and hardened as necessary and then it gets stored. So, do you ever drink someone's urine? No. Is there a pretty good chance that you are consuming water that in some way shape or form left the human body as a bodily fluid? Yes. If you live in a community that depends on glacial runoff the percentage is much lower than if you live in a community that depends on river water."
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dnffra | why are houses in Tornado Alley areas made of wood instead of concrete? | Engineering | explainlikeimfive | {
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"1) It's far cheaper to build a house out of wood and insure it against loss than to build a concrete house. 2) Getting hit by a tornado is actually an extremely rare event, such that many people live in tornado alley for *years* without even seeing one. 3) A direct hit from a strong tornado has the ability to obliterate brick and concrete, and EF5's have done significant damage to even reinforced concrete structures (e.g. hospitals, civic centers, and the like).",
"A tornado is a rare occurrence for any given house. The chances that your particular house will be hit over your lifetime is very small. My father's house in Salina, Kansas was really only threatened by a tornado once in 70 years. It destroyed the trees in the front yard but hadn't touched down yet. Drive through old towns in tornado alley, you'll see a lot of 100-year-old homes. They only get to be 100 years old because a tornado never hit them. You can't make people triple the cost of their houses because of a small chance they'll be hit by a tornado. That's not economically feasible. I remember a week I spent in Wichita and they were discussing (on TV) a law they were considering: every trailer park must have a tornado shelter big enough to shelter all the residents. Sounds reasonable, right. That's where the discussion went from happy-world to real-world. The cost of a tornado shelter that size was more than the trailer park made in rent in 2 years in most cases. More than 2 years in many cases. Maintaining that shelter was also a significant added cost. The residents really couldn't afford to pay for the shelter via increased rents. The landowner can't afford to be without the income for 2 years. So the only recourse the trailer park owner would have was close the trailer park and find another use for the land. You essentially pass a law to help people and end up displacing thousands of people who have nowhere else to live. Same with the wood to concrete conversion. It's too expensive. Not only that, but even a concrete house isn't going to withstand a semi-truck trailer being dropped on it",
"Even in tornado-prone areas, the odds of a particular house being hit by a tornado are extremely low. Tornados are devastating, but their damage is confined to a very small area (by natural disaster standards). It’s just not economical to build every house as a tornado-proof concrete bunker, plus they’d be horribly uncomfortable during the winters.",
"Because concrete is extremely expensive; not to mention is terrible for the environment to make. Houses with concrete, stucco or brick exteriors are held up by wood.",
"Think of the Amish barn scene from family guy. Easier to rebuild than concrete. Nothing cost-reasonable can really stand up to a tornado.",
"If you are talking being in a building hit directly by a tornado, neither structure would survive. Generally buildings are meant to resist everyday damages but it is simply unrealistic to build structures meant to survive extreme conditions.",
"Besides what other people have already said, concrete homes are not feasible in areas with clay soils that expand and contract depending on soil moisture. The walls will only crack as the foundation shifts up and down or flexes more at one end than the other. Wood frames flex. Concrete walls don’t.",
"Concrete houses are more expensive than wood houses. Many time more expensive. For example a tornado resistant above ground bunker has 2-4 ft thick concrete walls that are steel reinforced. Now you would only need this for the exterior walls, but that is still insanely expensive. Standard concrete cinderblock, brick, or stone walls are all easily dismantled by a tornado and so do offer much additional protection without the special reinforcement a storm bunker gets. In addition to this it is actually rare for a house to be destroyed by a tornado statistically. Houses in Tornado Alley generally last several generations of use.",
"While extremely strong, poured concrete building is significantly more expensive and time consuming to build vs wood frame. Sure, it’s possible to do if you have lots of time and money, but it’s often cheaper just to build a storm cellar. There are houses made from concrete block and they’re sturdier than frame and they’re not as expensive as poured concrete, but faced with 150+ mph winds from a tornado, they’re not strong enough, especially when the roof gets peeled off."
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dnlgp6 | How does a turbo charged engine in a car work? How do cars with smaller engines that are turbo charged go faster than cars with bigger engines? | Engineering | explainlikeimfive | {
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"None of these answers are ELI5 so I'll give it a shot. Your engine eats two things to make your car go: air and fuel. If it eats too much air and not enough fuel, the engine gets cranky and could potentially break itself. If it eats too much fuel and not enough air, the engine becomes lazy and ends up wasting fuel and making less power. So when your engine is pulling in as much air as it can on its own, it eats a certain amount of fuel and reaches its peak performance. The only way to make more power is to eat more fuel but without more air it can't eat any more fuel or it will get lazy That's where the turbocharger comes in. The turbocharger force feeds the engine more air than it could ever eat on its own. To compensate for all this extra air, your engine eats more fuel and it's peak power is now higher than it could achieve without the turbocharger. So even though your engine is small and normally has a smaller appetite than the bigger engine, the turbocharger shoves extra air in so that the little engine can eat more fuel and make more power.",
"The turbocharger uses exhaust gas to compress combustion air so that more air can be forced into the compression chamber, which means more fuel can be introduced, too. More fuel + more air = more power. Many turbocharged engines can produce the same amount of power as naturally-aspirated engines of twice the displacement.",
"Turbochargers use exhaust gas to spin a turbine which compress the air going into the engine. More air equals bigger boom boom inside the engine.",
"TLDR: A turbo-charger is an air compressor, more air means more power. Turbocharges and superchargers are basically air compressors. These technologies were developed in the aerospace industry to help power aircraft engines at higher altitudes. The problem with an internal combustion engine is that it can only take in air at atmospheric pressure. So at Sea Level and slightly above where we live there is plenty of air, but at high altitudes the air is much thinner so engines can't operate. The super charger is basically an air compressor that is attached the drive shaft of the engine to power it. It sucks in air to compensate for the lower pressure outside. This has the knock-on effect that at ground-level it brings in much more air than an engine normally gets so you can make the engine work much much harder. A turbo charger works under the same principal, but unlike a super charger it isn't directly powered by the engine. Attaching a super charger steals some HP from the engine (parasitic draw), because it needs HP to operate. You get a net-benefit to using one but its not very efficient. A turbocharger is powered by a turbine in the exhaust, so it's spun up by the exhaust gases rushing past which is otherwise wasted energy. This makes it much more efficient at sucking in air than a supercharger, but it has the side effect that it takes a while to spin up when you hit the gas. This is called turbo-lag. On a side note during WW2 engineers got the idea of squirting gas directly into a turbo charger. This basic concept is what developed into Jet engines. With a charger you are able to make much more power. In racing weight is everything so it's often better to run a smaller and lighter engine with a turbo than a bigger engine with the same amount of power.",
"To add to what others have said, the awesome thing about turbocharging is that *with a strongly-built enough engine* you can create as practically as much power as you want. Check out the 1.5 liter, 1300 horsepower 1986 Benetton B186 F1 car. With all the components strong enough to support each other, it's an exponential positive feedback loop. With the turbocharger spinning fast enough and creating enough boost pressure you can get ridiculous power levels from tiny engines. However there are major drawbacks too - driveability, reliability and expense. There's no such thing as a free lunch and often a big, lazy non-turbo engine is a better solution.",
"There is a small-large fan incased in metal. It is connected normally close to the engine and connected to your exhaust, the little pipes that stick out the back. As your car pushes air out the engine through the exhaust it will get filtered and redirected to the turbo and back out into the exhaust again. This air flow makes the fan spin. As the fan spins faster it then begins to blow air into your engine instead of your engine sucking the air in. This forces more air into the engine, triggering the car to use more fuel to compensate. More air = more fuel = larger combustion/explosion = faster zoom zoom. This is why there is \"turbo lag\". That is the time between starting to accelerate and the exhaust to reach the turbos fan and have enough umph to spin it to force air into engine."
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dnm8f1 | How does a gas stovetop not blow up the gas line when its turned on? | Engineering | explainlikeimfive | {
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"I believe there would need to be oxygen in the line as well as gas for it to do the boomies.",
"Your gas line is full of gas. Your room is full of air. Fire happens where gas and air touch. Since there is no air inside of the flames or back in the gas line (or at least there really shouldn't be), the fire does not enter the gas line. As a bonus fact, in things where gas and air *are* mixed while still in the line, such as an oxy-acetylene torch, the mixture is blown out of a nozzle so quickly that the fire cannot spread fast enough to burn the gas inside of the torch. If mishandled, the fire actually can climb up into the torch, and this can be a big 'ol problem."
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dnrxsv | Why is California the only state that has to cut off electricity to prevent fires? | Engineering | explainlikeimfive | {
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"The power utility is a private company and they took the blame for a very large fire last year I believe. They were assessed billions of dollars in fines and has been nearly bankrupted. As a result they are incredibly risk averse and now cut power to lines that they think might pose a danger of sparking fire.",
"We have the most fire risk and infrastructure in the same place. Plus the infrastructure hasn’t been properly maintained over the past decades for it to run safely. The ecosystem of much of the state is also such that wildfires are part of the deal. The landscape actually relies on wildfires to stay healthy. Certain plants only seed after a fire. Fires are needed somewhat regularly to clear brush from the forest. We have many fire-resistant (and some fire dependent) plants. In this ecosystem there is no problem with having wildfires except the fact that humans are here with our structures and we don’t want them. So we try to “prevent” and control them when they happen, which only leads to the overgrowth of brush and raises the likelihood of a more catastrophic fire in the future. California is meant to burn. We are the ones fucking up the cycle.",
"California is full of eucalyptus. It’s an introduced species, so other parts of the US that are also prone to fire don’t usually have to work around it. I grew up/still live in the southeast corner of Australia, where the trees California introduced grow naturally. We cut the power when conditions are bad enough too. Eucalyptus is a bit...unique in how it burns. It doesn’t actually need to come into contact with a flame to catch fire. The leaves are so full of oil that the air around a thick stand of eucalypts will almost shimmer, and static in the air (of the kind electrical wires can give off) can ignite that vapour. Instant, completely spontaneous fireball. Eucalypts are also prone to dropping branches unexpectedly, and releasing a lot of debris when they burn. That debris can bring lines down and (if power is running through the fallen line) start a second fire further along the front.",
"Some of the reasons: **Human incursion into forested regions**. The Western forests are 'designed to burn' in the sense that the ecology is structured around frequent fires in the forest. Normally, this isn't a big problem because sections of the forest burn and everything goes on as normal thereafter. However, if human beings are living in that section of the forest, you need an aggressive fire response that puts out the fire before it burns out - and preserves all the flammable growth for the next fire. Over time, that flammable growth builds up to the point where you get catastrophically large fires. **Bad forest management**. In general, if you plan to have human beings living in such forests, you need logging to compensate for the effects of aggressive fire control. Logging companies have an incentive to prevent those catastrophic fires, so they work very hard to reduce the flammable undergrowth. However, when you ban logging (as California does in many places), there is no one maintaining the forests to prevent massive forest fires. **Legal liability**. California is unique in that its laws assign liability for forest fires to the power company if there is any involvement with the power system at all - whether or not the power company was acting irresponsibly. This creates a huge liability risk for the power company and incentivizes it to simply turn off the power rather than run the risk of a power line getting knocked down."
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dobsix | how did people first discover water could cut through steel? | Engineering | explainlikeimfive | {
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"Waterjet cutters are abrasive, just like abrasive wheels that are often used to cut steel. As waterjet cutter technology was developed, it was tested on many materials, including steel. It's not really unusual in terms of its behavior.",
"Waterjets don't really use water to cut - they use sand to cut. We've known that fast-moving abrasive can wear away at material since before we even had spoken language. One day, somebody had the novel idea of using water to make the abrasive move really fast - if I had to guess, this was invented by someone who looked at sandblasting with compressed air and thought \"I can do this better\"."
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doh46y | How did old fashioned land line telephones carry voices so well especially when the lines were so long and prone to breaking? | Engineering | explainlikeimfive | {
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"They didn't used to be. There's a reason why phone calls are so low fidelity. Analogue audio needs a lot of bandwidth. There's a finite number of calls you can fit simultaneously down a copper wire. So they kept the quality as low as possible to squeeze as many calls in as possible. Basically narrowing it down to only pick up the specific frequencies of the human voice. The other problem is that copper wire has internal electrical resistance, so signal fades the further you get from the telephone exchange. So quiet, garbled long distance calls were a major problem with the early system. The invention of the vacuum tube amplifier alleviated that. At least for intercity calling. Overseas calls still suffered from major issues. In fact, the ability to make transatlantic telephone calls is a surprisingly recent one. The first cable was only laid in 1956, and could only handle 35 calls simultaneously. Making a call from the US to Europe was extremely expensive as a result. Even then, call quality was poor. So most didn't bother. It was actually substantially cheaper to record what you had to say on a cassette tape, and mail it over seas. Indeed, a lot of people did. My grandmother used to exchange tapes with relatives we had in Australia. The era of cheap, high fidelity overseas calling really didn't come about until 1988, when the first fiber optic cable was laid. As for breaks, the telephone system has a lot of redundancy built in. There's a bunch of nodes (those boxes you see along the street) that connect to the local exchange. Even if one node goes down, it'll just knock out phone service to the immediate area. It's rare that you'll get an event that knocks out the entire exchange. Again, there's usually backups in place. But it can happen. Tends to be a bit more common with the cable TV system, since it's not quite as robust. Most cable outages involve accidental line cuts."
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dojp6x | Why is it so hard to get someone out of a borehole? | Engineering | explainlikeimfive | {
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"I'm unfamiliar with the case in India, but the general problem is, to be blunt, that the rescuers need to widen the borehole without killing the person within. This is almost impossible. It's usually also impossible for the person trapped in the hole to be able to secure a rope about themselves to be pulled up. So instead, what is normally done is dig another borehole nearby, big enough for rescuers to go down, that is slightly deeper than the trapped person. One then cuts over to the other hole and can then rescue the trapped person. I can't answer your question about why the whole couldn't be dug in time, though soil conditions as well as weather would clearly affect things. Also, if soil conditions are *really* bad, digging the adjacent hole might cause the other to collapse. Edit: I finally found a link concerning the boy in India. Apparently drillers hit a rock shelf when digging the nearby trench.",
"The problem isn't digging down, it is digging sideways. If you dig too close to the hole, you risk collapsing it and suffocating the victim. You have to dig a parallel shaft then tunnel sideways or diagonally and hope you've done all your calculations right and don't auger the victim. Also, there isn't much call to dig a 100-foot hole a human can fit into. There is plenty of equipment that can dig big shallow holes or very narrow deep ones, but doing both at the same time is less common and requires complicated rigging that takes time to build. Finally, if the rescuers had a plan for 30 feet and the child suddenly went deeper, they'd have to pretty much start from scratch."
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4,
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dojppx | What is a stress strain curve? | Engineering | explainlikeimfive | {
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"f5ojfq1"
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"text": [
"Stress is force applied for a given area. Strain is the amount of change in shape. As you would imagine, when things are stressed, they demonstrate strain (they squish a bit). Different materials have different relationships between stress and strain, and these relationships play an important role in the kinds of load that a material can deal with. For instance, glass has a very tall and very steep stress vs strain curve. This means that it can take a lot of force, but that force cannot be delivered suddenly or else it will shatter."
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3
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dolleq | The difference between neutral and ground in electricity. | I installed my hard wired oven and there is two hots and a neutral, which is also serving as a ground. When are they the same and when are they different? | Engineering | explainlikeimfive | {
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"f5oryv1"
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"Neutral is return in an AC circuit. Without it, the appliance will not sense the potential and it will not work. On an AC appliance ground is attached to the housing in case of a short between the circuitry and the housing. This keeps the appliance from electrocuting someone who touches it while it is shorted. It should also trip the breaker on the circuit the appliance it plugged into. It is a separate line from neutral. If you connect the together the circuit will still work, but your protection is gone. You are also putting current in you ground circuit and that can have unintended consequences. In a DC circuit ground can be a source, depending on what the voltage of the circuit is. It can also act as the negative pole of a battery."
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11
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dpahml | Why do Seatbelts lock after you tug on them for a while | Engineering | explainlikeimfive | {
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"f5tiz4a",
"f5to9mt"
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"text": [
"It is to prevent your body from flinging forward in the event of sudden breaking. If you slam the brakes your bodyweight shifts forward pulling on the seatbelt so there is a locking mechanism to prevent you from hitting the dash. If you tug on it to abruptly it activates the lock due to that being similar to a body being pulled forward quickly.",
"If you’re asking why they lock up when pulled all the way out and allowed to slowly retract back in, it’s to secure child seats."
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5,
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dpb5a7 | If your car is constantly running with gasoline in it why is it dangerous to gas up with the car running? | Like the title says, ELI5 please! | Engineering | explainlikeimfive | {
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"f5tpiru"
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"text": [
"Gasoline is flammable, gasoline vapors are explosive. There is a risk of static electricity when you’re filling up your car and the air from the fuel tank is voided when the tank is filled. This air/vapor could explode although it’s unlikely. Most fuel pumps have a vapor return system to prevent emissions and it sucks this air back into the underground tanks."
],
"score": [
8
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dpcnyu | Why do front doors on homes in America (and I'm sure lots of other countries) open IN instead of OUT? | Engineering | explainlikeimfive | {
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"Popping the hinge pins out of a door that opens out will allow you to remove the door with almost no force. It's a security choice to keep the hinge pins on the inside where the evildoers aren't.",
"It's a safety issue. If the door opens outward, in cold climates there's likely to be a lot of times when there would be several feet of snow blocking the door from opening. Even in milder climates, trees and debris from storms could do the same thing. That's not good when your house is on fire and you need to quickly evacuate.",
"A door that opens inward means that the hinge pins are inside the building (so you can’t just pop the pins out to open the door), the frame prevents direct access to the bolt (it’s harder to shim the bolt open), and the person inside can brace the door if necessary (e.g. the trope of piling heavy objects in front of the door). At least with a traditional door design, you would lose all of that if your door opens outward.",
"Hey! Something I am qualified to answer! So doors swinging in is the norm, but there are absolutely doors in North America that swing out, its just not common, and heres a few reasons why: 1) As already mentioned hinge pins would be exposed. This is bad for a few reasons. Although there are more secured hinge pins, it is still a security weak point, and secondly is that alot of manufacturers dont want to warranty exposed hinges, especially if they are a specality hinge like spring assist for example. 2) Energy effency. When the door swings outward, it sits on the outer most part of the frame, which is less ideal for energy efficiency (for a whole bunch of reasons i wont get into here) North American window manufacturers are governed by a regularitory agency called NAFS. They set the standards for all exterior doors and windows, and when a door swings out, they require additional measures be taken to increase effiency. 3) Weather sealing. When the door swings out, the weather stripping seals against the inside face of the door, which allows weather to penetrate further than the standard in swing door, so additional weather / water control measures must be taken. 4) Price, all of the above points come with a price tag. And it ain't cheap. Usually 30%-50% more for an outswing. This is the bigest reason why outswing doors are not nearly as common as in swing in North America."
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dq8rvi | Why is it during storms that the lights flicker and then the power goes off? Wouldn’t the power either stay on or get cut off? (Hope what I said makes sense)? | Engineering | explainlikeimfive | {
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"The lights flicker because there is a fault, either a short or a surge. They stop flickering because a protection device has cut in to protect your electrical equipment from harm. Alas, it does this by disconnecting you from the power plant. Most protective devices are auto-resetting, they turn themselves back on after 10-15 seconds. If they trip again, or 3 times in 5 minutes, (or other rules) then they stay off until a human lineman resets them (hopefully after removing the tree that's causing the short).",
"My understanding is that most electric power is provided by overlapping service grids. (Theoretically) if one area is served by four grids each running at 75% of grid capacity, then if one grid goes down the area is instead served by three grids running at 100% capacity. The flickering lights and power is caused by the loss of power to one or more grids, and the subsequent shuffling of the load around onto other grids. This is why sometimes the lights flicker and do not go out. Sometimes when extra loads are moved onto the overlapping grids, the load is too much for the remaining grids, triggering a safety response (like a circuit breaker reacting and tripping) that stops sending electricity to an area. This causes another round of shuffling through the remaining grids, causing another flickering of lights. This will repeat until either the load is shared out among remaining grids safely (and the power stays on), or until no grids can handle the load and power goes out. There are more complex issues and features involved, but I don't have the experience necessary to elucidate them.",
"The power provider is constantly trying to provide just enough power for everyone, since it needs to be instantly consumed or wasted. During a storm, you can have a sudden influx in power if a lightning bolt adds power to the system, or a sudden drop if a neighborhood goes dark. The flickering may be a response to the sudden change in load needed.",
"Actually it is the electrical distribution system protecting its self. There are several types of devices such as reclosing breakers, fault interrupters, fuses and various overload protection devices. The wind of the storm can cause the power lines to touch each other or a tree for example that causes a momentary short ( connection of the power line to ground). The moment the wires touch (fault) the current in that part of the system increases dramatically. That can cause your lights to dim. Then the system acts to protect itself by opening the circuit and shutting off the flow of electricity. This can cause your lights to go out momentarily. This happens in a fraction of a second. The device will then reclose and allow the flow of electricity. If the wires are no longer touching each other or ground, the device stays closed and all is good. If the fault still exists (like when a car hits the pole and the wires are now lying on the ground) the devices opens again to protect the system from the high current. Most reclosure device go three times. If the fault is still there on the third time, it stays open. There are monitoring devices on the system which alert the power company to the problem. The whole idea behind this is to keep your lights on and prevent a temporary problem from causing the system to shut down."
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dqblq5 | Why does a typical clock make the "tick-tock" sound and not "tick-tick" or "tock-tock" sound? | Engineering | explainlikeimfive | {
"a_id": [
"f61vbj0"
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"text": [
"The ticking sound comes from a pendulum attached to a ratcheting mechanism. That mechanism only turns one way and the sound changes due to the differences in the shape of the teeth that engage that mechanism."
],
"score": [
30
],
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dqfi5r | Why our phones don't make any machinery noise like laptops and pcs make? | Engineering | explainlikeimfive | {
"a_id": [
"f62xqbp",
"f62yfla"
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"text": [
"Our phones have no moving parts whatsoever besides the vibration mechanism. Laptops and PCs have fans to cool the CPU, etc while cell phones will use a simple heat sink for cooling.",
"Because their battery is so small, phones use electronic components designed to really minimize power use. This means they don't need a cooling fan."
],
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13,
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