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d8or66 | How do planes land in a straight line? | I don't get how planes always land in a straight line, especially at those high speeds? When an engine fails, drag is different wing to wing, wear on tires etc... Are the controls on landing really that good or is there something I'm missing? | Engineering | explainlikeimfive | {
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"When there is an engine failure, it results in both more drag from the dead engine side, pulling that wing back and more thrust on the good engine side (you have to increase thrust on the good engine to keep it flying) which pushes that wing forward. These two things create a turning motion into the dead engine. You keep the plane flying straight by counteracting this with RUDDER. You can further assist it by turning the control yoke slightly towards the good engine, which causes the plane to fly with one wing slightly up, but in a straight line flight path. Colloqually known as 'raising the dead'. [When you're dealing with crosswinds, the approach begins in what's called a CRAB, where you point the nose of the plane off centerline into the wind, while maintaining a straight ground track. Flying slightly sideways. You can land like this up to a point, with the nose of the plane not pointing down the runway but a few degrees off centerline. Because the entire plane is traveling down the centerline, when the wheels touch, the plane will straighten itself out on rollout.]( URL_0 ) Typically, pilots transition from the Crab, where the nose is pointed off centerline to what's called a Slip at touchdown. A slip is where you keep the nose pointed down the centerline and lower the upwind wing slightly to maintain ground track, flying slightly crooked. [A proper crosswind landing using this method will result in the upwind wheel touching the runway first.]( URL_1 )"
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d8pit9 | Why two mirrors don't have the space between the finger and its reflection? | I always see people talking that if you want to know if the mirror is a two-way or a simple one, just put your finger on it. 1- is this real? 2- y tho? | Engineering | explainlikeimfive | {
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"There is no physics reason for this, both one-way and two-way mirrors could be front-reflectorized to eliminate the space. However, almost no mirrors of either application are made this way, only super reflective telescope mirrors are front reflectors. From an engineering perspective, rear-reflectorization is better, because the glass protects the thin reflective film from damage. Both normal wear-and-tear and criminals looking to mess with the mirror would have a much easier time if it was on the front where they could touch/scratch/... it. To reduce double reflections, one-way consumer mirrors are made with a rear reflector on a thin sheet of glass. This might leave a gap too small for you to notice. However, to make a sheet of glass bigger, you have to make it thicker. To make it protective from criminals who might throw their chair at you, you have to make it even thicker. This thick mirror would show a bigger space between the finger because the glass before the reflectorized layer is thicker. The effect would be identical for one-way and two-way mirrors. There is a simpler way to tell if a mirror is two-way. If you are in a police station, the mirror is two-way. OK, maybe not the mirror over the sink in the restroom, but all the other mirrors are two-way.",
"Real mirrors are silver plasted glass. Two way mirrors have a reflective coating on top of the glass. (Which means when there is light shining inside the room with the reflective side and not the other side, you cant see through (like your windows at night)"
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d8zpn2 | what is the difference between C, C sharp and C++? (Programming) | Engineering | explainlikeimfive | {
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"First we should get out of the way is that C++ and C# have C like syntax. This means that they share some keywords and idioms. When C was designed and created there was no such thing as Object Oriented Programming(OOP). Which is where you treat everything like an object in your program. C++ came later with the idea of making C with objects, allowing OOP. Since then it has changed a lot, but the underlying C with objects has remained for C++. C# is just Microsofts version of Java. Java was a programming language created with a Virtual Machine that takes the Java code(bytecode) and runs it on the computer. This Virtual Machine known as the JVM(Java Virtual Machine) allowed it to manage the memory of the program itself instead of the software developer having to write code to manage the memory. The JVM also allowed Java developers to write their programs once and run it on multiple different systems and hardware. When Java started to get popular, Microsoft decided to add extensions to it that only worked on Windows OSs. Sun Microsystems, the creators of Java, didn't like this and sued Microsoft. They settled, Microsoft took their Java version and created C# out of it. TL:DR version C++ is C with objects C# is Microsoft's version of Java, which is a C like programming language with a Virtual machine."
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d995mb | why do the metal prongs that plug into outlets always have circles cut out of them? | Engineering | explainlikeimfive | {
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"Many outlets have small bumps inside them that fit into those circles. It helps the plug stay seated."
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d9b7gm | Why are car batteries so huge? | The other day my car battery died and I got the car running using a portable battery pack designed for jumpstarting. The thing is only a little bigger than a large smartphone. This got me wondering, if a device this small is capable of starting the engine, why does a normal car battery need to be 50 times the size? EDIT: thanks for the replies and explanations. To add to the story, my car battery was dead to the point that the starter motor wasn’t turning at all. The jumpstarter was able to start the engine basically like normal after being attached for only 30 sec. I’m pretty sure the jumpstarter provided all or most of the energy used to start the engine. | Engineering | explainlikeimfive | {
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"Cranking the starter takes a *lot* of juice. Not high voltage, high *amps*. And it has to provide those really high amps consistently. Others are mistaken - car batteries are not designed to last for a long time, they are designed to provide a *lot* of power for as long as it takes to start the car, and not much longer. That's why you can't run your electronics for long without the car running - even the relatively low voltage of your radio will quickly drain the battery. Your battery does not run the electronics after the car has started - that's what the alternator is for. Car batteries are lead-acid batteries, which have some advantages over other kinds. One of the biggest ones is safety. Lithium batteries can provide a *lot* of juice but they also, well, [explode]( URL_0 ). Lead-acid batteries are a lot more stable and won't explode if they overheat. They are reliable, too, and will work when they're pretty cold or pretty hot. They can sit for a long time without much danger of going bad. They can discharge and recharge very often without dying, as long as they don't discharge too much. But mostly it's because turning over the engine draws *hundreds* of amps. Compare that to the breaker in your house for *high powered* devices like your dryer, which will trip at ~50 amps. Few battery types are capable of providing that much power without falling apart, permanently dying, or exploding. The small device that charges your battery doesn't charge it as fast as the battery is capable of discharging. It also doesn't need to fully charge it, just enough to get your car started so your alternator can take over and charge it the rest of the way up. Your battery charger also doesn't have to deal with the weather and constant charge/discharge multiple times per day or sit for a long time and still hold charge or not explode in an accident. So it can be a more compact battery type like a kind of lithium battery.",
"Cars use lead acid batteries, which are ancient technology and not as efficient as storing energy as newer lithium battery technologies. There's little reason to change because lead acid is extremely cheap, isn't fussy about how it's charged, isn't hazardous in a fire and won't catch on fire by itself, and since you're not carrying it around the extra weight and bulk isn't much of a drawback.",
"A jump starting battery only has enough power to operate the starter motor for a short period of time to start the car. The full sized battery has enough power for a few hours of running lights, AC, radio, etc with the engine off. Now you cannot run them all night or the like, but you can spend a fair amount of time with the engine off if you are waiting in a parking lot or something."
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d9dbir | Why are some LED colors cheaper to produce than other colors? | Engineering | explainlikeimfive | {
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"Blue LEDs in particular are more expensive. They use gallium nitride, which is naturally a more expensive material, and then for the longest time they weren't stable. People in 2014 literally *won a Nobel Prize* for making a stable blue LED. White LEDs are even more complicated, and for a long time were also quite expensive."
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d9izgy | How do tasers work? | Engineering | explainlikeimfive | {
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"Gun fires 2 prongs into a person. Prongs attached to wires, attached to gun. Electricity runs from gun to wires to prong. Electricity fucks with muscles cause our body runs on electrical pulses. Person disabled. No one dead. Probably. Most likely."
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d9kn8d | Tickling the Demons core? | URL_0 In regards to the above, what was the actual purpose of using the screw driver, what were they testing? | Engineering | explainlikeimfive | {
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"> The standard protocol was to use [shims]( URL_1 ) between the halves, as allowing them to close completely could result in the instantaneous formation of a critical mass and a lethal power excursion. Under Slotin's own unapproved protocol, the shims were not used and the only thing preventing the closure was the blade of a standard straight [screwdriver]( URL_0 ) manipulated in Slotin's other hand. A shim is just a thin wedge of some material designed to act as a spacer between two objects. In this case, they are keeping two beryllium spheres separated. Instead of using shims as designed, the experimenter used a screw-driver instead. The purpose of keeping the spheres separated was to prevent the demon core (made of plutonium) from going critical. Critical, in terms of radioactive material, is the point where there is enough of a radioactive material in close enough proximity that the radioactive emissions are high enough to start a nuclear chain reaction. Basically the emissions cause the radioactive material to split (at an atomic level) apart releasing more radioactive emissions causing more material to split apart, etc. This cause big boom. Why were they doing this? Basically they were trying to get the demon core as close to the critical point as possible. The purpose of the beryllium spheres is they act as neutron reflectors. \"Neutrons\" are part of the radioactive emissions as described above. Normally radioactive material will send neutrons in all directions. The purpose of a reflector is to bounce them back toward the radioactive material. The closer the reflectors get to the material, the more neutrons are reflected back, increasing radioactivity and bringing it closer to critical. So they have a ball of radioactive plutonium (the demon core) that is sending neutrons in all directions spontaneously. They want to surround the plutonium with beryllium which reflects those neutrons back into the plutonium, causing more radioactive emissions. The closer the two beryllium spheres get to the plutonium, the more neutrons are reflected back and the higher the radioactivity gets. If they touch, the radioactivity reaches a point known as critical and the thing becomes a nuclear bomb. The only thing stopping that outcome was the tip of a flat-edged screwdriver in the hands of a scientist."
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d9px44 | Why did we ever think putting asbestos into buildings was a good idea? | My uni recently smashed into the stuff during some redevelopment work and as such had to seal us in the library till they sorted it. Begs me to ask, why did we ever think it was a good material to use? | Engineering | explainlikeimfive | {
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"It's one of if not the best naturally occurring fire retardant materials in the world. While also have very good insulating properties. Oh, and it's cheap and relatively easy to work with. It's basically a miracle material. We didn't even know it was dangerous until the early 1900s and when you have miracle material people dont exactly stop using it overnight.",
"As it relates to the building trades Asbestos was first used because it was 1. Naturally occurring and abundant 2. Easy to work with 3. Fire retardant 4. Relatively inexpensive However, at the time (19th century) it was not well understood that it was a carcinogen. There were some rumblings but no solid data. So there was no good reason NOT to use asbestos. Eventually (20th century) the carcinogenic properties started coming to light....but by the time it was banned there was ~100+ years of asbestos use in construction",
"It was too tier technology of the time, cheap, easy to cut or shape, non flammable, weather resistant, - so perfect for cladding.",
"We didn't realize how bad it was so at the time it was miraculously easy to use, affordable fire retardant material. The idea that tiny razor blades would release and slice up your lungs didn't occur to anyone.",
"Because it was a very good idea. Asbestos has saved far, far more people than it has killed. It is an amazingly good and inexpensive fire retardant in an age where an errant flame could take out a city block and most people didn't live long enough to develop asbestos-related illnesses. Only when effective fire codes, professional fire departments, alternative flame retardants and increase life expectancies became a thing did the drawbacks of asbestos outweigh its advantages.",
"We used asbestos because it is a material with some very useful properties - for example it is very flame retardant and a great insulator, so it was used in many items that needed those properties. At the same time, when it was being used, we didn't realise quite how dangerous it could be - for example we didn't have the same knowledge and research we do now, we didn't have the same safety measures and record keeping to spot the causes of deaths, and since asbestosis can take decades to occur after exposure, it was even harder to relate asbestos related deaths to something a person may not have dealt with for years. Along similar lines, if you go back in time substances like mercury or radium were not understood to be dangerous and handled very carelessly - something that seems mad given how dangerous we know these to be now, but at the time nobody knew... This does lead to the question of what current substances or devices do we use in the belief they are perfectly safe, because we don't yet have the technology to discover the dangers of them?"
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d9v7pp | Why can't we put a metallic grille on plane reactors to keep birds from getting to the engine? | Engineering | explainlikeimfive | {
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"We can’t do this because if a bird we’re to hit the grille and damage it, it could cause more damage as the grille will become entangled in the engine. Most engines only take ~20% of air through the core of the engine, everything else is bypassed and creates thrust",
"There are a few reasons why this is impractical. It would cause resistance for air flow. Would add weight. If hitting a bird at high speeds, it would just fail and get sucked into the engine causing more damage than a bird would. Increasing the strength to withstand a bird strike would further increase the weight and further reduce air flow. But bits of bird would still get through and sucked into the engine.",
"That seems like the easiest solution, actually. But there's a problem: Most passenger planes have a cruising speed of ~800km/h. If just about any kind of bird, no matter if it's a goose or a pigeon, hits a metal grille at 800km/h, it will just be shred into pieces that then still get into the engine. Even if you make it very fine, which you can't because it will become too heavy and restrict air flow, bits of bones will still go through and damage Parts in the engine."
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d9vv19 | why do batteries perform worse the less power there is left? | Why haven't we made batteries that perform equally well no matter how much power is left? | Engineering | explainlikeimfive | {
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"Picture a water tank with a tap in the bottom. When it’s full, the pressure of the water means you get a strong net of water, right? But when it’s almost empty, it’s a dribble. It’s essentially the same thing. You’d have to find a way of separating parts of the battery and draining them one at a time, which would make the batteries smaller, but batteries are more efficient the bigger they get, so we go for bigger and more efficient rather than smaller and more consistent"
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da39d6 | Why are cars that are yet to be released painted in that weird black and white pattern? | Engineering | explainlikeimfive | {
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"Its to stop people from figuring out the exact dimensions of the vehicle. Cars used to be driven around with giant bags covering them to make it harder to tell what they looked like while they were still in development but for obvious reasons this was less than idea. The swirls make it hard to figure out exactly where the car curves, bends, bulges, etc making its shape harder to distinguish.",
"This is known as [dazzle camouflage]( URL_0 ). It makes it difficult for people (And especially automotive paparazzi) from seeing the exact shape and look of the car. It was created in the early 20th century to camouflage warships, and is now used for peaceful purposes. It's exact intentions are not to hide the ship, but rather to make it difficult to see what kind of ship it is and to make range finding and determining its heading more difficult.",
"When rival companies or paparazzi try to take pictures of the car, the colors make it harder to tell details about the shape of the car, because it tricks the camera's zoom so it can't focus. Car companies don't want their cars revealed until they officially announce them, but they want to test them."
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da8mgr | How do ice hockey rinks stay frozen inside and able to keep people ice skating / playing hockey on them if the temperature inside is in the 70s. | Engineering | explainlikeimfive | {
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"Refrigerated floors. The ice is actually a layer built up on top of a huge cooling exchanger. Plus, there's a lot of thermal mass there, so it wouldn't melt quickly even on a hot day."
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daahal | Why do semis engines tend to last longer than cars and pickups? | Engineering | explainlikeimfive | {
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"* diesel engines run at lower rpms and tend to last longer than gasoline engines * most of the miles will be steady highway miles, which is easier on the engine than stop and go traffic * they are operated by professional drivers who are less likely going to drive careless or have bad drive habits that increase wear * most countries have regulations that require regular maintenance * most semis are a part of trucking fleets, which will have full-time mechanics on staff * owner/operators depend on their trucks to make a living, and are highly motivated to keep them in good repair * semi's retain their value better, and are more economical to repair than replace for longer * semi's are engineered to be on the road for 15-20 years, and will be built with more expensive materials, components, and techniques (beyond just size) * people who buy are more cost-conscious than corporations that buy trucks, cars are built more cheaply to better compete on price"
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db2tq3 | How does water in a water tower stay "fresh" and not stagnant? From what I've read about them they're supposed to hold a days supply of water for the town they're in. Is there a backup for this backup? 1 day? That's it? Some of them look so small! No way that's enough water for the whole town. | Engineering | explainlikeimfive | {
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"Generally a water tower should be designed to only hold enough volume to accommodate the difference between the average daily pumping rate, and the peak usage. It’s not intended to be a backup supply. It’s intended to maintain pressure on the lines. The water in the tank cycles often enough that stagnation should not generally be a problem. Edit: punctuation Edit: Setting aside your specific examples, remember this is ELI5. There’s no need to document every variation on my explanation. Also, I held a PE license in water resources, so I actually know what I’m talking about and have designed water, sewer and storm systems.",
"Think of a water tower as a water pressure regulator not a back up supply, it takes water when the city pumps are running but not many people are using it, when too many people are using water for the pumps to keep up, the extra water comes from the tower. URL_0",
"> How does water stay \"fresh\" and not stagnant? Water towers aren't just for storage. Most towns have their water systems set up so that the water in the tower circulates regularly. At peak usage times, the tower drains into the system to contribute to the supply and keep water pressure up. When usage falls off, the tower's pumps kick in and refill it. > 1 day? That's it? Generally, they do hold enough to supply the town for an average day. I doubt that's a matter of regulation, although it could be. I think it's just a standard engineering practice. > Some of them look so small! Most water towers hold at least a million gallons. For scale, a regular sized bathtub holds about 80 gallons when full to the top. So the tower holds enough water to fill about 12,500 standard bathtubs.",
"Stagnation can actually be a problem for some water tanks. In general the chlorine in the water counteracts any negative effect of \"old water\" but some tank designs are a problem and the water doesn't get mixed enough so steps are needed to be taken to counteract the problem, these might include installing baffles in the tank for better mixing or setting up a program where the water is occasionally just dumped on the ground to refresh the tank. There are regulations for how much water a town needs to hold in reserve for fires or emergencies, it's usually not a full days worth of water. The tanks are to provide pressure in the system button also regulate the demand during the day and night cycle. The tanks drop during the day while people are up and using water and ate refilled at night when most people are asleep. The system I work in, the plants need to run 24/7 to provide water and if they would stop they would be out in a few hours.",
"Lots of water tanks have turbines in the to keep the water circulating to avoid this, but chlorine will usually do the trick.",
"In cities and large towns, the water is pumped into the towers for distribution at a higher pressure than the pumps alone can provide. The usage rate is high enough that the water doesn't have time to stagnate. Now, in a rural community, there isn't as much demand, so measures are taken in the design of the tower itself. Most newer rural water towers are of the standpipe variety. Think of the tall, skinny water tower that has the name of the town painted on it. These standpipes have passive mixing systems where the water coming from the inlet is directed through check valves in a manner that makes the water swirl in the tower to keep air mixed in so that it doesn't stagnate. Some ground storage tanks (large diameter, but not very tall) will have an aeration system that will spray the water coming in down from the roof that mixes air into the water preventing stagnation."
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db55x4 | what’s the difference between AWD and 4WD | Engineering | explainlikeimfive | {
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"AWD means each wheel can operate independently of one other. ie the drive train can power each wheel independently. 4WD means that the front and rear axels are powered independently so that both front wheels are powered together and both rear wheels are powered together but the front and rear can be powered independently. generally, there's no way to disable AWD. but for cars/trucks with 4WD, it's possible to disable 4WD."
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db5xd6 | What is the difference between inches, millimeters and caliber when talking about guns? | Engineering | explainlikeimfive | {
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"Caliber is a measure of the internal diameter of a gun barrel. Inches are an SAE unit, and millimeters are a metric unit. Either can be used to describe the caliber, based on the manufacturer's preference.",
"Oh boy, this one is a rabbit hole because there's no real standards and *a lot* of marketing nonsense. First, the basics: When a firearm is described as \"thirty caliber\" (.30) that means that the internal diameter of the barrel is .3 inches. Since the late 1800s most of the world outside of the UK and USA has used metric measurements (millimeters) for the same thing. So a gun that is \"nine millimeter\" (9mm) has a barrel with an internal diameter of 9mm. Now comes the confusing stuff, because there is another set of important measurements, cartridge length. For example, the most common 9mm cartridge is 9x19mm, and it is 9mm wide and 19mm long. It is commonly known as 9mm Parabellum or 9mm Luger. It is not interchangeable with 9x18mm (9mm Makarov), 9x20mm (9mm Browning Long), 9x17mm (9mm Kurz), etc. Imperial measurements play an even dumber game, because they are often just lies. .38 Special is actually .357 inches in diameter, the same width as .357 Magnum. .45 ACP is actually .451\" and .45 Colt is .452\". It gets real silly. Then you have OLD stuff, which was often measured in caliber and charge. So you get .45-70 Government (.45 caliber, 70 grains of powder) and 30-30 (.30 caliber, 30 grains). Don't even get me started on naming conventions for artillery because they are twice as insane."
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dbdlcf | How does braking your car not destroy your engine? | Engineering | explainlikeimfive | {
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"If you have an automatic transmission, your engine and drivetrain are not mechanically interlocked; they're independent. You have this device called a torque converter - it's a drum filled with oil, and a turbine. The drum is bolted to the engine crankshaft, the turbine is attached to the transmission. As the engine spins, it rotates the drum, which gets the oil sloshing. This creates hydraulic pressure, which runs over the turbine. This pressure CAN rotate the turbine, and move the car, but the brakes can overcome this pressure. If this is the case, the oil just flows around the turbine. In a manual car, you have a pair of friction plates held together by a spring. The clutch pedal pushes the plates apart. The engine and drivetrain are physically linked, but if you're going to stop, you have to step on the clutch pedal and disconnect the engine from the drivetrain, or you'll stall the engine and it'll be a bumpy stop, as you imagine.",
"It depends on if your car is an automatic or a standard (manual) shift car: Automatic: Your engine isn't connected straight to the wheels. The engine turns a sort of fan in this thick goopy stuff that blows on another fan. That makes the second fan turn, and that one is connected to the wheels. So when you stop the wheels only that second fan stops. Manual: There are no fans and no goop, but there is something called a clutch that does the same thing. Clutches are like a bunch of dinner plates stacked up. If you spin the top one, it will spin the bottom one too, but if you lift the top one a bit first it won't spin the bottom one. The bottom one is the one connected to the wheels just like that second fan in the automatic car.",
"In an automatic car, your transmission is connected to a torque convertor which just absorbs the rotational motion of the engine without transferring power to the wheels",
"If you've ever driven an automobile, you'll no doubt have noticed that it has either two pedals (automatic transmission) or three pedals (manual transmission). In the first case, which concerns the overwhelming majority of cars in the US, you have two pedals: one for the gas (to accelerate) and one for the brake. Because you're only supposed to drive an automatic car with one foot, you can't technically apply pressure to both the gas and the brake at the same time, unless you're doing it on purpose. In the second case (manual transmission), you have an extra pedal,n for the clutch, and you need to use two feet to operate the car. One foot either brakes or accelerates, just like an automatic transmission, and the other foot applies the clutch when you need to change speed on the manual gearbox or when you are stopped. Consequently, any time you remove pressure from the gas pedal to apply pressure to the brake to slow the car down, the engine immediately slows down. Conversely, when you want to accelerate, you remove pressure from the brake pedal and apply pressure to the gas pedal. This system was designed so that you wouldn't both brake and accelerate at the same time, or the other way around. Furthermore, as a general rule, the engine is going to be more powerful than the brakes, so if you gun the engine and brake at the same time, the car will still keep moving, so it's very hard to actually break an engine this way, especially when you take into account the fact that the grip of the rubber tires of an automobile isn't sufficient to prevent the engine from turning the wheels to begin with. Hope this helps.",
"Automatic cars have no mechanical connection between the engine and transmission. It uses a torque converter. The simplest way to explain it is it's like putting a fan face to face with a wind mill. The fan is the engine when it turns on the air blowing into the windmill turns it. But if you put a brake on the windmill the air will blow over it and not turn it. Then when you let the brake go it starts turning again. In practice it uses oil instead of air and is way more complex.",
"it comes down to the fact that the cars engine is not connected to the wheels in a \"solid\" way there is something that will slip or give in-between. in an automatic it is the torque converter and in a manual it is the clutch. the closest you could get to damaging the engine would be in a manual car with a really grippy clutch. even then you just force the engine to stall and while the engine doesn't like it, it doesn't hurt it. this last example happens 100's of thousands of ties a day when new people are learning to use a manual car or when someone stalls trying to go up a steep hill, same effect as braking, in that the engine is forced to come to a sudden stop.",
"Imagine your engine running, and that crankshaft is just driving a huge fan. Now, a few inches away, your transmission is there motionless, but also has a big ass fan attached. Rev the engine, fan blows harder, and starts to push that transmission fan, so the transmission starts to move your wheels. That's pretty much an automatic transmission, but it pushes fluid through the fans, not air."
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dbebwn | When a bridge was built to carry a river over a road and a boat passes over the bridge, what affect would the boat have on the load bearing capabilities of the bridge? | I saw a meme of a bridge in The Netherlands that carried a river over a road and someone raised the above question. | Engineering | explainlikeimfive | {
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"\"Effect\". And the boat would add load to the bridge as soon as it was placed in the water, and no additional weight as it passed over the bridge. The boat displaces its own mass in water, and that mass is spread over the entire river.",
"Only a very minor effect caused by the change in dynamic load. The static weight of the boat is equal to the amount of water it displaces. The water that it displaces is displaced out into the lake/ocean/body of water connected to the river, and has no significant impact on the actual weight of the bridge. The dynamic forces are mostly the different wave patterns that will batter the walls of the artificial river. A moving boat (anywhere) will have a bow wave as the water swells upwards as it finds a flow path around the hull. This and the trailing wake of the boat cause forward and sideways waves that will batter the walls of the artificial river. This will have been designed for. The boat also creates drag currents behind it that depress the water level as it passes. The water that flows around the boat then washes in to fill this void, but is also pulled along behind the boat a bit. This could accelerate wear on the riverbed and walls depending on how deep and wide the river is relative to the size of the ships, and could even impart significant forces forward (the boat's direction) on the bridge. Typically, I believe river bridges like these are fed by locks, which means there's very slow flow through them. I.e. the water isn't gushing out the end. If the river bridge *is* handling quite a bit of flow, then there will already be lateral forces caused by the flow. The boat will temporarily change these (making them greater or lesser as it chugs along). The bow wave and depressed water behind the boat create a higher load in front (more water there) of the boat and a lower load behind (less water) the boat, and these forces move along the bridge with the boat. I don't know much about these rivers, but I'd assume that there's a speed limit for boats going through them based on how the river bridge is capable of handling this dynamic load. The faster the boat is moving, the bigger the bow wave, wake, and drag forces, and the more the bridge would have to handle. For an example: In narrow channels where personal watercraft might travel through, there's often speed signs like \"10 mph\" to minimize wake creation, or simply \"No Wake\" signs. These are there to preserve the natural shoreline, because battering them with waves speeds up erosion.",
"Boats float by displacing more water weight than they weigh, so the boat doesn't add any more weight the the river. It's displacing equal amount of water that it weighs, thus no difference in weight.",
"Look up the \"Falkirk Wheel\", a boat lift in Scotland. During the tour they pointed out that the number of boats in the gondola makes no difference as the boat would simply displace it's weight of water out of the gondola before they close the gondola door. So the answer, from what I can tell, it that it doe snot add load to the bridge. & #x200B; [ URL_0 ]( URL_0 )"
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eiliar | Why do nuclear power plants have that hourglass shape? | Engineering | explainlikeimfive | {
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"I'm assuming you're talking about the cooling towers. The purpose of the cooling towers are to remove excess heat from the process and regulate the temperature of the reactors. The cooling towers are natural draft, which means they don't use any fans to move air over the water circulating through the cooling loop; the \"hour glass\" shape as you call it is the ideal shape to create the natural flow of air from the bottom and up through the top of the tower in order to remove heat from the water. This video might help explain more: URL_0",
"The large hourglass shape tower, like the one from the iconic Simpsons nuclear plant is the cooling tower, and not all nuclear plants have them (and some non-nuclear power plants do). Some plants will pull in water from a nearby river, use it to cool the nuclear-water which doesn't leave the plant, and then pump the cooling water out into the ocean. The actual shape is like what others have said. Good for cooling, easy to build, large opening for steam to pass though..."
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einwd3 | How did we make precision tools? | We never just started off with precision tools, we had to make them with less precise tools. How did we do this? Edit: here's a good example to what I'm more looking for. Did we have rails and rubbery wheels to reduce vibration from the less precise rail? were lasers used to cut a really smooth, straight rail? what did we do to achieve this? | Engineering | explainlikeimfive | {
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"Precision came about long before lasers. To make a perfectly flat surface, we can take three different metal surfaces and sequentially grind them together to make them all flat. From there, we can make things with straight edges, and mark out precise distances. From there, rulers. Eventually, we got lathes and those allowed the precise manufacture of screws. Screws allow precision to be relatively cheap. They also allowed for simple micrometers, which meant that we could slowly carve things into precise shapes and we could measure that precision to know when they were the right size."
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eiq1fp | How has the Voyager travelled over 17 billion km and not crashed into anything? | Engineering | explainlikeimfive | {
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"Space is really empty. Only 0.0000000000000000000042 percent of the universe is composed of physical matter.",
"That's like throwing a base ball 20ft and asking how come it didn't hit any ducks. The ratio of ducks to atmosphere is such that you should never reasonably expect to hit any ducks. It would be a freakish coincidence it that were to happen by accident. The ratio of stuff to empty space is significantly worse.",
"Space is remarkably empty. If you scaled down the Earth to the size of a baseball the Sun would be about a kilometer away. The probability of Voyager hitting anything more than space dust is fairly remote, and the scientists deliberately plotted a path through the solar system to avoid anything we were aware of. But in all seriousness Voyager could smack into a basketball sized rock anytime. The chances of that are just low.",
"“Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.” -Douglas Adams",
"Fact is; there really isn't all that much stuff in space, and the distances between things are really, really large. Once Voyager made it passed Pluto (at about 2.4 billion km), there just wasn't anything else to run into, and there won't be for many more billions of km."
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eiql26 | How do mics at concerts dont pick up the sound of the instruments or fans? | Engineering | explainlikeimfive | {
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"The mics they use are designed to only pick up sound from a specific distance and direction. This has to do some with the pattern on the grill of the mic. In actuality, there will be some bleed-through of other sounds in each of the mics, but it’s negligible.",
"They do, but they're easily lost in the noise because they're so faint. When we hear things closer don't sound THAT much louder so you might think this would be hard to distinguish for microphones, but in fact things get much much less loud when they get further away, the signal strength is so much lower it's easy to either filter them out or if your equipment is not sensitive enough, they won't be picked up at all. It's actually quite difficult to make a microphone that is sensitive enough for things further out, but also not blow out with things happening close by.",
"Two reasons: 1) Distance. Sound follows the Inverse Square Rule: the same sound from twice as far away sounds 1/4 as loud. A sound that is three times as far away sounds 1/9 as loud. Ten times as far away sounds 1/100 as loud. And so on. A microphone two inches from your mouth basically can't hear a guitar twenty inches away at your waist. 2) Direction. My understanding of [Cardioid directional microphones]( URL_0 ) is that there are two sound pickups, and the 'back' pickup is wired in reverse to cancel out the sounds coming from 'behind' the microphone. That's an oversimplification, but it is a convenient way of thinking of it."
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eivq59 | Pumpjacks, how do they pump oil | so i recently looked up how pump jacks worked, i just couldnt understand it, so how do pumpjacks pump up liquids like oil. | Engineering | explainlikeimfive | {
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"There is a rod with a valve in it that doesn’t move. This is called the standing valve. The valve consists a ring and a ball that sits on the ring thus sealing it off. (Think of a beach ball sitting on a basket ball hoop. It is too big to fit so it just sits on top of it and seals it off. If you push up from the bottom you can easily lift the ball and break the seal, however if you push from the top you only put more pressure on the seal. There is also a rod There travels up and down with a identical valve. This is called the traveling valve. The traveling valve is beneath the standing valve. The rod goes down and the pressure from the fluid pushes up from the bottom against the ball in the traveling valve. The seal breaks and the fluid enters the rod. When the rod goes back up the distance between the traveling valve and standing valve decreases therefore the fluid is forced to push up on the standing valve and then enters the rod string above it. This is continuous so eventually you have constant pumps of fluid to the surface NOTE: this may not be a perfect explanation and some things I said may have been wrong. I do not work in the oil field but am looking for a job as I am studying petroleum technology in school right now. However this is the general concept of how a pump jack works. There is a standing valve and a traveling valve and due to the reciprocating motion of the rod string the fluid is forced into each of these valves and due to the design of the ball and seat valve, once fluid has entered through the bottom of it, the fluid can not exit through the top. Hope this helps you :) PS: there are many illustrative videos on YouTube you may find helpful"
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ej2mof | why can you drive the car with the parking brakes on but it seems to be able to completely block the wheels at high speed? | I've been able to drive and seen other drive the car while the parking brake was on, even when you feel the car is struggling to start moving you can drive it. But I've also seen in movies and then in real videos people pulling the parking brake at high speed and then the car will block the wheels and skid. Using the parking brake (slowly) when the regular brakes fail is also recommended. | Engineering | explainlikeimfive | {
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"It has to do with the gear ratio. When you engage the first gear, your engine rotates more times than the wheels on your car do. This lets your car “break” the inertia and also have a lot of traction, while going slow. Once you start going up the gears, the ratio goes all the way down to 1:1 and sometimes even lower (0.85:1, for example). That means that your engine doesn't need to rotate that much to keep your vehicle moving. On first gear, you have a lot of pull but low speed; at four or five gear, you have a lot of speed but low pull force (to the point the engine barely keeps the car at the same speed). That's the reason you cannot start moving in fifth gear. When you have your parking brakes on, you start moving in first gear. The engine can easily overcome the friction of the brakes. Now, when you switch to four or five gear, your engine might not be able to overcome all that friction, so the parking brake “wins” the battle. Sorry for my ugly English."
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ej4fhr | Why does a clutch have to come up slowly in a car? | As the title says, why can't cars be made to cope with a clutch coming up fast - What happens? | Engineering | explainlikeimfive | {
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"Well, a car with enough engine torque can do this, but the result is undesirable. I had a V8 Ford Mustang, back in the day, and you could rev it up and release the clutch (what was called \"popping the clutch\"). All that engine power was applied directly to the tires, vastly exceeding their coefficient of friction, producing a huge cloud of tire smoke and squealing, much to the enjoyment of the car's passengers. We were idiots. The tire rotation is 0 when the car is stopped. The engine rotation isn't, because the engine has a minimum idle speed. You have one part spinning at speed X and another spinning at speed 0, if you snap them together you get a huge jerk. The point of the clutch is to allow controlled friction between the two parts to gradually move them from different speeds to the same speed. Could the car do this automatically? Yes, it's called a torque converter and cars with automatic transmissions use them. They do consume energy, so you'd get better fuel economy for less cost and weight with a clutch."
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ej6fx7 | How can pianos stay in tune for so long while instruments like guitars or violins need to be tuned each time they are played? | Engineering | explainlikeimfive | {
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"The strings in a piano go between two giant pieces of metal. That metal frame can withstand all the tension for a very long time. Guitars and violins are made of flexible wood in quantities you can easily lift. That flexibility can't provide the strength of half a ton of iron.",
"A few reasons. That instrument isnt handled and moved around, so things don't get bumped out of tune. The strings are struck and not plucked, which is harder on them. Because they don't go out of tune as easily, the screws that hold them in tune can be tightened very tightly. If other instruments were screwed as tightly, they'd be a pain to tune."
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ejf2bp | Why do pipes attached to hoses and nozzles tend to rupture close to the end where they are attached to the hose? | Engineering | explainlikeimfive | {
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"Because whatever is bonding the pipe and fixture together is weaker than the integrity of the piping itself."
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ejf5c0 | how do scientists determine material properties, e.g tensile strength, hardness, thermal coefficient, etc? | Engineering | explainlikeimfive | {
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"They put them on a rig and test them, by pulling, heating, twisting etc. they can determine the properties of a known sample."
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ejycqx | Why is flying so safe? | Probably the wrong thing to ponder while taxiing, but as we sat there in a line of about 10 planes waiting to takeoff, I couldn’t help thinking how impossible it all seems. I did a quick internet search and it says there are 87,000 planes that crisscross the United States every day! How are there not mechanical failures where planes are falling out of the sky more frequently? How are pilots not screwing up more often when taking off and landing? How are Air Traffic Controllers keeping 87,000 planes straight every day? I mean, it seems plausible in 1st world countries, where safety standards are much more strict, but there are just as many planes flying in the rest of the world, it just seems there should statistically be more crashes than there actually are. | Engineering | explainlikeimfive | {
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"Back ups, back ups, and more back ups. Planes are absurdly complicated pieces of machinery, and a lot of that complexity comes from redundant systems that can take over in the event of a failure. If an engine breaks, the remaining engine can still fly the plane. If all engines go out, the plane can glide to the ground. If the pilot makes a mistake, the copilot or the plane's computers can correct for it. Air traffic control has back up controllers that can take over if someone can't do their job. Regional air traffic control can take over if an entire tower goes down. Pilots don't blindly follow instructions either. If traffic control gives them a command that doesn't make sense, they can get clarification. After all of that, its just a matter of practice. Before you're sitting at the controls of a commercial airliner, you've done tons of training, both in real life and in very detailed simulators. You learn how not to screw up, and you take your job very seriously.",
"For mechanical failures: The same reason being in a building or on a bridge at any given moment is so safe. The amount of life that could be lost in a failure is so great, that everything is over-designed. You take all of the loads expected and increase them by specific factors, and use those numbers to design everything. For example, while designing a bridge, we take the weight of everything attached to the bridge and multiply it by 1.25. Then, we take all of the loads expected from cars/trucks and multiply it by 1.75. We do this for all the loads and then add them together, and then design for that. We also try to design the bridge to be redundant, meaning if a beam fails, it won't immediately collapse. The same ideas go into designing planes to be as safe as possible. Design well beyond the forces that are expected, and add several levels of redundancy so if something does fail the plane can still fly. Source: I'm a structural engineer who designs bridges.",
"Most critical systems on an airplane are duplicated of not triplicated for redundancy. Even if the engines totally go out there is a generator that can be dropped down into the airstream to run critical components to allow the plane to lands. Planes are checked and rechecked to be in working order, and if something important isn't working, the plane doesn't fly. The pilots are extremely well trained, with thousands of hours of practice required to do their job. The entire system is extremely strict, meaning rules are there for a reason and are simply not broken.",
"Large commercial planes are extremely expensive so they're built to last a long time. 747s were built for about 27 years of service before metal fatigue forces them to be retired. Since the planes are already expensive, then adding redundant systems isn't a huge cost adder but greatly reduces the risks. You can see in this [map of hydraulics]( URL_1 ) that each control surface is controlled by an electrical system and two hydraulic systems and only one needs to be functional to control the plane. This means that the plane would need at least three separate failures before it lost control. Single failures are rare, double failures a extremely rare, and triple failures just aren't going to happen They're also built to still fly quite well in the event that something bad happens to the engine. Almost all planes today can operator for at least 180 minutes with a single engine failure, this will let them get to an airport, and newer ones can operator for over 300 minutes. In the unlikely event of a double engine failure the planes can glide 15-20x the amount that they fall. An A320 has a glide ratio of 17:1 so if it suffers a failure while cruising at 11,000 meters, it can glide to an airport or landable surface within 187 km with zero functional engines By far the most dangerous part of a flight is take off and landing, but most of the time a plane is up and cruising and this helps to drive the danger statistics down as they're generally normalized per passenger mile, and if you cruise 3000 miles with 200 people onboard with no incident, that one plane got as many passenger miles in a day as 50 cars get in a year. [Boeing put together some airplane safety data a few years ago and its got some interesting breakdowns of incidents by stage of flight and by aircraft type]( URL_0 )",
"Every system is tested before it leaves the ground. Imagine if you checked every fluid in your car, even the gas for water content, before you even started it. THEN you start it and run it through all the motions to make sure it’s running right. Only THEN you pull out of the driveway. Also the people piloting aircraft have years more training than 98% of drivers."
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ek6tbq | how come that if I tap my remote control against my hand (or something else) it suddenly works again? | You know when your remote control is not working, then I tap it, and suddenly it works again? | Engineering | explainlikeimfive | {
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"Batteries work by electrons being stripped from a piece of metal with acid and pushed through a wire. When the metal is degraded too much, there’s no more electrons to strip off it and the battery no longer works. When you slap the remote the acid hits parts of the metal that may not have been soaking as much and shakes any potential lingering electrons off making them available to be pushed through the wire. Imagine you vacuum a dusty rug and it looks pretty good but then you pick it up and slap it and some dust debris still flies off. Same idea.",
"Many times, when a device malfunctions, we try tapping or shaking it, in order knock the offending part into proper position. In this case, the problem may be due to a battery that has moved out of its mounting, perhaps from a previous accidental drop.",
"Battery bounce. The remote was off in lala-land but then the battery bouncing against the spring interrupts and restores the power causing a power reset and a software reboot."
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ekrix3 | Why are all rockets cylindric-shaped ? Would it be possible to have hexagon-shaped (or other) ones ? | Engineering | explainlikeimfive | {
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"Propellant in rockets is kept under pressure and cylindrical shape redistributes pressure evenly. It might be possible to make a rocket with edges but it will require substantial reinforcement of those edges leading to heavier rocket, which is never a good thing. For the same reason planes, submarines and gas tanks all have cylindrical shape.",
"Circles have the least surface area for their enclosed space. The result is that cylinders are good at containing pressure and have very good aerodynamic drag characteristics. Rocket fuel is under high pressure, and rockets want to minimize drag. Thus, cylindrical rockets. It's also easier to make a cylinder than a hexagon."
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el6ocw | How do firearms work under water? | Engineering | explainlikeimfive | {
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"Poorly. Water has a lot of mass and does not compress easily, so a bullet propelled by expanding gas isn't likely to travel very far, instead fragmenting and losing its energy after a very short distance. However if you are asking about the mechanism of action, firearms use burning gunpowder to produce quickly expanding gasses that propel the bullet. Gunpowder contains its own oxygen to combust (which you could probably deduce by it being contained in a sealed metal cartridge). Because of this there is no need for an external air supply and being sealed inside the cartridge prevents it becoming wet."
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el6t6r | What's the difference between AWD and 4x4? | I recently saw a post about a truck that said that the owner converted their truck from AWD to 4x4. I'm wondering how the two are different because they both follow the principal of all four tires moving. Does one work mechanically different from another? Does one provide more power? How do they work? | Engineering | explainlikeimfive | {
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"AWD has a fluid coupler between the front and rear wheels so it allows for some wheel speed discrepancy between the front and rear wheels. Therefore AWD can be used on dry pavement and make tight turns without cause damage to the driveline or chewing up the tires. AWD is meant to be used in any and all conditions, all the time. 4x4 is selectable between 2 wheel drive and 4x4. When you're in 4x4 the front and rear drive shafts are \"locked\" together - turn at the same speed. Therefore it's not safe to be used on dry pavement and making tight turns (like in a parking lot). 4x4 is meant to be used only in \"low traction conditions\". ELI5: 4x4 makes front and rear wheels turn at exactly the same speed - not good on dry pavement. AWD allows for small differences in speed of wheels so you can use it all the time (like making tight turns, and parking)."
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el82rc | How does electricity make a motor spin? | Engineering | explainlikeimfive | {
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"Explanation: Electricity is a current of moving charged particles. Charge particles have an aura around them which is called an electric field. When these charged particles with there electric field auras move, you therefore get a moving or changing electric field. Moving electric fields generate around them a magnetic field. This weird interaction between electric and magnetic auras is scientifically referenced as Farradays Law - You can watch many YouTube videos on this (not ELI5, this phenomenon is called induction, as the moving electric field which induces it's own magnetic field. Since magnetic fields and electric fields are both created from charge particles it makes sense that they can be created from each other. This magnetic field can interact with other magnetic fields within it's \"aura\" just as hand held magnets do - likes repel and unlikes attract. This is what causes an electric motor to spin. (not ELI5, called an electromagnetic force when magnetic field lines interact). Electric motors utilise this interaction by clever engineering as mentioned in the other comments; coils, rotations and shafts. This is my attempt, hard to explain without models or pictures and attempting to keep it ELI5 whilst trying to stay as scientifically correct without getting too detailed. (edit: some grammar)",
"It's a bit of tough concept to explain, but a passing electric current in a conductor generates a magnetic field around that conductor. Wire a lot of wire into a coil, and that magnetic field can be strong enough to do work on a permanent magnet nearby. Put that permanent magnet on a nearly freely spinning shaft, and then you have a motor. The reverse is also true, which is one huge reason why we can manipulate/generate electricity to begin with."
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elb1z9 | When designing tall or complicated buildings, how do architects and engineers know their design is structurally sound? | I’m wondering how architects and engineers know that the structure that they’re designing (and plan to build) will be safe and structurally sound, before they start building it. | Engineering | explainlikeimfive | {
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"The building makes force over itself, they decompose those forces into smaller ones that are easier to calculate, with a lot of math they can figure out how much force every point of the building needs to bear, knowing this, and every kind of \"strength\" of the materials they are using, the can not only know that it is stiff enough, they can also figure out the maximum capacity of it.",
"That's what civil engineers go to college for, to be able to make the calculations required to know a building will be structurally sound. They know how much materials weigh, how strong they are, and what sort of forces they are likely to experience, the rest is just a bunch of math. Also, people have been making buildings for a long time and most of the materials haven't changed that much. There is a joke about how a mathematician finds the volume of a sphere by measuring its diameter and applying the correct formula. An engineer finds the sphere's part number gets the volume from the manufacturer's specs. There are centuries worth of calculations that have already been done, new engineering is mostly about applying this old work in new ways."
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eli16n | How do transistors work, what do they exactly do and why do devices benefit from having more of them? | For what i understood, they are used as some sort of "Amplifier" for electric current and as switches, why would we need an amplifier exactly? | Engineering | explainlikeimfive | {
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"At the most basic level, they take some input voltage and turn it into an output voltage of near 0 or not near zero, which translates into a binary 0 or 1. Which can then be used as the foundation for all digital logic design. Having more of them allows for more complex designs. Let's say you want to make a calculator. A basic adder might need 500 transistors. A subtractor, 800, a multiplier 2000, a divider 3000. Then trig whatever 10000. Or you can make a combined ALU that can do all basic math functions for 50000. Etc etc. Then you need more transistors to route your outputs, read inputs, etc.",
"Transistors are switches. Instead of a manual toggle to switch something on and off, it uses an electrical signal to turn on and off. This allows transistors to be used in building blocks of electronic logic gates. Logic gates are simple circuits that can model simple logic functions like AND, OR and NOT. All mathematical (nearly all) operations can be broken down into a series of simple logic functions. Therefore computers which are basically mathematical engines are built out of logic gates which, in turn, are made out of transistor circuits. The more transistors, the more logic gates can be put together. Generally, more gates means that a computer is more \"powerful\", meaning it can do much more calculating in a shorter period of time.",
"Although transistors are basically amplifiers, we usually don't use them that way in digital logic circuits. If you feed an amplifier either a very minimal signal or a decent signal, what you get out is a small signal or a big signal. We call those 0 and 1 (or low and high, black and white, or whatever). The point is, if you manage what kinds of signals you feed an amp by only feeding the input a small signal or a big signal, that's what you're going to get out. Two choices....binary. In essence, hooking transistors up that way allows us to use them as if they were little switches. Except instead of being turned on or off mechanically, they are turned on or off with electrical signals. So you can use some transistors to turn other transistors on or off, and use those transistors to control other transistors, and so on. In this way, we build up things like \"logic gates\". Imagine two switches in a row. That performs the AND function....both have to be turned on for current to flow through. Two switches in parallel make an OR function, since either can energize the circuit. By making simple logic gates, we can then construct more complex logic circuits, and make those into even more complex logic circuits. It's like making Legos out of smaller Legos. Then using those to build even bigger ones. And of course, it's always better to have more Legos. The more you have, the more complicated your stuff can be."
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elij98 | Atomic Bomb effectiveness | If the A-Bomb and the H-Bomb had been allowed to reach the ground or been detonated just a few feet (like 5-10 feet) above the ground would the effectiveness have been altered in any way? Was detonating it at 600-ish feet above the ground the sweet spot? | Engineering | explainlikeimfive | {
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"The destruction comes from the pressure wave. You compress air, and it gets thick and viscous. Slam that against a building at super and hyper sonic speeds, and it's the ultimate wrecking ball. An air burst is just optimizing the pressure wave for the desired destruction. What also factored into Hiroshima and Nagasaki were the nearby hills and mountains, that were able to reflect these pressure waves back into the city. So ground bursting one of these weapons may be sub-optimal, but it depends on what you're trying to do. If you're trying to bust a bunker, concentrate or limit the scope of the force of the explosion, then a ground burst may be better. One consideration is what a ground burst will do. Pressure is opportunistic, and will go the path of least resistance, taking everything in that path with it. This can lead to some real problems. A 104 kiloton warhead can displace a 100m deep x 400m wide crater, depending on the soil, and now all of a sudden all that soil is going to mix in the mushroom cloud, get contaminated, and produce fallout - intensely radioactive and dangerous dust that will settle for maybe thousands of miles, depending on the wind. An air burst will disburse the fallout over a wider and thinner area, making it actually less dangerous overall. So ground bursts are considered very dirty and very undesirable, if you have to use such a weapon at all.",
"If you set one off on the ground, half of the energy goes into the Earth. The Earth is tough, so this just makes a scuff on the surface. The idea is to apply enough energy to destroy buildings over as large an area as possible. Too close to the ground, and you apply more than enough energy to the ground close to the detonation point, energy is wasted. Higher up, the energy is applied in a larger wave of lower pressure. This can be quite optimal if you consider the terrain, crushing all the buildings in a larger area. It also makes less intense radioactive fallout, in case you want to come into the area and run it after the war.",
"5-10 feet is not enough to make any real difference on the scale of a nuclear weapon. There are differences, however, as you increase the altitude. So the Trinity test was tested on a 100 foot (10 story) tower — that is high enough to change its characteristics a bit (for example, there was almost no real crater formed). The exact heights that \"matter\" vary with the size of the weapon. [Here's]( URL_1 ) what is called a \"knee curve\" graph that shows how the blast wave distance changes with altitude. It's for a 1 kiloton explosion, but it's relatively easy to convert that to other yields because the blast wave scales pretty simply. This graph is a little hard to understand without being coached so let me give you an explanation of how to read it. First pick the height of the bomb. Let's say 1,000 feet. Now we can go along that horizontal line and see where the peak blast pressures are going to fall. So 10 psi (pounds per square inch) will go to 1,500 feet at such an altitude, while 4 psi is a little more than 2,500 feet. Now if we look at those curves, you can see there are places where different pressures \"knee out.\" This is what a planner looks at if they are picking an altitude. Let's say that your target can be destroyed by 4 psi. To increase the size of that radius, you look at the place where that \"knee\" juts out the furthest, then see what altitude that is. So for 4 psi that's something like 1,100 feet. But if it takes 10 psi, the best altitude is more like 900 feet. These differences seem small (both in how far they jut, and the altitude values) but remember this is for a 1 kiloton bomb. As you increase the yield of the bomb, the conversion factors for these distances changes and the increase becomes important. That dotted line on the graph is the \"Mach reflection region,\" and what that means is that the downward directed wave of pressure is reflected off of the ground and is interacting with the expanding blast pressure. This is what is responsible for those \"knees\" that jut out on the graph, as you can see. You can actually [see that reflection wave in some photographs]( URL_0 ). A nuclear war planner is going to ask questions like, \"what's the amount of blast pressure I need to destroy this target?\" and \"what's the accuracy of my weapon?\" If you have a \"soft\" target (like a city, which can be destroyed with 5 psi or so) and a low-accuracy weapon, then you definitely want an airburst and ideally a high-yield one. If you have a \"hard\" target (like a bunker, which might take 100 psi or more), then you need a surface burst. If you have low accuracy, then you either need a GIGANTIC weapon for a target like that, or you need to aim a LOT of weapons at that target. Another effect of a surface burst (or near-surface burst) is that it mixes a lot more dirt and debris with the rising fireball, which results in a lot of fallout (radioactive contamination) downwind of the detonation. For historical perspective, the reason that the Hiroshima/Nagasaki bombs were fuzed at the heights they were was because they wanted to maximize the 5 psi detonation blast wave, because they were destroying a city and wanted the maximum destruction to demonstrate the power of the bombs. The reduction of radioactive contamination was a \"bonus.\""
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eljz9y | for central air- what’s the difference between the 70 degree cool setting and the 70 degree heat setting? | I would think that 70 degrees means 70 degrees... but it sure feels different!! | Engineering | explainlikeimfive | {
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"I believe one setting operates the furnace and will only provide heat to raise the temperature while in a cold environment. The other only operates the air conditioner to lower the temperature in a warm environment. It has more to do with the electronics to operate the correct machine (furnace or a/c) rather then the desired temperature."
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elpzo0 | What is the purpose of the black dots on the edge of the windows of cars and trains? | Engineering | explainlikeimfive | {
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"So though all of these answers have some truth to them they are not completely correct. The black stuff is called frit, and it \"fades\" from heavy at the edge to light due to the tempering process. If it were a solid color it would heat the edge too fast and shatter in the oven. The purpose is for the adhesive to have something to adhere to, and hide the glue edge for a more finished look. I used to run the tempering oven that made windshields and we had to \"print\" the frit on in a very specific way to prevent breakage while tempering.",
"They are part of what keeps the window in place. They could make a regular band of black stuff that abruptly cuts of, but glass covered in dark film heats up a lot more than transparent glass in the sun. You would have a very hot part of the glass next to an not very hot part and things like thermal expansion would make that a weak point. Ideally you could make the black stuff gradually transparent to get a better transition from all-balck to no-black, but the stuff apparently doesn't allow for that so they go with the dots instead.",
"It's to cover the adhesives keeping the window in place. The dots are mostly there to be pretty. It's called a frit.",
"Lots of correct answers - Also, to have the gradient effect (for aesthetics and glass strength) without the ability to 'print' greyscale gradients they have to resort to old-school [halftone dithering]( URL_0 ).",
"They prevent ultraviolet light coming in contact with the butylene rubber used to bond the window in place or bond the two panes of double glazing together. Exposure to UV would degrade the butylene over time causing it to become brittle and eventually leak water.",
"They have a dual purpose 1. They are making a window look nicer with soft edges 2. They make it so temperature at the edges is gradual, so the windows don't break when temperature at the edges (which are 100% black usually) is different then at the center which is transparent."
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elrpnd | Why are "home" toilet seats a complete circle and nearly all public toilet seats are split in the front. I get the dripped pee aspect on complete circle seats, so then why aren't all seats split? | Engineering | explainlikeimfive | {
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"I am a plumber in British Columbia, Canada, this is the code clause: BC Plumbing Code 2018: [2.2.2.5]( URL_0 ) Water Closets In Public Washrooms \\- - - 1) When a water closet is installed in a washroom for public use, it shall be of the elongated type and provided with a seat of the open front type. & #x200B; The reason we are given for this clause is that it is for assistants of disabled persons who may be required to help with the cleaning of their patient. At least that is what they told us in school.",
"I think I remember reading somewhere that the open front seat is more sanitary as appendages have a lower chance of touching the seat. For home use it’s not necessary as there are fewer people using the same toilet."
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elyfhv | why do steel buildings lean towards the sun? | Engineering | explainlikeimfive | {
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"I have spent the better part of 45 minutes intrigued by your question and doing some research and from what I've been able to gather, they don't. The heating of the steel in the structure is not enough to cause it to list to the side the sun is on.",
"Its unlikely that the entire structure is leaning towards the sun, but rather that the outermost walls of the structure are \"bulging\" due to the design of the structure itself. Most skyscrapers are built with core supports that run the length of the building, whether they be pillars, lattice structures, etc. Generally the structure is designed in a way to allow for multiple environmental affects. Gravity, Wind, Thermal Expansion, and certain natural disasters such as minor earthquakes. Since, generally, the outside of the structure is more 'flexible\" than the core support structure, the outsides will swell or shrink with temperature, taking that side out of \"plumb\". The side facing the sun is affected by thermal expansion at a different rate than the side in the shade, hence the affect you're seeing. Some references: The effect is shown with the Washington Capitol Dome, oddly enough, which is mostly constructed of metal. > However, at dawn, while the western side of the Dome still faces the cold darkness of the fading night, the sun heats the eastern side. As the sun rises through the day, the southern side of the Dome rises in temperature faster than the shaded northern portion. At sunset, the eastern side is already cooling while the western side is absorbing the last heat of the sun before night falls. > This uneven heating is what causes the Statue of Freedom to move slightly toward the west at dawn, as the eastern side of the Dome expands, to the north at midday, and the west in the evening, as the sun sets. During the night, the Dome returns to a uniform temperature and a neutral position. You can see from that snippet that the entire structure is actually moving \"away\" from the sun, due to the expansion of the sun-facing side. URL_0 Also mentioned in this book, a clip of which is available on google books. Architect and Engineer, Volumes 40-43 (Free E-Book by the way) URL_1"
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em21vp | how car engines work? | Engineering | explainlikeimfive | {
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"An engine cylinder is a solid piece of metal that can slide back and forth within a hollow sleeve of metal. Step 1 is to inject a little bit of gasoline mixed with air. Step 2 is to ignite a spark - this ignites the gasoline/air mixture. Step 3: the burning gas heats the air inside the cylinder, which forces the solid piece of metal to slide out. It pushes on a crank to give you power. Step 4 is to push the cylinder back in as the gas stops burning and the air cools. Then start all over at step 1. Those four steps are an engine cycle. The engine cycles over and over, the cylinders slide in and out, turning the crank around and around."
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em37d3 | How does the thickness rating work for things like disposable gloves and garbage bags? They're often advertised as anywhere between 5 and 11mm thick when they're clearly not even 1mm thick. | Engineering | explainlikeimfive | {
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"It's not mm as in millimeters. It's \"mils\" which is thousands of an inch. 1 mil is 0.0254mm.",
"You are confusing mm with mil. 1 mil is 1/1000th of an inch. 6 mil is a common thickness for bags. You see the same thing for sheets of plastic and other things.",
"Thanks for the answer guys. Silly Canadian me just assumed it was a metric unit",
"Plastic films are measured in mils, not mm. One mil (thousandth of an inch) is 0.0254 mm."
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em51ss | how exactly does soundproofing prevent sounds from getting in and out of a space? | I’ve heard home recording studios have used mattresses backed up against walls to soundproof a room, as well as purple coloured Sheetrock which is supposedly designed to be soundproof, but what qualities do these items have to dampen sound, and how does it compare to that in a professional environment? | Engineering | explainlikeimfive | {
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"Think of the sound waves as little fists trying to pound on the walls so people on the other side can hear it. That happens because you're slightly vibrating the wall causing new sound waves to be produced on the other side. If the wall is padded with something soft then when you try to pound your fist on that, the wall won't be moved nearly as much and thus won't create new sound waves on the other side."
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emdahs | - How can huge boats like cruise ships or aircraft carriers float?? | Engineering | explainlikeimfive | {
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"> I understand the basics of buoyancy and that some smaller boats like recreation boats have hulls made of plastic and other buoyant materials. Congratulations you have just proven you do not understand how buoyancy works. All materials are buoyant. Whether or not something float depends only on whether it weighs more than the volume of water or any other liquid or gas it displaces. Aircraft carriers float because if they sank they’d displace more water than they weigh.",
"Boats do not float because they are made of buoyant materials. That's how rafts float, but not boats or ships. Boats float because they are made of relatively watertight materials. These materials allow the hull to keep water on one side and air on the other side. Since air is much, much lighter than water displacing the water with air provides a lot of lift. Watertight materials, like steel, are relatively heavy, but their strength allows them to keep the water and air separate. > What doesn’t make sense to me is how huge metal boats can float when they are obviously extremely heavy. Water is also very heavy. By displacing a lot of water, you can make a lot of heavy stuff float."
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emdesl | how do manufacturers know how many batteries and what type of batteries a product needs? | For example, my son received a toy Yoda as a Christmas gift that takes 4 AA batteries. How did they determine it needed 4 AA?? Why not 2 D batteries? | Engineering | explainlikeimfive | {
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"AAA, AA, C, and D batteries all operate at around 1.5 volts. If your device requires 6 volts, you need four of those, no matter what type. The only differences between a AAA and a D are size and capacity. If you have a small device that doesn't need much juice, you use AAAs. If it is big and you want it to last a long time, you use Ds. If it is small and it needs a lot of energy, you change batteries a lot.",
"Batteries have two characteristics we care about: Voltage (potential energy difference between the + and - end of the battery) and capacity (measured in milli-amp hours). Both AA and D batteries provide 1.5 Volts each, but with different capacities. The circuit designer has calculated what the power usage of the circuit is, so they know how long a particular battery will last. Size and weight are also both factors, as 2 D cells are significantly heavier than 4 AAs. There's also that fact that by connecting the batteries in different ways, you can combine the characteristics in different ways. When connected in series (the + of one battery connects to the - of the next, so all the batteries are in a line), the voltages add together, but the capacity stays the same. So the 2 D cells could be made to provide 3V with the same capacity of 1 D cell. The 4 AAs could provide up to 6V at a single AAs capacity. The other way to connect them is in parallel (all the + connect together, and all the - connect together). This keeps the voltage the same, but adds the capacities together. So your 2 D cells, could provide 1.5V at twice the capacity of 1 D cell. The AAs would provide 1.5V at quadruple the capacity of a single AA. One thing you can do with 4 AAs that you can't do with 2 Ds is a combination of series and parallel. Connect the AAs into two 2-battery series, then connect those two in parallel. That gives you 3 Volts at twice the capacity of a single AA. Most electronics nowadays require a minimum voltage of 3V, so at least 2 batteries will be needed, regardless of AA vs D. The deciding factor comes down to how much capacity is needed to give a reasonable amount of playtime, while not being too heavy for a child's toy. A D battery provides 6 times the capacity of a AA, but also weighs 6 times as much. So 2 D batteries in series gives us 6x the capacity at 12x the weight of a single AA (remember capacity doesn't add when the batteries are in series). The AA hybrid circuit gives us 2x the capacity at 4x the weight of a single AA. So a 2 D-cell Yoda will last 3x longer, but will also be 3x heavier. If it's a motorized toy that will have to move that weight, that can make a significant difference. & #x200B; In short, you probably could power that toy with 2 D-cells, but it would be a lot heavier to get battery capacity that you may never need."
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eml3is | How do submarines survive the pressure of being in deep oceans? | I've seen submarines go to the Mariana Trench, Challenger Deep and other deep ocean places. My question is, how do they manage to get there? Do they have some kind of protection for the pressure? | Engineering | explainlikeimfive | {
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"Deep sea submersibles Have very strung pressure hulls designed to withstand the extreme pressure at depth. The Deepsea Challenger, for example, uses a 64mm thick steel sphere to protect the pilot from the surrounding water pressure."
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emlsr7 | How is it organised to change the side of the road your drive in at borders of countries that drive on opposite sides? | As seen in [this]( URL_0 ) infographic, there are numerous land borders between countries that drive on opposite sides, how does that changeover work? EDIT: title should read "side of the road you drive on" blasted autocorrect. EDIT2: I'd love to see photos of videos of it happening if available. | Engineering | explainlikeimfive | {
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"You can Google for some interesting pictures but the general principle is that the road splits, then one crosses the other (overpass), then they merge back together.",
"URL_0 The Hong Kong-China bridge is a good example. Most countries which share land borders drive on the same side, but if they don’t, one of these is usually used. As far as laws go, most countries which share this border don’t restrict what side the steering wheel needs to be on"
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empi36 | With lessons learned from building an oil pipe line through Alaska; why can't the USA build a water pipeline along I-70 from the Mississippi to relieve Midwest flooding AND droughts in the West? | Engineering | explainlikeimfive | {
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"The Alaskan oil pipeline is very tiny compared to the water involved in the floods and droughts. You would need hundreds if not thousands of pipelines with water in order to even make a measurable impact."
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emq5ak | What is the purpose of half-cocking a handgun? | I've googled it, and people keep saying its a safety, but I don't understand. Wouldn't uncocked be safer? | Engineering | explainlikeimfive | {
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"When uncocked the hammer is resting on the firing pin which is resting on the primer of the round in the chamber. If a sufficiently strong force is applied to the uncocked hammer (like say if it was dropped on the hammer) it'll transfer that energy into the primer and fire the round unintentionally. At half cock the hammer is resting on a notch that prevents it from contacting the firing pin. Half cock was a more common safety mechanism on really early single shot weapons and most modern striker fired pistols completely avoid this issue by not having a hammer. ELI5: if you drop the gun on the uncocked hammer it'll fire. EDIT: Please stop telling me about modern safety features. I know. The question was about half cock, a feature that was obsolescent by the 1890s. Modern safety features are not relevant to OPs question. If the gun has a modern safety it does not have half cock and therefore isn't what the question is about. Thing the 2nd: Double action and single action triggers are not half cock. A DA trigger is one where you cock the hammer back with your trigger finger as you pull the trigger, in one motion. An SA trigger pull is one where you first manually cock the hammer and then pull the trigger just to drop the hammer. This is not a safety notch, it is full cock. A DA/SA trigger has both options. Thing the 3rd: Firearms history is incredibly diverse, there are exceptions to everything. I must generalize or I would have to write several books to answer this question.",
"Because on revolvers, think like the Wild West, the hammer could rest on the primer of the round. If something hit the hammer hard enough it could then fire. By half cocking it, you remove the hammer from being in contact with that primer. Also they didn’t have a safety like modern firearms do today. I don’t think they had one at all aside from some kind of trigger safety.",
"It depends on the gun design, for some semi automatic firearms, half cock keeps the hammer locked back away from the firing pin so that a bump does not set off of the primer of the round. This makes the gun safer to carry and also easier to pull the hammer back... In some revolvers half cock allows you to rotate the cylinder.",
"Finally a question I can answer. I have a degree in gunsmithing technology so this is perfect. So many people here saying the firing pin \"rests on the primer\" which is completely not true on modern guns. The firing pin should never touch the primer unless the trigger is pulled and the hammer or striker drops. Modern guns have springs that rebound the firing pin away from the chamber but the spring is weak enough that the hammer drop overcomes it to strike the primer. A lot of guns also have firing pin safeties that only allow the firing pin to move far enough forward to fire when the trigger is actually pulled (apply this to striker fired). On old hammer fired weapons (black powder and some of the 1st guns that used cartridges) there was a huge safety issue because the firing pin was on the hammer making the guns not drop safe. They later fixed the issue by moving the firing pin off the hammer and adding a transfer bar that moves upward in between the hammer and firing pin when the trigger is pulled allowing the hammer to transfer the strike to the firing pin. If that transfer bar doesnt move (aka no trigger pull) then the hammer will never be able to strike the firing pin. A lot of people carried on an empty chamber to avoid the safety issue prior to transfer bars being introduced. I carry a semi auto hammer fired pistol on half cock. Modern guns with a decocker usually dont have safeties so the half cock is a way to make your gun safer to carry because on full cock the trigger pull is usually much lighter and shorter than a decocked trigger.",
"Several people in this thread talk about preventing drop fires, but half-cock will usually also be hooked so that just pulling on the trigger won't release the hammer and fire the gun, so it does also deactivate the trigger in a sense. For example, in this animation of the rolling block action, you can see the light cyan block is the hammer and the trigger is in dark red. URL_1 You can see that there are two notches at the bottom of the hammer, the back one is \"flat\" and the front one is \"hooked.\" When the hammer gets pulled back all the way to the back one, the notch is hooked, and the hammer can be dropped by just pulling the trigger, but the hooked notch mechanically stops the trigger from rotating to drop the hammer. Edit: The narrated version of the animation from C & Rsenal might be clearer: URL_0",
"A gun works by having a little button on the end of a bullet called a primer. (Look on a bullet and you'll see a circle inside the end of a bullet). If you hit this with a piece of metal it ignites and the gunpowder in the bullet in turn ignites creating pressure to push the bullet out of a gun. On certain handguns (older ones) you'd have a hammer (the part you see people cocking back on a revolver) which when the trigger is pulled releases the hammer to strike the firing pin or just hit the primer itself. When you half cock a handgun it keeps the hammer or firing pin off of the primer completely. So there is less of a chance of anything striking the primer. Full cock engaged the trigger mechanism so that can be pulled to release the hammer, but that does not work for half cocked, thereby making firing less likely. Modern guns do not work this way, and generally have a completely different mechanism. Remember though to always treat a gun as if its loaded, regardless of safeties.",
"The actual intention of the half-cock notch on hammer fired guns, especially single action ones, is it's a safety measure against you trying to cock the hammer fully, but it slips out of your fingers, flies forward, and fires the gun. Particularly when talking about percussion cap guns. If you cock a hammer with your thumb and slip before the hammer reaches the half-cock notch, it won't have enough energy to fire the cartridge, if you slip after you are past the half-cock notch, the half-cock notch is supposed to catch the hammer so it doesn't hit the primer. ie, if you need 75% of the energy of a fully cocked hammer to ignite the primer, the half cock is there to prevent no more than 50% hammer strikes unless you pull the trigger.",
"I’m going to add my two cents. In the early days of firearms, especially in the flint lock era, you couldn’t close the frizzen without the lock being out of the way. If you fully cocked it, bumping the trigger could either discharge the firearm or mangle the operators fingers. Same story on the cap-lock days, had to put a fresh percussion cap on the nipple this the hammer had to be out of the way. Thus the purpose of half cock. Revolvers were simply a progression of cap locks (cap and ball revolvers) into more modern revolvers. This was held through even into semi automatic pistols. Colt 1911 is a great example of a semi-modern pistol that has both a half cock and manual grip safety. Even with some modern hammer fired pistols, half cock is still present (though now it’s to prevent a misfire in the event of the hammer slipping).",
"I didn’t read all the responses so my post is purely my experience and training. As a 12 year USAF SF vet and former law enforcement, all the training I have ever been through only explained it 1 way. Half cock is somewhere between full squeeze :BANG: and “ok the threat may not be there but I was pulling the trigger on a target which is no longer a threat but may have to re-engage and don’t want to need a full squeeze at whatever lbs per sq in it takes.” That’s it... of course there may be a super technical reason they put this feature on a firearm but what I stated above is how every USAF CATM instructor explained it in any class I have been through, and it’s a lot of classes and hours on the range. :edit: I’ve read some responses about it being “safer” but any pistol I’ve carried for service to the military or public have a firing pin that either rotates out of line with the hammer or is blocked when the hammer is in the resting position. So the idea that half cock makes it safer than “resting” isn’t valid as the firing pin can’t be struck when the hammer is resting. Again, only on firearms I’ve carried for duty. I’m not a “gun guy” per-se so I dunno about models I haven’t been trained on."
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emrtzd | how do they test submarines for pressure and leakage before just plunging to the depths of the ocean? | Engineering | explainlikeimfive | {
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"We know the strength of the steel and other materials the pressure hull is made of, the specific design of the submarine, and how water pressure works, so it's not hard to use computer models and simulations to determine what depths a submarine can withstand. When built, submarines have a design depth, which is the depth they're designed to be able to withstand. Then during sea trials (sea trials are the usually lengthy period when a naval vessel is tested at sea before deployment to make sure everything works as intended), the submarine will actually be taken down to various depths to ensure it performs normally."
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emwvlg | Why do fire hydrants not freeze and burst in the winter? The water is theoretically above the freezing line, no? | Engineering | explainlikeimfive | {
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"> The water is theoretically above the freezing line, no? No, actually! The part of the hydrant you see is only part of it; there's a long pipe that extends underground to the water main, where it's connected with a valve. The valve is connected to the nut on the top of the hydrant (that you twist to turn it one) by a long shaft. The whole thing is kept empty until it's turned on. The reason for this is exactly the issue you're thinking of: to keep the working parts of the hydrant buried so deep that they never freeze.",
"If I recall correctly, there is a feeder pipe up to the hydrant from below the frost line, and an activator device which keeps the water below the frost line.",
"> **The water is theoretically above the freezing line, no?** In cold climates, it is below the freezing line. The design is simply a valve below ground and long rod that extends up and you can turn from the top. [Fire hydrant design.]( URL_0 )",
"Once you close the valve (which is located about two feet below the ground level, or more), that water that is in the top part of the hydrant drains away into the soil nearby. If you do not open the valve, and you drilled into the side of the part of the hydrant that is above ground, there would be only air in the part where you drilled the hole, and a rod that connects the top handle part to the subterranean valve.",
"One I can answer! They can and do freeze if there is water past the shutoff valve. We prevent this by systematically and regularly checking the water level of hydrants. If water is found, we turn on and then turn off the hydrant to make sure it is fully closed and not leaking. Then we pump out the water that remains in the space between the shutoff and the street. Hopefully, when they are checked two weeks later, they are still unfrozen and dry. I have tried to open a hydrant that was full and frozen and it was not fun. Fortunately another truck had a working hydrant. It may work differently in different jurisdictions. Source: am a firefighter"
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en7bg4 | how do hydrogen-powered cars work, and are they a considerable competitor to electric vehicles? | Are hydrogen-powered cars even a thing? | Engineering | explainlikeimfive | {
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"Hydrogen can be used as fuel for combustion engines that are very similar to the fossil-fuelled ones. But another, less mechanically complex solution is to use hydrogen as fuel for a *fuel cell* which creates electricity that powers the car. So in that case, the car is just a normal EV where the large battery pack is replaced with a hydrogen tank and a fuel cell.",
"Your car runs on compressed hydrogen gas, either by burning it or reacting it with oxygen in a fuel cell. The tricky bit is making and transporting the hydrogen gas. It takes a lot of energy to make, is often made in very “dirty” ways and is hard to get to the end-user. Right now, there is not a lot of adoption of this type of vehicle. So, they are not much of a competitor to either fossil fuel or electric powered vehicles.",
"Mix petrol droplets with air and light the mix on fire. It goes BOOM. This is what powers gas-powered cars, they just take the force of thousands such explosions every second and use it to move the car. Mix hydrogen with air and light the mix on fire. Mix goes BOOM... You get the idea. There's also hydrogen fuel cells. When hydrogen reacts with oxygen, some electrons move from the hydrogen atoms to the oxygen attoms. You can use this moving of electrons to make electric current, which drives an electric motor, which moves the car."
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en8rw1 | how exactly is the “stop/start” automatic engine feature in newer cars “better”? | Not quite sure what it’s called but when you stop a vehicle it sounds like the engine stops then starts again automatically when you put your foot on the gas Seems to me that would be more wear/tear on your vehicle | Engineering | explainlikeimfive | {
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"Barely any wear and tear, better for the environment as all that time you spend not moving while the engine running is time that CO2 and pollutants are spewing out when they don't need to be. Multiply all that time by millions and millions of cars and you have a significant CO2 saving. Saves fuel and thus cash too."
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enarf8 | In a car, why does the wheel and brake pedal lock up when the engine is off? | Engineering | explainlikeimfive | {
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"For two very different reasons: the wheel locks up because the key also acts as a lock so even if they can hot wire the engine, they can’t steer and drive The brakes lock up because there is a device called a brake booster that uses suction created from the engine to reduce the effort needed to stop, it still works, but it’s incredibly stiff, on some cars the brake is still soft until you press it once However if the car is still in gear and moving when he engine is tuned off, the brake booster will work since the engine is being turned by the movement of the car and thus still creates suction"
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enehqq | Why do electric cars not need gears, while combustion engine cars do? | Engineering | explainlikeimfive | {
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"Because internal combusiton engines operate most efficiently at a certain range of RPMs. Gears allow the engine to operate at that RPM range while driving at a variety of speeds.",
"Because combustion engine cannot turn at 1RPM or 15,000RPM, which you need to drive at 1kph and 200kph. If you don't got gears then you have to do a burn-out at every green light and then be limited to maybe 80kph driving at red line."
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enjo0h | How are the sequins on those reversible sequined pillows sewn? | You know those sequined pillows where they're one color first, and then you rub the sequins the other way and it's another color? I couldn't help but wonder how factories manage to get all of those little sequins in an orderly fashion on the fabric for such a cheap price as to be found in Wal Marts everywhere. Obviously, they're mass-produced, otherwise they'd be a hundred dollars and imperfectly spaced. I just can't think of a machine that can hold all those little discs of shiny plastic, pass through those holes without missing them and making an empty space, pass through the fabric back and forth, AND knot the work as it goes along to prevent mass unraveling. The only results I get on Google are about printing images on the sequins after the sequins are sewn. | Engineering | explainlikeimfive | {
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"I know you said you couldn't think of a machine that could do that, but fabric factory owners certainly can! It is basically exactly as you'd expect: vast numbers of sequins pour into the machine, which has multiple sewing needles. Each needle is supplied with a specific colour of sequin, and a computer controls which needle fires at any point therefore deciding which colour is placed where. Here are a couple of short videos that will give you the rough idea: URL_1 and URL_0 (neither are particularly high quality, unfortunately)",
"Essentially, take a solid, two sided sheet of the sequin material - solid colour or printed patterns; then use a sewing machine like a serger that instead of cutting the material, it holepunches out a piece of sequin and sews it on in one motion, then moves on to the next spot. Repeat.",
"If you're trying to do one yourself it's really easy ( but a time consuming pain) Just put a sequin on where you want it, and dont stitch so tight. Give it some slack to roll over"
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enldro | How is the B-2 Spirit mostly undetectable by radar? | Does the polygonal design of the plane itself play a role in this? | Engineering | explainlikeimfive | {
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"Imagine you're in a room with the floor walls and ceiling painted black. Now turn the lights out and imagine you have to find a mirror somewhere in this room with nothing but a headlamp. You'd think it'd be easy, just shine the flashlight around the floor walls and ceiling until you see it, however you will soon discover that you can't see anything on the floor walls or ceiling because the mirror bounces the light from your flashlight away from you. All you see is the black reflection blended in with the black background. You will only be able to see the mirror when it's facing directly towards you and you can see your flashlight in the reflection. Now take the same concept, except instead of a mirror, it's a pinball polished to a mirror finish. You should be able to find it almost immediately because there's always some part of the ball bearing that will reflect you and your flashlight back to you. As soon as you shine your headlamp on it, you'll see a twinkle from the reflection. This is the basic concept behind stealth; certain shapes reflect electromagnetic waves back to their source better than others. Flat shapes are typically the best at avoiding this, while spherical shapes and edges will almost always cause a return. *Edited for clarity/simplicity",
"Yes, because of the footprint shape and the sleek design there are very few parts of the plane where a radar signal would be bounced back to its source. Radar works by sending out a signal and then waiting for it to bounce off something and come back.",
"In the USAF I maintained the avionics on both the F-117A and the F-22 Raptor. The stealth characteristics of these aircraft are determined by shape and RAM (Radar absorbent material, F-117 terminology) / LO (Low observable material, F22 terminology). There are measures taken to reduce thermal footprint like exhaust. Any other answers that go deeper then that are guesses because even with the security clearance I had there are things that just aren’t discussed.",
"You are probably thinking of the F-117 which looks like a plan from a 1990's video game as opposed to the B-2 which looks like a very nicely rounded boomerang. The answer to the question though is that we understand how radar waves bounce off objects at a theoretical level and can thus engineer shapes with the objective of bouncing back as little of the signal as possible. Radar absorbent materials, and other strategies, are employed as well.",
"I was driving to St. Louis on Thursday and one of those big bastard flew over the highway at a relatively low altitude. We also see them flying very high over KC (where I live) occasionally."
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enntls | Why is there never a fan setting in your car for both the windshield and dash? | Engineering | explainlikeimfive | {
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"Our vw golf can. But it's buttons not a dial. Great for keeping hands warm and windshield clear.",
"That's just how the manufacturer decided to split off the various air flow ducts. See URL_0 , where, once the air is ready to flow into the cabin, the first split is between \"feet\" and \"windshield/dash\", and then the \"windshield/dash\" duct splits into either windshield or dash."
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env97f | Is it true that prosthetic limbs make you faster and if so why isnt the world record for events like the 100 meter faster than the regular? | Engineering | explainlikeimfive | {
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"To have the 100 m record broken due to a prosthesis, you'd need to have one of the fastest men in the world (like Bolt) *also* need a very specific type of prosthetic... a pretty unlikely event. The argument against Oscar Pistorius was that he would have been a borderline Olympian without the prosthesis, but that the 'blade' made him a little faster than he would have been otherwise.",
"It's true for very specific types of limbs and specific types of races. If you got whole prosthetic leg, then you can't run at all nevermind fast.",
"I just want to add a warning not to conflate a prosthesis with an orthotic. The former replaces a missing limb, the later augments an existing limb. You may have seen those crazy boots that prop a runner on spring-steel leg extensions, granting a person the ability to run upwards of 25 mph with no particular athletic training. That is an orthotic. And why there isn't a world record, well, you'll have to ask Guinness or other certifying bodies. This part of your question is too broad. There's all sorts of 100m records, you just have to define this one in a way that makes sense. What kind of prosthetic or orthotic? How about one with jet engines? See the problem? How do you define and constrain record categories so that they make sense without an utter proliferation of definitions?"
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enzrna | Why are the guns of today less extravagant and dolled up compared to the aesthetics of weapons from the the 1700's and 1800's | Engineering | explainlikeimfive | {
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"My guess is because they are mass produced now compared to the true craftsmanship of the earlier era."
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eo514a | Why do trains only have a single gear? | Trains accelerate incredibly slowly and often have a single gear that is optimised for high speed. Similar to trying to pull away in your car in too high of a gear this makes trains really slow and takes along time for them to hit their high speed. Most cars will often have 4 (for very old cars) - 6 gears to keep acceleration smooth and fast whilst still being efficient at high speed but trains don't. I get that electric motors have all of the torque available at low RPMs whilst ICE only have full power at a high rpm but wouldn't gears still allow trains to get to higher speeds quicker? | Engineering | explainlikeimfive | {
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"The problem is not the available torque in the engine. But there is a few issues with trains that make their initial departure very slow compared to cars. The first issue is that the train is very long. When the locomotive starts moving the back of the train does not immediatly start moving. As the locomotive moves forward it will start taking up the slack between the cars and also stretch out the frames of the cars a bit. Even a very tight passenger train will have a few seconds from the locomotive starts moving to the back cars starts moving. And first then will the momentum of the back cars make it lighter for the locomotive. This causes a jerking motion going back and forth through the train as it gets up to speed. In order to make this jerking motion much less which increases comfort and reduces wear on the equipment the engineers will start with the minimum of throttle and slowly increase it as they get up to speed. The second issue is that the train tracks and wheels have very low friction. This is very good for high loads and high speeds but terrible for acceleration and braking. If the engineer applies full throttle from a standstill the wheels will just slip on the smooth tracks and he will spin in place damaging both the tracks and wheels. So he have to get up to speed where he have some momentum before he can apply the full power.",
"Trains are amazingly underpowered compared to cars. Freight trains are somewhere around 0.5 to 2 horsepower per ton. You're average car is somewhere around 100 horsepower per ton. That's why trains can't go up hills more than about a 2% grade and accelerate very slowly. However, they are very efficient. Trains can move a ton of freight over 470 miles on a single gallon of fuel.",
"This is going to be a very basic explanation, I understand I'm leaving out a lot of important details here. Trains dont actually have any gears. A modern diesel electric train is essentially a generator. Inside the locomotive, a diesel engine is coupled to a gear box. Let's say 1:25 ratio (just an example). So for every 1 revolution of the crank (output) shaft of the diesel engine, there are 25 revolutions on the output shaft of the gearbox. The output of the gearbox is then coupled to a generator. Electric generators and motors are essentially the same thing. Apply mechanical movement to a generator/motor and you will produce electricity. Apply electricity to a motor/generator and you will produce mechanical movement. The power that is generated from the locomotive is used to power everything on the train. Lights, controls, heating, air compressors, and the motors that move the train. Those motors are located under the locomotive, between each set of wheels, much the same way electric cars work. The electricity that was generated in the locomotive is fed to the motors through a variable frequency drive (VFD). The VFD controls motor output speed by changing the frequency of the sine wave (Dont worry about this part). It kind of limits the amount of power applied to the motors. Those motors are then connected to each set of 2 wheels through another gearbox, this time stepping down, so let's say 25:1 ratio (again just an example). So for every 25 revolutions of the motor (input of the gearbox), there is 1 revolution of the wheel (output). Trains are slow because they produce an insane amount of torque, and the process I outlined is what makes that torque. As others have mentioned, slow speed, high torque is exactly what trains need, as they are very heavy, and have little friction between the wheels and the rails to work with. As they start to gain momentum, the frequency can be turned up on the VFDs to slowly add more speed, and less torque to the wheels, this is essentially what the throttle control on a locomotive does. (Again, very simplified here). Another benefit of this is the ability for trains to use regenerative braking, the same way electric cars do. (I wont get into this), Streetcars, subways, LRT vehicles, and some passenger/freight trains use the same process of adjusting frequency to drive motors, but instead of generating the electricity right inside the locomotive, power is fed to the train by an overhead wire, or a third rail.",
"Electric cars don't have gears, too. Electric vehicles don't need them. Trains accelerate slowly because of steel tires on stelle tracks. There is not much grip.",
"Some trains do actually have gears. Diesel multiple units (DMUs) are often used in the UK and other countries for short passenger runs. They consist of 1-3 train cars, with diesel engines underneath. The engines are coupled to the wheels either via a mechanical gearbox (usually with automatically changing gears), a hydraulic transmission (potentially also with a gearbox), or electrically via a generator and motor. The mechanical gearboxes are kinda rare nowadays, but there are still some trains (class 143/144 Pacers) in the UK that use them - albeit that they're on schedule to be replaced pretty soon. Also, smaller lighter trains can have better acceleration - underground trains in particular are pretty snappy. They're never going to be going all that fast, so I guess the motors can be optimised for high acceleration.",
"Trains are awesome in that they **only need one gear**. Train tracks have much stricter limits for grade and turn radius than roads.",
"The \"torque from the start\" isn't universally true for all types of electric motors, and it's also not entirely accurate. Many types of electric motors actually have a lot more torque available at a standstill, while others only reach their maximum torque once they spin up. Ceiling fans for example have barely any torque from a standstill, which is why they take so long to spin up and why you can easily stall them. However, you are not wrong: For maximum acceleration, electric motors can benefit from a gearbox in order to keep the motor running in its optimum RPM range. Porsche does that with their new electric sports car. However, this adds weight and complexity, and electric trains usually have plenty of acceleration anyways."
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eogprc | how were straight lines made? | in elementary school, my teachers would say, "if you need a straight line use a ruler," but how did they get a ruler to be straight? it must have come from a machine or something, but how did people get the first one? one way I'm imagining it is if you stretch out a string taut you can get a straight line, but how was this applied to everything else that uses straight lines? is there something in nature that is perfectly straight? | Engineering | explainlikeimfive | {
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"Take a string, tie it to two stick. Pull the sticks apart, string becomes taut. You now have a straight line. You can use shorter lengths of rope to decrease your margin of error. Source: We still run string lines today.",
"If you enjoy reading, check out The Perfectionists by Simon Winchester. He explores the earliest (and most crude) measurements of precision up through the achievements of millionths of a meter and a second in the contemporary age of GPS and aerospace. A fascinating and entertaining read!",
"Use gravity to help you make a straight line if you want to see such things in nature. Tie a string around a stone; hang the stone over a nearby branch; gravity gives you a straight line.",
"The simple answer to where straight lines come from is that, if you take three stones and alternate rubbing them against each other, eventually you will get three perfectly flat surfaces. Join two points on one surface, you will get a straight line. Use the stones as a reference for building the first ruler. Here is a link to a GREAT video about exactly where measurement came from, even asks the question about straight lines. Chanel is called Machine Thinking URL_0"
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eogsbs | Why are there 1TB micro SD cards that are the size fingernail and most 1TB hard drives are bigger than my hand? | Engineering | explainlikeimfive | {
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"A MicroSD card has very limited throughput (read and write speed) compared to a proper disk drive. A really good SD Card can write about 30 MBytes per second. A hard drive can achieve 100MBytes to 200MBytes per second. And a solid state drive can do 500MBytes per second. So SD Cards are small and cheap, but slow and not robust. Hard disks are cheap, moderately speedy, and robust. SSDs are robust, relatively expensive and very fast.",
"Other than the underlying technical reasons, there's also a matter of standardisation. Hard drives have to be big enough to fit in a drive bay and have the appropriate connectors.",
"They are different storage types. MicroSD and other flash storage are loaded on chips whereas inside a hard disk drive there are physical metal platters/\"disks\" which hold the information. These have also been designed to work across different desktop storage brands with universal mounting capabilities. They are also generally more resilient of a storage medium than flash memory, particularly when it comes to deleted/formatted data recovery. Portable storage has its merits but they are unpredictable at end of life.",
"In SD card technology software on the phone or device acts as the hard disk controller allowing the physical chip to be smaller. In an SSD hard drives a motherboard which also contains the NAND chips acts as the controller. The SSD NAND hard drive is also a slightly more advanced NAND storage medium (think extra features) requiring a larger die size.",
"Hard drives or HDDs are magnetic storage methods that are a few decades old. They are reliable and you can store a lot of data on them. Their size is due to the mechanical components in them and partially due to standardisation. For example, you can find laptop hard drives that are very small. No need to make them smaller as they are quite slow and the machines that use them are servers or desktop computers. Solid state drives SSDs have no moving parts and they store data in small memory chips. Their size is the same as a laptop hard drive and this is due to standardisation as well. If you open one of these up, you'll see it's mostly empty space with a tiny board a few chips in them. There is no need to make them smaller as most devices have bays where these need to fit. Standards again. M2 drives are the next gen of SSDs. They are faster than SSDs and as robust, but they are smaller than an SSD. They don't have any casing which saves a lot of space and the new standard allows for a smaller form factor. So thanks to technology, the industry has agreed to make drives a bit smaller for the next gen of SSDs. MicroSD cards are tiny yes, but this is because the devices that require them are fairly small as well like your mobile phone. Some of the hardware required to make them work is also on your device so that saves space. They are also limited in terms of input/output. Going smaller than this is probably not practical as they are delicate enough to crack or get lost. In short, storage gets smaller as technology advances but some of the form factors you see are due to industry standards. While I'm sure they could make an SSD the size of your fingernail, it all depends on what devices can support. It usually takes a few years for different manufacturers to agree/adopt new form factors.",
"On a computer chip, the only thing that matters is the surface layer. The silicon below the surface is just there for support, so the chip doesn't break apart during or after manufacturing. So what they can do is to grind away the bottom of the chip until there's just a paper thin surface layer left and glue multiple of these flattened chips into a single stack, which is still thin enough to fit into a micro SD card. The disadvantage of this is that this doesn't work with power hungry parts, since concentrating a lot of heat into a tiny area can easily set the thing on fire. But flash memory uses very little power, allowing it to be crammed into tiny spaces like that. This should not be confused with MLC flash. That means multi level cell, which is a technology that allows to store more than just one bit within each individual cell.",
"Different technologies. The former uses [flash memory]( URL_0 ) which is implemented with transistors which are really small. The latter uses [physical disks]( URL_1 ) which also need some sort of motor to spin them and a reader/writer arm.",
"With solid state hard drives, part of the issue is backwards compatibility. It has to match the size of a magnetic drive to fit security in the case, as well as it has to be large enough for the Sata connector.",
"Side question, but how much does it cost to produce a MicroSD card? SanDisk’s been dropping the price on their 1TB pretty violently, and it’s got me wondering just how much profit they’ve been able to make per unit."
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eoh9ye | How a wet-wing fuel tank on a plane works | I've read online on how airliners store their fuel in the wings of the plane, and especially how it's not in a tank, but its just inside the wings. How exactly does this work? Does this mean the wings have to be completely air tight? If the inside of the wings are mostly structural elements, how do they deliver the fuel while making sure that there aren't areas in the wing structure where the fuel gets trapped? | Engineering | explainlikeimfive | {
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"I might try looking at the [Wikipedia page for this topic]( URL_0 ), I think it answers your question fairly well. First sentence of the article: > A wet wing is an aerospace engineering technique where an aircraft's wing structure is sealed and used as a fuel tank. Wet wings are also called integral fuel tanks.[1] > A disadvantage of the wet wing is that every rivet, bolt, nut plate, hose and tube that penetrates the wing must be sealed to prevent fuel from leaking or seeping around these hardware components. This sealant must allow for expansion and contraction due to rapid temperature changes (such as when cold fuel is pumped into a warm wing tank) and must retain its sealing properties when submerged in fuel and when left dry for long periods of time. There's also this [Quora thread]( URL_1 ) that I found. The answer by Jayaprakash has a pretty cool picture of a tank-like object taking up almost the entire cross-section of the wing.",
"I'm not sure how it is on all planes, but on the ones I work on there are \"bladders\" inside the wings. The actual fuel would corrode the metal, and when airline manufacturers tried anti-corrosion coatings... it would come off and gunk up engines so bad they would completely stop running. So, they went to bladders inside the wings. Again though, this is only on planes I've worked on."
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eom4e0 | Why is airplane engine noise changing pitch every couple of seconds even when cruising? | I noticed that airplane engines often change pitch every couple of seconds in a rhythmic fashion, even at cruise altitude when they are supposed to work at a constant speed. If I had to visualize this change, it would be a like a sine wave. This is especially noticeable if sitting in a window seat and leaning against the wall. | Engineering | explainlikeimfive | {
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"It is likely that you are confusing the pitch and amplitude of the sound. This is something which happens whenever there is two sound sources with a very close pitch. What happens is that the sound waves interfere with each other and either amplify each other or cancel each other out depending on the phase between them. The phase changes with the difference between the pitch. So if there is a 1Hz difference in pitch of the two engines you will hear the sound becomes stronger and weaker in a sine pattern once every second. if there is a 1/2Hz pitch difference it will be once every 2 second, and so on."
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eom8ju | Why do passenger trains have those front headlights that swivel back and forth? | Engineering | explainlikeimfive | {
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"These lights are called Mars Lights, because once there was a company named Mars that developed them. They swivel because deer are **stupid**. OK, that's not the whole reason, but deer have a behavior that's undesirable to train operators. When you shine a bright light on them, they freeze. If you point the bright light at the tracks in front of the train, you cause more deer to stop on the tracks in front of a moving train, get hit by the train, and possibly scuff the engine. Instead, the moving beam surges bright and dim. When it's bright the deer becomes frightened and it freezes, and when the light dims they seize the opportunity to flee. It also works to get the attention of humans and other possible track-misusers."
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ep0ftq | How are rigid or solid life boats deployed on a cruise ship in an emergency? | Are there people inside? Do they get dropped into the water? How long would it take for evacuation? I'm quite interested but I can't seem to find any information, easy enough to understand. Thanks! | Engineering | explainlikeimfive | {
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"They are attached to the side of the boat on pulleys. When needed they are raised or lowered to the level of the people. When full they are lowered into the water. The time needed has many factors."
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ep193y | How come no matter how much water I pour in the toilet, the water level never rises? | Engineering | explainlikeimfive | {
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"There's a U-bend just past the hole where the flushed water goes. The water goes from the bowl backwards, up a bit, and then straight down to the drain. That's the U-bend. As you pour water into the toilet the water level in the bowl rises, but also on the U-bend side. The water goes over the edge of the bend and the water drains our keeping it in balance.",
"The water you can see in the toilet sits in a U bend. What you're looking at is one half of the U. Inside the pipe that you can't see, there's another water surface before it spills over into the drain. That edge is what stops the water level rising past where it is. When you add water, all you're doing is making the back side of that U bend spill over into the main drain, so the water level never appears to go anywhere. That U full of water is there so nasty smells from the sewers don't float up through your toilet and make your bathroom smell like rotten ass all the time.",
"[Once the water is high enough to go above the weir, is flows to the outlet.]( URL_0 )",
"Check your library for a book called Toilet by David Macaulay. It’s in the children’s section and I (not a child) found it fascinating. URL_0",
"Imagine a teapot with a low spout. The toilet doesn't overflow for the same reason that you couldn't keep pouring water into the top of the teapot and expect it to overflow at the top - the top of the spout lower than the top, so once the pot is full up to the spout level, any extra water simply runs out of the spout. Your toilet is basically the same. The toilet bowl is the top of the teapot; the U bend is the pot; the pipe out of sight round the U bend is the spout. And the pipe out of sight is (quite a bit) lower than the top of the bowl, so once the U bend has filled up to the pipe's level, any extra water just flows out that way."
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ep4s3g | why do cars not have regular outlets that people could plug chargers in? | Engineering | explainlikeimfive | {
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"110 volt AC vs 12 volt DC current. Different systems with different pros and cons. You need an invertor to convert between the two systems. Also, alot of cars do have 110v plugs these days. Usually they're good for 400-500w. They (unfortunately) allow me to work on my laptop while sitting at a jobsite waiting for the dickhead surveyor who's late.",
"A lot of new cars definitely do have normal 120v plugs and most if not all new cars have USB charging ports",
"Cars run on roughly 12 volt DC (ofcourse it fluctuates a little). All home appliances run on either 230 volt AC (most countries) or 110 volts AC (few countries including US). So you need an invertor to convert 12 volt DC to 230/110 volt AC. Why most cars dont have invertors for mains power? Few people use them. Not to mention car electrical system can only handle limited amount of power.",
"A car'ss electrical system runs off of batteries, and batteries produce direct current. Household electrical comes from generators that supply alternating current. AC loses less power over a distance and can be efficiently converted to DC as needed. DC can be converted to AC, but not as efficiently. Also, the only devices that really need AC are motors, most other either don't care (lights, heaters) or convert AC to DC internally (most electronics), so you aren't losing much not having AC. Many AC devices with motors use so much power it isn't safe or practical to run them through a car's electrical system.",
"Some cars do. A few older Pontiacs like the Vibe and Aztec have them, and some light/medium duty trucks do as well. It's not an option most people need, though."
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ep5vvg | How does torque work in wrenches? | Like why does a longer handle let you put more force on a bolt when when you’re putting the same amount of force on the other end of the wrench? Where is the additional energy coming from? | Engineering | explainlikeimfive | {
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"It’s coming from the increased distance of travel. To turn a 1-foot wrench 360 degrees (one full rotation), the end of three wrench needs to travel about 6.28 feet (pi * 2 * radius). To make the same rotation with a two foot wrench (which gives you twice the torque), the end of the wrench needs to travel twice as far. This is the same concept as an inclined plane and other leverages.",
"Torque, the measure of twisty-ness, is force times the length of the arm used to convert pushy-ness into twisty-ness. More torque isn't more energy, it's more twisty-ness. Longer wrench means more twisty-ness but less rotational speed, just like how a lever lets you lift a heavier load more slowly.",
"Not intended to be accurate, but understadable. Let's say you have a 1ft wrench. When you twist, your hand moves one foot in a circular motion, but the bolt only moves an inch but with 12 times the power. You are converting distance to power, like gears.",
"If you've ever been on a see-saw it's not tough to understand. Take 2 kids, with one heavier than the other, and move the heavy kid towards the center and the lighter kid away from the center and you reach a point where they are pretty close to balanced. Because the torque around the center of the see-saw is their weight times the distance. Now, this doesn't change the energy at all, because on a torque wrench, even though you are using less force on a longer torque wrench, you are also moving your hand a larger distance. If your wrench is double the length, it has to travel double the distance in making a circle. So, since the Work accomplished is Torque times distance, you are using the same amount of energy."
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ep9tc8 | why do high-speed trains have hydraulics mounted sideways on wheelsets | Seen here [ URL_0 ]( URL_0 ) | Engineering | explainlikeimfive | {
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"It's a yaw damper. Since the wheelset rotates passively and the train has a lot of inertia it has a tendency to overcompensate: if it has to steer to the left it steers too much so then it has to steer to the right, but again a bit too much. This goes back and forth for a short while making the wagon sway laterally. The damper counteracts this motion so it goes smoothly."
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epbf6w | what's the difference between an accelerometer and a gyroscope? | Engineering | explainlikeimfive | {
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"text": [
"An accelerometer measures straight-line acceleration. A gyroscope measures rotational velocity. Spin a basketball on your finger: high rotational velocity, low straight-line acceleration. Bounce it on the floor: low rotational velocity, high straight-line acceleration."
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epcuoo | How does the "push button to cross" improve a crosswalk? | Engineering | explainlikeimfive | {
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"If there aren't any pedestrians crossing, you don't want to hold up cars waiting for them to cross. The button lets the light timer know that there are pedestrians, so it only allows time for pedestrians if there are actually pedestrians. Similar reasoning to having car sensors in the road. You give more green light time to the directions with more traffic."
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epjgo0 | The difference between 1st and 5th gear in a vehicles transmission | Engineering | explainlikeimfive | {
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"First gear let’s the engine spin a lot of times for each spin of the wheels. This is easier to get the car started and to drive acceleration. Fifth gear turns the wheels more for each turn of the engine. This is more efficient but doesn’t allow for easy acceleration.",
"[I honestly just recommend watching this old video.]( URL_0 ) Seriously, the explanation is very good and is easy to understand. [I also recommend their explanation on differentials.]( URL_1 )",
"Combustion engines operate best in a pretty small RPM range, between 1,000 and 4,000 RPM. But we need cars and trucks to operate at a wide range of speeds, from less than 5 MPH to, say, 85 MPH. The gears in a car transmission are like the gears on a bicycle: You have small gear combinations that make the engine turn many times for each rotation of the tires... this is good for getting moving from a stop and for moving at low speeds. Then you have bigger gear combinations that make the engine turn only a few times for each rotation of the tires. This is what is used at high speeds. You can't really get going from a stop in high gear, because this would require the engine to move at only a very few hundred RPM under load, which it isn't strong enough to do. And you can't really make a car go 80 MPH in a very low gear, because the engine would be spinning so fast it would fly to pieces."
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eppyei | What bad traits when driving can damage the longevity and performance of your car? Why does it affect them? | Engineering | explainlikeimfive | {
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"Riding the clutch, riding the brakes, accelerating too hard regularly, braking too hard regularly, all cause damage from excessive wear/ excessive heat. Not performing maintenance causes damage to multiple systems. Not avoiding potholes and speedbumps can damage suspension components. Modifying your car in a variety of ways i.e. putting on oversized rims/ cutting into the electrical system to add electrical equipment/ lights."
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epujot | Just watched Ford v. Ferrari. How was the 1964 GT40 able to achieve a top speed of 210+ when modern supercars are still barely pushing 200? | Engineering | explainlikeimfive | {
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"First of all you're comparing a 55-year-old racing prototype to brand new road cars. The Ford GT40 was made to go as fast as it could, reliably enough to win a 24-hour race; a modern 'supercar' is designed to look pretty, be comfortable, meet government safety regulations, etc. reliably for years. A more apt comparison is between the GT40 and a new Le Mans prototype. Compared to a modern Le Mans prototype it was more advantageous for the GT40 to be as fast on the Mulsanne Straight as it possibly could. The course at Le Mans now has chicanes deliberately added to the straight, to force drivers to slow down. There's only so much space to reach top speed now, so carrying as much speed through corners and accelerating as quickly as possible is more advantageous. Last year's Le Mans winner, the Toyota TS050, had a top speed of 217.5 mph, which isn't all that much more than the GT40. However, it reached that top speed in much shorter straights and can corner much quicker than the GT40 ever could. Dan Gurney set the fastest lap in '66, at 3:30. Last year's fastest lap was 3:17, with two chicanes in the Mulsanne Straight. Overall the TS050 is **much** faster.",
"I don't know where you get the \"barely pushing 200\" from. 1993 McLaren F1 - 240.1mph. ... 2005 Bugatti Veyron - 253mph. ... 2007 Shelby Supercars Ultimate Aero - 256.18mph. ... 2010 Bugatti Veyron Super Sport - 267.857mph. ... 2014 Hennessey Venom GT - 270.49mph. ... 2017 Koenigsegg Agera RS - 277.87mph. ... 2019 Bugatti Chiron - 304.77mph. Plus as others have said, these are production cars. The GT40 was a purpose built race car. Modern NASCAR race cars are purpose built to do 210ish mph on the top end and average about 180 mph for 500 miles(depending on the track.) Lemans cars average around 150 mph over the course of the race. Funny cars and dragsters are purpose built and regularly hit 330+ mph in under 4 seconds. By comparison the 2017 Bugatti Chiron took 32.6 seconds to reach 249 mph.",
"It's not that they can't. We have Hennessy making road cars that can hit 270. A lot of factors and a certain degree of risk/reward comes into play when you're going that fast. It's not worth it for a lot of manufacturers.",
"A modern supercar is quite different from a race car. race cars are spartan, lightweight, have no emissions (to speak of), and are designed to go very very fast. Also, most of them will kill anyone who's stupid. The 1964 GT40 was technically a prototype car. Yes, there were several, but they were all hand built, hand tested, hand tuned and driven by very talented pilots. supercars are *production* vehicles - they're designed for the roads you and I drive on. top speed of a supercar isn't really relevant - and if you're gonna take one to the track, then you're probably rich enough to afford the modifications necessary for it to compete on the track. If you look at even modern day 24h Lemans races, you'll note that any of the production cars there are not what you'd see on the showroom floor. They're purpose built race cars (for example, the Corvette C7 R and C8 R)",
"The technology that makes cars fast is pretty old: burn as much gas and air as you can, as quickly as possible. We’ve improved on that since the 60’s by making it more efficient, but the land speed record was already well over 200 mph by the late 1920’s. The modern supercars you’re talking about also have fancy amenities like airbags, a passenger seat and a radio, and they’d probably go much faster without all that extra weight.",
"> are still barely pushing 200 I'm sorry, what?",
"Top speed isn't all that important in racing. Acceleration, mechanical grip and aerodynamic downforce allowing for fast cornering are what matters.",
"There does come a point where a car cannot maneuver safely and a point where a person can not react fast enough. There are purpose built cars that go 500+ miles an hour, but they only go in a straight line. Car tires can only maintain their grip to a certain point as well. At a certain speed turning the wheel would be useless as the force pushing you forward would overpower the tire grip and you would still continue in a straight line. That is provided the car didnt disintegrate in the process. Lastly you have G forces. Assuming we overcome the grip issue, the G force put on the driver during a turn may become to much for him/her to keep their hands on the wheel. They could even lose conciousness or blurred vision which would be disastrous at those speeds. So it's not that we cant get above those speeds, it's more that the faster we go the less control we have.",
"Am I the only non-american around here who was like: What are you talking about? I can go 220 in my 1.8 Honda Civic. And after a few moments I got it.",
"Along with what everyone else is saying, there's a certain speed at which a car stops being a car, and becomes a plane. Downforces help with this, and if I'm not mistaken, the downforces are now at such a level that an F1 car could absolutely drive on an upside down track (wouldn't that be awesome to see!). But that downforce also adds a ton of ~~weight~~ drag.",
"Modern cars have downforce. This will hinder you on the straights. Also weight is a huge factor. modern cars are built to regulations on safety for drivers and pedestrians . This adds weight and weight is the enemy of a racecar.",
"Lots of people giving roundabout answers, but the true answer here is gearing. You have a lot of control over performance with gear ratios. You can make a supercar that will absolutely hammer the 0-100 sprint but run out of rpms in top gear at 110, or you can build a supercar that will take a lot longer to get to 250+. The truth is that 99% of buyers wont see 200+, so manufacturers choose to make the \"sprint to 100\" car for better driver satisfaction.",
"Because of a lot of reasons: 1. Why? What purpose does a street car that can go 200mph have? Not much. So why build one? 2. Most cars that would otherwise be capable of 200mph are electronically limited because street tires aren’t designed to handle more than 186mph. 3. ‘64 was about the time when governments started interfering with car design. These days, you have to have backup cameras, deal with crash and pedestrian impact standards, etc. when you build a car. There is no technological reason why every company can’t build a 200mph car.",
"I read an article in the Robb Report last year that seemed to imply there are 3 or 4 companies that are nearing the 300mph top speed mark. I think that what holds back most modern supercars from the over 200mph top speed are mandatory safety and milage/emissions standards. Also the fact that most modern supercars are luxury statement machines and not really meant to be race cars.",
"Because we would rather only have professional drivers breaking over 200mph. But it is pretty funny watching rich guys ball up expensive cars.",
"Modern supercars are not barely pushing 200, they can hit upwards of 250 or even faster, with some upcoming ones apparently aiming for 300.",
"The Bugatti Chiron has done 300+ and the Koenigsegg Jesko as well. which is insane compared to 200mph when you start to understand how exponential curves work",
"You will also see a lot of modern supercars with top speeds of 211 mph. This is due to the tires they run only being rated for 211 mph.",
"most modern cars are also electrically modified to limit speed to like < 200mph, which is a good idea considering there's still people street racing and we don't want someone getting T-boned at an intersection by a tuned supra going 200 mph. wouldn't end well.",
"Today there are production vehicles with air-conditioning, luxury seating, bluetooth connectivity and TFT screens pushing 200. And they do not require a rebuild after each race. Some can carry 5 passengers. Think of it this way, the first car to break 200mph was in 1927! Would you make the same argument? Obviously not because it was a specific machine for the specific job. The same principle applies to your question.",
"As /u/nostromo7 already stated, the GT-40 was only built to do one thing - Go stupidly fast. Safety was barely on Ford's radar back then, beating Ferrari was more important. Modern supercars today vary wildly. You've got more luxury minded supercars like the McLaren GT or the Ferrari Roma which don't NEED to go 200 MPH, just look good doing anywhere around 120+ with little effort. Then you have the record-breakers that happen to do it all - Go well over 220 MPH, look good while doing it, and likely way safer than anything Ford built in the 60s. Which is also why they're hundreds of thousands, or in some cases, millions of dollars. High-speed cars have existed for as long as cars have existed. One of the fastest land-speed cars, pre-WWII was projected to be the Mercedes-Benz T80. Had a modified engine from a Messerschmitt BF-109 in it with over 3,000 HP and was projected to hit over 450 MPH, but it never did. In 1939, Goldie Gardner held land speed records in a modified MG streamliner that could do close to 210 MPH. You can engineer anything to go silly-fast speeds, but the real challenge is when you figure out how to stop it safely or make it handle while being able to do that."
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eq2491 | How do they extract natural gases? | It may be possible for me to understand the extraction of natural gases in our modern days, by using such a cutting edge technology, but what I don't understand is how they did it back in the 1800'? It is not like they could just go in a pit and catch the gases with a jar... Or could they? | Engineering | explainlikeimfive | {
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"There is a difference between Methane, the natural gas in the pipes that your stove uses, and all the other gases - which are also naturally gases. Methane is found underground with oil deposits, it;s what causes a \"gusher\". You drill a hole into the Earth, and cap it with a valve and pipe. The refining process also produces more methane, and all that goes into the pipe. Other gases, like Oxygen or Nitrogen, are extracted from the air through distillation. Instead of heating a liquid, the air is cooled. Different gases condense at different temperatures, and can be separated that way."
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eq7q8d | How do kevlar vests work? | Engineering | explainlikeimfive | {
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"Kevlar has a high tensile strength. They weave it into a fine mesh, so for a bullet to push through it needs to push the strands out of the way, but they’re interlocked so can’t go anywhere. With enough layers this will both stop a bullet from penetrating and dissipate the load across a wide area so you don’t die from the mesh stretching enough to go into you. You’ll still very likely take some injury, including possible broken ribs. There’s a limit to how much energy it can absorb before the strands break. Most Kevlar vests are good for common handgun rounds but will not stop a rifle round (not counting 22LR/.17HMR) or shotgun slug. You can get armor piercing bullets too with steel tips, these do not deform so quickly as a plain lead / copper jacketed bullet so go through Kevlar at lower energy. To resist against rifle rounds you need more sophisticated things like ceramic plates inserted into the armor to be able to absorb enough energy from common infantry rifle rounds (you’re out of luck against something like a 50 cal BMG). These plates are one time use. Similarly - a plain bullet proof vest won’t be stab proof as th knife can cut the fibers. So stab proof vests often use a mesh of little plates inside the best that can resist a certain level of impact.",
"The reason a bullet penetrates your skin and other things is that all of the energy it has because of its speed is concentrated on a tiny area. A force creates smaller pressures on larger areas. This is why needles hurt even when you touch them very lightly. Kevlar and other types of bulletproof vests work by spreading the energy of the bullet to a larger area. It's similar to placing a bottle cap between your finger and a needle. You still feel the same force but the needle can't penetrate your skin as its pressure is spread over larger area. Edit: I understand some physics and have a vague understanding of how Kevlar vests work but I have never used them or researched them well enough to tell you about different types of vests, how a bullet shot feels through them, etc.",
"Kevlar (as a fabric) is supposed to keep the bullet from penetrating any part of your body. Many people hear this and think that kevlar makes them bulletproof. It does not. While a kevlar vest will (in most cases) save your vital organs from being ripped to shreds, it does nothing to lessen the impact of the bullet. Here is an interesting video demonstrating the \"punch\" of a bullet on kevlar: [ URL_0 ]( URL_0 ) Hope this helped!",
"Kevlar is a very strong synthetic fiber that can be woven very tight and is really, really resistant to stretching and tearing. The biggest danger of a bullet is it's ability to penetrate you - it's going so fast with so much energy, that it can tear right through your clothes and skin and hit the stuff inside of you - organs, muscles, bones. Bad stuff. Because Kevlar is so tight and so resistant to being stretched or torn, when a bullet hits Kevlar, the energy from that impact is dissipated - it's spread into many, many of those tiny Kevlar fibers, making them stretch slightly. So ideally enough energy is dissipated that the bullet is stopped completely without penetrating, and any impact you feel is from the material of the vest itself being pushed into your body. This will mean that the energy hitting over a wider area - so instead of penetrating through you, it'll just \"punch\" you, hopefully leaving nothing worse than a bruise. Of course, the more powerful the gun, the more energy that's going into that bullet, and heavy-duty calibers can potentially penetrate most protective gear. But for smaller, more common bullets coming out of your typical handgun, they'll provide a lot of protection."
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eq8wqn | what does fixed wing plane mean. Are there planes without fixed wings | Engineering | explainlikeimfive | {
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"It means the wing doesn't move. In a helicopter, the thing providing lift (the blade) moves. A helicopter is not a fixed-wing aircraft.",
"Fixed-wing plane isn't a thing. It's a fixed-wing aircraft. Which would be a plane. The other option is a rotary wing aircraft, such as a helicopter.",
"3 types of flying machines. Fixed wing, rotary wing, and ornithopters. Fixed wing doesn't mean solid and unmoving wings. As planes with folding wings or variable angle wings still are fixed wing, but rather that they do not move to produce lift, hence \"fixed\" Rotary wings are like helicopters, where the movement of the wing surface creates lift Ornithopters are old school creations where people would flap wings to try and achieve lift",
"The best counter-example for fixed wing aircraft is an [autogyro]( URL_0 ). It looks like a helicopter but works like a normal (fixed wing) aircraft: the engine drives a propeller and the (free spinning) wings are unpowered with the lift coming from the forward motion of the airframe.",
"Helicopters = rotary wing aircraft (the blades are shaped the same way a wing is and operate in the same fashion with minor differences to accomodate and they rotate above the aircraft) F-14 Tomcat (the jet from Top Gun) = variable wing aircraft (called swing wing, but not really, the wing adjusted forward or swept back to either provide additional lift/ maneuverability or was swept back to provide less lift/ drag to go faster) Everything else = fixed wing (wing doesn't move, it is the center of everything for the aircraft, with some exceptions) V-22 Osprey = freak of nature. Do not trust this thing. It floats with magic and is usually filled with pissed-off crayon-eaters.",
"Yes, there are aircraft without fixed wings. Helicopters have rotary wings, for example. There are also aircraft with variable wing geometry, the most famous of which is the F-14. At low airspeed, the wings stick out almost straight, but as the aircraft crosses the sound barrier, the wings sweep rearward essentially forming a delta wing. As an interesting side note, NASA experimented with an oblique wing aircraft in the late 70s, where one wing sweeps forwards while the other goes back, but that didn't go anywhere.",
"Fixed wing is what you normally think of as an airplane. Helicopters are referred to as \"Rotor-wing\". The blades of a helicopter are wings, that are not fixed.",
"There are swing wing aircraft like the F1-11 and F14. Also gyrocopters have unpowered rotors.",
"Aerospace engineer here! The term is usually fixed wing \"aircraft\" which is a plane, as opposed to a rotor craft (helicopter) or blimp. All are aircraft, but only one has a fixed wing design."
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eq9uhs | How are dams/structures in contact with water maintained? | How are structures that are constantly in contact with water repaired or maintained? I imagine there has to be some sort of preparation or specific process for this? | Engineering | explainlikeimfive | {
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"Concrete 100% submerged is significantly less susceptible to deterioration than concrete that goes through wet/dry cycles which would be in shallower depths. For shallow depth repair work, you can install temporary cofferdams around the work area to dewater the immediate area to complete the required work. Also divers can do many repairs, albeit slower/costlier in general.",
"There is lots of work that can be done under water. For more serious work you'd set up a barrier around the work area, then pump out the water, perform the work, then fill it back up and remove the barrier. For something like a dam you can drain the water if needed.",
"Modern day dams have sensors and a team of people who ensure everything is working correctly. Smaller dams are checked and maintained by third party companies like “Damwatch” who specialise in this very task.",
"What about dams and other structures that aren't made of concrete, i.e. earth?",
"There are hydrophobic, crystalline additives that can be added to the concrete mix that fills the voids and strengths the concrete. There are also coatings and cementitious grouts that can be applied underwater for spot repairs."
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eqe76o | how does a battleship accurately target ground targets? | What with waves etc, and before GPS. | Engineering | explainlikeimfive | {
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"Well, battleships don't exist anymore, but back when they were last used, in WWII (ignoring the 4 recommissioned in the 80's) they had complex mechanical computers that could calculate firing solutions by collecting information from radar and the ships own movements. Hitting stationary targets on land was not that hard. Hitting moving targets like other ships was possible but harder. GPS wouldn't really help because the guns are unguided, meaning they follow a ballistic trajectory. GPS wouldn't give you any more information than radar. It's essentially the same as a person with a rifle shooting a target. Your aim determines where the bullet goes. If you aim in the right spot, the bullet goes where you want to. Also waves weren't really a factor, These ships were huge and extremely heavy, they really didn't move around that much. Wind and weather was a much bigger factor than waves."
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eql5r3 | Explain the physics behind removing a load-bearing wall and replacing it with a giant heavy beam. How does something waaayyy heavier placed higher support the home? Visuals are welcomed. | Engineering | explainlikeimfive | {
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"It's really not about the weight, the weight is all getting transferred to the foundation eventually through the framing that the ends of beam are resting on. It's about deflection. Having a dozen vertical 2x4s spread across a distance will keep the structure above it from sagging. But if you don't have that, you need something rigid enough to span the distance without deflecting.",
"Beams and arches \"channel\" the weight to their supports. It's like a chair, the weight of YOU is channeled by the wood of the chair to the 4 legs of the chair. Beams and arches serve the same function for construction. [Visuals]( URL_1 ). EDIT: Walls are usually not \"solid\". They're made of [load-bearing studs]( URL_2 ), and if you want to get rid of some, to put a door in, for example, you can replace the ones near the door frame with thicker (more load bearing), and remove the ones in the middle to make room for the door. A beam on top prevents any weight from pressing down into the door space, and instead redirects that weight tot he thicker posts on the sides of the door. [Visual]( URL_0 )."
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eqlo96 | stacked shipping containers on railway cars; how do they stay put, not topple? | Got stopped at a railroad crossing yesterday, and saw lots of shipping containers go past. How are the top ones held in place? And are the bottom ones held on the rail cars somehow, or just sitting there? | Engineering | explainlikeimfive | {
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"Shipping containers have holes in the corners and a locking pin mechanism holds them down to a trailer or rail car, and to each other.",
"There are [twist locks]( URL_1 ) on the corners of the rail cars and double ended [twist locks]( URL_0 ) between the containers.",
"It’s quite an interesting story how these containers shape (literally) our world, I highly suggest a google. As for your question they are a standard size and construction and each of the eight corners are equipped with a locking mechanism that allows them to be secured to trucks trains crains and each other."
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eqpi39 | What are the bristle brushes on the sides of an escalator used for? | Engineering | explainlikeimfive | {
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"They're a safety feature to stop things getting dragged into and caught by the workings of the escalator."
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eqrohf | how refrigerators and freezers make things cold. | Engineering | explainlikeimfive | {
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"You have to know three things: - Heat moves from where it's hot to where it's cold. - If you compress something, it gets hot. - If it expands, it gets cold. A refrigerator has a system of pipes with a gas (they choose certain gases that evaporates in the ideal temperature), connecting a compressor and a special valve twice, dividing the pipes in two parts, one inside of the refrigerator (where the gas pressure is low), and the other on the outside (with high pressure gas\\*). Because the low pressure segment is cooler than the objects inside, it absorbs heat from them. And in a similar fashion, because the compressed gas\\* is hotter than the air in the kitchen, it releases heat. If the gas doesn't circulate, eventually will be the same temperature as the outside, so you need the compressor to pump the gas around and the valve to create a low pressure pipe segment. \\* I said \"compressed gas\", but maybe the compressor forces it to become a liquid. I don't know."
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eqt6nv | Why are there different types of screws that require different screwdrivers? Like why do they make star shaped or flathead type of screws and not one universal type screws and screwdriver? | Engineering | explainlikeimfive | {
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"Flathead used to be the most common, but they're annoying to use because the driver pops out easily. Phillips are most common now but they strip out easily. Robertson (Square) are easy to stick to the bit before installing. Allen heads (hex) are great but you need a different size tool for almost every size of bolt. Torx (star) are great because they're stronger than Allen, but have the same drawback and are more expensive. Security Torx are for anti tamper applications and are designed to be hard to unscrew without the specific tool. There are more but I think I hit the basics.",
"It’s a matter of their purpose. Some screw heads like square, star, torx, triangle, and hex can handle a lot more torque than other types. Other screw heads are better suited for reuse. Some screw heads like Philips and flat head are super cheap to make. Some screws are good for use by robots. Patterns like hex and torx can easily and quickly have the driver meet up with the head. There are others that are “security” screws that are bit patterns that are harder to find and can’t as easily be operated by an improvised tool.",
"Different designs have different flaws so they are chosen for specific purposes. Some of them slip easily: bad for automated assembly lines Some of them strip easily: wear out if reused too much Some of them can be unscrewed: bad for preventing tampering Some of them are overly common: see tampering Because there isn't really one screw that outright wins every scenario, you see a variety of them. Some are definitely more common due to versatility it isn't universal."
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eqvvk8 | What is wet bulb temperature? | What does wet bulb temperature mean in thermodynamics? And what do I gain from knowing the degree of wet bulb temperature? | Engineering | explainlikeimfive | {
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"Wet bulb temperature is determined by a thermometer with a piece of wet cloth that is fastened around the bulb. As the water in the cloth evaporates, heat is pulled from the thermometer bulb and the temperature reading goes down (this is the same reason sweating cools you down). The more humid the air is, the less the wet bulb temperature will differ from the dry bulb. Another way to think about this is that when the air is more humid, less water from the cloth will evaporate because the air is already more saturated with moisture. Once you know the dry bulb temp and the wet bulb temp, you can use a [psychrometric chart]( URL_0 ) to determine a whole lot of other information about the state of the air. The most common information being the relative humidity and the dew point. Other information would include the humidity ratio (mass of water vapor per mass of air) or enthalpy (the heat capacity per unit of dry air). All this stuff is important if you are designing an HVAC system or are a meteorologist or something like that."
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erbfti | What's the noise/happening inside a propeller engine before the propeller starts rotating? (example of sound below) | It sounds like it is warming up but the propeller is not turning does it have a clutch? it's only when the engine properly fires up does it sound like the engine is on and the propeller will be turning. Example of sound: [ URL_0 ]( URL_0 ) | Engineering | explainlikeimfive | {
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"I can't speak for every prop aircraft, but the Bf-109 in that video uses an inertia starter. An electric motor spins up a flywheel (probably the sound you're referring to) before the flywheel is connected via reduction gearbox and clutch to the crankshaft for starting. This allows for a smaller electric starter motor, since it only needs to spin up a flywheel rather than the entire engine.",
"What you are hearing is an inertial starter. The electric starter motor is not connected directly to the internal combustion engine, but rather to a flywheel, like a heavy rotating weight. The electric motor spins the flywheel up very fast, to nearly 20,000rpm over several seconds. The flywheel is then able to connect to the main engine and spin it quickly to get it started. The reason for this is that the electric motor can be made smaller and lighter, (which is good for a fighter plane) because now the motor can work for a while putting energy into the flywheel. Then the flywheel can then put all that energy into the engine all at once. Maybe think of it like money: it costs $10 to start the engine, and you can’t do it on a layaway plan. So the electric motor is putting money in a savings account at $1 per second over ten seconds so that the flywheel can make the $10 withdrawal in one second and start the engine. Not all propeller driven aircraft have this kind of starter. Most piston engines just have a direct drive like a car starter. These WWII aircraft had very very large engines and needed creative ways to get them going. This was one of several different solutions."
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erf4d9 | how do mess free markers/paint work? | ELI5: My kids just received a Crayola mess free paint set for their 3rd birthdays. Once batteries are installed you basically just have to tap the brush on the lid of the "paint" color you want - then dip the paint brush into this gel stuff - then when you paint the gel onto the special paper it comes out as the color you activated. All the gel is the same. It really just changes by what color you have activated by the paint brush. I am a very tired mom of twin 3 year olds. Please explain how this magic works! Also just a quick side note - it is not in fact "mess free" if it involves a gel that you have to wash off your hands and every inch of your kitchen counter tops with afterwards! [ URL_0 ]( URL_0 ) | Engineering | explainlikeimfive | {
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"Probably a photochemical reaction. The light (photons) emitted from the end of the brush have an effect on the gel compound.",
"It appears that the gels contain both a [Leuco dye]( URL_0 ) to color on the paper, and a redox indicator that is actuated by the pen. You don't have to tap the pen to activate the color, but maybe to turn on the electrode in the pen. Edit. ELI5: There are 2 parts of the gel, one changes color when touched to the paper and is permanent, the other reacts to an element of the pen to show the color on the brush returns to clear when off the brush."
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erhbc8 | If vehicle engines use sparks to ignite fuel, how do they ensure that cars will never blow up? | Engineering | explainlikeimfive | {
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"They make the place where the spark happens out of sturdy metal that can contain the explosion without failing.",
"The fuel is ignited in a combustion chamber that has limited air and fuel. In the wrong percentages, fuel will not burn well at all, so only by getting a fairly close percentage of fuel and air will the mixture burn at all. The combustion chamber is mean to handle this combustion of fuel, with a safety factor of many times. That said, in engines which burn self oxidizing fuel like nitromethane (top fuel dragsters are a perfect example), the fuel can and will explode, causing engines to blow themselves apart spectacularly."
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erhwjj | how do subway tunnels handle rain water and flooding? | Edit: answered | Engineering | explainlikeimfive | {
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"If they're lucky the subway tunnels are above the water level, so natural drainage can happen. But most subways and underpasses are not. So these drain into cisterns at the bottom-most part and are pumped from there into the normal storm drain/sewer system. _Wait, if there's a flood won't there likely be power outages?_ Yep. Most will have a backup system in place, likely more than one, but sometimes your tunnels just fill up. In any case, if the power is out and a subway tunnel pump stops, you're not going to be running electric trains anyway."
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erznrn | When on a submarine, how is noise from the kitchen reduced? | So I was watching a youtube vid of a sub kitchen. And it was pretty noisy while cooking with all the banging of utensil. Movies usually make it a point to depict that any internal noise will carry over outside. So, how is the kitchen noise mitigated? Or do movies just exaggerate the noise levels? | Engineering | explainlikeimfive | {
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"You only go silent on a sub when you absolutely have to. It halts all operations. But movies generally don't show a sub going about its daily business because it is boring.",
"As I understand it, there’s some installation of the sounds. Plus, if another sub were to be detected, they would tell everyone to be quiet. The ocean is a pretty big place, there’s what, maybe 500 subs by all countries combined, so it’s not like you have subs on top of each other all the time. So, even if you have a kitchen in full operation it’s not usually an issue. Edit: I should add, there’s a lot more noise than just the kitchen. Mechanical engineering, people working, guys working out, dropping / banging tools, etc. The kitchen is just one of a lot of things going on that make noise.",
"Movies exaggerate many things, and leave out all the boring stuff. There are two kinds of submarines, and they are mostly the opposite of each other. Missile submarines spend all their time on a mission avoiding being detected by anybody. They do this by going to the middle of nowhere in the ocean and avoiding anything that comes by. This does not require the \"silent running\" of a WWII movie, as those submarines had a totally different sort of design. It simple involves long range sonar and actively staying in water that makes you hard to detect. Attack submarines are on patrol with a surface navy flotilla. They patrol the outer edges and keep out enemies. This might require silent operations for an hour or two to sink something. However, a) this almost never happens; b) this is frequently drilled, so sailors know how to do it; and c) they simply stop doing things that make noise - sandwiches for lunch today."
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es3l01 | why doesnt a lighter work after being left outside in the cold? | Engineering | explainlikeimfive | {
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"The boiling point of butane is about 30F / -1C. If it is colder than that, the butane liquid in the lighter will not turn into a gas and come out the top to burn (liquid butane doesn't burn). The trick is to hold it in your warm hands (or keep it in your pocket) to keep it well above the boiling point.",
"Most Bics or Cricket lighters have no wick for the liquid butane to wick up. When you press the little red lever it simply opens a valve and the gaseous butane comes up. But there needs to be a relatively high air pressure inside the body of the lighter, and when they're cold, well, air pressure inside drops accordingly and voila. Hold it for a few minutes in your hand and the temperature should rise enough to create enough pressure inside.",
"Not too positive on this, but I am pretty sure it has to do with the pressure at which the fuel inside the lighter vaporizes. For example, when it is hotter outside, puddles of water dissappear more quickly than when it is colder outside. The same is true for the fuel inside the lighter, as the fuel itself does not come out of the lighter as a liquid, but as a gas. It is harder for this fuel to become a gas when it is colder, as with water. You can experience this concept at home by smelling a strong alcohol or a bottle of hydrogen peroxide, as those liquids evaporate easier and turn into a vapor easier than water.",
"Boiling point is below the threshold for vapor pressure. Same as when you want to BBQ in winter. You have to warm the regulator and tank with boiling water to get it going if the outside temp is below the boiling point of -43°F",
"There is liquid butane in a Bic lighter that acts as a reservoir to fill all the empty space up with gas the gas itself is what burns which is why liquid butane burns pretty quick when you spill it (gets hotter making more gas making more heat etc). The Colder the lighter the less of the fluid is likely to be pressurized and evaporating",
"Its the pressure. Basicly a lighter is a tube full of pressurized lighterfluid. The pressure helps push the fuel out to be lit. Without pressure, the fuel doesn't get pushed out. When it gets cold, the fuel becomes more dense and has less pressure to push it. Think of a baloon. When its cold, the gas inside of it condenses and the balloon shivles. Same concept."
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es7ro0 | Why do certain turns on the road have signs instructing how fast you should go while others do not? | I ask because certain turns seem a lot sharper and you might be going 60 mph around an easy turn and no sign is to be seen yet other times going 45 mph around a corner and their is a sign telling you to go 35 mph. Hope its not a dumb question. | Engineering | explainlikeimfive | {
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"The geometry of the road determines the speed you can travel on it. An old country dirt road that’s been paved is going to have sharper turns, and be more irregularly contoured than a newer road, that has contoured/banked turns and longer, smoother approaches into those turns. The smoother, more engineered road can be driven at higher speeds than the irregular one. And in some cases those specified speed limits may be to help with a bad spot that wasn’t accounted for in the past. If the traffic engineers figured out that there are lots of incidents there, the lower speed limit may be to reduce incidents.",
"There is a speed limit on every road. If there is no sign stating a safe recommended speed, then the safe recommended speed is the speed limit. On the local roads I use, there are signs warning of turns with recommended speeds, and also signs warning of turns but there is no recommended speed sign attached. On those turns (and any others with no sign at all) the traffic engineers have determined that the speed limit is safe for all vehicles in most conditions. Snow and ice notwithstanding, you can go around those turns at the speed limit."
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esea9t | How does a gasoline pump know when the tank is full and automatically shut off? | Engineering | explainlikeimfive | {
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"There is a little tube inside the handle called a pitot tube. That pitot tube senses the vacuum created by the fuel flowing into an empty tank, and when there is less vacuum on that tube (from the tank being almost full and the fuel not drawing as much vacuum as it goes by) the handle clicks off and the pump quits pumping."
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esi9rv | Why does all the small led lights found on laptop charger keep glowing even after unplugging? | Engineering | explainlikeimfive | {
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"Not all chargers show this phenomenon. But when they, it is caused by the electrical energy still stored in the capacitors inside. The capacitors in chargers are used to \"filter\" unwanted ripples when converting from AC to DC. They help smooth the electrical output curve by storing energy and work a kind like a overhead water tank e. g. in a water tower. In normal condition, this tank is drained by the device plugged into AND to a tiny amount by the control led. When unplugged, the stored electrical energy is only drained by the tiny led. And this led uses only a fraction of the energy that the capacitors still hold. Depending on the electrical characteristics of both the led and the capacitor, the glowing effect will last shorter or longer. A small mobile charger led will go dark sooner than the one on a laptop charger. Still some chargers led will NOT glow, when unplugged. It depends a lot from the circuits construction. Hope that brings a little glowing to this question.",
"Many electronics store or hold electricity without being plugged in. That's why things like TV'S have warnings about self servicing. I'm not an expert by any means but have been told this. So whether its accurate or not I don't know."
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eskoy1 | In cities where the tap water is clean enough to drink, how does the water stay clean throughout the whole process to get to the your faucet? | Engineering | explainlikeimfive | {
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"The pipes are clean. The pressure in them keeps things out of the pipes as well. You’ll see water companies issue “boil water” notifications if they lose pressure or have a break in the line that could let nasty stuff in. After that, they’ll flush the lines and usually add some chlorine to the water to kill nasty germs.",
"So this quote came from an uncle who was a civil engineer in the 50's and 60's, \"the solution to pollution is dilution.\" In other words, he was saying if enough water is running through the pipes, the crap's not noticeable. Probably not today's best practice.",
"Water towers create positive water pressure in the entire system at all time, meaning when there are any cracks or leaks the result is that clean water goes out rather than dirty stuff going in. Since people are always using water the system is always having more clean water go through it. In the circumstances where they have to work on the pipes and contaminants can get in, that's what \"fire hydrants\" are really for. They open them up and let clean water flush anything out, spilling into the street for as long as is necessary to ensure it's clean.",
"In addition to what's already been mentioned. The water is treated with Chlorine, or Chloramine to kill bacteria, viruses and the like. It takes surprisingly little."
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espga1 | How does the Bullet ballistic study work? How are they able to tell this particular bullet was fired only from this particular gun, something like DNA test accuracy? | Engineering | explainlikeimfive | {
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"Nowhere near as scientific or accurate. The inside of the barrel *(the tube a bullet is pushed through)* has pieces which stick up very slightly *(called lands, and 'slightly' is in the neighborhood of 5/1000\" 0.005\")*, and the bullet is just barely enough smaller than the inside of the barrel that they scratch the sides of the bullet *(which is much softer than the metal of the barrel)*. Their purpose is to set the bullet spinning, like an American football, so it's more stable in flight. Some brands have arrangements for the lands which are specifically characteristic to that brand - how many there are, how fast they twist, how deep they are, the relative size of the lands & grooves *(space between lands)*, or how they're formed in the barrel. So that can give a good clue about the bullet being fired from a gun of that brand *(or more often, it excludes certain brands)*. Each barrel also gets its own wear marks, though this part isn't as clear as TV shows make it out to be. So kinda like a shoe worn by someone who drags his right outer heel makes an identifiable print, those imperfections in the barrel can mark the bullet. URL_0",
"They take the gun and put another bullet through it, and then compare that bullet to the one they have as evidence. The rifling inside gun barrels leaves it's own distinctive fingerprint on the bullet, so you compare the two to see if a bullet came from a specific gun."
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Subsets and Splits