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7cc4hv | What makes Japanese cars (Honda/Toyota, etc.) more reliable than German (Mercedes/ BMW/ Audi, etc.) and American cars (Ford/ Chevy, etc.) | Engineering | explainlikeimfive | {
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"When Japanese companies first tried to break into the American auto market, their quality and reliability were horrible. They literally were a joke. So they decided to get very very serious about improving. They brought in American quality experts (Juran and Deming, most notably) and actually listened to what they said. They didn't worry about profits; they believed the quality experts when they said that profit would come if they did things right. This drive to improve became ingrained in the culture of some of the auto companies. They started to see success in the market as a result, and that just reinforced the culture. They pursued various different techniques to drive continuous improvement, and some of them worked very well. American and German car companies have tried at times to replicate this, but they can never seem to muster the guts to commit totally and not worry about short-term profits. They have improved, but simply not as much. And some companies (I'm looking at you, Ford) have never been able to get past the idea that if you save $1 per car on a million cars, that's a million dollars of pure profit. They can't seem to internalize the inevitable degradation that happens with this thought process. The penny pinching is ingrained too deeply in their history and culture. Not to say that Honda and Toyota never have issues. Cars are incredibly complex beasts, and sometimes a supplier can screw you. But in general, those two companies have learned to walk the walk and have reaped the rewards."
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7ch1x2 | Why is the current in a series circuit with multiple loads constant throughout the circuit? How do the electrons "know" to send less current in a circuit with more loads? | My teacher explained this concept using Kirchoff's Current Law: since all the current entering a load must come out, a series circuit cannot have different currents in different parts of the circuit. Here's where I get confused: If I hook up a 5V battery to a 1 Ohm resistor, I get 5A of current in my circuit. However, the moment I add another 1 Ohm resistor "downstream" of the first resistor, I get 2.5 A of current everywhere in the circuit, even "upstream" of the first resistor. Since I'm thinking about this with the water analogy, I'm confused by how the battery "knows" that there's an extra load and it better send only half the current. Essentially, I guess my question is how the electron flow automatically changes to fit whatever load is in the circuit. | Engineering | explainlikeimfive | {
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"The battery doesn't know anything, the amount of current is determined by the load If i put a 20 pound backpack on you and tell you to go for a run as fast as you can you'll make it in some certain speed If i put an 80 pound backpack on you and tell you to go for a run as fast as you can you'll be going much slower Would it be because you thought \"I am moving more load so i should run slower\" or because you aren't strong enough to move that larger load at the same speed(hint, its the second one) Same thing happens for the electrons. If you put more resistance in the circuit it is harder for them to squeeze around the circuit. Series devices all share the same current because an electron can't move forward without pushing another out of the way so you end up with the same number moving into the battery as out of the battery That last bit works like a straw. If i give you a big straw to blow down its not too challenging, there is some resistance. Now pinch the middle and try to blow, its a lot harder and you can't get as much air through even with more force, that's increased resistance. You're not choosing to get less work done because there is resistance, the resistance is causing you to get less work done",
"> Since I'm thinking about this with the water analogy, I'm confused by how the battery \"knows\" that there's an extra load and it better send only half the current. Batteries don't send current. Batteries are a voltage source and (ideally) provide a fixed voltage regardless of the load. For the water analogy, how does a watter tower or pump station know to send more water to your garden hose when you swap the nozzle from the from the wide open high flow one to the narrow low flow one? It doesn't, it just provides a fixed pressure and whatever happens with flow happens. If you open a really really big valve, you will greatly increase flow and cause a pressure drop all the way along the pipe back to the source. The pressure wave travels at the speed of pressure waves, also known as sound. > Here's where I get confused: > If I hook up a 5V battery to a 1 Ohm resistor, I get 5A of current in my circuit. However, the moment I add another 1 Ohm resistor \"downstream\" of the first resistor, I get 2.5 A of current everywhere in the circuit, even \"upstream\" of the first resistor. It doesn't the moment you add a new resistor. He skipping what is known as the transient state and going to the next steady state. He's skipping the actual change process and telling you how it will end up. Let's say we swap a very very fast acting instantaneous switch that makes the resistance somewhere in the circuit higher. At that instant, the electrons in that new resistor, and that new resistor only, start to encounter more resistance and slow down. The rest of the circuit is unaffected. Next, the electrons before the new resistor start to catch up to the slowed down one. Current is higher upstream than it is down stream at this instance. They are like charges, so they repel each other. Them getting closer together is building up a voltage repelling then from getting any closer to each other. This creates a kind of back log, and will propogate backwards upstream all the way to the battery. On the downstream side, thanks to these slowed electrons, the ones ahead of then get further and further ahead. This gap of missing electrons (which are negative) is positively charged and wants to pull these electron back. This also propogates further downstream all the way to the battery. They'll be tugging back and forth and the current will fluctuate up and down all along the circuit as all things try to equalize. Current will not be equally everywhere, in places you will be building up or depleting charge to accomodate this. Eventually, this rippling in the current will settle down to zero and you will have a new steady current in your circuit, the one your teacher is telling you about that obeys KCL. How fast do these changes propogate at? Effectively the speed of light, that's why you can't notice them. The electrons attract and repel each other because they are electrically charged. Electrical chrages interact through voltages and electric field. Speed of light is just the speed a changing electric field propogates at in a vacuum. Inside of a wire is a little slower, but not far off. How long does this transient period last for? Probably millionth or billiotnths of the smallest time frame your brain can proceess. These aren't effects noticable to humans. It's really no different than a traffic jam in a highway. Just because one guy has to slam on the breaks to slow down for something doesn't mean the entire highway will instantly slow to his speed. There's a human reaction time and breaking time and this will propogate backwards as some speed until all the cars behind him slow down. Replace electrons with cars and human reaction time with the speed of light, and you have the circuit. Is your teacher wrong about KCL? No, KCL can be made to hold at all times. What he's skipping over is another two electrical circuit components, capacitors and inductors. Capacitors store charge and let through a displacement current, so all this minute charge build up and seperation would be represented by capacitors. Even if you don't have an actual physical capacitors in your circuit, you resistors and wires have inherent capacitance and inductance and to properly model it would have the actual physical resistor replaced with an ideal resistor and an ideal capacitor. Why did he skip capacitance and the actual change process when you add a new resistor? Because you'd need advanced mathematics including calculus and imaginary numbers to deal with it. In addition to being not all that important or even noticable. Also, as I've stated these changes happen way to fast for a human eye to see. However, if we stuck an actual capacitor in the circuit, a one that can hold a lot more charge than a resistor can, you could actually see these changes happening on a human timescale."
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7ci6c3 | How are tunnels that go underwater built? | I don't understand how they are built underwater at all especially the older ones. | Engineering | explainlikeimfive | {
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"Under the water is some kind of ground. If you're lucky, that ground is sturdy clay or chalk that's easy to mine through by digging underground, and holds its shape long enough to quickly reinforce the tunnel with brick walls or with iron rings. If you're not lucky, that ground may be unstable silt, or really hard granite. In some situations, a river can be diverted through a temporary channel and the tunnel can be dug and covered over with a brick or concrete roof while the riverbed is dry. Sometimes you can do half the river at a time. For some crossings, it turned out to be easier not to tunnel at all, but just to put down big pipes at the bottom of the bay or river, hook those up at both ends to the approaches, and pump out the water.",
"The most common way is to dig a trench on the surface below he water where you want the tunnel. Then you sink a bunch of large tubes in to the position you want them in the trench and bury the tubes in rocks so they dont move around. Finally you attach the tubes together and use pumps to drain the water.",
"The Transbay Tunnel (Oakland to San Francisco, used by BART trains) was built by dropping in segments of the tube into the (soft, silty) bay floor, and then welding them together (using caissons) and pumping the water out.",
"Pretty sure the tunnels from Norfolk to the Easter shore of Virginia were built in Louisiana and towed to their current location, sunk, sealed and pumped out. So, lots of different ways.",
"A bit late, but I wanted to add: For larger tunnels for trains and such (the Chunnel) the process is usually digging below determined thickness of rock that can support the weight of the overlaying water. Sometimes the sediment is enough if it is a clay or silt but I think it’s usually preferred to go through bedrock/heavily reinforce the tunnels. For smaller tunnels like cables or pipelines there is a process called horizontal directional drilling, or hdd for short. Here’s a [video]( URL_0 ) with the basic idea.",
"Sometimes the tunnel is over-pressurized with air to stop the water from seeping in until the walls are properly waterproofed.",
"A cool documentary on a tunnel here in Vancouver built in the 50s. Assembled, sunk and drained URL_0",
"You place a full tunnel of blocks of your choice then hollow it out to eliminate the water 👌🏼"
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7ckznh | Why Do Wind Turbines Always Have 3 Blades? | Wouldn't having more than 3 blades mean that you can catch more wind? | Engineering | explainlikeimfive | {
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"I just learned this in class last week. The number of blades chosen is a function of the tip speed ratio, which is the velocity of the tip of the blade over the velocity of the wind. If you have a high tip speed ratio, you have a low number of blades. Tip speed ratio is actually chosen for design purposes to be usually about 5 to 7 (i didn't copy the graph down in lecture), and the chart says to use 3 for that range. Basically I guess it's because it's cheapest to produce and transport blades of that length versus how much power you'll get out of the turbine",
"Adding more propellors increases weight and turbulence, and can actually make a wind turbine less efficient. Also, the object isn't to catch the most wind, it is to keep the turbine spinning within it optimal range. For a given amount of propellor length, fewer blades are usually better. One blade is obviously impractical because the propeller would be unbalanced. The jump from two to three doesn't lose a lot of efficiency, but it does save a lot of space, allow more shorter towers. More than three doesn't give you as good of a tradoff.",
"Three blades is calculated to be the right number to catch the wind but also not too many such that they run into the turbulence of the last blade that passed through that space. Too many blades and they block air rather than catch it."
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7co604 | why did municipalities use lead pipes for water in the first place? At what point was that a good idea? | Engineering | explainlikeimfive | {
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"Lead is easy to form, very stable, and cheap. Especially if we are talking about 100+ year old technology, it is just one of the easiest and chepaest way to make a pipe. If lead was safe, it would probably still be used today in a lot more applications. And 100+ years ago the dangers of lead were not understood like they are today.",
"Lead pipes were used in ancient Rome, because lead is an abundant metal that's easy to form into things. No cities in the last couple of centuries used lead pipe, but they used lead in solder to fit their copper pipes. It is a fine idea because the lead corrodes quickly to build up a protective coating and after that it's remarkable stable and safe. Things only go bad, like in Flint MI, when the people operating the water supply make mistakes. Mistakes can cause the protective lead oxide to be removed and much more lead to be leached into the water supply. The Flint folks made a mistake because they wanted to save money.",
"Lead forms a small layer of corrosion when water flows through it. Protecting the user from the effects of lead poisoning. This is why it’s no longer allowed to be worked on (where I live) and is usually replaced. It can never be soldered. Source: I’m a plumber",
"Lead pipes are actually much safer than people think and many houses still have them, though it's much safer to replace them I you have kids especially. By keeping the pH high in water lead is highly insoluble and in the case of high traffic plumbing the water tends not to have the time nor the agitation to dissolve the lead very readily. It's the equivalent of just pouring salt in water. It would take quite a long time to dissolve completely with no agitation, despite a high solubility. These two facts generally mean that lead piping can be used with few measurable side effects in humans. TL;DR-Though it's not great, there was little health incentive to do so because very little lead actually leeches into the water."
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7corfe | How do hand brakes work on automobiles? Do they use the same mechanism as normal brakes? | Engineering | explainlikeimfive | {
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"They typically operate the rear brakes mechanically as opposed to hydraulically. This is why they have been and continue to be called \"emergency brakes\" by some people, in theory they should still engage the rear brakes without brake pressure. There is an actual cable going to the rear brakes as well as a brake line full of brake fluid. This varies vehicle to vehicle of course.",
"Hand brake was a cable that you pull to apply mechanical pressure on the pads. Now a lot of vehicles have it on a push button because most people never use it for a decade, it rusts. Then use it and it becomes jammed. Electronic e brake is the same principle except there is a motor that will work when you push the botton and it will activate itself at time while you are in park. Your normal brakes are hydraulic powered. You press on a piston that pushes fluid in all the system.",
"They got a few different ways of doing it, but it is two separate systems, even when parts are shared. You used to have a single channel master cylinder feeding all your brakes with proportioning valves used to set brake bias. This system was designed for a drum brake system with two seals in every wheel cylinder, for a total of eight, plus at least 3 rubber hoses (could be four or five) as a point of failure for the entire hydraulic brake system. The emergency brake was for when one of those points failed (cause surprise! People back then didn't take care of their shit either). It was a mechanical system used as a back up by operating only the rear brakes because they want you to be able to steer after you lock em up (Subaru WTF? Or was that the hill hold brake? Still WTF Subaru?). Someone who's brakes were held together with rust failed and could stop their car, now we're blessed with the parking brake. We also have a split system master cylinder that only controls half the brakes, either both front/both back or left front right rear/right front left rear. Now only half your brakes fail at once due to a hydraulic failure. A few different systems came around after 4 wheel disc brakes. My favorites are the hat on drum or the separate mini caliper styles. You still should use them regularly, but you're not fucking up the calipers by not using them (thanks Ford). The other kind is built into the caliper. It uses a ratchet and lever to apply the brake and self adjust. It's perfect on paper. The problem is when you don't use it, all the nasty stuff in your brake fluid settles on the bottom and when you finally do use it, it's not only way out of adjustment, but it's now scoring the surfaces with brake fluid gunk (cause why would you bleed the brakes? They work a don't they...). They all have their pros and cons, but as long as you maintain them, they'll work great."
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7cpb82 | Why do tires “go off” or get slower over the course of the race? | Why do race tires degrade over the course of the race? I get that rubber is removed by friction of the track but shouldn’t the same rubber be exposed? | Engineering | explainlikeimfive | {
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"New tires have smooth, consistent contact patches. As the race goes on, the tire is subject to extreme asymmetrical loads. This changes the contact patch shape and wears different areas of the tire more quickly. The tire also sees abrasion from the track surface or debris, and if the rubber exceeds ideal temperature the tire can bluster, which causes vibration and additional wear. Therefore, all of these causes lead to a significantly altered contact patch and increased wear. This hurts lap times.",
"Race tires are made of somewhat uncured rubber and are VERY soft and sticky rubber, great for the best possible grip while racing. But that also means they have an extremely short life. The both wear out very quickly because they are such soft rubber against an abrasive surface and once they fully heat up the rubber itself starts to cure. One heat cycle and they are no longer fully soft, they have begun to cure to a harder rubber with far less traction."
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7cpen9 | Why did the boats used by American’s on D-Day have a front hatch? Why didn’t they use side/ back hatches and use the front as cover, instead of running head first into machine gun fire? | Engineering | explainlikeimfive | {
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"Doors on the front let you unload into shallower water, ideally straight onto the beach. Remember these guys were carrying a lot of weight, if they got into water over their head they were going to drown, there was no way to swim and many soldiers did drown after going over the sides or leaving boats that couldn't get close enough Unloading from the rear is no good, you don't want people getting into the propeller as that might damage the boat and block a section of the beach If you're landing thousands of soldiers on a beach you're going to have heavy casualties. The best way to reduce them is to end the battle quickly, the best way to do that is getting as many men on the beach as fast as possible. That means quick egress and the safety of the boats are paramount even if it results in higher starting casualties",
"To add to the above they also had vehicles as well. Hard to drive those out the side and up the beach. Far better to dr8ve straight out."
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7d05r0 | Why are the vertical stabilizers on combat jets slightly angled horizontally and not strictly vertical? | If they already have horizontal stabilizers, why do the vertical ones have to be angled. [Example F-22]( URL_0 ) | Engineering | explainlikeimfive | {
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"It depends on the plane and the goals. The [F14]( URL_0 ) has twin vertical vertical stabilizers, while the [F/A-18]( URL_1 ) is similar to the F-22 with twin angled vertical stabilizers. The vertical stabilizer has a control surface on the back of it. On a vertical stabilizer like that on the F-14 it is only a rudder, but if you tilt the stabilizer now it serves as a bit of a horizontal stabilizer too and the control surface is now both a rudder and an elevator. More control surfaces means more force for maneuvering which results in a more maneuverable aircraft which is useful in a fighter"
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7d4dxm | Why is there one standard for music and movie playback that multiple manufacturers use but not for video games. | Example: Sony, Samsung, LG etc, all make VHS, DVD, BluRay players etc, that all play one standard of media, but Sony, Microsoft and Nintendo all have proprietary gaming consoles. Why hasn't gaming gone the way of other media consumption? | Engineering | explainlikeimfive | {
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"Actually they do. Both PS4 games and Xbox One games use Blu-Ray discs. The reason why you can't play a game for one console on the other however is twofold. First, the encoding done on the disks at the time they're burned which includes code that prevents \"unapproved\" disks from being used. This is why you can't just download a ripped version of a next-gen game and burn it to a blu-ray, then stick it in your console. Second, each console has a very strict list of hardware that is used to create the console. This line has gotten blurrier in recent years as consoles have gotten closer and closer to being basically just PCs with specifically tailored operating systems but the hardware limitations still exist. If you were to put a disc from one system into the other and it *were* capable of reading it, the code on the disc still wouldn't necessarily be compatible with the hardware in use by the system. To use a highly generalized example the code on the disc may be expecting a specific graphics processor that isn't used on a competing console. tl;dr - Code on the disk is tailored to the console it's used for, both for antipiracy measures and in order to get as much power as possible from the machine. The fact that this forces consumers to use a specific console for specific games is just a bonus as far as the console manufacturers are concerned."
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7d6u35 | How much different is the $50 high performance air filter from the $5 OEM replacement air filter? What are the benefits and drawbacks of each when compared to one another? | Engineering | explainlikeimfive | {
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"There could be quite a few differences, depending on what the air filter is for. Can you clarify? Is this for an HVAC system, for a car, for a car interior (aka cabin filter), for a PC fan grill, or something else? A general rule would be to find the size of the particles each allows for. Depending on how many nanometers (nm) the filter \"spacing\" is, that will determine the size of particles that can get through. In the event of cabin filters or HVAC filters that can make a huge difference for allergy sufferers. If we are talking about for a PC, then that determines the size of dust particles that can or can't ge inside your system to gunk up the works."
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7d9ta6 | what is the function of a water heater expansion tank? | And why does it have to be pressurized? | Engineering | explainlikeimfive | {
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"When water is heated it expands, the expansion tank prevents over pressurization of your plumbing lines. It’s an overflow tank basically."
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7djvrn | What is the point of humanoid robots like Atlas? | Engineering | explainlikeimfive | {
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"The world we live in has been designed over hundreds of years to fit humans. Machines, homes, offices, cars, trucks, our living spaces, tools, etc. Sure, you could design a robot that can function in each of those spaces to replace a specific human job. But make one that can function in all of them and you have yourself a winner. A robot that can slip into any human role and replicate a human at a fraction of the cost. Years ago we would have to carry watches, calculators, cameras, notepads, games machines, music players, laptops etc. Now we have one device that does everything and has dominated the global market. Imagine if we could make a robot that could also fit into any role."
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7dkmi9 | How does Boston Dynamics stay in business? They're cool, but they haven't made products that are in use. | Engineering | explainlikeimfive | {
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"They probably get research grants through DARPA and about a dozen other govt. agencies nobody's ever heard of. I kinda think they did something for the space program too (not sure why or what though).",
"They receive research grants and funding from DARPA, the US Military, and various private sector companies looking to get into the field of robotics. Boston Dynamics is a pioneer in the field, far and away better than any other US based organization, so they get a lot of attention from the US Government when we're trying to figure out \"what the next big thing is going to be\". We know that robotics is going to be a huge, extremely important field in the future, once we can work around some of the kinks, so it's important that we invest in it now, so we can be leaders in the industry once it becomes a viable, standalone industry."
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7dl6vg | Why do Large Planes Require Horizontal and Vertical Separation to Avoid Vortices, But Military Planes Fly Closely Together With No Issue? | Engineering | explainlikeimfive | {
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"Military pilots are securely attached to the plane and willing to tolerate much more extreme maneuvers than commercial passengers. To reduce the \"fear of flying\" and avoid spilling drinks, commercial aircraft desire a much more stable ride.",
"It's like watching a race car go down a track at 300 kilometres an hour, compared to a bus full of 50 people doing forty on a city street. If a bus went 300 down the street with another bus a half-second behind it, I don't think any of the passengers would be getting on that bus ever again.",
"We’re much, much more aware of where those vortices are, and can adjust our position to avoid them. Also, separation between aircraft is also for safety during abrupt, unexpected maneuvers, not just for vortices. I’m talking directly to the other aircraft in my flight so I’m aware of when abrupt turns will happen.. if I’m in front, I know the position of the other aircraft to be able to avoid turning in a way that endangers them. Commercial aircraft don’t have a lot of direct communication with other aircraft without preplanning frequencies.",
"Aerodynamic factors of vortices can be avoided by flying in proper formation (either with step up or step down, depending on the situation (step up being vertical separation from cockpit to cockpit)). Military aircraft (including helicopters) fly tight formation to decently sized aircraft (C-130s, KC-10s, etc) all the time during in flight refueling and that's generally not too problematic (as long as it isn't too turbulent). Legally, it's because the FAA requires something like... I think at least ~~1~~ 3 miles laterally and 1000 feet in altitude (I should know this) between each aircraft on an instrument clearance (which pretty much all commercial flights have). Military aircraft can declare MARSA which is \"Military Assumes Responsibility for Separation of Aircraft.\" This relieves ATC of the 1 mile/1000 feet separation requirement and so the military pilots are now flying formation off the lead aircraft. Edit: After some google-fu, it looks like lateral separation is 3 miles in a terminal environment and 5 miles en route, with (generally) 1000 feet of vertical separation. Source: FAA order 7110.65, 4-5-1 and 5-5-4.",
"I'm not an expert, but from what I've observed, the military planes like the Blue Angels fly side by side, so all the vortices are behind them. Second, the military pilots are trained to deal with an injured/damaged/partially disabled aircraft so their skills might be more oriented in that direction. Plus the plane they are flying is much more maneuverable than a jumbo jet. Third, commercial airlines want to avoid vortices because they cause bumpy rides, and then passengers complain. Plus the horizontal and vertical separation just makes more sense from a safety standpoint also.",
"Normal aircraft separation under Instrument Flight Rules is defined in FAA Order 7110.65. Either 1000 feet vertical or 3 miles lateral separation (more if dealing with Heavy aircraft and aircraft on final approach if different weight classes). You can have less than this if flying under Visual Flight Rules (see and avoid). Military aircraft flying in formation use a rule called MARSA - Military Assumes Responsibility for Separation of Aircraft - in which they fly under one flight plan as essentially one aircraft for purposes of ATC. Military planes doing exercises in restricted airspace are flying under VFR, and are responsible for their own separation. Source: former military and civilian RAPCON/TRACON controller. EDIT: pointed out by another user that MARSA is Military Authority Assumes Responsibility for Separation of Aircraft. I accidentally a word. Been out of the aviation industry for about 7 years.",
"Wake turbulence is a by product of the lift being produced by the wing. The heavier the plane, the more lift that needs to be generated, therefore the wake turbulence will be greater. An aircraft will produce the most wake turbulence while flying at a heavy weight, at a slow speed and in a \"clean\" configuration (no flaps, or minimal flap settings). Once the aircraft is accelerated in cruise flight for example, the wake turbulence is still there but it is dramatically reduced. Military aircraft flyijg in tight formation are usually in this cruise phase of flight. We have seen a couple high profile wake turbulence upsets at cruise altitudes recently, the Challenger 604 vs the A380 over the Middle East had garnered a lot of attention from the industry because it highlights the risk of wake turbulence upsets outside the terminal area. In the arrival phase, ATC provides anywhere between 3-6 miles of lateral separation. Certain pilot techniques can be applied while landing to avoid wake turbulence but it is invisible so there is only so much you can do. While flying an approach behind a 767 or 747 used to keep me on my toes, now I'm also worried about the wake turbulence more and more at altitude. I will be crossing the Atlantic at 40,000 or 41,000 feet which usually puts you above most large airliners exceptttttt now the 787 Dreamliner can be found anywhere between 40,000 -43,000 ft. It's a lot harder to shit post on Reddit at 40W when you have to worry about Dreamliner McDreamliner face coming opposite direction 1000 ft above you.",
"Air traffic controller here, this won't exactly be ELI5, but I'll do my best. The way this question is being asked is actually confusing two separate things that we have procedures for in ATC. The vertical and horizontal separation that we use doesnt have anything to do with wake turbulence, it has everything to do with not letting the aircraft physically collide with each other. If we allow two IFR aircraft to get closer than that without ever establishing visual separation, then we get in major trouble. Wake turbulence is not to be confused with Jet Blast or prop wash. Wake turbulence is a horizonal vortex emanating from the wingtips that starts as soon as the aircraft's nose lifts off the run way and stops when the aircraft touches down for a landing. The vortex can persist for several minutes, descends slowly at about 300ft per minute. In still air, the vortices will move away from the aircraft as they descend in opposite directions, but a crosswind can cause one of the vortexes to stall over the runway. This is where the danger lies. If a small aircraft flew into a strong vortex created by a large heavy aircraft, it can cause the entire aircraft to rotate along the axis that runs from nose to tail, flipping it upside down and causing an unrecoverable crash. We do protect for wake turbulence, but not by using the 3 miles horizontal or 1,000ft vertical rules. We restrict the distance that aircraft can follow behind another aircraft as they are coming in to land on the same runway. The larger the aircraft in front is, the larger the distance we put between them. We also restrict aircraft from taking off behind a departing aircraft by a certain time interval, the heavier the aircraft in front, the more time we wait before authorizing a takeoff. Once the appropriate time interval has passed, the vorticies will have had enough time to dissapate and will no longer be a factor. BTW, this can be confusing, the terms, small, large, heavy, and super that we use dont refer to the SPECIFIC weight of the aircraft at that time, those aircraft fall into their respective catergories regardless of how much they are carrying. A fully loaded C130 will still just be a large, a completely empty C17 will always be a Heavy, etc etc. Further more, when talking about \"military aircraft flying closer together\", its important to distinguish between things like refueling operations, standard/nonstandard formation flights, and aerobatic airshow type blue angles formations. For things like refueling operations, wake turbulence absolutley is a factor and the aircraft flying behind to get the fuel have to take it into account as they approach the tanker. When the only aircraft involved are fighter type aircraft like F18s, wake turbulence isn't strong enough to really have a noticible effect on the aircraft trailing in the formation. As for the blue angles, those guys are just plane nuts.",
"I've done formation training in military planes. There's several factors at play here that allow us to fly several Feet away from each other: 1. We've briefed with the crew in the other plane. We know them and have established a general plan for how the flight will go, and what manuevers we will do. 2. We use standardized hand signals to tell the other crew what we're doing, and look for the standard reply that they've acknowledged our signals. 3. We have a discrete radio frequency that only the two of us (or however many are in formation) are using. The primary method to pass info is hand signals, but we have the radios as backup. 4. We're trained in how to avoid prop/jet wash, and how to recover from it safely without hitting another plane in the event that we do encounter it. 5. We maintain proper positioning. It sounds crazy, but when you're flying in formation, the closer you are the safer it is. If I'm flying wing, and I'm tucked up close in parade position, I can detect if anything is wrong sooner and easier than if I'm farther away in trail or chase position. 6. Lastly, if anything goes wrong, we've briefed and thuroughly discussed just about every contingency plan. We know what we need to do and what the other plane will do if we lose sight of each other, lose radio contact, fly in to the clouds, have an engine failure, run in to each other, etc. We spend hours talking about this before each flight. Of course if things get too bad (like we ran in to each other and lost a wing) there's always the option of pulling the ejection handle. Obviously it's not the first choice, or the second, or the third, but it's there. Airliners that are flying at altitude, and being routed on instrument flight plans have none of this. 1. Theres no reason for flying closer than they already are. 2. They're less manuverable. 3. They likely have zero training in formation flight 4. They've probably never met each other before, and certainly didn't discuss flying formation before they took off. It's an FAA requirement that any aircraft that will fly formation with each other have prior knowledge before takeoff.",
"The larger and heavier an aircraft is the more severe the wingtip vortices and turbulence are from the disturbance of the air moving over the aircraft's surface. Military aircraft (fighters and attack) are heavy and big, but not on the scale of a 747 or A380. Those aircraft are literally hundreds of times heavier and larger, and produce stronger and larger effects behind them. In addition to comfort mentioned by someone else, large transport aircraft are not very maneuverable compared to military aircraft and would struggle to recover from the effects of other aircrafts wake.",
"You know how people like to say routine flying is safer than driving? Not the case in the military. Even at peacetime.",
"Most of the required horizontal and vertical separation rules are for collision avoidance more so than turbulence avoidance. Also, the military aircraft flying in formation (for example) are not flying in each other's wake turbulence even though it might appear that way.",
"Military planes don’t always fly together without issue. Allow me introduce you to the tragedy of my favorite plane, the [XB-70 Valkyrie Mach 3 Strategic Bomber]( URL_1 ). According to [Wikipedia]( URL_0 ): *The accident investigation also pointed to the wake vortex off the XB-70's right wingtip as the reason for the F-104's sudden roll over and into the bomber.*",
"This was the cause of a famous mid-air collision between the XB-70 experimental bomber and one of the chase planes. URL_0",
"You do not really see them close, and ot the aft of AF1 for example, mostly I believe it is the size of the plane. That is why commercial airports hate GA light aircraft, I think you have wait 3 minutes after a heavy takes off to completely screws up the timing."
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7dldw6 | I don't understand how it is possible to cool a car engine with air alone. So how does air-cooled engines work? | Engineering | explainlikeimfive | {
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"There is a lot of air, and the average temperature of the air is suitable for long-term engine operation. It's simply a matter of exchanging the engine heat with enough air. Even liquid cooled engines do this, they use a fluid to move the heat to a radiator and from there into the air. An air cooled engine simply does it directly. It uses fans to circulate a lot of air over an engine structure with fins to provide enough surface area to exchange the heat.",
"The main difference you'd see on air cooled engines are [fins]( URL_2 ). [Lots]( URL_0 ) and [lots]( URL_1 ) of fins. What these do is increase the effective surface area of the engine by a *lot*. This is all surface area that's going to heat up when the engine gets hot, and so air going over those fins will far more effectively be able to pull heat away than if the engines were smooth and finless. On a motorcycle, they just rely on the forward motion of the bike to send air over those fins and keep the air cool. In cars (like the old VW Beetle) there are ducts and fans that send air over those fins to pull heat away. The problem is it's not as controllable. With liquid cooling you pull the heat away from the engine in a medium, in the usual case engine coolant, and then the heat is dumped into a radiator, which can be easily cooled either by movement of the car itself, or with a dedicated cooling fan. Plus water has far more capacity for heat than air. Air cooling once you get beyond a certain point (power level, heat output) stops being effective, which is where liquid cooling comes in.",
"All engines are cooled with air. Engines with liquid radiators just take the heat from the engine into a liquid then into the air via a radiator. Air cooled engines just invite that air to blow directly over the metal of the engine. To increase the ability of the (air) fluid to remove heat from the lump of metal, the engine's metal will be finned to create increased surface area for the air to blow over and absorb heat from. Also, something I think people don't think about: the car engine will still be really, really hot. It will just be 'cool' enough to not overheat. So yes, the air is 'cooling' the engine--the engine is 'cooler' than it would be otherwise. But you hear 'cool' and think \"cool to the touch\". That's not happening, same as with water-cooled engines."
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7dmy5c | How can Tesla's new truck have a lower drag coefficient than a Bugatti Chiron when the surface area is so much larger? | Engineering | explainlikeimfive | {
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"There is a difference between \"drag\" and \"drag coefficient\". The equations to calculate the drag coefficient account for the surface area. If you have two spheres, where one is 1 inch in diameter and one is 1 foot in diameter, they will both have the same drag coefficient, but the bigger sphere will have more drag.",
"Coefficients are normalized to remove such things as size and surface area, which aren't important to a coefficient. The number represents how streamlined a particular aerodynamic object is. An airplane can have a very low drag coefficient while experiencing substantially more drag simply because it's either bigger or moving faster. Many performant cars actually have really high coefficients because they have air inlets for intake and cooling, as well as aerodynamics that provide downforce at the expense of drag. The [Aerocivic]( URL_0 ) is a 1987 Honda Civic covered in a sheet metal shroud salvaged from a disused shed that has a really low drag coefficient though it's not really all that fast, and it has far more weight and surface area than the original car. It's drag is so low it leaves a minimal wake, so drafting this car (following closely behind it so your car has to displace less air) is pointless.",
"It's not just the surface area that you have to account for. A large amount of air is actually captured into the Chiron to use to cool the engine. This is what is creating most of the drag, not just the surface area."
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7do6tf | How is it possible that a bomb can be dropped from a plane and hit the ground and NOT detonate? | Engineering | explainlikeimfive | {
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"Some explosives are highly unstable. Those type of explosives would be terrible for making bombs. Some explosives are very stable. They build bombs out of those. Stable explosives require a lot of energy to be added before the explosion happens. Think of all the times you hear about 'detonators'. You know how in old cartoons the TNT needed to be hooked up to that box with the handles? That handle was spinning a little generator that made an electrical pulse that went through the wires making the ~~TNT~~ balsting cap in the TNT explode. Modern plastic explosives are so stable you can set them on fire and they won't blow up! They need an even bigger addition of energy before they go off.",
"Bombs have fuses and bits inside that need to function correctly in order to explode. They don't blow up by simply hitting the ground really hard."
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7dqqg0 | How do they build Jettys and Piers and other structures where the supports are underwater? | Engineering | explainlikeimfive | {
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"You have a pile driver which rams the initial supports into the mud sand or whatever is under the water. URL_0"
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7dscet | How do plastic tyre bikes work? Why did we use rubber ones (requiring air to be pumped in) in the first place? | Curious after seeing the post about the airless tyre bike. | Engineering | explainlikeimfive | {
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"Well, the material itself is somewhat stiffer, while still retaining a certain flexibility. The holes are there to offer room for the material to flex, avoiding the uncomfortable hardness you'd experience if the tires were solid- it fulfills the same purpose as the pressurized air inside a regular tire, as well as lowering weight. The classical, inflatable tires fulfill the requirements to enjoy a smooth ride, while remaining simple to produce, but early on, bike tires were, in fact, solid rubber."
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7dt5p7 | why do tires have to have air? Why can’t they just be solid rubber? | Engineering | explainlikeimfive | {
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"Tires absorb impact, they work with the suspension to make for a smooth ride. They also need to be soft to grip the road ALL at the same time!",
"Solid rubber is very... Solid. Filling a tire with air gives it some cushioning to make it a comfortable ride. Hitting a bump with a solid tire would wreck havoc on your vehicle's suspension - so either greatly shortening the life of it, or transferring that force to the cabin and its passengers."
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7e48r6 | Why do guns that fire the same caliber of bullet have different muzzle velocities? | I'm looking at the wikipedia for the M16 and the M4 carbine. They are both listed as firing 5.56 x 45mm cartridges. The muzzle velocity for the M16 is listed at 960 m/s. The muzzle velocity of the M4 is listed at 910 m/s. Is this a function of barrel length? Some other variable? My understanding was that the same cartridge would have the same amount of gunpowder, leading to the same amount of force propelling the bullet forward. | Engineering | explainlikeimfive | {
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"Bullets are accelerated by the expanding hot gasses behind the bullet. If your barrel is 3 inches long, the bullet leaves the barrel before all the gas is finished pushing on the bullet. If your barrel is 5 feet long, the gas pushes all it can and then the bullet slows down due to friction in the barrel. Somewhere in the middle is the perfect point where the expanding gas gives the bullet all the energy it can with the least energy lost to friction. The M16 has a longer barrel, giving the bullet more speed when it exits the barrel. The M4 is shorter and more maneuverable in tight spaces, but a little bit of power is lost when you shorten the barrel."
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7ecp01 | How is it that an intermittent drip from a faucet can keep the entire pipe system from freezing ? | It’s cold in the winter, and people always say just a slow drip from your faucet will keep home pipes from freezing. Is that true? | Engineering | explainlikeimfive | {
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"Leaving a faucet on a slow drip keeps pipes from bursting when they freeze, not to keep pipes from freezing. If you put a can of soda in the freezer, when the soda freezes, the can might burst because water expands when it freezes solid. If you open the can beforehand, there’s more room for the soda to expand out the opening, the rest of the can will stay intact. Opening a faucet, even a little, makes more room for the water inside your pipes. So if any of that water does freeze and expand, instead of pushing on the pipe, that expansion pushes more water out of the faucet.",
"Imagine a house with pipes and those pipes are full of water and it's a closed system - all the valves are closed. If the water freezes in one portion, water expands as it freezes pushing the water in the rest of the system. If there's no outlet, then if enough water freezes the hydraulic pressure can rupture the pipes, or more likely the joints. If you leave the faucet 'running' you no longer have a closed system and allow an outlet for the water when one portion freezes."
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7edj8w | How do Radio Jammers work? | Engineering | explainlikeimfive | {
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"Radios talk on a frequency, like morse code but so incredibly fast that no human could ever hope to listen and keep track of everything. If there's another radio screaming on the same frequency, even a computer can't hear through the noise. If you've ever found a radio station where you can sorta hear two channels but can't really hear either one, then the two radios were jamming each other.",
"You and I are in a restaurant having a conversation. The information encoded in our speech is sent by one, and received by the other to relay the information. Even with general background chatter, we can usually error correct and understand what is being said. Radio jamming would be like somebody standing next to us playing pink noise very loudly so that we can't hear each other above the noise (the signal to noise ratio drops until the signal is lost). There are more sophisticated jammers that spoof the source signal and garble it and retransmit. This would be like us having a conversation and a 3rd person uses a synthesizer to record what we're saying, add a delay and randomizer, and then spit it back out to us mangled so that intelligent speech is woven with gibberish in the same tone of voice and accent, making it hard to distinguish one from the other. The advantage of this is that it doesn't need to be very loud, just sophisticated and adaptive."
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7egps0 | Why do the air from air conditioners smell different when you turn it to fan mode? | Engineering | explainlikeimfive | {
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"Humidity. Ever wonder why farts stink so bad in the shower? Humidity. Air conditioners take humidity out of the air, so you can't smell the scents in the air as well. When you switch to fan mode, no humidity is lost, so you can smell all of those scents."
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7ejqeo | Why do ladders have one or two steps towards the top that you are not supposed to climb? | If you are clearly not supposed to use them because they are unsafe, why include them? Seems like a waste of material. | Engineering | explainlikeimfive | {
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"1. They still provide structural integrity in holding the ladder together; 2. When those steps are flat (as with folding ladders) you can rest tools and materials there. 3. They still provide support in the sense you can lean forward a bit and have something you can rest your legs, knees, or shins against. 4. If you got rid of those top two steps then the next two steps would be the top two steps and therefore unsafe to stand on. Repeat until there is no ladder left.",
"The extra steps on top of a ladder, that you aren’t supposed to climb are there for structural integrity and stability. This is so the climber doesn’t fall.",
"The fact that it is not safe is because it is the top step. No matter how tall the ladder is, there has to be a top step, and that step will be unsafe."
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7ek16a | How do mines handle snow? Like several meters. Aren't they pressure sensitive and wouldn't the added pressure just blow up the mine? | Engineering | explainlikeimfive | {
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"It depends completely on the design of the mine. The simplest mine you can imagine just has a trigger weight. 20 pounds say. Your question then becomes \"If I put 20 pounds of snow on top of that will it go off?\" Yes it will. If you ask \"if i put 15 pounds of snow on it then a person walks over will it go off?\" the answer is again yes even if a footstep is distributed down to only 5 pounds on the mine. If your question is \"is there some amount of snow that will spread out enough weight of the person not to set off the mine?\" The answer is completely dependent on the trigger weight of the mine. Now... that said. I think on the DMZ the kind of mine you need to use is a bit more sophisticated. I know nothing about mines but I do know that in korea in the winter the ground can freeze. When things freeze, they squeeze. Thus, to avoid being set off I imagine a landmine need to measure the rate of change of weight on it as opposed to the actual weight on it. And that would really help avoid going off in the event you were walking over it when it was covered by a very thick layer of snow.",
"Even a few meters of snow isn't heavy enough to trigger most landmines. Though mine fields due tend to suffer attrition over time due to things like bad weather, landslides, fires, etc. and need to be constantly maintained.",
"The mines are designed for the pressure of a human foot stepping on them, or a tank rolling over them, depending on the type of mine. For the same reason, APOPO's rats (a Reddit favorite topic a few years ago) don't set off landmines when they go scurrying over terrain trying to sniff out mine locations. URL_0",
"it all comes down to the type of fuse and/or the mine itself. if its a [tiltrod trigger]( URL_3 ) than yes, snow could potentially set it off by shifting it more than 20 degrees out of center. if its a [tripwire]( URL_1 ) attached to an M16A2 or \"Bouncing Betty\" the snow will pile up and just cover the wire and it wouldn't build up enough weight to set it off. its its the typical [pressure fuse]( URL_2 ) on an M15 then the snow would have to build up enough weight directly on the pressure plate, added bonus some of the early fuses used a liquid filled ampule akin to a chemlight...and it would freeze. The new fused need something like 300lbs to go off. As far as walking over snow...if the snow was thick enough and displaced enough of your weight, then yes it \"Could\" be safe. Cold isn't great for mines but it doesn't stop them. Mines are very dangerous but also oddly specific in the triggering. If a [toe popper]( URL_0 ) goes off under 4 meters of snow you would probably barely hear it.",
"Snow is not very heavy and landmines aren't that sensitive. They are rated to explode for people/vehicles, but you don't want your minefield revealed because a mouse decided to run across it. So its fine (sadly I guess)",
"Bonus question: why do minefields not become forests? Like I would not like to be the guy to cut the grass there.",
"A quick and dirty answer, based on nothing but high school Physics. I did the Math for a [PM-1 antipersonnel mine]( URL_0 ). Diameter: 0.112 m Activating weight: 5.8 kg Density of water: 1000 kg / m^3 Height of a column of water needed to trigger the mine: 5.8 / ( & pi; * (0.112 / 2)^2 * 1000) = 0.59 m Or about 2 feet, if you're a Merkin. (Don't forget that snow is mostly air, so 0.59 m of precipitation would be several metres of snow.) Looking at [a climate graph for Seoul]( URL_1 ), which isn't too far from the DMZ, we see that winters are pretty dry, so there's unlikely to be anything like that much precipitation. As for your question about walking over a minefield. The snow will distribute the walker's weight, so the mine will feel less pressure from the walker. But if you're unlucky, that slight pressure increase, in combination with the weight of the snow, might be enough to trigger the mine. I wouldn't chance it."
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7eqrci | What's a cellular automaton ? | The Wiki just led me to a million different places and now I'm lost. I am trying to understand the history of computing and I came across cellular automatons. They look really cool but turn out to be really complex too. Thanks in advance. | Engineering | explainlikeimfive | {
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"For starters, it's not really an essential part of understanding computing. The idea came about as more of an entertaining thing to do with computers. At best, they're a footnote in the history of computing. Take a grid. Each cell of that grid can either have a thing in it or not. Now, given some rule based on the neighboring cells, you can decide if that cell has a thing in it for the next generation. The original example was [Conway's Game of Life]( URL_0 ), the rule were fairly simple: 1. A live cell with 1 or 0 live neighbors dies of loneliness. 2. A live cell with 2 or 3 live neighbors is happy and survives. 3. A live cell with more than 3 live neighbors dies of overcrowding 4. A dead cell with exactly 3 live neighbors gets colonized with new life. With these 4 simple rules, it turns out that surprisingly complex patterns can arise. There's a virtually limitless number of different rule sets you can come up with, all of which have different patterns of behavior.",
"To build on what /u/ameoba said, a cellular automaton does not have to be a 2D grid and doesn't have to rely on just immediate neighbors. The second most famous series of cellular automata are the one dimensional ones by Stephen Wolfram. Basically, you have a single line made up of cells. Each cell can be on or off. You draw a line below that based on a simple set of rules determined by a cell and its immediate neighbors. There are 255 possible rules and some produce some really [pretty mathematically significant patterns]( URL_0 )."
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7er5ki | What keeps vandals from stealing train tracks? | Engineering | explainlikeimfive | {
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"In the US, train rail is welded. This makes it more stable, albeit at some increased heat trouble, and very hard to steal. A typical welded piece could be a mile long and when you cut it the sensors for continuous train control (where it is deployed) would detect a loss of signal. It's a lot of work to steal, a federal crime, and the scavenge price of steel is low. Plus, if you do it once, every scrap yard in the area will be contacted by the police and when you show up with your second batch it's going to be an angry confrontation with a burly scrapper who'd rather call the cops than pay you.",
"Heavy, dangerous and who the fuck is going to buy a train track?",
"They're just made of steel and steel isn't overly valuable. They're also very firmly attached to both the ground and the ties (the traditionally wooden parts that run perpendicular to the rails).",
"Well, that actually happens: [Article in german]( URL_0 ) German railroad company \"Deutsche Bahn\" has an annual damage of around 17mil euros because of stolen train tracks.",
"Most vandals aren't particularly interested in doing things that will cause massive property damage or loss of life. Secondly, most vandals can't get a team of a dozen people together to move around a 50+ foot long section of steel track. Thirdly, once you start fucking with railways, it becomes a federal crime and people will be all over finding you.",
"The sheer weight and otherwise uselessness of them. Not to mention the fact that they are bolted down. They are really hard to remove without some serious tools and a lot of people. Vandals don’t have that sort of dedication when there are so many easier targets",
"TL;DR: If I have the equipment and team necessary to move a section of train track in the middle of the night, I probably also have the equipment and team necessary to do some other sort of heist with a much higher return on investment. As for \"those who just want to do bad to others\", again, there are much easier methods of doing so than removing chunks of train track."
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7etbb1 | How are dams built? Wouldn’t the water get in the way if it was built ground up? | Engineering | explainlikeimfive | {
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"They divert the water by digging a tunnel around the dam, or if the river is wide enough, they will close off part of the river and force all the water to flow through the other half, creating a dry area to construct. Then, they flood the half-built dam and make a dry area on the other side to finish it."
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7evrgk | Hysteresis? All the online explanations are too complicated for my tiny brain. | It seems it has something to do with the history of an object impacting the current/future performance of that object, but I’d like to be able to better explain it to others. | Engineering | explainlikeimfive | {
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"Try this one on for size: You have an intelligent self driving car. One of the things it does is turn on the headlights when it gets dark. (Ignore local laws about driving with your lights on, OK?) Well, imagine it's dusk, and it's just on that level where it's nearly dark enough to turn the lights on. So the detector that decides whether it turns on or off is constantly flipping on and off, and the headlights constantly flash. Introducing hysteresis. We set two different thresholds, so that when it gets dark, we turn them on. However, it has to be a lot brighter before we turn them off again. Now the headlights will be staying on until morning. That distance between where we turn something on or off is the hysteresis of the system.",
"Hysteresis is a tendency to \"stick\" in a certain state. Consider the a toilet tank. When the tank is empty a float drops which opens a valve which fills the tank. When the tank fills it lifts the float until the water shuts off. But what if you remove just a little water? If you remove a couple of tablespoons, it won't turn on. You probably have to remove a few cups before it turns on. Once it does turn on, it won't stop until it reaches the original water level. That's hysteresis."
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7ewert | Do blender setting really make a difference? | Engineering | explainlikeimfive | {
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"If you don't have your lighting right then the render will be completely useless. And depending on your graphics card this could waste hours of time to rerender.",
"Yes the settings make the blender spin at different speeds. Typically slower speeds will make something more chunky and faster speeds will make it more liquid."
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7exken | SOAP versus REST for web development | I've read that SOAP is like putting your letter(request) in an envelope to make it more secure, while REST is just sending is the letter(request) as is. Also that REST apis should use unique urls for each 'item' with the adequate verbs, ie. if i want to create an item i would just do a POST request to 'server/items', if i want to modify it i would do a PUT request to 'server/items/id', and CREATE and DELETE to the same URL 'server/items/id' or if i dont pass an id to the 'server/items' i would get all. But what goes on in a SOAP api? | Engineering | explainlikeimfive | {
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"> I've read that SOAP is like putting your letter(request) in an envelope to make it more secure, while REST is just sending is the letter(request) as is. This is complete nonsense. The difference has absolutely nothing to do with security. Neither SOAP nor REST have encryption as part of the protocol. Someone apparently heard that SOAP has something called an \"envelope\" and completely misunderstood what that means. SOAP has a very detailed (and complex) specification on the message format level, and it is completely based on XML. It doesn't actually need to use HTTP, anything that can deliver text messages and responses works. The important difference is that with SOAP, the messages themselves contain all the information. When used over HTTP, a SOAP web service will have a single URL, and you use it by making a POST request to that URL and sending an XML which says something like \"this is a GetOrderList request and here are its parameters\", and in response you will receive an XML with says \"this as a GetOrderList response, and here is the data\". The advantage is that you have these exact specifications that tell you how requests and responses look like, and they will always follow the same basic rules in every system that uses SOAP. The disadvantage is that the specification (and the XML itself) result in quite a lot of overhead and complexity even if you want to send very simple messages. It is *not* enjoyable to use. In contrast, REST does not really have a standard, it just tries to use the elements of the HTTP standard (URLs, different HTTP methods, some of them previously almost forgotten) directly as part of the \"web service\" concept, the way you described it. This means that it is not really possible to use REST via anything but HTTP. The *format* of REST messages is actually not specified at all, it is typically JSON, but can be XML or even something else. The advantage is that it's simpler and much more pleasant to program, but since there is no real standard, every REST API works a little different, there can be surprises hidden, and in fact many APIs that call themselves REST actually don't follow the basic REST concepts at all - anything where one URL is used for different kinds of requests based on the *content* of the message is not \"RESTful\"."
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7f2047 | Why do archers say you should not fire a bow without an arrow? | Engineering | explainlikeimfive | {
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"Dry firing in archery refers to the loosing of the string of a bow or other weapon without ammunition, which should never be done. Without the arrow to absorb the energy, the energy is instead dissipated through vibration of the string, limbs, etc. ... It can even cause the bow to shatter.",
"Pulling a bow back fills it with energy, energy that is supposed to be dissipated into the arrow. Without an arrow the energy is dissipated into the bow."
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7feo30 | Say you acquired a (small) bit of a spent nuclear fuel rod. How feasible would it be to heat a residential home with it? | Engineering | explainlikeimfive | {
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"Not feasible: * The heat output of a piece of non-treated, isotopically unseparated spent fuel is low. It depends on what type of fuel we are discussing, but none of it is very thermally warm. * It is thermally the warmest when it is at its highest level of radioactivity. * The dose rate coming off any piece large enough to be of interest as a heat source will be high enough that it will kill you in short order. The shielding and special handling equipment required to safely store this heater in your home would be enormous and expensive. * [The heat generation and dose rate both go down rapidly over the first couple of years]( URL_0 ). So, you'd either need to build a large shielded vessel to account for the first few months of high activity, which would then be overly large and useless for the rest of the time, or you'd need to swap out chunks fairly regularly, which would be expensive and a huge hassle."
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7ffnja | How is it so easy to open a refrigerator from the outside, but so difficult or nearly impossible to open from the inside? | Getting trapped inside a refrigerator was a real danger that we were repeatedly warned about as kids. I never tried it myself, but the warnings made you at least curious. If I'm strong enough to pull open a fridge with my arm, why wouldn't I be strong enough to push it open with my legs or arm from the inside? | Engineering | explainlikeimfive | {
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"Old refrigerators had latches to keep them closed. New ones don’t. So you are fine in a new fridge, but not an old old one from which the warning comes.",
"Fridges used to have locking handles. Nowadays it wouldn't be an issue to open, but if you wait too long you risk suffocation if the seal is still good.",
"Because refrigerators from the early 1900's had latches. Now they don't. An armless child could free themselves from a modern fridge no problem.",
"As Indiana Jones taught us, even a nuclear explosion wasn't enough to open old fridge doors.",
"* older refrigerators had mechanical latches that could not be opened from the inside * if a refrigerator was on its back, a small child might not be able to lift the door against gravity * children playing could potentially tip a refrigerator so it lands door side done, or so something else lands on top of it"
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7fi4vn | How does the escapement of a mechanical clock maintain a constant rhythm even though the force from the spring/motor will vary? | Engineering | explainlikeimfive | {
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"A pendulum swings for the same beats per minute regardless of the distance it swings. The timing of a pendulum is related to its length only. The escapement ~~transfers energy to the pendulum~~ is restricted by the motion of the pendulum, and as long as the pendulum keeps swinging, the motion of the clock is dependent on the length of that pendulum. As the force in the wound spring decreases, the distance a pendulum swings will decrease but it will continue to swing the same number of beats per minute until it comes to a stop."
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7fj8at | Why are dams, like Hoover Dam, built in arc shape instead of straight line? | Engineering | explainlikeimfive | {
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"It is designed with the arch upstream so that the pressure of the water strengthens the dam as the water presses it against the foundations and abutements. Edit to add: PBS does a better job than I \"The Hoover Dam is a curved gravity dam. Lake Mead pushes against the dam, creating compressive forces that travel along the great curved wall. The canyon walls push back, counteracting these forces. This action squeezes the concrete in the arch together, making the dam very rigid.”",
"Cement/concrete is really strong against things that try to squish it and crush it, but really weak against things that try to pull it apart, we put rebar in it to help but it's still wayyyy stronger against squishing The curve of the dam means that as the water pushes on it it will try to become straight, but becoming straight would require it get wider but there are cliffs that push back against that. This puts the load on the dam in the way it is strongest since pushing in the middle creates a squishing force on the dam materials If you have a straight dam then the water behind would be pushing the middle outward, this will try to stretch the dam forward and it is very weak to that so it won't be able to hold as much water back",
"Arches and domes are much better at spreading force than straight lines are. The arched shape of a dam lets the force of the water run along the dam and into the supporting structure.",
"If you want a tactile analogy, try squeezing an egg along the long axis. You’ll find that it’s surprisingly difficult to crush. The curved shape of the egg distributes the load similarly to how the curve of a dam distributes the load. (Not an exact analogy, but close enough for illustrative purposes.)"
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7fkovv | With all the technological advances of the current millennium, why are flights from Point A to Point B the same duration as decades ago? Why aren't commercial airplanes flying significantly faster today than decades ago? | Engineering | explainlikeimfive | {
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"Commercial airliners fly just under the speed of sound, and have for a while now. They do this because supersonic flight is *much* more expensive. It requires very expensive new aircraft, burns a ton of fuel, and is outlawed over land in many places because of sonic booms. A few supersonic commercial aircraft have been tried, but they were not successes. Turns out most people aren't willing to pay five times as much to get there twice as fast.",
"The short answer is this, when a aircraft flies faster then the speed of sound it suddenly becomes a lot more expensive to operate. Even though it spends less time in the air it still costs a ton more to fly above the speed of sound then just below it, so in the interest of reasonable ticket prices airliners stay just below the speed of sound. Here's the long answer, sorry if I don't simplify this enough as it is more advanced. Sound is pressure waves moving through matter, whether that be wood, rock, metal or air. This speed at which this pressure waves moves can vary between material and how tightly packed that material is but what's important for us is that it's always around 343m/s in our atmosphere. Now well that is the speed the wave moves at, what's more important is why it moves at that speed. That reason is because a pressure wave (sound) moves by particles being hit in a constant direction and hitting other molecules of air and transferring energy to them, in a way similar to a [Newtons Cradle]( URL_5 ). The closer, or more dense the air is the faster this can occur. The way this all relates to aircraft is that, wave speed is constant regardless of how fast the input (an aircraft) is moving, this in effect makes it so that the speed of sound also is the speed at which air moving away from an object it collides with. This result of this is that as an aircraft goes faster then the speed of sound the molecules of air move away slower then the plane moves forward. As a direct result of this, the pressure of the air in front of the wing suddenly increases dramatically because instead of moving away in sound waves it's now bunched up in one spot forming an area of much higher pressure which creates the distinctive sonic boom. Higher pressure does more then just create a nice boom though, it also creates considerably more drag since the aircraft now has to push more air out of the way then it did before, which results in higher fuel costs since the engines have to burn more fuel to maintain said speed. There's also a lot of other REALLY weird effects that going supersonic has but that's the jist of it. The above is why airliners don't go supersonic, however most airliners actually cruise at Mach ~0.85 (mach is % speed of sound) instead of literally just below it (Mach 0.99). The reason for this is actually pretty straight forward, wings work by having lower pressure on top of the wing and lower pressure on the bottom of the wing. The way this pressure differential is achieved is by having the air on top of the wing moves faster then air on the bottom of the wing (The physics behind that are far beyond the scope of even this wall of text, so just [watch this]( URL_2 ) if you need to know more). This means that the air on top of the wing will go faster then speed of sound before the entire aircraft does, along with the increased drag that incurs (and even worse stuff like control surfaces tending to not work). This are is called the transonic region (trasnition and supersonic combined) and well planes can and do fly in this speed range it's simply easier and slightly cheaper to keep them that little bit slower. *deep breath* ok, so that was the aerodynamic side of the issue, here's the governmental side of the issue. Sonic booms are loud. really loud. Like 60,000ft isn't enough to not make them annoying loud. Therefore it's illegal to fly supersonic over the US (except in certain designated corridors). This means that any supersonic aircraft could only service overseas routes like the Concorde's New York to London route. In reality this isn't actually *that* bad since long haul routes are what really fast planes are good for, what does make it bad is that it cuts off the entire trans-continental market. Even then though it wasn't, and isn't the sonic boom that makes supersonic transports non-viable. The other two hurdles that aren't the laws of physics (at least not directly) are the ozone layer and takeoff noise. Supersonic transports typically need to fly really high in order to reduce drag and dissipate heat better, that alone isn't a problem. What is a problem is that they fly so high that, at least in the 60's when the Concorde was coming out, people believed that they might make a hole in the Ozone layer. I don't know if there's scientific truth to that statement, but it did hurt the development of the Concorde so I feel it should be listed if only briefly. The other factor was noise at takeoff and landing, long story short the Concorde used jet engines that were much louder then jet engines used on normal airliners (and it goes to reason any new supersonic plane will need them as well), to be more specific there were straight up afterburning turbojets which is as loud as you can get jet engine wise (I used to have a old ELI5 for this but I can't find it). These engines were so loud that pretty much every airport the Concorde was planning on servicing banned it, minus the ones on the trans-atlantic route between New York/London, and a few other cities (not that they didn't try). This caused everybody who wanted a Concorde to suddenly not want a Concorde and Air France/British Airways got stuck with it. So those are the problems facing a supersonic transport as well as why we don't have them anymore. But why is nobody trying to make one? Well, some people are, but those companies are dubious at best (not saying they won't succeed, just saying we'll see the Spanish inquisition coming before I predict it). Thankfully this is the one part of this topic that is simple (hallelujah). Developing an airliner, or even a business jet is stupid expensive. For example, Boeing barely turns a profit from it's 787 (at least I think they did) and that's a conservative guaranteed to succeed type design. A supersonic transport is a massive gamble in an industry where there is no room to gamble. Bar government funding of some sort or a massive improvement in drag reduction (among other things) making these things feasible we're really not likely to see a supersonic transport anytime soon. That's why airliners stay at about that max speed, but why don't we go even slower to save more money? Simple answer: people thought that would happen but it's all around better to go faster until drag starts getting exponentially higher. Long answer: it's time for a history lesson. After WWII and the invention of the jet engine it didn't take long for people to figure out how to make very powerful propeller engines that used a jet engine to spin the propeller instead of a piston engine (commonly known as a turboprop). When these came out aircraft manufacturers thought that these engines would simply replace the increasingly complex piston engines airliners were starting to utilize since the turboprops were far more reliable and all around simpler. They were aware of straight jet engines (called turbojets) but simply thought they were to unreliable and inefficient to catch on. This gave rise to planes like the [Lockheed Electra]( URL_0 ). Inevitably people tried putting turbojets into airliners but, that ended up in the [Comet]( URL_1 ) which was... not a good plane (it had an issue of breaking apart mid-air). So people still thought turboprops would be the way of the future, then Boeing released it's [707]( URL_3 ) which was hugely successful and resulted in other companies rushing to catch up with planes such as the [DC-8]( URL_4 ) (To be fair, this was just as much a way for Boeing to make a jet tanker as well). After the success of both of these planes a turboprop simply couldn't compete since jet's were faster and about as expensive, though they still are sometimes used on short routes where speed is less of an issue .",
"Flying faster uses more fuel. Fuel is by far the most expensive cost to the airline. They fly at the planes best fuel efficiency. This is the \"same\" as decades ago because as you get close to breaking the sound barrier, it's physically more difficult to go faster (uses more fuel than its worth). The deciding factor for the vast majority of travelers is cost, not time. Airlines care about costs, to keep prices competitive. Edit: I'm half asleep, and poorly summarized [this video]( URL_0 ) check out their other videos on airplanes and airlines. It's fascinating (to me at least)",
"The Concorde was a super sonic jet that cut travel time significantly. However, the cost to operate was a huge drawback. Also, flying super sonic above the continental U.S. isn't allowed by passenger planes. The Concorde required an additional engineer in the cockpit raising operation costs. Overall it's cheaper to operate as is. I believe that Boeing is pursuing super sonic flight again though so maybe in the future that will change!",
"You’ll be suprised because the majority of commercial planes flying today were designed before the new century. But that’s irrelevant. The biggest reason is that on shorter and more dense routes with higher traffic, regardless of how fast a plane can go max speed, they may need to slow down for sequencing. However on longer and less populated routes these routes have gotten faster. Edit: Cost index is something to look into also.",
"We could fly faster but it would be so expensive to the passenger that only few could afford it. The few that can afford it fly smaller private jets that do fly higher and faster than large commercial planes. The duration of a flight on a private jet is very quick when you consider you don’t have to bother with airport lines, security, baggage claims, etc."
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7fl04c | How does plumbing actually work? Why can toilets, sinks, and showers work without electricity (minus having hot water because electricity of course)? | Engineering | explainlikeimfive | {
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"Gravity and water pressure, that’s the basics of plumbing. But everyone knows being an electrician is better than being a plumber. Source: I’m an electrician",
"> How does plumbing actually work? Why can toilets, sinks, and showers work without electricity (minus having hot water because electricity of course)? Sinks, showers, and toilets work because the water supply has pressure behind it. This is achieved with pumps (which require electricity) but can also be maintained by simply placing a reservoir of water in a high place. Water towers are a common example of this method of achieving a constant pressure on the water supply. In the same way that blowing on water in a straw will force out out the other end, the pressure of the water supply will force it out of the shower or faucet.",
"Someone else mentioned water towers. I'm gonna expand on that a little. Under normal operation, a water tower has electric pumps moving water up into them throughout the day. We *could* just not build a tower and use those pumps directly to move the water into homes, but the water tower provides an advantage: we don't need electricity to make water flow downwards, so if the pumps lose power, the water in the towers will still flow down and out to the homes and businesses where it's needed. Most water towers contain at least a day's supply of water, so it would take a pretty major disaster for one to run out of water."
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7fpavw | What could go wrong to lead an electrical outlet to spew water? | Engineering | explainlikeimfive | {
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"Plumbing in the wall either bursts or a connection fails, which leads to water pouring out into the interior of the wall. In most cases your wall is just some drywall boards nailed to an open wood frame, filled with insulation. An outlet is usually just a plastic or metal box with holes on every side for wires to feed into, and the outlet itself is just mounted onto this box, most of which is sitting inside this inner space by cutting a hole in the drywall. The holes you see on a wall outlet are really just there to guide plugs into position to make contact with metal and complete the circuit, so they aren't sealed or anything special, and a cover plate forms a loose seal against every other part of the outlet. Long story short, if a pipe bursts in your wall, water pours in and, unless it has a path to drain straight down (not usually the case because of the aforementioned insulation), it will look for the path of least resistance - in this case, into your outlet box and out the plug holes.",
"Not electrical fault would cause that of course, but it does mean you have an issue. My first thought would be **turn off that circuit at the breaker immediately**. Is it against an outside wall? It could be you have a leak from outside that is making it's way into the box. Either way that is a safety hazard for sure. Please stay away from the water coming from the outlet as it could give someone a shock."
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7fu4ro | How come a mechanical watch changes it’s time over a long period of time | Found an old watch from a few years ago and it’s around 20 minutes behind what my current time is. | Engineering | explainlikeimfive | {
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"For starters, if it's been running nonstop since \"a few years ago\", I doubt it's actually a mechanical watch. A mechanical watch, one driven by a wound up spring and gears, can only run for a few days without needing to be wound. It's far more likely that you have some sort of quartz watch - where a battery run electricity through a crystal and count the vibrations - that simply has an analog face (ie - moving hands instead of an LCD display). Quartz watches can run for years on a single battery without a problem - smart ones can even stop moving the hands to save power & just slide them back to where they belong once motion is detected. That out of the way... Watches and clocks all do one simple thing internally: they make something happen periodically and count how many times it happens. A grandfather clock tracks swings of a pendulum, a mechanical watch tracks the oscillations of a spring loaded weight & quartz movements run electricity through a quartz crystal, causing it to vibrate and they count those vibrations. If the speed of that periodic thing isn't precisely tuned, the timepiece might run slightly fast or slow. Being off by only 20 minutes in a year means that it's 99.996% accurate - if it's been multiple years, that's even more accurate. One source I found online says that typical mechanical watches are typically only accurate to within 10 seconds per day & quartz watches are generally within about 15s per month. If you want to spend more money, you can get more accuracy - as little as 5s per year - but even a $100-150 watch can do 15s/year (as long as you prioritize accuracy over flashiness). All of that pales in comparison to sensitive scientific instruments. Atomic clocks can *easily* be accurate to within a second every million years. The current record is [accurate to 1 second every 15 billion years]( URL_0 ).",
"It's essentially impossible to create a mechanical watch that counts time completely accurately, especially given that the internal mechanism changes its timekeeping based on the temperature and other environmental factors. The best mechanical watches typically keep time to within a second per day, which is only an error of about 0.001%, but can easily build up to being off by many minutes if you let them run for long enough."
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7fv273 | Why does the temperature surrounding an LED affect the brightness? | Engineering | explainlikeimfive | {
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"text": [
"Because how much light it gives off depends on the amount of electricity goes through it. Changes in temperature can change the resistance of a material, which changes how much electricity goes into the LED, which changed how bright it is."
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7fx5jo | How can using a cheap charging cable mess up a small or a tablet? | Manufacturers recommend you only use the cables that come with your phone / tablet, and I've heard other people say that they won't use anything but factory approved cables and charging blocks. What's the worst that can happen? Edit: Smartphone. Not Small. | Engineering | explainlikeimfive | {
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"text": [
"The worst is that they are defective, either catching on fire, or sending way too much voltage into your device causing damage. More often, the voltage may be a bit off, causing your touch screen to misbehave while plugged in. Good ones work fine."
],
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5
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7g1tew | Should I turn off my scooter engine while waiting at a stop light? | Engineering | explainlikeimfive | {
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"Below a certain amount of time, it's more efficient to allow an engine to idle than switching it off, since restarting uses a small chunk of fuel. For a car, it's about three minutes of idling per start, so unless you're waiting longer than that, just leave it running. I don't know what the equivalent amount of time would be for a scooter, but I'm sure someone has done the maths. (the above doesn't include cars with the auto-shutdown/fast-start capabilities, and is a general rule of thumb)",
"No. Starting and stopping an engine puts a lot of wear & tear on the engine & results in a bunch of unburned gas getting ejected into the exhaust. Replacing worn-out engines also has a big environmental impact. When you see hybrid vehicles shut themselves off, they're designed to do it safely & efficiently."
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7g2he8 | We are hearing alot about electric cars and scooters, why do we seldom hear about electric motorcycles? | Engineering | explainlikeimfive | {
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"text": [
"There are actually electric motorcycles: URL_0 Most people buy a bike for the cool factor, others buy them to have the freedom feeling. In my country of origin their a great mode of transportation because you are allowed to filter trough traffic. Its less interesting for a news outlet to provide news about electric motorcycles compared to cars, but there are bike related news outlets that report on them \"often\" enough.",
"Maybe you are referring to America in your statement. In China, particularly Shanghai most of the motorbikes I saw were electric, and very quiet. Cultural differences mean in China they are bought for a functional purpose rather than ego extension.",
"A large reason behind someone buying a motorbike is the emotional response it invokes - it is loud, and cool, and about freedom. People don't buy motorbikes because they are cheap to run - they are, but it isn't what inspires someone to buy one."
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7g5jyh | Why does a car door get stuck if the driver unlocks the door at the same time that the passenger is trying to open it? | Engineering | explainlikeimfive | {
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"text": [
"There's a latch that connects the door handle to the actual opening mechanism. When the door is locked the latch is disconnected. If you lift the door handle when unlocking the latch doesn't engage."
],
"score": [
13
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7g6js1 | How do movie prop belt-fed machine guns cycle through rounds that appear to have real projectiles in the casing, and eject an empty casing? | Engineering | explainlikeimfive | {
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"They probably do some shots firing blanks, but not many because blanks cause jams very frequently. After that it's probably just good camera angles to make you think they are ejecting.",
"It depends on the shot. Sometimes that might actually be a gun firing at a target that's off-camera somewhere. Other times, it could be a prop that's specifically made to have a shell spit out and a belt of fake bullets fed in through some hidden mechanism (if it were me, I'd use a \"sandbag\" to have a tub of casings and somewhere for the belt to go, for example) Other times, it could be a blank or squib round. Part of the job of the crew is to figure out what's appropriate and safe for the shot they're doing."
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7gexqi | how does a catalytic converter reduce emissions? | Engineering | explainlikeimfive | {
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"text": [
"It reduces the amount of incomplete combustion results from leaving the engine. A catalyst works by decreasing the activation energy of a certain reaction, in this case the decomposition/combustion of fuel into CO2 and water. Its usually a metal block (what it is made of depends on the catalyst) with a lot of pores. The exhaust passes through the pores, and any unreacted fuel, CO, or NO, gets reacted with oxygen on the catalyst to break it down into the desired products. Without the catalytic converter cars put our a lot more soot and harmful gases than otherwise. So its more so about changing the emissions than strictly reducing them (it often reduces the ones that are measured though)"
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7gf1y7 | How does an Internal Combustion Engine work?? | Specifically the ones in cars. I have tried reading up on it but the terminology is way above my level of expertise and want something more bare bones. | Engineering | explainlikeimfive | {
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"[Here]( URL_1 ) is an image from the side. The blue valves open and let in fuel, then close. The piston goes up which compresses the fuel until a spark ignites it. This creates an explosion which pushes the piston back down. The red value opens and lets the exhaust out. All of the pistons are connected by a shaft in the middle, but are staggered so one or more pistons going down helps push the other pistons up, repeating the process. [Here]( URL_0 ) is another image from another angle, showing how a piston moving up and down can nevertheless turn a rod around and around.",
"Modern engines have 4 strokes to generate power. 1) intake, piston moves down and draws in air and fuel (newer engines don't always draw in fuel but use direct injection. I'm not covering direct injection here. 2) compression, the piston moves up and compresses the air and fuel mixture. 3) detonate and expansion, a spark is generated which lights the fuel and air mixture. The heat generated expands the gases in the chamber creating power by pushing the piston down. 4) exhaust, the piston returns up pressing exhaust gases out the exhaust port and then starts back at the intake. Up and down motion is converted to rotary motion via the crankshaft. Valves for intake and exhaust are controlled by camshafts to ensure consistent timing of the opening and closing."
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7gfqj9 | How do automatic transmission cars know when to shift? | Engineering | explainlikeimfive | {
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"Through a computer. Prior to the advent of digital computers in cars, the very first automatic transmissions used hydraulics in maze-like constructs in order to implement logic through what was essentially an analog computer. If you want to see something interesting, look for 'automatic transmission valve body' - these things implemented the logic for gear changes."
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7gmm27 | How do Wind Turbines work and how do they compare to other sources of producing electricity? | Engineering | explainlikeimfive | {
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"Pretty much all generators use the same principle. **Magnet** \\+ **copper coils** \\+ **motion** \\=\\ #electricity. The differences between how the electricity is generated comes from the motion part of the equation. The largest turbines put out around 1000 megawatts and are fueled by natural gas. A single wind turbine puts out less than 1/100th of that volume of electricity, but the fuel cost is zero.",
"How: Wind makes blades spin, blades rotate a turbine, turbine generates electricity. Compare: The largest individual wind turbine is 9 Megawatts. Turbines powered by other means can be hundreds of Megawatts in rating. In addition, they only spin when the wind blows, and will only spin as fast as the wind can push them. Their benefits are being environmentally friendly and not requiring fuel."
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7gxyzs | Why do some engines have a much lower redline? | What determines how high an engine can rev? On one end there is the S2k which goes to something ridiculous like 9k, then there are some diesel trucks that sound like they are about to explode at 4. | Engineering | explainlikeimfive | {
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"Most of it comes down to mass in motion. The more mass that has to be moved further gives a lower red line. I have a 8.3L V10 in my car. The redline is at 6k. The pistons are large and they move quite a long distance. By the time you make it to 6k it sounds like the gates of hell have opened up. Smaller engines like my motorcycle can rev up to 17k or more. They are smaller with little stroke movement. The valve train moves very little also. With that combination it can move much faster without parts breaking. Mass in motion. The more mass the less motion. Parts can only withstand a certain amount of stress before breaking.",
"It's important to understand, the goal of burning fuel is to produce hot expansive gasses; the consequential pressure pushes on a piston as it tries to find equilibrium. This action is called force, and when it's transformed from linear motion to rotational motion, it's thus called torque - torque is force about an axis. Gasoline engines have a fixed charge of fuel and air that enters a cylinder and is burned, whereas a modern diesel has an initial mix of fuel and air entered into the cylinder, which is ignited by the heat of compression, and then subsequent injections of fuel are introduced during combustion. As a consequence, gasoline engines only produce torque for a fraction of the power stroke, and diesel engines produce torque for the entire power stroke. No diesel engine will run higher than ~5k rpm because diesel fuel *doesn't burn fast enough*. If you ran the engine faster, which is certainly possible, you'll be dumping still burning, still hot and expanding gasses out the exhaust manifold. I'd be a flame throwing explosion past the valve and down the manifold. Diesel engines, then are designed around the advantages of energy density and stability of the fuel and the speed at which it burns. Gasoline burns very fast and is very volatile. Gasoline used to be a waste product before the invention of the internal combustion engine because it was too evaporative and explody. F1 race cars engines are are speed limited by rules, because it was, like, 2008 Ferrari had a 22k rpm engine and they were working on a 25k one. None of the competition had the money or resources to compete, hence the 18-some-k rpm limit. That's how fast gasoline burns. And F1 cars run off (highly refined, if not pure octane) gasoline. Here, the limiting factor is not the fuel, but the mass of the engine components. I think they're running 1.6 L v6 engines, normally aspirated (vacuum pressure sucks charge into the cylinder when the piston strokes down on the intake stroke), and they're still making +700 hp, which is a measure of work. What is work? Think of it this way, if you want to move across a room in half the time, you need to perform twice the work, you don't need twice the force. How do they do that, when my sports car 3.5 L v6 makes 300 hp? If you want to make 700 hp, you need to breathe 700 hp worth of air. These engines are over-square, which means the pistons are wider than the length of their stroke. They move up and down less than an inch. The pistons and rods are made of aluminum, or titanium, or I've heard of connecting rods now made with some sort of carbon fiber type material. All in the name of reducing weight. My sports car engine's pistons at 7.5k rpm are moving the same top speed at the half way point down the cylinder as an F1 engine, but mine has several inches to speed up and slow down, and the rate it has to stop and change directions is much lower. The F1 engine has to stop and change directions more often in the same period of time, at 18k rpm, and less than an inch to do it. The connecting rod actually stretches and compresses, I think up to 1/2\" in some cases. If their components were made from the same shit as my sports car, the engine would explode from the weight alone. Can gasoline burn faster? Yes, but at this point it's a materials problem. Understand that high speed engines are a means to an end. F1 started building fast engines only because the league banned turbochargers in the 80s. If they had that option, these engines would run much slower, because there are more efficient ways of making power. Also be aware that these engines are at the fringes of what's physically possible. F1 regulation requires an engine to last 2 races. They had to impose that because the top teams were pushing their engines so hard they only lasted one race, and smaller teams couldn't compete. This is why street cars don't run stupid fast engines, because the fuel required to do so is expensive as hell, and the maintenance requirements are unsustainable."
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7gz2lz | Why do abandoned houses fall apart structurally? | Engineering | explainlikeimfive | {
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"Basically, because abandoned houses lack residents/caretakers to maintain and protect them. When an occupied house develops a leak, or a tree branch falls through the roof, or termites infest it, or it suffers minor fire damage, the owners fix it. An abandoned property, conversely, is simply victim to the vagaries of time and nature. Similarly, homeless people, vandals, and thieves are less likely to intrude and damage/take from a house with residents. If someone lives there, you’d worry about getting shot/arrested/etc. if you broke in.",
"In addition to /u/Abdiel_Apollonius's comments, an abandoned house won't have any heat or AC on, which will massively contribute to the problem, particularly if it's in a wet area. I'm in the UK, and a friend of mine moved into a house that hadn't been occupied for 9 months, over winter. By the time he moved in, round the walls were black with mould, the wallpaper was hanging off, the whole place smelled dank and fusty. Imagining it carrying on like that for, say, ten years, I could see the place being in an absolute state by then."
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7gzth5 | Why is there no warning light for when your headlights or breaklights are no longer functioning? | I feel like a lot of people get pulled over for that sort of thing bc they just don't realize it was broken in the first place. | Engineering | explainlikeimfive | {
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"In some newer car models, if your indicator is clicking faster than usual, you have a bulb out. Not really an answer to your question but a small tip.",
"If a headlight is out, it's usually pretty obvious. If a taillight is out, the parking brake indicator usually comes on.",
"Depends on my car, my old VW golf absolutely did give me a light when my license plate was out (actually, shop installed wrong replacement that made it think it was out). Expensive/newer cars will generally tell you if it's out. Older/cheaper cars frequently do not because it's just a lot of electronics. The right way to check a light is every light needs to be wired individually, then power tested (it needs to use a well known amount of power), this requires electronically switching the light onto the power testing circuit. The circuit doing this needs to be able to actually power the light for test, so it can't be just the 1 cent circuits, it's going to be a rather substantial circuit, and it needs to interface with all the switches. In practice, adding the function to the car probably adds a couple dollars to the price which is rather significant."
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7h1meb | How do electronic devices use so many watts of power considering Ohm's Law? | Let's say I have a computer doing a heavy task, nearly maxing out on it's power supply of 600 watts. If the computer is running on 120 volts, then that means it's using 5 amps of current. But then that would mean, according to Ohm's Law, the total resistance in the circuit inside my computer is only 24 ohms. This doesn't seem reasonable. Am I missing something here? | Engineering | explainlikeimfive | {
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"> then that would mean, according to Ohm's Law, the total resistance in the circuit inside my computer is only 24 ohms Ohm's law is valid for passive resistive loads, and your computer is full of things that actively switch, amplify, store, and release electricity in a lot of ways that are not anything like passive resistance, which means Ohm's law doesn't apply in the way it applies to resistors. Sometimes when working with systems you *do* use that kind of calculation, in a black-boxish kind of way, usually to figure out what a source or load 'looks like' to the other parts of the system. Like: if a transistor output is leading into a current-hungry circuit, the intrinsic resistance in the transistor will cause the output voltage to drop (as Ohm's law says it will)... so you might add an emitter follower (aka common collector amplifier) stage that has a large input impedance (high 'resistance') that won't load down the previous circuit and a small output impedance (low 'resistance') that can drive the 'hungry' circuit without sagging... it's basically the opposite of a resistor. It all adds up to that 600 watts in the end, but it doesn't do so in a straightforward Ohm's law manner."
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7h813h | Why do bomb/mine defusers wear bulky armor and face masks? If that actually going to protect them? | In various movies, documentaries, and videos, i've seen bomb defuses from all over the world is these giant bulky suits of armor when they go to defuse a bomb or a landmine or something. But i'm almost 100% sure that no suit or armor we can currently produce will protect you from a landmine at point blank range. What are those suits for, and how effective are they? Also, they don't have any gloves? Are you just expected to get your hands blown off? Edit: Thanks for all the responses! The wikipedia page on Bomb Suits is frankly horribly written and uncited, so I thought i'd give that a shoutout if anyone experienced wanted to take on that burden. | Engineering | explainlikeimfive | {
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"The suits (called EOD suits) are for protecting against debris from hurting the EOD tech if an explosion goes off near them. Unfortunately if a bomb goes off while someone is working on it the suit most likely wont help since the pressure wave from the explosion will kill them, even if the suit stopped shrapnel from hurting the technician. EOD suits protect against debris, but not pressure waves. EDIT: They sometimes wear kevlar gloves but most dont because they need the dexterity when working on a device, and those gloves wont protect against much anyway",
"It will protect against small explosion, partial explosions, and explosions at a distance. There is still risk, but they will increase your survivability. > Also, they don't have any gloves? Are you just expected to get your hands blown off? They are far more likely to die if they mishandle a bomb wearing clumsy gloves. Risking their hands is the tradeoff.",
"Everything is a balance. The safest place is miles from a suspected bomb, but that's not where a bomb technician is needed. The suit attempts to protect critical organs while still being light enough that they can move. Gloves might prevent the tech from using precision tools in certain situations, or getting a good grip, reduce sensitivity. URL_0 If a bomb went off, hopefully the suit will protect the chest, head and neck. They can still live with out an arm or legs.",
"Follow up question: Are there documented cases of the suit likely saving someone's life? How many?"
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7hb8u1 | How to the fountains in Rome (or elsewhere) get the water pressure to flow on such a grand scale if they were built hundreds or thousands of years before electric pumps? | Engineering | explainlikeimfive | {
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"text": [
"The fountain is located downhill from a river. A pipe from the river to the fountain lets water flow there under the natural pressure of gravity. The river could be miles away, thanks to excellent Roman engineering.",
"Gravity. This is the same way that water towers provide water pressure in towns that use them. The source of water is at a higher elevation than the town/fountain and thus the pipe connecting it has pressure."
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7hdxae | How does camouflage work on ships? | What do people do to camouflage a ship? There has to be something more than just painting the blue or something right? What to Navies do to hide their ships from the enemy in the ocean? | Engineering | explainlikeimfive | {
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"Today ships aren't particularly camouflaged, they're painted gray so they blend in with the haze on the horizon from far away but they don't have a particular camo pattern they're using today because your ship is likely spotted by radar longggg before it is seen In WW1 and WW2, ships did have camo. This could range from a basic gray to blend in with the haze to [Dazzle camouflage]( URL_0 ) which was designed to make it hard to identify a ship, as well as making it hard to estimate range and speed when looking at it through a periscope trying to aim a torpedo. The [dark and light banding]( URL_2 ) cut into edges and shapes that would usually be used to identify what class the ship was. This was sometimes combined with a [painted on bow wave]( URL_1 ) to make it look like the ship was going full speed even if it was only going half",
"You don't really camoflage ships at all. It is rare to spot them visually, normally they are detected by radar and sonar systems so giving them camouflage painting is for the most part pointless. Ships use the sheer size of the ocean to stay hidden as long as possible by sailing to different spots outside of radar/sonar range and approaching from unexpected directions.",
"At night they use deceptive lighting to give the appearance of a different kind of vessel. Another method is to control the RF and sound being emitted. An example might be only using a commercial radar and keeping the other radars off. Because they operate in different frequency bands, it will appear to someone monitoring RF emissions that the contact they see on their radar is most likely not a military vessel."
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7ho4gi | Why are light sources (incandescence lamps, neon lamps, led lamps ) so inefficient? Why can't we produce light instead of heat? | So I had to wrote this essay for school and I wrote a comparison between combustion engines and electrical engines. I knew electrical engines were more efficient but i didn't know that they were about double the efficiency, this because they generate a lot less heat ( cus, you know, no combustion). This had me thinking about light sources and well... where does all the heat come from? Incandescence light bulbs I can understand, but LEDs? I've read wikipedia's entry on [luminous efficacy]( URL_0 ) so, long story short: Is there some physical reason it's hard to create light without creating a lot of heat? Will we ever be able to produce visible light with an efficiency over 50%? | Engineering | explainlikeimfive | {
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"Atoms in lighting devices, when stimulated with energy, produce light at several frequencies. Unfortunately these generally include both the ones we want, and some other ones such as infrared (which we perceive as just heat). Scientists are working on ultra-high-efficiency LEDs that don't have this flaw, but it's hard -- no one designed atoms for this task.",
"To excite something enough to emit visible light, you're probably also going to pour a certain amount of energy into the material as heat. That could be via the excitation process (e.g. tungsten filament bulbs are heated by the electricity passing through them until they emit light like any blackbody radiator) or it could be via photons hitting other atoms on the way out (thermalisation of photons as happens in the sun). LEDs are much, much better than filament bulbs, but still put out a surprising amount of heat - if you pick up an led and filament bulb, the led will be much heavier because of the heatsink. The other factor is that luminous efficiency is determined by how well our eyes can use the light, and they aren't equally good at all frequencies. That's why the 100% line in the table is for a particular frequency of green light. We wouldn't want all our homes illuminated in green (if you've got an RGB LED bulb you can try it, it's not very pleasant. People used to sell green bulbs for fridges so the food within wouldn't look as appetising)."
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7hociw | How does depleted uranium help penetrate armored vehicles? | Engineering | explainlikeimfive | {
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"Same way lead does. It's just heavy. As a result, it has more momentum once you get it going, making it harder to stop. Its relation to radiation has nothing to do with it. Uranium is simply heavy, even heavier than lead.",
"The force or the kenetic energy or momentum of the bullet all depends on the mass. Uranium is very heavy, which means that it has a high mass (and greater density). So a lead bullet and a uranium bullet of the same size will have the same wind resistance characteristics, but the uranium bullet will pack a bigger wallop. Which is important if you're trying to penetrate two inches of steel.",
"I actually quoted this earlier: > To understand why depleted uranium (DU) makes a good anti-tank weapon you have to enter the Alice In Wonderland world of high-energy collisions. When metal meets metal at five times the speed of sound, hardened steel shatters like glass. Metal flows like putty, or simply vaporises. A faster shell does not necessarily go through more armour, but, like a pebble thrown into a pond, it makes a bigger splash. > Armour penetration is increased by concentrating the force of a shell into as small an area as possible, so the projectiles tend to look like giant darts. The denser the projectile, the harder the impact for a given size. DU is almost twice as dense as lead, making it highly suitable. The other metal used for anti-tank rounds is tungsten, which is also very hard and dense. When a tungsten rod strikes armour, it deforms and mushrooms, making it progressively blunter. Uranium is \"pyrophoric\": at the point of impact it burns away into vapour, so the projectile stays sharp. When it breaks through, the burning DU turns the inside of a vehicle into an inferno of white-hot gas and sparks."
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7hs4ui | How is Elon Musk's The Boring Company allowed to start boring tunnels under LA? I know they need permits from Bureau of Engineering but this is something that affects every resident, why isn't this a joint private/city project with local government oversight? | Engineering | explainlikeimfive | {
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"You own your house, and the grass on your lawn, but you don't own that wedge all the way to the center of the Earth. In most places, there is a line where your ownership rights ends, well before you get to the water table. You need to get permits from your local government, because they own the land that you might want to tunnel through. In general, city governments are unwilling to fund/participate in speculative ventures like this - being good stewards of the tax payer's money and all.",
"When you own land you only own a few feet into the ground and about 30-40 feet into the air. If you own mineral rights you own deeper but few people in cities own mineral rights. What this means is that once you go deep enough it is the Government that owns and controls that space and they are the ones that give permission for all activity done there. And the Boring Company getting their permits is them dealing with the local government."
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7hzqa2 | Why does OSHA say that breaking concrete requires a hot work permit? | Engineering | explainlikeimfive | {
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"\"Hot work\" includes anything that can provide a source of ignition. Concrete frequently contains metal, and jackhammering this can produce sparks."
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7i44ma | How does ink get removed from paper in the recycling process? | Engineering | explainlikeimfive | {
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"text": [
"After the recycled paper is turned back into pulp, a bleaching agent is used to remove the pigment from wood fibers. No matter what was on the paper, it all ends up white!"
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7i4wvw | How does a cars heat and cooling system work | Where is the heat & cold air generated from | Engineering | explainlikeimfive | {
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"Heat is generated by the combustion engine transferred into the coolant through a heat exchanger (radiator) then pumped to another heat exchanger ( heater core) where it is transferred to the cabin air by a blower motor",
"Heat is generated from the engine. The engine is using gasoline or diesel and making fire. It has alot of heat. It uses a water system to keep it cool. Some of the heated water is piped into a heat exchanger and fans blow out hot air. Most of the heat goes out on the cars exterior radiator. Cold air is made by an air conditioner. Compressed refrigerant is cold and absorbs heat, and evaporates into a gas, pumped outside, then compressed back into a liquid."
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7idwwt | How does a nuclear bomb explode before hitting the ground ? | I always thought that a bomb was meant to explode when hitting the ground | Engineering | explainlikeimfive | {
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"Bombs are made for different purposes, those which explode on impact are designed to destroy what they actually hit, whilst those that explode in the air cause damage to a wider area. Nuclear bombs are an area effect weapon so they work best when detonated at altitude this can be triggered in a number of different ways one of which is using the difference in pressure at different heights.",
"While some bombs are crude enough to rely on simple kinetic energy (slamming into the ground) to ignite, many — including nuclear bombs — are not set up that way and require a fuze. A fuze is just a little device that triggers the detonation. For a ground burst, a fuze might be a little button that gets pushed, which would set off a tiny explosion, that would set off the larger, main explosion. For an airburst, there are different kinds of fuzes that can be used either individually or together to make sure the bomb goes off at the right height. All of this applies to conventional as well as nuclear weapons, though with nuclear weapons it is especially important to have good fuzing because they will not typically explode in a nuclear fashion if they just slam into the ground without a fuze (they are complicated devices that require precise timing to work) Examples of aerial fuzing might include: * A timer that activates when the bomb leaves the plane or whatever. This is a very crude method but if you know it will take 45 seconds to fall to the right height, you can just time it. * A device that uses barometric pressure to detect how far up or down it is. The precision on this kind of thing can be tricky, but they have been used. * A device that serves essentially as a little radar antenna to detect where the ground is, by bouncing radio signals off of it. And so on. In the first atomic bombs, all three of these were used in a sequence to make sure that the bombs were more or less at the right altitude before they exploded (the timer made such the bomb was outside of the plane, the barometric switch made sure it was below a certain altitude, and the radar antenna did the final check). You could even use GPS for this sort of thing, though there are errors that might be added. Depending on the nature of the target you are trying to hit, there are pros and cons for setting the bomb off in the air or on the ground. Generally speaking if you are trying to hit something relatively small and \"hard\" — like, say, an underground bunker or silo, or a single building, even a large one, like the Pentagon — you want your bomb to go off as close to the target as possible, because it concentrates the most pressure on it. If you are trying to hit something relatively large and \"soft\" — like, say, a city — you want your bomb to go off in the air, because it disperses medium and light pressures over a larger area (for a variety of reasons). It is just a small note that for many years the nuclear weapons of the USA could _only_ be used as airbursts because it is actually much harder to make a heavy, complex, delicate instrument that goes off at exactly the moment it hits the ground (and before the collision breaks it) than it is to make one that detonates several hundred feed above the ground.",
"i believe he is asking \"how\" in terms of how the process flows so the device always explodes before hitting the ground rather than \"why\" we let them explode above ground",
"Blowing up a bomb in the air has a different effect than one hitting the ground, or even below the ground. An explosion in the air covers a much wider area instead of directing energy down into the ground. Of course, if your target is a subterranean bunker, then they have ground penetrating bombs to take are of that.",
"They have their own trigger mechanisms so impact with the ground doesn't have to be the trigger. They explode before they hit the ground because people either remote detonate them or set a timer that makes sure they go off that way (*why* air detonations are used has been explained by other users a bit). Separately, for a nuke to work a *very* precise series of firing mechanisms has to take place. The explosion has to start in a very specific, controlled way. If you just hit a nuclear bomb with another non-nuclear bomb to blow it up, it wouldn't go nuclear. It's not as simple as \"hit something and the reaction goes nuclear.\""
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7ijhqw | How are ships able to maintain electricity for the entire ship, be it military or cruise line, for such extended periods of time? | Engineering | explainlikeimfive | {
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"That have generators onboard that make electricity. So long as they have fuel they have power. I am confused as to why you think this would be an issue?",
"The engine rooms of large ships are a lot like a power plant on land. Only small ships today have their engines connected directly to the propellers. Larger vessels instead use diesel-electric transmission. [Massive diesel engines]( URL_0 ) (you could actually stand up inside some of them) are connected to equally large electric generators. These drive powerful electric motors, which spin the propellers. These huge generators can produce more than enough electricity to power the ship's secondary systems like passenger accommodations, water filters, navigation equipment, etc. Diesel train locomotives work the exact same way, just on a much smaller scale. They act as a generator powering both the wheels and passenger cars. There are other setups too. A lot of military vessels use nuclear power. In this setup, you have a nuclear reactor boiling water that turns a steam turbine. A lot of other vessels will use gas turbines. Basically a stationary jet engine that runs on natural gas or other fuels. They all drive electric generators though. Ships docked in port can also be connected to the local power grid via huge cables, which is often cheaper than running the engines. They use electric motors because they maintain even turning power (torque) regardless of RPM. So do steam engines, but they're very large. If you've seen a cutaway of the Titanic, half the ship is just the engines and boilers. Electric motors are more compact. They can also be mounted in [swiveling pods]( URL_1 ) under the ship, which make big vessels more maneuverable. As for fresh water, it depends on the vessel. Short range ships will carry potable water in tanks. Long range ships will often have a desalination plant, which removed salt and purifies water either through evaporation or reverse osmosis filters. Nuclear ships will typically have these as well to provide fresh water to operate the steam turbines."
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7ik638 | What is a monolithic structure in operating systems? | Follow up question, what kind of structure do the most popular OS use? Windows - ? MacOS - ? Linux - ? Android - ? iOS - ? | Engineering | explainlikeimfive | {
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"In software, a monolith is a single program which performs a large number of tasks, but is still a single binary. The alternative to a monolith is to break the large program up into several smaller programs, each of which has a more narrow list of responsibilities. The alternative to a monolith is called (especially if you like buzzwords) something like \"Service Oriented Architecture\" or \"Microservices\". Monolith: One big program which performs a number of tasks Microservices: Many little programs, each of which only does one thing Operating systems are just programs, although they have a very specific set of responsibilities and are treated specially by the system. Some of these responsibilities are things like memory management, process management, thread management, disk management, device drivers, event handling and routing, I/O processing, interrupt processing, security and access, etc. Despite the fact that something like \"Process management\" is a pretty different beast from \"I/O processing\", these two things are still compiled into a single Kernel binary. In a monolith, if you need to update one of these, you have to update the whole shebang. This is a monolith. The alternative to a monolith would be to break all the different responsibilities into their own binaries, and those would communicate over some kind of messaging channel. This is called a \"Microkernel\" approach. Microkernel seems nice because each individual service can be smaller, simpler, and able to be managed/updated independently. The problem comes because the messaging channel which allows these services to communicate starts to become very complicated: handling enough bandwidth, keeping messages properly ordered, serializing and deserializing, keeping message contract versions in sync between services, etc. And then there are all the complexities where you have to keep track of which order the services are initialized in during boot-up, and all the million possible scenarios of what the system should do if one of the services crashes while the other ones are still healthy, etc. Linux, BSD and Android are pure monoliths. Windows basically uses \"hybrid\" approach with three basic parts: HAL, \"the microkernel\" and \"the executive\", where \"the microkernel\" is basically a normal monolithic kernel, and \"the executive\" contains several services which operate in kernel mode. Darwin (iOS, OSX) also uses a hybrid approach, though it's harder to describe. There's a kernel called \"Mach\", but there are components borrowed from FreeBSD like I/O and POSIX, and there are also a number of microkernel-like services though many of them run in user-space to avoid context-switches. The only \"real\" microkernel system I can think of for modern OS's is GNU Hurd, which has been under development for nearly 30 years and STILL isn't ready for reliable wide-spread use because of the inherent complexities of the approach. Symbian was a microkernel, but a lot of complexity there disappeared because symbian ran on single-core phones, and Symbian isn't really a thing anymore with the advent of iOS and Android.",
"A monolithic structure means the kernel (all OS's have them) are set up so that the kernel does everything. This was the way it was first done in Linux - back when you had to compile your own kernel if you wanted support for sound (for example). As kernels got more sophisticated, they became too large to go with the \"one size fits all\" things, so they evolved to use modules, that could be added in whenever needed, and to accomodate more complex and sophisticated hardware. Nowadays, I don't know what Windows uses, but as for MacOS, I'm pretty sure they use a monolithic kernel, since the hardware is pretty narrowly defined. Linux used modules, but I'm not sure about Android or iOS. Soneone else could give a better better idea about these things. Hope this helps."
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7ildqz | Why do washing machines have, "cold cold; warm cold; hot cold" settings instead of cold, warm, hot. | Also...why do you need different temperatures to wash clothing? Edit: Wasn't sure which flair to use for this topic. | Engineering | explainlikeimfive | {
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"The first temperature reference is for the wash cycle, the second is for the rinse. In these cases, you would have: * Cold wash, cold rinse * Warm wash, cold rinse * Hot wash, cold rinse For the most part, rinsing can be done cold; but some types of cloth or types of clothing can't be washed hot or they will suffer damage. In other cases, some types of stains might only be removed with hot water instead of cold water.",
"It's telling you that the wash cycle is cold/warm/hot, and then the rinse cycle is always cold. I **think** there's some benefit to stains and other substances dissolving more readily in hot water, though it can harm or shrink some fabrics. However, once the stains are gone and the clothes are just soapy, cold water gets rid of the soap fine so there's no need to heat it."
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7iovim | Why don’t modern ships use sails to increase efficiency? | Engineering | explainlikeimfive | {
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"Modern ships are much larger than old sailing vessels, and would require much larger sails in order to be moved properly by the wind. They would therefore be heavier and harder to move by crews when stowing or unfurling them, and would be enormously more dangerous in a storm.",
"This is starting to become a thing. Check out the [BBC Skysails] ( URL_0 )"
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7it6ev | what is ‘proof-of-work’ and ‘proof-of-stake’ and what is the difference | Newb but not a n00b | Engineering | explainlikeimfive | {
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"Proof of work: I trust this because it was \"carved in stone\" and accepted by the community. The work involved in crafting it makes it near impossible to tamper with it. Proof of stake: I trust this because it was created by, and signed by people who have a a personal stake in it.",
"\"proof of work\" is how bitcoin transactions are secured: basically they are recorded in blocks that are \"signed\" with a number that is very hard to find, so you know that statistically, a lot of computing power was spent on finding one, and someone trying to use their computing power to maliciously manipulate the transactions will be foiled when the next block is signed by someone else who did not include the manipulated transactions. The manipulator can only win in the long run if they control more computing power than everyone else together. The downside is that the \"work\" is basically wasted electricity - by now a pretty large amount being constantly wasted, and it has to keep increasing with the total value of Bitcoin. It is inefficient by design, unless you change the design to make the work something useful, but that complicates everything a lot (there are such designs, e.g. to have miners do protein folding simulations which helps with medical research). The solution to this problem is \"proof of stake\" where people pledge some of the currency they have to be allowed to confirm transaction blocks. Here as well, only if you pledge more than anyone else together will you be able to sustain a manipulation, otherwise you will only lock up your capital for no gain (and in some variations of proof-of-stake you can even lose it)."
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7iuagj | Why does the snow only accumulate on the outline of the bricks in this photo? | Engineering | explainlikeimfive | {
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"The more dense stone of the bricks holds heat longer, especially since it's darker and will absorb more sunlight. The snow on the brick melts, allowing the stone to absorb sunlight, which keeps it warm, which keeps the snow melting. The grout, masonry sand, or dirt between the stone doesn't hold heat as well, so some snow can accumulate. That reflects sunlight, which keeps it cooler, so more snow accumulates."
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7ixfsb | How Do jet engines work and what’s the benefits of using them over a standard internal combustion or Wankel engine? Is it possible to use a jet engine in a car? | Engineering | explainlikeimfive | {
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"Jet engines are faster, they output more thrust. Both operate by reacting gases to make an explosion. the jet engine uses that explosion itself to push forward, while a standard engine/wankel engine uses the explosion to spin a wheel. propeller planes and almost all cars just spin wheels because the materials are easier to get (cheaper), the control is better, and they don't use as much fuel. Jet engines are faster. you can put a jet engine in a car (thats how the land speed record was set). Its just hard to control and unsafe to drive on normal roads.",
"Jets take in air, compress it, add fuel to it, light it, and shoot the heated gassed out the rear. Benefits? A jet turbine can be built to be immensely more powerful that a standard internal combustion engine. They can work at much higher altitudes than standard reciprocating engines (you won’t find a prop powered plane at 50kft). And sure, you can put a jet turbine in a car. It’d be serious damn loud, though. You can’t really muffle a turbine without performance impacts. Plus, I wouldn’t want to be sitting behind a turbine powered car when the light turns green. It’d burn the paint off my hood."
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7ixzox | How do Lego bricks not lose tension/force through repetitive play? | Engineering | explainlikeimfive | {
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"It's how the bricks are made. The LEGO term is \"clutch power\". You have the top cylinders on the top of the bricks and then on the inside you have inner cylinders. They do give out but after a LONG time. Here's an example URL_0",
"Basically, they do - but it takes A LOT of repetitive play and the design is such that it's meant to counter this as much as possible. The shapes click together, and are meant to click together so that the least amount of the plastic is abraded away (this is why you should stick them together straight up and down and not break them apart by wiggling them side to side!). Something that people have noticed as Legos change in composition is certain aspects of their quality, particularly the weight/plastic content (up to 30% less plastic in some shapes!), and one of these factors is typically the height of the little button/\"cylinder\" on top that clicks (or clutches) together with the underside of the top block. The taller that cylinder is, the longer the Lego lasts. Newer Legos will not last for as many click-togethers as old ones did, but the precise decline in quality can't be accurately predicted yet."
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7iztik | Where does the air that causes an organ to produce sound come from and how is it carried to the pipes? | Engineering | explainlikeimfive | {
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"text": [
"In the Olden Days, it was provided by a set of [bellows]( URL_3 ), that some [assistant]( URL_0 ) had to keep pumped to keep the organ supplied with air. Nowadays on most organs, it's done with an electric air compressor, though there are still organs with bellows that exist. There's an organ in a church near me that still has bellows, though there's a compressor too. You can use either to get it to start working, though obviously you can't play and pump at the same time. *edit* Just noticed I only answered half your question. Hold on and I'll finish up. *edit2* for some reason I've just noticed somehow the system didn't save the second half of my message. There's a [air reservoir]( URL_2 ) inside the organ which is basically a box which has a weighted top that can move up and down on a sort of accordion kind of arrangement. As the bellows assistant pumps, or the compressor compresses, the top of the reservoir lifts up, and the idea is that while the top of that is floating (not topped out so it can't go any higher and not bottomed out), the organ will be provided with a steady flow of air. This is necessary otherwise the organ will only sound each time the person gives a pump, which obviously will make playing anything decent difficult. Beyond that there are valves controlled by the keyboards, which allow air to the appropriate pipes. To modify the sounds, there are [stops]( URL_1 ) which are basically selections to adjust which set of pipes the air goes to (or does not go to). Volume is controlled using pedals which open or close mechanical shutters to limit how much sound can come out of the pipes. That's about it in basic terms. Obviously you could literally write books on the mechanics of pipe organs."
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7j55go | Where did anchors get their shape? | According to my (limited) nautical knowledge, an anchor is just a heavy object that holds a boat in place by sitting on the sea floor. Is there a reason why anchors are generally shaped with upturned hooks, besides making them look badass? | Engineering | explainlikeimfive | {
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"Ahoy, matey! 'Tis a fine piratey question. Yer not alone in askin', and kind strangers have explained: 1. [Why are anchors shaped the way they are? ]( URL_0 ) ^(_15 comments_) 1. [ELI5: Why are boat anchors shaped like that and how do they work? ]( URL_1 ) ^(_23 comments_) 1. [ELI5: How do anchors work? ]( URL_2 ) ^(_7 comments_)",
"When the ship starts to move it will drag the anchor. If you just drag a cube of steel nothing but it's weight will stop you. If you put some hooks on it then when you drag it the hooks will dig in and fight your motion more than just the weight of the anchor",
"This is more complicated than one would think, actually. & nbsp; First off, understand that anchors are not actually what keeps the boat from moving. It's actually the cable that connects the anchor to the boat which keeps the boat or other sea vessel stationary. The cable uses the movement of the water to form a natural catenary. The catenary is the curve of a cable. This occurs because the cable is fixed to the boat and the ground so the weight of the cable causes the catenary. This curve of the cable absorbs the sudden movements of the water, such as waves, and allows the vessel to remain in place. & nbsp; Now, the catenary only works properly if it is safely secured at both ends. This is where the anchor comes into play. There are various types of anchors that are used. It all depends on the material that makes up the sea bottom. Some areas are extremely rocky and require a different anchor than if it were a soft sand. Their shapes are all different. For example, the [Mushroom Anchor]( URL_0 :) is used in a soft sea bottom for vessels that are going to be there awhile. It is designed so that it goes straight into the sea bottom and uses suction to remain. This is for more permanent mooring. The more traditional [Fisherman's Anchor]( URL_1 :) is designed so that the two flukes (points) can dig into a soft sand bottom."
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7j5bqu | How genetic algorithms learn and how new generations are made | I've seen videos like [this]( URL_0 ) that shows genetic algorithms "learning" how to do things like walk or fight. How does this work? And how are the newer generations made? | Engineering | explainlikeimfive | {
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"It's all up to the programmer. They write some arbitrary \"fitness test\" to assign a score to each member of the current generation & then breed them. Breeding, likewise, is up to the programmer. They figure out a way to represent attributes/parameters of the algorithm as numbers and then \"shuffle them up\" while possibly adding in some \"mutations\" to get variability. In this case, score is a pretty obvious fitness test & [it looks like they used a relatively simple neural network]( URL_0 ) to model behavior. This means that the numbers would be the weights of the connections in the network."
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7ja3qu | Why can't you put a charger into the wall socket when the switch is on but you can plug a cord in to the base of a USB charger when it's on without any problems? | When I put a plug into a wall socket I have to make sure it's off or a could potentially be electrocuted but if I switch the plug on while I have a USB charger base in its completely fine to put the charging cord into the USB base without fear of electrocution. Is there a particular safety implementation that stops this or is it just that the current becomes to low? If it is a safety implementation why is this not just the norm for wall sockets? | Engineering | explainlikeimfive | {
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"You must be using UK style outlets. On this side of the pond our outlets aren't switched UK style outlets are super duper safe with the switch to cut power and hot and neutral pins that are fully shielded before they're energized. The risk of shock with a UK style outlet is exceptionally low In the US there aren't switches on outlets and the pins are completely metal so they are live before they're completely in. The risk of being shocked by a US outlet is fairly low, you have to get your finger between the prongs, and the risk of dying from a 120V shock is also fairly low so they're safe enough Your USB charger is using much lower voltage and is isolated from the mains. At 5V you could lick it and nothing bad would happen, but feeding all your devices from low voltage is impractical. You'd start using pipe as wires to carry enough current to run a vacuum or kettle Tldr-usb uses way lower voltage and has low power available so it can't hurt you"
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7jco0x | Do municipal water towers freeze during the Winter? | Engineering | explainlikeimfive | {
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"No, they're filled at night and drained during the day. The turnover alone should be enough to keep them liquid since the water is always moving. In areas with extremely low temperatures they may be equipped with insulation or even heaters/agitators to ward off extensive freezing. Frozen water expands with incredible force, and this will destroy all the equipment if the tower is allowed to freeze significantly.",
"There’s enough “turnover” in the system to keep non-freezing water in the tank. The water that comes up out of the ground is around 50-60F. If you pumped it in there and let it sit, it would freeze. But there’s always enough demand on the system that it’s always being replenished by ground-temperature water."
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7jevhg | Why is it bad if firearms overheat? Why do they stop shooting? | I don't have much experience using real guns, so I'm not really sure. Also, in the games I play, you re-chamber a round after your gun overheats. Why is this? | Engineering | explainlikeimfive | {
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"I have torture tested some guns for quality assurance. When a cartridge is fired, the gunpowder is converted to very fast moving and hot gases. The barrel (among other firing components of a firearm) \"eat up\" some of this heat from the expanding gases (you can YouTube this stuff, there are hilarious but real demonstrations of people \"cooking\" bacon on an AK-rifle barrel and such after a segment of rapid firing). When these metals get very hot, they increase their chances of deforming to stress; stress such as very fast gas expansion can ultimately break internal mechanisms and/or cause your gone to \"explode.\" This is a very basic explanation, I don't do metallurgy so I can't explain the nitty-gritty of what's going on in a metallo-atomic scale.",
"it's extremely rare for most guns to overheat to the point of malfunction. heavy machine guns are the exception to this because they fire many thousands of rounds a minute and are used in sustained fire. URL_1 URL_0 a repeating gun requires the cycle be completed to fire the next round. that cycle depends on mechanical linkages to move parts back and forth. when metal gets hot, it expands and becomes softer. once the metal gets really hot, it may not move the parts back to same position in order for the cycle to complete. obviuosly in guns that have plastic parts rather than all metal, the plastic will melt and deform before the metal parts.",
"Metal changes shape when it's hot, and it gets soft. Bullets have a lot of energy behind them. If the barrel is too hot, the bullet could stick in the barrel, burst the barrel. The cartridge could burst in the magazine. None of this is good.",
"Guns are made up of a bunch of metal pieces that need to slide around each other fairly closely. If pieces heat up at different rates, they'll expand differently and may have trouble sliding around in the right way. Inside a bullet is a tiny explosion that gets directed down a tube. This tends to heat up the bits involved in bits that handle those hot gasses. As long as you haven't physically damaged the weapon, this will likely just result in a round not getting pushed into place right & not firing. If you give it a second to cool off a bit, you can just reload it & you're fine.",
"others have answered the question about guns, so I'll take the part about videogames. videogames often embellish for gameplay reasons. it would be extremely unwise to continue firing a weapon that had overheated to the point of malfunctioning, but the game isn't interested in real gun behaviors, it's a game mechanic. you have a tool that can only be used for so long. the game is challenging you to balance the use of this tool with your interest in completing your task quickly. this creates opportunities for decision making. the gun can be used a bit longer if you pay the price of not being able to use it at all for a period. when are those periods, how long are they, etc. the animation simply serves to tell the player that they can't fire and give a general sense of how long they will be paying for overusing the weapon. using a character animation as opposed to a timer helps immersion and reduces the player's mental load.",
"Most people have gotten pretty close, but not quit hitting the nail on the head. Many (not all) fully automatic firearms work using a direct impingement or similar gas-powered mechanism for pushing the bolt back, ejecting a the spent case, and loading a new round. Generally, this is the mechanism that fails in an automatic rifle. Basically how it works is there is a \"block\" near the end of the barrel that directs some of the gasses created by the ignited gunpowder up and into a tube that directs it back to cycle the bolt. Normally either the block will get so hot it \"melts\" or deforms and stops directing the gas the right way. Or the gas tube can do the same thing. Now, there's another mechanism. Good old fashioned blowback. It uses the gasses created by the gunpowder as well- but instead of traveling down a tube from the end of the barrel, it's designed so the bolt is quite literally blown back by the expanding gas in the actual chamber. So for this design, everyone else has already explained it. Usually a barrel can \"droop\" from getting so hot the metal is malleable, or other miscellaneous parts can be malformed or break from getting hot. As for how it's done in a video game- well, it's a video game. To be honest they're never very accurate about firearms. But if it were always 100 percent accurate it wouldn't be fun right?"
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7jgjuk | What is AC grounding as opposed to DC grounding? | Engineering | explainlikeimfive | {
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"They aren't any different. In both cases you're just establishing a common point to reference voltages against For AC your signal will vary between above and below ground regularly. For DC your signal will either be above or below ground and stay there",
"Unlike a typical household protective earth ground (AC), in a DC circuit, the \"ground\" is often somewhat of a misapplied but extremely common term for what's a reference point in the circuit. You'll notice your multimeter usually says COM where plugging in one of the probes; that's what it's for. Voltage is measured as a *difference* in potential, so you need a 0 to base it from. This is that reference point, and everything else measured-from and returning-to there in the end. It gets more complicated with AC/DC coupling setups where noise is an issue; in this case AC grounds refer to grounds with a capacitor which blocks the DC signals.",
"For AC, you are actually connecting to an earth ground through a cable or rod. For most low volt DC, you just connect to a reference ground. Think negative battery terminal in a car. In both cases you are just completing the circuit, so current can flow.",
"Electricity = angry pixies that want to go home. AC = angry pixies who want to get back in to the earth (home) we are using the neutral that is bonded, connected, to the earth to get them there. DC= angry pixies want to get back to the negative terminal (home). Some people say the pixies go from negative to positive but we will ignore them for now. Some times you will have a DC power supply that has the negative terminal bonded to the earth, this is for safety. In both AC and DC you ground the system to provide a path home for the pixies and that path should be a Low resistance path so it is easier for the protection, fuse/breaker, to trip and stop the pixies."
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7jjbug | How are magnets in the road used to regulate traffic lights? | I was at a traffic light and I could help but notice that there were a bunch of metal lines in the road. I 'm assuming that the lines are magnets or something that helps the traffic lights to know when to switch. I was just wanting more detail on exactly how the system is supposed to work. | Engineering | explainlikeimfive | {
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"Traffic light sensors, at least historically, are specialized metal detectors embedded underneath the road. They don't use magnets specifically, but they detect how the presence of metal in a car above it alters the electromagnetic properties of the space around the coil. However, most municipalities are moving away from these kind of sensors though, in favor of miniature radar based sensors that mount to the signal post itself. Under-road type sensors are extremely expensive to fix when they do fail, as this usually requires tearing up the road for at least 24 hours (and thereby obstructing traffic.) Having a small radar sensor that mounts above ground, which you can replace in 20 minutes with a cherry-picker? This is a no-brainer.",
"The system was first used in Australia. The Sydney Coordinated Adaptive Traffic System, abbreviated SCATS, is an intelligent transportation system that manages the dynamic (on-line, real-time) timing of signal phases at traffic signals, meaning that it tries to find the best phasing (i.e. cycle times, phase splits and offsets) for a traffic situation (for individual intersections as well as for the whole network). SCATS is based on the automatic plan selection from a library in response to the data derived from loop detectors or other road traffic sensors. SCATS uses sensors at each traffic signal to detect vehicle presence in each lane and pedestrians waiting to cross at the local site. The vehicle sensors are generally inductiveloops installed within the road pavement. The pedestrian sensors are usually push buttons. Various other types of sensors can be used for vehicle presence detection, provided that a similar and consistent output is achieved. Information collected from the vehicle sensors allows SCATS to calculate and adapt the timing of traffic signals in the network. SCATS is installed at about 42,000 intersections in over 154 cities in 25 countries. In Australia, where the system was first developed, the majority of signalised intersections are SCATS operated (around 11,000)."
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7jlwsr | Why are the typical designs of small drones and helicopters not more alike? | Engineering | explainlikeimfive | {
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"Quadcopters are actually very unstable aerodynamically, and require lots of fine-tuning and adjustments by the computer that is interpreting user input and turning into a device response. Quadcopters get around this by having electric motors with very light rotors that can speed up and down very easily. Helicopters have *extremely* heavy rotors that don't change speed very easily, and they carry human cargo. Hence, stability (i.e. safety) is prioritized, so only one rotor (plus a tail rotor) is utilized."
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7jmp99 | How do automatic flush toilets detect or "decide" when they should flush? | ...because sometimes they seem really aggressive and other times impossible to activate. My office bathroom, for example, has 2 of the same model toilet. One of them invariably flushes while I'm still sitting down, while the other is inconsistent - sometimes it flushes at the right time and sometimes I need to push the manual flush button. Shouldn't the mechanism/timing be consistent if it's the exact same model of toilet? | Engineering | explainlikeimfive | {
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"they would be...if the components were both maintained and working perfectly. the sensor is an infrared motion sensor. bad sensor = no flush. some crap blocking the sensor = no flush. janitor comes and wipes the sensor clean = flush. janitor comes and wipes the sensor with a dirty rag = sometimes flush."
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7jmujk | Difference between single phase and three phase electricity | Engineering | explainlikeimfive | {
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"AC electricity has waves of voltage. Essentially the electricity arrives at the destination alternating in peaks of high voltage and peaks of low (or negative) voltage. [Here]( URL_0 ) is what it looks like. Now normally there is one peak and one trough in your flow. In three phase power you actually have three different waveforms in one stream. [This]( URL_1 ) is what it looks like. Three phase power is advantageous in that if you take the sum of the power of all three phases at any time, it is always the same value. This is good for things like ~generators~ motors that need a constant, smooth supply of large amounts of power.",
"Yarr! Yer not alone in askin', and kind strangers have explained: 1. [ELI5 how three-phase power works ]( URL_4 ) ^(_7 comments_) 1. [Fuckin' three-phase electric power -- how does it work? ]( URL_7 ) ^(_6 comments_) 1. [ELI5: 3 Phase Power ]( URL_8 ) ^(_18 comments_) 1. [ELI5: 3 phase power. ]( URL_0 ) ^(_6 comments_) 1. [ELI5: What exactly are three-phase power systems? ]( URL_2 ) ^(_12 comments_) 1. [ELI5:Phase/Neutral/3-Phase How does electricity work ? ]( URL_3 ) ^(_1 comment_) 1. [ELI5: Explain 3-phase vs. Single Phase power ]( URL_5 ) ^(_7 comments_) 1. [ELI5: AC/DC, Single Phase and Three Phase Power ]( URL_1 ) ^(_2 comments_) 1. [ELI5 Three-phase power ]( URL_6 ) ^(_8 comments_)"
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7jpau9 | Do planes have speed limits? | Engineering | explainlikeimfive | {
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"Planes are not allowed to fly at the speed of sound over land (at least in the US). It causes an unpleasant sonic boom.",
"If you mean to ask whether airspace has speed limits that aircraft must adhere to, yes! It varies by region, and can be a bit complex, but typically airplanes have to slow down the lower they are flying. For instance, in Canadian Aviation Regulations 602.32(reference number if you want to look online!) Specify that below 10,000 feet speed shall be not above 250 knots, below 3,000 feet no faster than 200 kts... but it depends where you are and how busy it is. In reality commercial aircraft sometimes exceed those speeds and it is rarely reported or punished in areas with low traffic volume and low oversight.",
"Types of speed limits for airplanes in the U.S. * No faster than the speed of sound over or near land * No faster than 250 knots (about 300mph) below 10,000 * Certain types of airspace have a 200 knot limit * Certain departure/arrival procedures have a published limit saying \"cross XXX position at XXX sped\" * Air traffic control can assign specific speeds example: \"snoo 1234 maintain 300 knots\" Physical limits: * vNE/vMO: Never exceed or max operating speed, the speed at which physical damage may occur to the aircraft * mMO: Similar to the above but measured in % of the speed of sound. Usually mMO limit exists to prevent some of the effects encountered when flying near the speed of sound (mach tuck, buffett) * vFE/LE/Whatever: Similar in nature to vNE but lower due to a specific situation like having the flaps out. These can vary depending on the type of plane you're flying, for example my current airplane has a limit saying that you can't exceed 160 knots with the window open.",
"Both answers posted so far are good but it should also be mentioned that planes will literally start to fall apart if they are flown faster than their intended top speeds."
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7jsa6r | how do engineers make sure wet surface (like during heavy rain) won't short circuit power transmission tower? | Engineering | explainlikeimfive | {
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"They space the wires far enough apart (for their voltage) that rain won't cause an arc between phases. If you were to hover between them, it might cause an arc. There was a lot of testing done to determine how close is too close for electrical safety.",
"If you look closely at a picture of a glass insulator like they use on the towers, you'll see that it's designed to have both a long surface area between one end and another, plus the bell shape helps ensure it's hard for rain to reach or remain on the inside surfaces. Power companies also wash the insulators as part of regular maintenance, using boom trucks that spray deionized water that's a very poor conductor. By keeping the insulators clean, when rain hits them it doesn't become a good conductor and voltage leaks are minimal.",
"Power System Engineer here: Insulators are used to keep circuits separated. Insulators are typically ceramic and are structural supports that don’t conduct electricity. The insulators have groves on them so a complete coat of water cannot exist to create a circuit from phase to phase or phase to ground. Here is what an insulator looks like URL_0",
"Isn't fresh water not really that conductive? While salt water is. EDIT wording - rain is still conductive, just less so than you might think.",
"Question for you guys who know electricity: There's a path my wife and I have hiked a couple times that goes under some high voltage transmission lines. Both times we went under them they were making a sound like a steak sizzling. Why is that? (The first time we heard it we were like, uh, do we really wanna walk under that?)",
"A rough rule of thumb is that air will insulate about 1000 volts per millimeter.. Or 10k volts per centimeter. That means that on the highest 330kv lines, you only need the cables to be at least 33 cm apart.. And they make sure that the earth cable (usually the single cable) is the closest cable...and that other cables are at least a meter away. Next, when the cables pass a transmission tower, the insulator that holds the cable up is at least a meter long... And there's a cable that jumps the join, so the path of least resistance is always away from the tower. Next, the single most common mode of failure for hv lines is that they oscillate when struck by strong gusts of wind.. ^1 Which is why cables in exposed areas are either bundles of cables separated by plastic spacers, or have large plastic \"football's\"on them to stop resonant vibrations. ^1 The cables on one side \"skip\" one way, and the cables on the other in the opposite direction, and in some cases get close enough to touch... So you make sure they are under enough tension they don't touch, or don't oscillate. The next most common reason for hv line failure is either joint failure.. The cables have to be joined at some point, and the joins fail because the cable is flexible. You can spot the fault with a thermal camera, and the fix is simple. (Cut, weld)... Or bird damage. Big Bird lands on cable, stretches wing, gets close enough to next cable for the voltage to jump.. And the arc runs away.",
"Just to add, we go into the 138kv switchyard in the rain sometimes to open up gas circuit breakers and isolate the buss from the grid in the pouring rain. It's actually not an issue to be around high voltage to certain distances.",
"They sometimes make insulators out of a special rubber that is hydrophobic which means the water beads and runs off. In areas where pollution is higher - such as near the sea or near mines or bush fires area - they increase the distance across the surface of the insulator with a wider insulator or ribs to more surface on the underside. Heavy rain isn’t as much of a risk as any salt/pollutants/ash that might conduct will wash off with the rain. Fog is a bigger risk where the insulators are just wet enough to dissolve the pollution or salt on the insulators make a conductive path.",
"Pure water is a very good insulator. Rain water directly from the sky is pretty close to pure. Even fresh water (rivers/lakes) isn't all that as a conductor of electricity. So the issue doesn't come up as often you'd think. Not even at modest voltages (e.g. 120 AC line voltage or even at 100s to 1000s of voltage). To be a good conductor you need \"ions\" from dissolved salts. So salt water conducts better. As others have said - given ENOUGH voltage (\"electrical pressure\"), even a shitty conductor will start to conduct. But then the trick is distance: the amount of conduction (aka \"current flow\") depends on distance (R = 𝜌 d/A, I = V/R, increasing d increases R which decreases I)."
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7ju9v2 | How does military camouflage work? | How does it allow for snipers to be invisible. Or how does it allow hunters to be invisible while hunting deer? | Engineering | explainlikeimfive | {
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"Camouflage patterns don't actually make you invisible, it just makes it harder for someone to spot you. My 7th grade bio teacher had a great demonstration to explain camouflage in nature, you can try it yourself if you have different colored construction paper. Everyone gathered around their black table/desk top. Scattered around the surface were tiny squares of paper cut up, all a variety of colors. Red, blue, black, white, yellow, purple, etc. All the students were told to pick up as many of the squares as they could in 30 seconds, but you could only pick up one square at a time and then put it into a cup. By the end of the thirty seconds, there were almost as many black squares as there were when we started, a lot of dark purple squares, and almost no white or yellow squares, because the darker colors matched the black table top. That's how camouflage works (and how natural selection gives lots of animals natural camouflage). The reason for this is that eyes are very good at spotting changes, things that are different from the world around it, but are more likely to skim over things that are blending in with their background, like black paper on a black table or camouflaged snipers in the bushes. Military camouflage has similar colors to the leaves/dirt of the territory the wearer is working in, and it's arranged in round-ish blotches because it more closely matches the patterns of the leaves around the wearer. Because the clothes (and sometimes face paint) blends in with the area, it's much easier for someone looking in the direction of a sniper or hunter to miss them entirely, their eyes passing over the clothes without seeing a change from the environment. tl;dr: Eyes are good at noticing change, and when camouflage matches the area around it, it blends in, so eyes are more likely to skim over it and not notice.",
"Mostly it works to disturb the human outline. We're very good at spotting humans and human-ish shapes. Have you ever seen a person in the dark but there wasn't actually anything there? That's your brain constantly searching for the shapes, proportions, and other features that mark \"human\" to you. Camo works by messing with those shapes and adopting the coloring of the plants you're trying to blend with. Like [this]( URL_1 ) or [this]( URL_3 ). If you wear the appropriate suit that messes with the patterns that mean \"human\" to your brain, the brain won't register the shape as human. If you do it right, you can get things like [this]( URL_0 ) and [this]( URL_2 )"
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7jww2a | why is it bad for planes to have a full tank of fuel? | My pilot tonight held us up by an hour on the tarmac waiting for the fuel crew to pump out excess fuel. The fuel crew never showed up so the pilot announced we’re going to fly at a lower altitude to burn 1,700 lbs of fuel. Why would the pilot do that? Isn’t it a waste and harmful to the environment? | Engineering | explainlikeimfive | {
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"Aircraft have different maximum take-off weight and maximum landing weight. The forces on a aircraft is higher when you land compared to when you take off. It is quite obvious when you are take a flight. A takeoff are smooth but there is/can be a significant bump when you hit the ground and land. For a Airbus A 380 the maximum take-off weight is 575t and the maximum landing weight is 394t or 68% of the take off weight. and it can load 323,545 L ~300 t of fuel so a significant part of the withe of the plane when fully loaded is fuel In normal operation this is not a problem because the plane used fuel during its flight and you have the minimum amount of fule that you need to travel to the destination + a bit extra for diversions or other emergency. The result is that the plane when landing on a normal flight is as light as possible. You what to minimize weight because that will reduce fule usage. So if a aircraft have to do a unscheduled landing for a emergency it might need to dump fuel into the air so it can land in a safe way. I would guess that the aircraft you was incorrectly fule a mistake or fueled for a different destination that changed. So the alternative was to pump it out or fly in a less fuel efficient way so the weight of the plane is below the maximum landing weight when you land at the destination",
"Was it a short flight? Maximum takeoff weight (MTOW) is higher than maximum landing weight (MLW). On a long flight, the plane will always burn enough fuel to be below MLW, even if it starts with a maximum fuel load. On a short flight, that isn’t always true. Say you have a plane going from NYC to London. It’s fuelled to near capacity. Some issue arises shortly after takeoff that means that the plane cannot continue across the Atlantic Ocean. If the issue is urgent enough, you can land right away, above MLW. But the plane will have to be thoroughly inspected as it may be damaged. If you can stay in the air and dump fuel or just circle to burn it (some planes don’t have dump valves) till the plane crosses under the MLW threshold, that is preferable. What I am speculating happened with your flight is that it was a short one and was overfuelled. With a normal course, the plane would arrive at its destination over the MLW and that is no good. Dumping fuel is an emergency measure, worse for the environment than burning it, and not possible on all planes. So instead the pilot purposefully flies a less efficient (lower) route so that more fuel is burned in flight and by the time it arrives at the destination airport, it is light enough to land without risking structural damage. You probably also made better time than scheduled. Cruise speed is a balance of fuel burnt and time spent. If you have to waste fuel, you might as well use it to go as fast as possible.",
"Fuel is heavy. Commercial planes can have higher maximum take-of weights than their maximum landing weights. Your plane can weigh a lot more when it starts then when it lands. This usually makes sense because you burn fuel and it gets lighter during the flight. However when you need to land before you thought you would or started with more fuel than needed for some reason you have a problem. Normally it makes sense to fly with as little fuel as needed because fuel is expensive and even just carry around fuel that you don't use ends up wasting fuel to transport the extra mass. In an emergency situation there is also the added consideration that you don't want to crash with giant tin can fuel of highly flammable fuel."
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7jzmn9 | How is the sound of a blinker made? | A tiny speaker? A sound chip? Something mechanical? | Engineering | explainlikeimfive | {
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"It used to be a relay that was purposefully noisy to remind you it's on. I would say in the last 5 years it has been mainstream that all lights are controled by a computer so now the sound is from a speaker and a computer decide if the blinker sound should ring or the safety belt warning or the door open or other warnings.",
"It is mechanical. There are a few types of blinker mechanisms, but the one most commonly used in modern cars essentially is an electromagnet that pulses on and off moving physically moving components that open and close the circuit, making a clicking sound as they do. [Link]( URL_0 ) with better explanation and video",
"No expert, but if I remember 7th grade physics it used to be mechanical and now it is artificially made to recreate the sound. In early days to make light blink in a circuit there was a metal piece that would bend as it got warm from current. When it bends it breaks circuit and light goes off, metal would cool down and snap back into place connecting the circuit and getting warm again. Since it's small and thin piece of metal it can do this rapidly and relatively fast. Thank clicking sound was metal bending back and forth. Now? i know it is electrical though I am not sure where exactly from is the sound coming from. I will wait more educated responses for that :)"
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7k1gwz | Why do Ford trucks have manual hubs, but nobody else's do? | Simple enough question: why do Ford F250s and up have manual hubs that require you to get out and flip them after shifting to 4x4, while to the best of my knowledge Dodges, Chevys, etc. don't? Is Ford just cheap? | Engineering | explainlikeimfive | {
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"The reason why the 'super duty' ford trucks have manually locking hubs is by design....for a vacuum actuated hub to work, the wheel needs to spin min 3/4 of a turn. SuperDuty trucks are 'work trucks' so they are often off road, plowing snow etc...if you were in 2WD and got stuck in a vacuum actuated system it would be near impossible to have those hubs lock in if they truck cant move. Ford recognized this and put the manual locking hubs on all trucks (except the F150). Plus, if you had a vaccuum acuated system and that vaccuum fails then what? manual locking hubs wont fail like an accutated system.",
"I am not an expert. But it looks like auto locking hubs is a vacuum actuated addon feature and not standard package. So if you bought a base model...then that explains that. Also seems even with the auto feature in the console, the manual lock is still retained if you need to. And since auto lock requires the wheel to spin before locking, manual override may be needed if you're binded stuck. Or yet auto vacuum system is damaged."
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7k1hnu | How is cable “run” underground? How does it get from there being no cable underground to there being (for example, a mile of) said cable underground? | Engineering | explainlikeimfive | {
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"text": [
"A trench can be dug to lay conduit. Or use a boring machine to bore a line underground. Then cable is pulled through the raceway created."
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7k2aka | Why can’t small models be scaled up directly? | For example, why not scale up a small caliber rifle to get a huge, anti-vehicle rifle? Or, scale up a small engine to make an engine with more power? | Engineering | explainlikeimfive | {
"a_id": [
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"drbbzuw"
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"text": [
"[The square cube law.]( URL_0 ) When you double the scale, all surfaces are multiplied by 4 and all volumes by 8. Strength is proportional to the surface, mass is proportional to the volume. This means that when you scale something up, your strength to weight ratio (and also heat to cooling) goes down dramatically.",
"5yo answer: Imagine you build small sandcastle. It's made of sand grains and water that bonds them together. Now build same looking castle with same sand grains and water but 10 times bigger. You will notice that walls start to break easily. This is because much stronger forces (sand weight) affect your construction but you still use water of same bonding power. Lets try use glue instead of water and your construction will be much stronger. Let's go to different scale. Sand grains become atoms and water/glue becomes chemical bonding. Many material properties like durability or elasticity change with scale because forces goes up or down but bonding stays the same. Explosion that pushes bullet out of a barrel need to be bigger when scaling up and forces it produce may be too big for bonding of metal you use. Everything may explode into pieces instead od pushing bullet out."
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7k4ctd | Why should a car engine be left to idle for a few moments after starting? | Engineering | explainlikeimfive | {
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"For the most part you don't need to, in reality on a modern engine the oil pump is pumping the oil where it needs to be within a couple of seconds, and the oils required by the engine are selected such that they work properly in your engine when cold (that's why there are two numbers in your engine oil spec, 5W-30 means it's a 5 when cold and 30 when warm, so the engine designer would design it to run ok at 5. You shouldn't run your engine hard when cold because things are not the optimal size and it doesn't lubricate optimally. But idling your car for a minute isn't going to get the oil meaningfully warm, you really need like 5-10minutes of actual driving to get there. But if you're planning on racing, yes, you could drive a few miles before you actually start racing.",
"All the comments are one sided here, and only focused on oil. Your engine has multiple moving parts that are made from different materials, each of these materials expand and contract at different rates as the temperature changes. The tolerances( gaps between these moving parts) are precision machined to be the correct size at the engines operating temp. Oil plays a roll, but as a mechanic (in Minnesota)I've have rarely been concerned about oil as the reason to warm it up. I'm concerned about aluminum pistons slapping an iron block at 10° or below.",
"TL:DR It's good for your engine to idle at least 10 seconds before driving off after a cold start. More than a minute or two is unnecessary. Other comments seem to be based on letting it idle *minutes* instead of *moments*. I define a few moments as anywhere from 10 seconds to 1 minute. Most everybody has been correct about one thing or another. Modern oils are very good. A 0w20 used in most cars nowadays has a \"winter\" or cold viscosity (how quicky or slowly it flows) rating of 0, and a hot viscosity rating of 20. Oils did not use to be so good, which is where the 3k miles or 3 months change schedule came from. You changed oil viscosity with the seasonal change in temperature. Manufacturers use and recommend oils with a range of viscosity because the vehicles will be used in a very large variety of temperatures, and have learned how to build engines that can better tolerate them. Race cars usually use a single viscosity oil that performs really well at high temperatures, and let their engines warm up fully before use. More manufacturers are going to Direct Injection fuel systems, where the fuel is injected directly into the combustion chamber, when older cars injected fuel into the intake manifold where it was then drawn into the combustion chamber. These new engines tend to not run as well when cold, as the fuel soaks up heat when it is injected. My Focus stumbles a little if it's freezing outside and I drive off right away. If I idle for about a minute it's fine. When the parts in the engine heat up they expand and fit together tighter. If you drive aggressively before they do, they can wear out faster from the excessive movement. This is not something that will happen right away, but over years. With modern vehicles the electronics will likely crap out before the engine, unless you are *ruthless* to your car. Example, there was a guy I worked with who had a Camaro. He would be in gear and moving before the starter was disengaged, even if it was 5 degrees F. That is *horrible* for the moving parts. Letting your engine idle longer than 2 or 3 minutes is just a waste of fuel; engines actually warm up better when driven gently for a few minutes. It creates more heat than idling but doesn't create excessive wear. The higher the RPM the more wear.",
"Modern cars don't need as long of a wait as you'd think. 90 seconds is almost always more than enough except for really extreme environments. If you start your engine then check your mirrors and fasten your seat belt, adjust the radio, and put it into gear, it's ready when you are. [This guy]( URL_0 ) goes into a very high level of detail about oil. There's a bunch of car related stuff there that can get you lost for hours. Most of waiting after starting is so things get flowing, but there's more than just wear on parts that aren't slicked up yet. Until the cylinders are lubricated fully, some of the exhaust gas can slip past the piston rings into the crankcase. This is called blow-by. This exhaust is colder than it should be, so the burn isn't as clean as it could be, and full of water vapor. The vapor mixes with the nitrogen compounds in a dirty burn to form nitric acid, which gets into the oil. Normally, this happens to a small degree anyway, and there's additives in the oil to help neutralize the acids so they do no harm. But romping on the throttle right when the car stats dumps a LOT more acid-forming gasses into the crankcase, shortening the oil life greatly. There's also the soot that forms and gets into the oil, and other crud that forms, all of this leads to why oil that's used looks and feels the way it does: it's runny and gritty. Runny means it can't stick where it should, so it doesn't lubricate as well, then the grit and crud in it makes for even worse lubrication, it's like liquid sandpaper! Running on oil that's like this just chews up your engine. Another benefit to the short idle is you can hear your engine at rest, and make sure it's not making strange noises before you put it to task. Stopping the engine just as it makes a strange sound can save you a lot of money in repair bills."
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7k4yb0 | What's the difference between naturally aspirated and forced induction in a car engine? | Engineering | explainlikeimfive | {
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"drblwwm"
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"text": [
"In a naturally aspirated engine the pistons moving down draws air in from the outside. In forced induction, or supercharging, an air pump pressurizes the intake manifold so that when the intake valve opens air is pushed into the cylinder."
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7k50yf | USB Power Adapters | I'm just curious about why usb adapters all come in different shapes and sizes. I'm talking about the piece that converts the outlet to a USB port. URL_0 I recently bought a laptop that charges through micro usb and the adapter is about 4x the size of the one for my phone, while i can still use either charger to charge either one of the devices. Why is this, when the size/shape of the adapter has no real difference on what can/can't be charged and the rate it charges at? | Engineering | explainlikeimfive | {
"a_id": [
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"The power adapter for your laptop is likely designed to provide much more power to the device (likely using usb-c power delivery, or Qualcomm QC) than the one for the phone. The adapter for the phone will work the laptop and vice versa, but if the adapter for the phone is used to charge a laptop, the laptop will charge at a slower rate due to the limitations of the phone adapter.",
"If the 120volt 60 cycle power of a wall outlet were sent directly into the USB port it would light up like a candle as it burned. So it can't directly use electricity from the wall. It actually *could* if they designed it that way, but it wouldn't be as safe, would waste a LOT Of energy as heat, and there is absolutely no reason to do it like that. But what it can do is siphon some of that power off in the form of electromagnetic waves, re-convert those into electricity, and use that to power the circuit. We do this with a component called a transformer. And the larger the transformer is, the more current it's capable of siphoning off of the 120volt AC power. A transformer is made up of 2 or more electric coils, and something like a ferrite magnet. Energy in the primary coil, from the wall, is converted into electromagnetic energy just like an electromagnet made by wrapping wire around a pencil and using it to pick up paper clips. This electromagnetic energy then flows into the second coil where it is converted back into electricity much like the coil in a generator converting magnetic flux back into electricity, except there are no moving parts. Another way to think of it is like connecting an electric motor, to an electric generator, except instead of spinning a motor shaft, it spins magnetic fields. the thicker the windings of metal coils in the transformer are, the more power it can send to the secondary coil. This thicker coil means a larger transformer which means a larger component. In a small phone charger, the coil is tiny and easily concealable and capable of sending a few hundred mah of power. The larger transformers in a laptop power supply are capable of sending thousands of mah of power, but they are necessarily larger. The VERY large transformers in use in something like a stereo amplifier, are capable of sending tens of thousands of mah of power to the circuit board."
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7k83bx | How can concrete support the weight of an entire building yet it is still possible to break it apart with nothing more than a hammer? | Engineering | explainlikeimfive | {
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"A swinging hammer has more force in psi than a building at rest. Also, there are a lot stronger types of concrete than your typical cinderblock that you may be imagining.",
"Concrete’s strength lies in compression strength. You can pour concrete and set an very large amount of weight on it without failure. Any other kind of force directional (lateral, vertical, or torsion) and concrete fails very quickly. Hitting the side of a concrete column Is not compression force.",
"Concrete is extremely strong in compression, but weak in tension and especially weak in shear stress. Moreover it has poor elasticity, that is, the ability to absorb forces by compressing or stretching temporarily like a spring or rubber band. That means even slight deformations will cause extreme buildup of forces in the material. If shear forces exceed a certain level, the concrete can't absorb excess energy by denting or crumpling, and the energy can only be absorbed by fracturing. You'll notice that glancing, angled blows with a hammer are most likely to cause chips/ cracks. This is due to shear failure. Note that hardened steel is much stronger in shear and in tension than concrete is. You wouldn't want the cables of a suspension bridge made out of concrete for example.",
"Many materials have a tradeoff between hardness and brittleness. A diamond is much harder than steel, but you can crush one with a hammer (**trust me, don't actually do this**). Concrete is a huge range of materials, it's not just one thing. Building concrete is flexible enough that an Earthquake won't cause a collapse but it's still hard enough that you can shatter little pieces off with a hammer.",
"Concrete is about 10% as strong in tension as compression. If you swing a hammer hard and fast you can generate a force of like up to 10,000 Newtons... which is a force of like 2,250lbs(f). If you smack concrete thats 6,000psi concrete, it can stand approx 600pounds of tensile force. When you smack with the hammer you will internally generate compressive and tensile forces in it and it will chip off a piece."
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7kb2i4 | Why can't we build an airplane that have something like bird wings, like the one illustrated by Da Vinci? | Engineering | explainlikeimfive | {
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"drd2g6b",
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"text": [
"For the reasons lithium mentioned plus the fact that building a bird that functioned like a natural bird wing would be very mechanically complex. There are real flyers that do fly like birds they're called ornithopters, most of them are small light and don't have a long run time. Because of this, it isnt practical to upscale them and use them for anything meaningful.",
"Bird flight isn't very efficient, they're *extremely* light to enable flight at all and pretty slow. The fixed-wing flight our aircraft use is much faster and can carry much greater loads."
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7kblgp | Haven't rockets always been reusable? | Engineering | explainlikeimfive | {
"a_id": [
"drd1rli"
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"text": [
"They *crash* land in the ocean and are obliterated on impact. Much of it sinks. Theoretically you could fish it up, piece the shattered husk back together, and reuse it, but it's cheaper and safer to build a new one."
],
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7kfhw4 | Why do products that claim to block radio frequencies from cell phones actually increase your exposure to radio frequencies? | According to the new California Public Health [guidelines]( URL_0 ) on reducing exposure to radio frequency energy from cell phones, you should avoid "products that claim to block radio frequency energy" because "these products may actually increase your exposure." Why is this the case? Does the phone actually "work harder" to stay connected if there's a blocker in the way? | Engineering | explainlikeimfive | {
"a_id": [
"drdw1gz",
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"text": [
"There’s also [this fun video from LinusTechTips]( URL_0 ) that suggests a tin foil hat (or equivalent reflective surface) could act like a satellite dish, focusing radio waves directly into the center of your head.",
"> Does the phone actually \"work harder\" to stay connected if there's a blocker in the way? Yes. Cell phones adjust the power of their transmitters as needed to maintain a connection. If they have a hard time being detected by the tower they will increase the energy they output so they can be heard. So if you are trying to block their output it just means they yell louder.",
"The short and simple answer is that a cellphone will, to preserve battery, never use a transmitting power that is higher than necessary. Kind of, why the heck shout if you can whisper? If you install a blocker, you'll end up making it worse for the phone to communicate. And the obvious response from the phone is to transmit at the highest possible output in an attempt to reach through. That said, a directional shield *might work* without making the phone scream its lungs out, but that also requires that you ensure that the blocking direction is not the same direction as the cell tower.",
"They have to work harder, as you said, because they are constantly trying to get past the blocker. However, the concern with cases that claim to do this, or *try* to do this, is that they can't cover the screen. The case can't form anything like a faraday cage (and without being grounded it doesn't do anything), AND because you hold a phone up to your head screen-forward, it may simply direct more of the energy towards your head when you're talking."
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