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7wxr80 | How do engineers get different amounts of horsepower out of engines that have the same number of cylinders without adding turbochargers etc? | Engineering | explainlikeimfive | {
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"The cylinder count isn't what defines the power, the displacement has a far large effect on power than cylinder count does. There are a bunch of knobs that can be turned to dial in the performance of an engine. Cylinder aspect ratio, engine displacement, cylinder count, piston weight, and valve timing all have significant impacts. An engine with 6 small skinny cylinders with light weight pistons can get up to extremely high RPMs and generate a ton of power as seen in F1 engines. Its going to be stupid expensive and short lived but it'll do it An engine with 8 big fatter cylinders with heavier pistons is going to be a much larger engine and can't put out as much power as the super fast one, but will put out good numbers while having good longevity as seen in many V8s out there.",
"Many different combinations as already mentioned. Displacement is only one factor, valve size, valve timing, valve lift, airflow through the ports, type of airflow regarding runner length of the intakes affect power at different RPM's, cam lobe separation and duration, spark timing, spark intensity, air/fuel mixture, and even air temperature will affect horsepower numbers.",
"Power is roughly proportional to displacement * RPM. So you can either optimize the engine to run faster(lighter reciprocating parts, better balanced rotating parts, tighter tolerances, more wear), or make the cylinders bigger(this also usually decreases the maximum RPM the engine can tolerate, but not as fast as displacement increases). So you'll get racing engines that are small and light hitting 20k RPM, and you'll get ship engines that are huge and heavy at ~100RPM. More cylinders lets you increase displacement with less of an impact on max RPM, and turbochargers let you burn more fuel and air without increasing displacement."
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7wy34p | Why do bottles of liquid have a dent/semi circle at the bottom of them? | My brother told me a while ago that it prevents it from exploding or something. Is there an act | Engineering | explainlikeimfive | {
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"Its to make the plastic stronger. Without it they would have to add much more plastic to make it stable, which is more expensive. The bottle wouldn't explode, but it would cause the thinner areas to sag and deform. That would increase the chance of it bursting apart when force is applied. But with the divot, that sort of outcome is essentially impossible.",
"It can be for strength, if the contents are under pressure, or it can just be so it will sit flat on a surface without rocking. You could in theory do that with a perfectly flat bottom, but that requires more precise and expensive molds(have to account for distortion as it cools too). Or it can be to make the bottle look bigger, compared to its volume.",
"If you had a flat bottom it would simply bulge out. Now you would have a shitty bottle that can't stand. The dome simply distributes the forces evenly to the outside ring of the dome. It's the outside ring that has a bit more ridigity that prevents that from deforming. I suggest you look up the making and design of a soda can on YouTube. It explains the engineering behind it. Pretty cool stuff.",
"The dent and curved rim on the bottom of a can gives it the strength to be stacked on without bursting",
"I can't speak much for glass bottles, but plastic bottles and aluminum cans have these features so that if they freeze, the plastic will bulge out and pop into a new position that gives the liquid more free volume to occupy. This prevents a sticky mess on the consumer's garage floor"
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7x0cm5 | How planes get AC working at such high altitude going so fast? | Engineering | explainlikeimfive | {
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"It takes superheated air off of the engine known as bleed air. The air is then sent through a heat exchanger that takes the cold ambient air and mixes them to the desired temperature.",
"They use what's called an air cycle machine, they take hot compressed air from the engine and blow cold air overtop of it to cool it off. They then compress it more which heats up the air more so they can once again use outside air to cool off the pressurized air. After that they let it expand through a turbine to power the compressor within the machine, while the air expands it further cools off. At that point you have very cold air that can be mixed with the hot pressurized air from the engine to get the desired temperature."
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7x0sde | With car engines, what is it (X-amount) litres of, exactly? | Engineering | explainlikeimfive | {
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"It's the volume of the engine cylinders. So if it's a 2 litre engine and has four cylinders, each cylinder will have a volume of about 500cc.",
"It is the size of the engine cylinders. Volumes below 1 litre are usually expressed in cubic centimeters (cc), which is equivalent to 1 ml. Bigger cylinders allow you to take in and combust more fuel, giving more power at the cost of more fuel consumption and emissions. Modern technologies like turbochargers can make a small engine more powerful, but in general the bigger engine size has the most power. Also for the same size, petrol engines consume more fuel than their diesel counterparts. [Source]( URL_0 )"
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7x1bwj | Why are printers always f***ing up? | Engineering | explainlikeimfive | {
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"Yeah it's the worst. I see it as the most major part of a PC setup that has multiple large moving parts and consumables (ink and paper). They are typically infrequently used too which means ink dries etc. I know that doesn't answer the technical side of it, but that's what I think makes them unique and problematic."
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7x1gbz | How does a car know to activate the anti-lock brakes when it is sliding? How do anti-lock brakes release the brake if the user is still pressing it? | Engineering | explainlikeimfive | {
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"Speed sensors detect rapid deceleration in tires to see if they're locking up. The ABS controller then opens/closes valves that control the brake lines to increase/decrease pressure on the brake so that the tire keeps accelerating (doesn't lock up). You'll feel it yourself on the brake pedal as a sort of \"pulsing\".",
"The anti-locking system depends on sensors. The car measures the rotational speed of all four wheels, constantly. If you hit the brakes, the car will start to slow down. But, here is the thing, the wheels will slow down EQUALLY. If one wheel accidentally finds a patch of ice while you brake, it will slow down faster than the other wheels. And that is what the anti-locking system detects. Once it has detected that a wheel is locked (i.e, because it's not rotating despite that the others are still slowing down) it starts to control the brake of that wheel individually. In pulses. Releasing. Braking. Releasing. Braking. Releasing. Braking. The whole idea is that if the wheel starts to turn again, it will have more braking power once the brakes get on again than it had being locked. Thus making it safer. Well. Somewhat safer, anyway. But accidents happen because people are sort of on the wrong side of their margins, so sometimes this is what it takes to get back on the right side again. The same sensors are also used for the anti-spin system. Again, measuring the rotational speed of all four wheels. This time, they measure when the engine applies force to the wheels. If you loose traction on one or a few of the wheels, the car will swiftly and automatically force down the rpm in an attempt to give you back control of your car. Because, you know, that is literally how you do it if you do it yourself too. Let go of the gas and hope for the best. The only difference is that the car can detect it long before you feel it yourself so the car has reacted long before you can yourself. EDIT: spelling"
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7x3niu | Has there ever been a serious attempt to unify the types of electrical plugs worldwide with a universal standard? | Engineering | explainlikeimfive | {
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"Not really. There really isn't a good reason to do it -- such a move would be incredibly expensive with little benefit. Imagine if this happened, and you were told that all of the electrical outlets in your house had to change, and all of your appliances had to be modified to use the new standard. You're told this will cost about $500. What benefit would that bring to you? How is your life better, in any way, for having spent that money? It would only be useful for people who travel internationally on a regular basis, and we deal with that with a $20 converter in our bag.",
"No because the electric grids are fundamentally incompatible You at least need different plugs for 120V 60 Hz and 240V 50Hz plus all the variations from them(3 phase, split phase, twist lock, high current) Once you're already going to have a dozen versions there's no sense in standardizing The best solution has been detachable power cords so you just get one for your country",
"That would be a serious undertaking and which standard would you use? If you used the US standard then the rest of the world would have to pay to adapt all of their electrical systems to match. If you used the standard from Europe, then everyone else would have to pay. It was a tremendous deal for power generation facilities in California to switch to a national standard, imagine that on a global scale and you realize the immensity of the problems that would entail."
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7x5vl0 | How does Velcro retain its strength through continuous use? | Given the basic hook-loop mechanics, I would think it would become significantly less effective extremely quickly. | Engineering | explainlikeimfive | {
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"There are a couple factors that explain this. First, most loops don't find a hook each time the Velcro is sealed. Only a minority of the loops need to find a hook for the Velcro to hold. Second, not every loop tears when you rip apart the Velcro. There are many times more loops on the one side than there are hooks on the other, so this combination of factors gives Velcro an appreciable use\\-life. If you notice your Velcro is getting less sticky, you'll get more bang for your buck by trying to clean lint out of the hook side first."
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7xcc58 | Why can't tech companies make copiers that don't jam? | Engineering | explainlikeimfive | {
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"There's no such thing as a machine with moving parts that never jams. Besides the condition of the paper you put in, there is wear and tear on the components. But certainly some copiers are more reliable than others, which is a selling point.",
"There was an article all about this problem recently that went very in depth on the problem: URL_0 . The short version is that paper is an organic product that varies wildly in properties, printing is a complicated precision operation, and the paper has to travel a very long path, and when you combine all that together it's very difficult to ensure that the paper makes it through every single time without fail."
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7xdgnv | Why does road construction take years to complete? | Engineering | explainlikeimfive | {
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"Politics. Union workers. Manufacturing time. Weather. Laws not allowing you to work during certain times of the day. Cooling time. People messing around on the job. Errors. Source: I worked for an architecture agency. Always annoyed by exactly this."
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7xgx22 | Why was hitting 400km/h so hard to achieve in this age of technology? | I don't get why it is so hard to make a car that would be able to hit 500 km/h. Or 600 etc... Besides the tires can not hold the force, what other aspects make this task so hard for automobile factories? | Engineering | explainlikeimfive | {
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"There's no issue at all with making a VEHICLE hit those sorts of speed. But that's a vehicle without wheels, clamped to a rail. Just mount a gigantic rocket on the back end of 'er and there ya go. The issue starts to occur when you specify that the vehicle has to be a CAR. First, there's the mode of power. Drives that are based on internal combustion engines have many more moving parts than the simple action/reaction chamber of a rocket. There's pistons, a crankshaft, and a transmission system that connects them and translates their \"up and down\" movement into a rotational movement some feet away and at a different angle. That introduces a great many more points of possible failure. > [I used this online calculator for the analysis about forces on the car.]( URL_0 ) It's not so bad when your wheels are rotating about 12 times a second (at 100 km an hour for standard cars)... but multiply that by 5 to get 60 times per second at 500 kilometers per hour, and those gears are going to be under amazing stress. Then there's the wheels flying apart. At 60 miles/100km per hour, a tire's surface experiences 213 gravities of outward force. A normal tire can take that. But go to 500kph... and now your tire surface outward force is *five thousand gravities*. For most materials, if those tires touch an irregular surface or have any sort of wobble at all, the driver's dead from shrapnel. Next let's talk about the car's staying on the ground. The tiniest bump at 500kph will launch the car like a rocket, and air resistance will tear it apart. And finally... cost. Overcoming all of these challenges would cost way way way more money than they'd ever get back from sales of the resulting vehicles, let alone any place to safely drive them. Aside from the occasional ultra-rich collector, there'd be no market for them.",
"The problem isn't the speed. 400 km/h was broken in 1932, 500 km/h in 1937 and 600 km/h in 1947. The problem is making a car that can hit those speeds and still meet the requirements of automotive regulatory bodies. Emission restrictions, safety, usability, and noise levels all add their own unique engineering and financial challenges for auto manufacturers. Does your car cost over a million dollars or pounds or euros? Guess what? You have to smash 5 or 6 of them to perform impact safety testing.",
"Think of it the other way around. Think of how much engineering would have to go into a building that would need to withstand 500 or 600KM/h winds. The air at that kind of pace has a **lot** of force in it. It presents no different a barrier than you trying to punch your way through it at that rate. It's air resistance at high speed that means spacecraft have to have thermal shielding and perform the be-you-own-meteor re-entry on the way back to Earth. As you increase in speed, air resistance and drag become a real pain in the ass, to put it mildly. That's the main thing stopping progress. With the Bugatti Veyron, the design of the door mirrors meant the difference between stability and not. The door mirrors. Think about that. As you get faster and you have to punch through more and more mass of air per second the design of the car becomes beyond critical. It's really not an easy thing to solve.",
"The faster you go, the higher the resistance from wind, from internal friction (like your axle bearings), and from external friction with the ground. You need more and more power to overcome it. Notice how much easier it is to make an airplane go this fast -- that's your clue that the problem is with having wheels that move along the ground.",
"I think it's mostly because there is no demand for such a vehicle. Public roads dont allow these kinds of speeds, and even *race tracks* can support this kind of speed. I think the current land speed record is over 1,200 kph. But it is far from a road worthy vehicle. It uses jet engines, solid wheels, and requires an entire team to prepare a single run. The Hennessey Venom F5 is in the works and will supposedly approach 500 kph. But it's really kind of pointless. There are cars like the Hennessy Venom GT, Koenigsegg Regera, or Bugatti Chiron that max out around 450 kph. But it is *extremely* rare that the situation presents itself. You need an enormous, straight, ridiculously flat road, the fuel and tires deplete at an enormously accelerated rate. So most of those cars will never actually see that speed. It's basically stat porn. You have the privilege of saying you have the fastest car or that it's capable of these speeds, but most people never intend to actually attempt it. So when you consider the special gearing, cooling, aerodynamics, tires, braking, etc that is required to do this, it ends up being kind of pointless. The vast majority of hyper car customers value lap times way above top speed. So that's what most manufacturers try to achieve. But even at that, most of those cars never lap Nurburgring. The owners just want a car that can produce shocking levels of acceleration from 0-240 kph or so. And top speed gearing is often at the expense of acceleration."
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7xi5lo | Visually, rocket engines look simple to design, so why are they so difficult to produce? | Looking at pictures of rocket engines there doesn't seem to be too much to them, but given that companies/states are still using those produced decades ago, one would imagine they're not that simple at all. | Engineering | explainlikeimfive | {
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"Simply put you have thousands of pounds of liquid oxygen and kerosene (or other fuel) arranged in pressurized tanks that are brought into contact and burned at enormous rates. This creates massive amounts of heat and pressure that has to be directed out the nozzle to produce the thrust of the rocket. All the while the very combustion is creating massive vibration that is shaking every part of the rocket. Thus you have to construct a vehicle filled with explosive propellants, that are burned precisely in the right proportions. The rocket has to be strong enough to withstand the vibrations and also as light as possible to maximize payload. The current rockets for the most part were developed after many massive failures and once a working solution was found in a rocket design, then payloads were developed to match the capabilities of the launch vehicles. Probably the worst such failure was the Soviet Nedelin Disaster.... URL_0",
"Rockets are simple! You need fuel(fuel tank), a way to get the fuel to the place you burn it(turbopump), a place to burn fuel(combustion chamber), and a nozzle for the hot gasses to expand in(the bell) The trick is making a combustion chamber that can withstand 20 MPa while at a toasty 3200 C! The core principles are simple, building a big rocket engine is much more of a material sciences problem than a fundamental layout challenge",
"It’s really an engineering problem: it’s hard to make one that stays in one piece and goes where it’s supposed to.",
"There are several problems: * Pumping: You need lightweight pumps that can move enormous amounts of reactive (and possibly cryogenic) liquid. For large rockets they partially combust the propellants and drive a turbine at high speed with the (hot, reactive) gas, and try to run the pumps off the same shaft (tricky, because of cavitation issues). The pumps and plumbing must be also robust with respect to the motions of the vehicle (unexpected oscillations in the plumbing were part of why the soviet moon rocket N1 failed). * Combustion Stability: The combustion phenomena are prone to feedback. The injectors need to be designed in such a way as to prevent vibrations from developing and destroying the engine. AFAIK this is very much a trial and error process. This was famously a problem for the V2 and the (Saturn V booster) F1. Basically each time they scaled up the technology. * Cooling: Good rockets run at absurdly high temperatures and flow rates. The chamber and throat of the engine are usually actively cooled by running propellant through passages in the engine wall. This is an easier problem than the others, with less trial and error involved, but still annoying. IIRC, the fuel for the V2 had to be watered down to be a better coolant, and on the N1 part of the reason the soviets used super-toxic propellants is that the kerosene they had available wouldn't work to cool an engine for prolonged periods.",
"It's simple to design a working rocket, a rocket that's efficient enough to make it to space is a different story. You could just do the math and make an aluminum pipe of sufficient strength, stick a nose cone on it, weld in some plates to mark off the tanks and then build a simple rocket motor and stick it on the end. That's a simple design right there, but I'll tell you right now it won't have the delta-v to make orbit. The cheap way of designing a rocket results in something with the weight of a ship. To make an efficient rocket you need to do lots of simulations and lots of studies to determine exactly how much strength you need and where, and then you design the rocket to have just a hair more strength than needed. This is required to get the weight down to the point that you can make orbit without some gigantic rocket.",
"The hardest part would be the turbopump, which has to tolerate extremely high temperatures at one end, and extremely cold temperatures at the other, while spinning extremely fast. It's basically a fuel pump and engine that's several thousand to tens of thousands of horsepower, just pumping fuel into the main combustion chamber. In some designs the hot end is also in a highly corrosive environment(hot oxygen) The combustion chamber has to be designed to burn all that fuel and oxidizer uniformly, without melting or corroding. Basically, the design is relatively simple, but you're really pushing the limits on what the materials are capable of. You can make a simple rocket engine in your garage, but making a high performance low weight one is an entirely different beast. If you want a bit of perspective, the F1 engine on the Saturn V is the size of a car, and its fuel pump alone is something like 50,000 horsepower.",
"Partly it's because of how few are produced. Internal combustion engines are also very complex, and designing one that will last work reliably for 100,000+ miles with minimal maintenance, that costs less than $1000, will cope with a wide range of operating conditions and can meet modern fuel efficiency and emissions standards has taken a vast amount of work. Cumulatively, much more than rocket development. Designing a new car engine from scratch is as expensive and difficult as designing a new rocket engine. The difference is that millions of them are made so the development cost is spread over more units. As far as I can tell, the cost of a single rocket engine is not particularly high when compared to large aircraft engines. The overall 'difficulty' of making a rocket engine is probably lower. The major difference is that rocket engines are only used once (or sometimes a few times) for a total operating time measured in minutes while aircraft engines operate for many years. Basically, all engines are difficult. We're just very good at making the common ones so it looks simple. Rocket engines aren't as well developed as car engines or aeroplane engines.",
"Essentially, a rocket is a controlled bomb. A cat looks like it would be easy to manage because they're cute and small, but have you ever tried giving one a bath?",
"The hard pices are not visible: The turbo pumps that deliver the fuel & oxidizer look like your cars turbo charger on the Hulks steroids; the \"shower head\" combustion chamber mixer where fuel and oxidizer are injected into the combustion chamber for burning. These components and many others are insainly complex do to presurization, metalurgical interactions, thermal gradients... It's really, really hard engineering."
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7xm0c3 | Why do some airplanes have a third engine at their rear stabilizer? | Engineering | explainlikeimfive | {
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"Are you referring to trijet planes with a full rear engine in their tail like the [Dassault Falcon 900]( URL_0 ) and the [MD-11]( URL_1 )? There exists a standard called ETOPS which defined how far away from a backup airport a plane is allowed to fly to enable it to make a safe landing in the event of an engine failure. Early on, many planes could only travel for about 60 minutes with a single engine so planes with just 2 engines weren't allowed to be more than 60 minutes away from an airport which meant they couldn't cross the ocean. This is clearly an issue, but strapping a third engine in the tail means that they can travel significantly further from an airport because the odds of a double engine failure are much much lower. These days engine failures are far less common and engines are significantly more powerful. Most planes have ETOPS ratings of 180 minutes so they only need to be within 3 hours of an airport which is totally doable even when crossing the ocean so they don't need the extra engine which adds weight and drag just so they're allowed to cross the ocean. Some big planes like the 747 have multiple engines because they needed the thrust, but most planes get plenty of thrust from two engines these days and with regulations permitting them to operate with just two engines there is no reason to add a third",
"> Why do some airplanes have a third engine at their rear stabilizer? That is the APU or Auxiliary Power Unit. It is a small jet-powered generator which is started by battery or hydraulic accumulator and then provides enough power to start the main engines. It can also provide power to keep the cabin comfortable during boarding, and in the rare event of total engine failure can provide power to critical systems."
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7xmu5v | How do they make some new muscle/pony/sports cars sound like a carborated "gas guzzling" car, and what are the advantages/disadvantages to this modification? | Engineering | explainlikeimfive | {
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"In some cases they intentionally reduce the efficiency of the muffler. In same cases, amazingly, they even add an actual speaker that plays a recorded fake engine noise inside the car. I wish I were joking.",
"The sound has nothing to do with how the engine receives its fuel air mixture. (Carborated vs fuel injected) It has to do with the exhaust. The quicker the air and fuel come in, the quicker the exhaust must get out. Generally speaking less restriction means more noise."
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7xu2fw | Why do your earphones make noise when you walk through thief detectors at the doors of libraries? | Edit: I am not stealing headphones. I walk in and out of the library regularly while listening to music. | Engineering | explainlikeimfive | {
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"Those detectors use magnetic sensors to detect specialized tags placed on or in big-ticket items. If you've ever seen one of those stickers with a big metallic spiral on the back, that's one design for them. Meanwhile, speakers work by using small electrical currents to turn an electromagnet on and off incredibly quickly, each on/off cycle corresponding to a single wave of sound produced by the speaker. Magnetic and electrical fields are really the same thing (hence the word electromagnetism), and as a result sufficiently strong magnetic fields can cause a current to move through a wire passing through them. The presence of the magnetic field essentially transforms a tiny percent of the energy involved in moving the wire into electrical energy. This electricity is very weak, but earbuds also work with tiny electrical charges. Try moving through those detectors at different speeds to see how it affects the sound. I have no idea what it will do, but I suspect faster speeds will make the sound higher pitched.",
"You can induce current by moving a magnetic field past a conductor. Electricity and magnetism are the same thing, and the magnetic field causes the negatively charged electrons to move, which is electricity. The detectors use a magnetic field to induce a current in a tag, which essentially powers it remotely to send a signal back saying that it can detect. The detectors induce a current in any conductor. Your headphones too. This powers the magnets that move the speakers.",
"I have a similar issue at work. Our copy room has a light that turns on when you enter. When I get close to the sensor, I get all sorts of static from my hearing aids picking up the em waves."
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7y3q9a | The differences between the various specialties within Engineering. | Computer vs Software vs Mechanical vs Electrical vs Civil vs Environmental vs others? | Engineering | explainlikeimfive | {
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"They apply engineering processes to different parts of the system's design. Computer = processing hardware Software = code Mechanical = moving parts Electrical = circuits Civil = built structures Environmental = natural structures",
"The ones you listed: **Mechanical Engineers** are concerned with designing physical thing. They'll study the properties of various materials, how mechanisms work, fluid flow, etc. They're what many people would think of as just an \"engineer.\" **Electrical Engineers** work with electricity. This may be power transformers for electrical energy distribution, down to circuit boards going into just about anything that runs off of electricity. **Computer Engineers** are a specialization of electrical engineers, just different enough to get their own heading. They deal with microelectronics, often dealing with circuitry at the silicon level. They're the ones who design the chips that electrical engineers put on their boards. Often an electrical engineer can do a computer engineer's job and vice versa. **Software Engineers** write software that runs on computers. All engineers have to study some programming, but software engineers will specialize in it. **Civil Engineers** are all about building the building blocks of society. They study how to construct buildings and roads. They design traffic systems to efficiently get people from A to B. **Environmental Engineers** are closely related to civil engineers but specialize in how civilization maintains the environment. Managing water resources is a big part of this. Some other fields: **Aerospace Engineers** are closely related to mechanical engineers, but learn about aerodynamics more than general fluid flow and learn more about aircraft design. **Controls Engineers** work with control systems--anything that has some value that it tries to achieve and maintain is a control system. This could be a thermostat in your home (very simple) or the system that keeps a fighter plane from going unstable and crashing. Often controls engineering isn't a separate degree but it is a separate job title. **Industrial Engineers** work with human factors like ergonomics. They also look at processes and make them more efficient. Toyota is famous for their processes; that's industrial engineering at work. **Chemical Engineers** work with chemical plants (they are *not* chemists, although they do take chemistry courses). They will take a process that have been proven on a small scale and ramp it up to a massive scale then monitor said plant and make sure it doesn't screw up. **Petroleum Engineers** are a mix of chemical engineers and geologists. They learn all aspects of petroleum extraction and refining. **Architectural Engineers** are closely related to Civil, focusing primarily on the design of buildings. They will go into more detail on building components (e.g. learning more about HVAC than a civil might). **Robotics Engineers** (that's me!) are the intersection of mechanical, electrical, controls, and software engineers. They design robotic systems. And finally, **Systems Engineers** are engineers with a wide enough body of knowledge to manage a project that covers a wide range of different fields."
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7y43jv | How do spray bottles produce a mist | How do spray bottles produce such fine mist instead of just spraying water everywhere? What makes something spray super fine and something less fine? | Engineering | explainlikeimfive | {
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"The holes in a mister are very small. As liquid gets pushed out, the stream tends to break up because it’s so thin that surface tension is significant. So the stream quickly becomes tiny blobs of liquid suspended in air: a mist."
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7y8kn2 | How do they remove the temporary lines when they are finished with road construction? | Engineering | explainlikeimfive | {
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"text": [
"They used a torch to burn them off on the road by me a couple of months ago. The temporary stripes were tape-like and they shriveled up under the flame.",
"They have a machine that comes through and just sands the paint off, ever notice once construction is done somewhere it looks like there's tiny rumble strips in some spots, that's where the paint was"
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7yhc1p | How does a car engine keep working when the car is still. | Like shouldn't you have to unclutch? | Engineering | explainlikeimfive | {
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"manuals do have to push the clutch in of course. automatics have a torque converter, which the easiest way to describe it is 1 fan blowing on another fan. Except they look nothing like an air fan and they are submerged in fluid. but at low RPMs, 1 fan can spin while the other stays still. Or 1 can spin faster than the other, multiplying the torque allowing the engine to spin faster than the transmission side does. at a certain RPM/Speed, modern transmissions will lock the 2 fans together to minimize the inefficiency lost.",
"In a manual transmission you do press the clutch to remove the engine power from the rest of the drive train."
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7yjtf1 | cities build into/inside a mountain | I was wondering how people managed to build house/cities inside of a mountain.Especially without the help of modern tools. | Engineering | explainlikeimfive | {
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"People have been building stuff out of stone since, well, the stone age. This involved cutting rocks into smaller rocks (or sometimes not that small). Look at the size of the bricks they used for the pyramids. If you can cut bricks out of a mountain, you are obviously able to cut *into* the mountain. You don't need modern tools - you just need time and people to do the work.",
"If you're talking about places like Mesa Verde, the chiseled out area the city's sitting in was largely already there. We don't get massive build-in areas that aren't mines until much latter."
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7ynxz4 | What is a “defeat device”, and how does it work? | I’ve just started watching Netflix’s Dirty Money, and the first episode on Volkswagen’s emissions scandal revolves around defeat devices, how the NGO that discovered the lies couldn’t believe at first that the company had put in place such a device, and that while tests showed that the cars complied to emissions standards in the labs, the same can’t be said when they were tested on roads (I hope I’m right). I just need to wrap my head around the concept. Thanks! | Engineering | explainlikeimfive | {
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"In this case, it's just a device that senses when an emission test is taking place. When it senses the test is taking place, it alters how the car runs to conform with emissions requirements. In practice, this usually makes the car accelerate less aggressively. So when it isn't sensing that a test is taking place, it runs in a way that does not conform with emissions standards or even its own test result. It's a way of gaming the emissions testing system.",
"It's the computer changing the engine configuration to change emissions, one for the regulated testing and one for normal operation. I just watched a documentary on this last night. What VW did was recognize if the steering wheel was being moved (because the wheels are strapped down when on the dyno in testing), if this was the case the computer would go into super efficiency mode, which in turn would reduce emissions and voila, they pass. However, in normal driving mode the steering wheel is moving, the computer (\"defeat device\") would go into normal operating mode and significantly raise emissions, on average by over 400%. Once caught they basically went into defense mode for like 5 yrs, they would tell the testers \"try this\" \"do this\" \"check this\" etc, and each round would raise more questions. Eventually it got to the point that the EPA said either admit your defeat device or we will not approve ANY of VW's next year's cars, so the truth came out. It is interesting though in Europe this is common practice. There is a loop hole in the rules that says that the computer is not a defeat device if it is \"protecting the engine\". Which in this case it technically is (apparently they are pretty brazen with how fast and loose they play with the loop holes there). The super efficiency mode wreaks havoc on the engine and exhaust systems, so that is why they turn it off in normal operation. I also found it interesting that in the EU most member states will take the results of the first state to test a car. So the manufacturers will actually shop around for which state they want to test their car first. If France is being nitpicky lately, they might try Spain, etc."
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7yr8rj | How do cars have rain-sensing windshields? | Engineering | explainlikeimfive | {
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"text": [
"They are not actually rain sensing. There is a small laser that is projected up the window. If the light is broken before it reaches the top, it assumes that water is on the windshield, which activates the \"rain-sensing wipers\"."
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7yy7o7 | why do combustion engines hum instead of sounding like a high rate of fire machine gun? | Engineering | explainlikeimfive | {
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"The more cylinders you have firing, the smoother the sound. Take the muffler off of an old lawnmower and listen",
"They *do* sound like a machine gun. But we attach a *muffler* to the exhaust, which removes the sound.",
"They're idling enough cylinders quickly enough that the sounds blend together. Most modern cars are well muffled too, and idle quietly. Cars with extremely agressive camshafts and large cylinder bores built for drag racing do have an audible chop when idling. It's quite obvious in top fuel dragsters, you can even see the individual exhaust plumes from each cylinder at idle.",
"This question needs a few things to adequately address it. 1. The way combustion works in a car engine versus in a firearm. In a firearm, the charge is ignited and accelerates a projectile to supersonic speeds (unless you are firing subsonic ammo), which produces a loud snapping sound as it breaks the sound barrier. Anyone who has shot subsonic rounds with a suppressor can tell you just how much of a difference not breaking the sound barrier makes. In a car, cylinders fire off in a specific firing order, often designed to limit the vibrations of the engine itself. Commonly, this can be seen (or rather heard) in the difference of the sound of cross plane versus flat plane V-style engines, with the former giving you more of that muscle car burble and the latter sounds more like a naturally aspirated Ferrari. Add to this the fact that most cars are tuned in such a way there is never/rarely any unspent fuel dumped into the headers of the exhaust, so it never backfires. 2. Cars are designed to muffle sounds, whereas unless a gun is suppressed, it cares far more about the flash and recoil of the muzzle than the sound it makes. While cars have mufflers made out of materials that specifically channel the air flow in such a way that the sound is canceled out as much as possible as it exits the muffler. Without the muffler, your car engine will sound completely different. If you have ever stood next to a straight-piped car, you would notice it instantly once it is turned on or if the gas is tapped. 3. The rate at which things are moving and producing sound. This ties into the first point in that in most cars, the cylinders are all firing in a predetermined sequence, often optimized for a low-vibration, smoother operation. Compare that with automatic fire on a machine gun where there is no such consideration. Now compare this with an incredibly high rate of fire weapon such as a Gatling gun and you will find that it emits a very steady sound, something like \"BRRRRRRRRRRRT!\" instead of the usual sound associated with machine guns. Keep in mind that many machine guns can fire at a relatively fast rate such as the MG42 which was nicknamed Hitler's Buzzsaw. **TL;DR:** Cars do not have to accelerate a projectile to supersonic speeds, car engines have multiple cylinders designed to fire in a specific order to lessen vibration, cars are required to have mufflers designed specifically to cancel out sound waves. You can make a machine gun hum, just get yourself a MG42 and lay down on the trigger for a few seconds."
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7yz31l | Why do some street lights buzz? | Engineering | explainlikeimfive | {
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"text": [
"For the same reason that some fluorescent lights buzz. They contain a circuit to generate a high voltage (because that is needed to pull electrons off the gas in the bulb and ionize it). That high voltage circuit is oscillating, and the rapid current changes cause rapid magnetic field changes. The leads to [magnetostriction]( URL_0 ), where the magnetic field changes cause some materials to slightly deform rapidly. They literally get longer/shorter or bigger/smaller as the magnetic field effects them. Just not by very much. Or they can simply get moved back and forth. That can cause buzzing, whining, rattling, etc. It might not even be the thing undergoing magnetostriction that makes the noise; it could be shaking something else and that thing makes the noise. The same phenomenon can cause other gizmos to make noise. Transformer whine/buzz, capacitor whine. etc."
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7z1knl | why aren't more efforts made to go deeper into the earth? | Engineering | explainlikeimfive | {
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"What for exactly? Its expensive and dangerous (high pressure and heat). We already do it for mining because that is literally the only time it makes sense fiscally",
"Going deeper gets exponentially more expensive, and most resources we gather, like oil, aren't found at much greater depths. There are some scientific projects that deal with the sole goal of going deeper inter the earth's crust, the current reccord being about 12km or 7.5 miles deep (wells for oi land gas I'd say get as deep as 2 miles on average). They made some geological/biological discoveries, but nothing practical, even for geothermal energy wells are preferred to be shallow to reduce the cost of pumping fluids."
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7z1r2p | Why do airplane engines rev up so fiercely upon landing? | Engineering | explainlikeimfive | {
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"That's the sound of the reverse thrusters deploying which creates the additional sound and sensation of engine power increasing.",
"What you're hearing is probably the pilot deploying thrust reversers on landing. Even with the engines at idle reverse they can sound quite loud in the cabin since the thrust is now being pointed forward and to the side rather than straight back.",
"Or, if you're talking about an aircraft carrier, the engines revving up so fiercely as soon as the wheels hit is actual forward thrust, which will be very helpful in case you miss the wire that normally stops you. You don't have room on a carrier deck to stop yourself, and by the time you could notice that you missed the wire (or if the arresting gear otherwise fails), you'd likely be too late to recover. So, every time you land you pretend that you're just going to touch down and then take right off again, and consider yourself lucky if you stay down. Source: probably watched some show on PBS 30 years ago.",
"Those are the thrust reversers. On landing, planes almost always deploy the thrust reversers and increase the throttle a bit to slow down the aircraft faster and use up less runway. The reversers are just ducts on the engines that redirect the flow forwards. If you have a window seat near the wing (or the rear in case planes with tail engines) you can usually see them deploy just after touchdown.",
"What's even cooler is standing 50 ft in front of an engine at full power with thrust reversers deployed. Shakes your insides more than standing next to a subwoofer at a concert.",
"This video explains it well in about 5 minutes time URL_0 Good channel in general (relating to planes, anyhow).",
"There are flaps that can be opened and closed around the engine to reverse the engine thrust. They use this, along with air brakes and brakes on the landing gear, to slow down so they use less runway and reduce wear on the wheels."
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7z3tom | What's the difference between welding and soldering? | Engineering | explainlikeimfive | {
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"text": [
"Soldering uses a (relatively) low temperature to melt solder to join the two metals. The metals themselves don't melt and remain separate. In welding, the metals that are being joined melt and resolidify around the weld, effectively forming a single piece of metal after the weld has been made. Although it's common to use a filler material (often in the form of a consumable welding electrode) it's not a strict requirement.",
"Get two metal bars, melt a bit of chocolate and let the chocolate set to hold them together. This is like soldering. The chocolate is easy to melt but not as strong as the metal. Get two metal bars, melt them together. This is really strong but you have to make it very hot to melt the metal."
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7z5y88 | Why do so many mobile phone manufacturers sell their devices in different regions of the world with different processors? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"It's typically a licensing or patent issue. If you're referring specifically to how Samsung uses either Exynos or Qualcomm, you can read about that more in depth here. URL_0"
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7z8rxv | Why does wiring two speakers in parallel reduce the resistance by half? | I'm currently putting together an audio system for my car, so I've been learning a lot about basic wiring and electric stuff. One thing that I've found I don't really understand intuitively is how resistance works. I could sit down and learn the equations and do the math I'm sure, but I'm asking for a more basic answer than that. Let's say that I have a single speaker at 4 ohms, and my amp is giving it 300 watts at 4 ohms. But if I add a second speaker and wire them in parallel, the resistance is now 2 ohms, and the amp is now capable of giving ~600 watts total, or 300 watts each. I don't understand how you can add a second speaker and they should each still perform as well as a singular speaker. My intuition is that each should now be receiving half the power. | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"If you have a sink full of water and one open drain at the bottom. Opening a second drain will cause the sink to empty twice as fast. As long as you can fill the sink fast enough, both drains will drain at full speed. the Amp you have needs to have enough capacity to power both speakers at full blast. as long as it does, both speakers have full capacity",
"Electrician here. It's about the path the current flows through. In series the current has to pass through every resistor. This means all the resistors add together for the total resistance. In parallel circuits the current has paths around all the resistors so the total resistance can not be the sum of all the resistors. The math tells you precisely how the numbers work but basically series circuit resistors are added together as a sum. Parallel resistors are a division of the total.",
"Electrical engineering student here - I'll try my best. So what's happening here is by wiring them in parallel, you've created two equal loads that the amp is delivering the same amount of voltage to (parallel circuits recieve the same voltage, not always the same current). Because power(watts) = v^2 /r, both loads, your speakers in this case, *should* recieve the same amount of power under ideal conditions. In practice however, it's dependent on your source. Let's look at your example. 300w into 4ohms, then we add a second one in parallel. The amp is going to *try* to deliver 600w because it's attempting to deliver the same amount of voltage and current (v=i(current)*r) to both loads. If the amp is capable of delivering 600w or more, then it should work fine. If you have a 300w amp, then you'll run into issues, and there won't be enough power for both speakers - I think because it won't be able to deliver enough current. Does that make more sense?"
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7zb1f4 | Why are electric cars able to speed up so quickly compared to a car that uses gas? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"A electric engine can provide full torque (the force that cause the rotation of the tires) from then start to rotate from stationary. A combustion engine have low tork at low RPM and max torque and a quite high RPM To be more correct the electric engines torque will change when the rpm changes but is it a flatter curve then for a combustion engine so in most/all RPM depending on the combustion engine a electric engine have more torque and therefore faster acceleration"
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7zbsnh | if warm light is better for the eyes at night, why are most new LED streetlights a bright white color as opposed the orange sodium lights? | Engineering | explainlikeimfive | {
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"text": [
"The new LED lights that have appeared in the past few months are bright, and they’re white, but the color temperature is a warmer, more neutral white, rather than a harsh, almost blue-white that I think you are describing. The light that Is first mentioned yields far better color rendition, meaning that colors appear to the human eye to be far closer to how they would look in daylight than in the harsh orange glare of a sodium light. Even if the lights you are asking about are that almost blue-white that is described above, the color rendition would be **far** better than sodium lights."
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7zf76p | How do cold and hot water taps work? | Engineering | explainlikeimfive | {
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"text": [
"I'm not entirely certain what you are asking here, so from a basic standpoint: If you open the cold tap, it opens the way for water from the cold source (pipes in the ground). If you open the hot water tap, it opens the way for water from the hot source (like your water heater). If you open both, then you get a mixture, with the final temperature being determined by the ratio of hot/cold you are mixing. The initial water out of either may be closer to room temperature, as it is sitting in pipes exposed to the environment inside your home, rather than in a water heater or in the ground."
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7zg471 | If my router has 200Mbps and has 4 computers attached via Ethernet, does it split the 200Mbps between all Ethernet cables and has a separate 200Mbps for Wifi, or does it split the 200Mbps between Ethernet AND wifi? | Engineering | explainlikeimfive | {
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"The 200Mbps is the maximum rate at which the router can transfer data. So whether the data is being sent via ethernet or WiFi, the total maximum data rate is 200Mbps. If only one computer is using the Internet, then that computer can use up to 200Mbps (though it usually doesn't, unless you are streaming high quality video or something). If you havr two computers using the Internet, each computer can use up to 200Mbps, but eachis limited by how much traffic the other is generating. This applies for three and four computers as well. This is becauae of how the TCP and UDP protocol work. Each use packets, which is just small chunks of data that is manageable by most routers. TCP (ie file download) makes sure each packet arrives in the correct order without corruption, but UDP (ie video streaming) doesn't (because who cares if a 1 of a billion pixels is wrong, or if 1 frame of 60 fps is lost). If all computers are sending and receiving just as many packets as the others on the same router, then the data rate will be shared among all computers equally. This is because the packets have to be temporarily stored on the router before forwarding, meaning that sometimes the buffer gets filled and can't hold anymore data. Once packets are lost, each TCP connections slows its transfer rate, to give the buffer time to make more room. This phenomena is call TCP Sharing, which also means if one computer on the network is downloaded 10 files and the other computers are only downloading 1 or 2 files, the computer with more connectiona/downloads will get most of the data rate. UDP doesn't care about slowing down, so when the buffer of the router gets full, the packets are lost forever. YouTube gets arround this by taking into consideration whether or not you receieved the UDP packets (through ACKs, or acknowledgements, part of the protocol), and sends less high-quality data (aka less packets or packet size) so that you still get some kind of video stream. This is what is happening when you are watching a video and the quality suddenly drops. TL;DR 200Mbps is the maximum rate for data leaving or entering your house, and it is split evenly among the number of connections (ie how many file downloads) each computer is using, not split evenly among ethernet or WiFi nor split evenly among each computer intentionally.",
"The 200Mbps is for all devices. So one device could take up the entire 200Mbps over wifi or ethernet.",
"It depends on the router somewhat. Usually that is throughput between the LAN and WAN. That means from machine to machine on the LAN side you may get 200 or you may get 100. The WiFi is counted towards the total throughput. So let's say you had 3 computers, all downloading, 2 wired in and one on the WiFi. The one on the WiFi is limited by WiFi speeds, so it'll max out at 54 Mbps (IIRC), but realistically you'll see ~40 Mbps. The remaining 160 Mbps will be split by the two wired machines. Then multiply everything by .8 for network overhead and other inefficiencies. Now, your router might have advanced controls, like Quality Of Service. That would let you prioritize or deprioritize specific machines, so you could tune that to get better performance on your pr0n/gaming machine, and worse performance for your Nest (or whatever low-priority stuff you have)",
"Network Engineer here. I'm going to assume that you mean you have an Internet connection at 200Mbps. That 200Mbps is split between all four computers. Each computer will be able to use ***up to*** 200Mbps of that bandwidth. If you begin downloading things on each computer, they will be fighting for bandwidth, and it will be split among the four computers - it won't be split evenly, either. If you mean the actual router itself, then that is most likely talking about the Wi-Fi connection being 200Mbps. If you are connecting via Ethernet, those Ethernet ports are dedicated 10/100Mbps or 10/100/1000Mbps Ethernet ports. Meaning, they will ***always*** provide the maximum bandwidth possible. Where it gets hairy is the Wi-Fi. Technically, you will each get a 200Mbps connection to the router over Wi-Fi, however, it can vary wildly due to the nature of wireless signals. The wireless signal can be disrupted by pretty much anything - microwaves, refrigerators, TVs, anything that has electronics, and thus, a magnetic field of its own. This is one reason why it's very difficult to fully saturate a wireless connection (i.e. it's very difficult to get max speeds off of Wi-Fi). The router itself will be able to provide maximum bandwidth to all connection types, since they are handled differently and it more than likely has the processor capability to handle it all. Once you start getting up into A LOT of connected devices, it gets pretty clumsy and starts slowing connections down, dropping clients, gives priority to the Ethernet connection, etc. I hope this helps!",
"This depends on what you're meaning by the 200Mbps speed. Is it your internet speed? If so the router can receive 200Mbps from the internet, and has to divide the speed on those devices. Every combination is possible (it doesn't matter if 4 PCs get 50Mbps each, or, one could get all of the 200Mbps etc). . Now your router has other kind of speeds as well. E.g. it may have 600Mbps Wifi and 4 Gigabit Ethernet ports. Ethernet: Every device that's connected via gigabit ethernet can receive and send 1000Mbps simultaneously without slowing down other devices. (that means PC1 can send 1000Mpbs to PC2 while PC2 can send 1000Mbps to PC1 and at the same time PC3 can send 1000Mbps to PC4, and PC4 can send 1000Mbps to PC3). . Wifi: Unlike ethernet wifi can only be used by one device at a time. So you could send a file from an ethernet device to a wifi device with 600Mbps. But you could only send a file from one wifi device to another wifi device with 300Mbps (Device 1 sends it to the router, the router sends it to device 2) (however there's an exception if one of the devices uses 2,4 Ghz, and the other 5 Ghz)."
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7zgx1a | Why does vegetable oil work as a substitute for traditional fuel in a diesel engine? | Engineering | explainlikeimfive | {
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"Diesel engines work by compression ignition; you add a small amount of hydrocarbon fuel during compression, it eventually combusts and the energy of combustion raises the pressure and drives the piston downwards, producing power. However, this autoignition isn't a magical property unique to diesel fuel; **every** hydrocarbon will autoignite at some temperature, and it just so happens that vegetable oils have similar energy contents, viscosities, and autoignition temperatures to standard ultra-low sulfur diesel. That being said, you typically want to run the vegetable oil through what's known as a transesterification process to lower the viscosity and make the fuel a little more stable, and help with cold-starting the engine. You add either ethanol or methanol to the oil, along with bases like soda ash or lye to keep the acidity down, and then mix it to produce fatty-acid ethyl/methyl esters (so, FAEE or FAME), along with glycerol that must be removed during processing. The FAEE and FAME products are known more popularly as biodiesel."
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7zk6v4 | How percolator style coffee brewers work. | I have a general idea, but my mind can't seem to put the details together for me. | Engineering | explainlikeimfive | {
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"Expanding steam forces the hot water up onto the basket of coffee grounds, where it drips down and mixes with the water in the pot. Once the steam has lifted the water, it rushes out and is replaced with more hot water. Some of that water boils into expanding steam and the process repeats."
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7zlmer | How Dubai's rotating skyscraper gets it's electrical and water supply? | Engineering | explainlikeimfive | {
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"The standard method for transferring electricity across a rotating joint is the [slip ring]( URL_5 ), in which sliding contacts on one piece runs along conductive tracks on the other. The standard method for transferring fluids across a rotating joint is the [rotary union]( URL_1 ), which is kind of similar: port holes on one part connect up with coaxial U-shaped channels on the other. However, I suspect this project is \"vaporware\" -- a sales pitch without the technology to back it up. Wastewater plumbing, in particular, is going to be a nightmare: it's going to be hell to keep liquid with solid chunks in it moving smoothly through a rotary union, and even worse to clean it out if the system gets clogged. It's not clear that they're using the standard methods listed above: the link /u/axz055 provides describes utilities that \"attach via flexible pipes to the building’s central core at various points \", which is just a shocking number of moving parts, valves, and seals that could fail. But none of the core technologies are described anywhere, we only have cool CGI animations of the final results. And even those don't make sense: the articles say that each floor will rotate independently at the owner's voice command, but the images show a smooth coordinated transition between each floor. Google Maps marks [a location in Dubai for the project]( URL_4 ), but looking at satellite images in Google Earth, I see apartment blocks being constructed on this site from 2016 through late 2017 that are definitely not a rotating skyscraper. The architect has never designed a building before, and the company building it seems to consist of three people. And most importantly, I found a bunch of articles describing the launch of the project from 2008, with construction \"set to complete in 2010\", then postponed to 2011, but never built. There are a bunch of identical-sounding articles from 2017, with construction \"set to complete in 2020\" -- just far enough into the future to sound plausible without having to show any progress. This could be real, but my bull*** senses are tingling. I don't know if it's fraud, a PR stunt, empty promises by people in over their heads, or what, but this looks to me like a classic case of designers working without engineers. URL_2 URL_3 URL_0",
"According to the wikipedia article, there’s a special system integrated into the core of the building which supplies clean water and carries out sewage, similar to an airplane refuelling mid flight. Based off that I’d assume that there would be “breakpoints” between the core pipes and the apartment pipes. The apartments would have a septic tank and clean water tank or something like that, and from time to time they’d have to rotate to certain fixed positions to cycle water in/out. As for electricity, that could be supplied to each apartment via wireless induction. Exact same concept as a wireless phone charger, except it would be constantly “connected” to the grid.",
"Electricity would probably work similar to the third rail in a subway system. That would be the easiest way. For water, based on the descriptions from the architect, it sounds like it wouldn't be a continuous connection, but would disconnect when it's moving and reconnect to various points along the core with flexible hoses and some sort of [quick-connect]( URL_1 ) fitting. URL_0"
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"https://en.wikipedia.org/wiki/Rotary_union",
"https://www.newyorker.com/magazine/2008/07/21/shape-shifter-paul-goldberger",
"https://gizmodo.com/5019323/dynamic-tower-skyscraper-every-floor-self-rotates-powered-by-wind-and-sun",
"https://www.google.com/maps/place/Dynamic+Tower+-+Sheikh+Zayed+Rd+-+Dubai+-+United+Arab+Emirates/@25.223442,55.2738046,14z/data=!4m5!3m4!1s0x3e5f42ecad9e6bc5:0x48b21b75d8791b0a!8m2!3d25.2204768!4d55.2860838",
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809iip | How does Trellis Coded Modulation actually work? | I'm trying to understand how trellis coded modulation works, I think it might be a little over my head though, does anyone know how it works? | Engineering | explainlikeimfive | {
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"I used to. Hopefully I still remember enough to describe it correctly. Don't think of the transmission as being a series of binary bits; think of it as states, like 8 possible states. This can be via something like Quadrature Amplitude Modulation, or simply by looking at each triad of bits as a state (000 is state A, 001 is state B, etc.). The key is that, like a state diagram, each state only has certain states that can lead to it, and certain states that it can lead to. So that means that some sequences of states are permitted, and other sequences are not. This is valuable for error correction. Since the transmission channel is imperfect, sometimes the receiver will detect a wrong state (a state other than the one transmitted). Ideally, that state won't make sense in the context of the preceding and following states, and the receiver will be able to insert the correct state instead. On a continual rolling basis, the receiver will look at the last few states and calculate the maximum likelihood sequence, given the states that it *thought* it received. This has the effect of correcting occasional errors. Of course, if there are too many errors the whole thing falls apart. And calculating the decision tree inserts a bit of delay. It takes a bit of time to do the calculation, and you want to wait until N+1, or N+2, or N+x arrives before you decide what the state at time N probably was (depending on how deep of a trellis you want to work with). But for things like sending voice over a cellphone, a bit of delay won't hurt you. Note that trellis coding is by nature imperfect. If there are too many errors, it will still come up with a \"best guess\". Again, probably OK for cellphone voice info. If you need better results, you can combine it with other error detection/correction methods."
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809wsv | How do cars measure their own speed | I mean, is there sensors or something that can do measure speed? Help. | Engineering | explainlikeimfive | {
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"When the engine turns over, the driveshaft turns to make the wheels spin round. The speedometer cable, powered by the driveshaft, turns as well. The cable spins a magnet around at the same speed inside the speed cup. The magnet rotates continually in the same direction (in this case, counter-clockwise). The spinning magnet creates eddy currents in the speed cup. The eddy currents make the speed cup rotate counter-clockwise as well in an attempt to catch up with the magnet. Remember that the magnet and the speed cup are not joined together in any way—there's air in between them. The hair spring tightens, restraining the speed cup so it can turn only a little way. As the speed cup turns, it turns the pointer up the dial, indicating the car's speed. URL_0",
"They use magnets. They are attached spinny bits. And there are magnets attached to the non-spinny bits. When the spinning magnets pass by the non-spinning magnets they produce an electric pulse, the faster the pulses. The faster the car is going.",
"If you use a bike speedometer, you attach a magnet to the tire, and another one to the frame, and every time the tire rotates the magnet will pass the other magnet, signaling the tire went around one time. Depending on the size of your tire, they calculate that distance.",
"Most of the responses I've seen so far are close, but not quite. The response from u/alek_hiddel is great and very accurate (except the speed gear is usually in the transmission rather that on the drive shaft) but it's for older speedometers. Modern ones use a tone ring (not a magnet as some have mentioned, though you could do it that way as well). A tone ring is a ring that has either gaps in it or has bumps on it. This is generally mounted at the wheel of the car. The tone ring rotates with the tire and the sensor is mounted stationary. The sensor senses a change in the field (a very slight magnetic field, but not a magnet) as the bumps/gaps pass by. That signal is conveyed to the computer which then calculates the speed of that wheel and sends a signal to the speedometer, which then turns a tiny motor to the right location on the dial (or just displays it digitally). The advantage of doing it this way is that many modern cars have a tone ring on each wheel so the computer knows which tires are rotating faster than the others (indicating a loss of traction) and can either apply the ABS (if the wheel has stopped turning) or apply traction control (if it's spinning faster)."
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80a0e4 | When your car stalls, what causes it to shake so violently? | I always wondered what happens between the motor and transmission that causes it to shake when you stall the car. | Engineering | explainlikeimfive | {
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"The engine does not just stop, some cylinders are firing and others aren’t for a few seconds. The unbalanced firing shakes the engine and thus the car until all cylinders stop firing. Much of what makes a modern car drive smoothly is the design that goes into a balanced firing of the cylinders in the engine where the shock wave from each cylinder is balanced by another.",
"In the case of killing the engine while shifting, you're releasing the clutch quicker than the engine is spinning. Releasing the clutch puts more friction on the flywheel and crankshaft, meaning it's going to be harder to spin. The cylinders firing are usually using the piston to push on the crankshaft. If there's too much friction, it won't turn and and spin in a balanced movement. You end up with a few explosions that are out of balance and rattling the whole engine. The engine is several hundred pounds and connected to your frame, so the whole car shakes until the engine stops from not being able to turn the crankshaft at all. Also, with the clutch partially or fully released, the rough movement is transferred to the transmission. So you're getting uneven and slipping movement of all the gears down the line. This turns into a rough jerking motion from your drivetrain as well."
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80hf9p | How has the Roman roads lasted for thousands of years but we can't make our roads last no more than a few years without it starting to erode? | Engineering | explainlikeimfive | {
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"Mostly because they didn't have thousands of motor vehicles weighing 2000+lbs every day. Roman concrete was also a bit of a mystery until somewhat recently. Why does ancient concrete that is not reinforced with rebar like modern concrete last so long. Apparently it's because they used seawater, not freshwater.",
"Modern roads are built of asphalt, which is a surprisingly soft material. It's essentially tar with sand and gravel mixed in. It's cheap, pliable, quick to lay down, doesn't require any extended curing time before traffic can start driving over it, is recyclable, and easy to remove. However, because it's soft, it's not all that durable. It'll crack as the roadbed settles or as heavy vehicles (especially overweight trucks) drive over it. Water then seeps into those cracks, which freezes come winter, and widens the gap or causes a big chunk to come loose, creating a pothole. It can also melt under high heat, like in a vehicle fire. We can and do build roads out of concrete. Ones that need to last a long time, like major highways. However, it's more expensive and labour intensive to lay down, and it requires an extended curing time. So they're not used everywhere. Asphalt remains the most cost effective road building material we have. Roman roads are actually not much different than a modern road. You have layers of crushed stone to make a foundation, then you put your paving material on top. They used stone blocks instead of concrete. Stone is a hard and very durable material, so it can last a very long time. It's not as porous as concrete or asphalt, so it'll handle freeze/thaw cycles better. The Romans did have a legal weight limit for their roads. A cart could weight up to 1,000 Roman librae, or 723 modern pounds (328kg). Plus horses, you might be talking something equivalent to a modern subcompact. Certainly nothing like the big rig tractor trailers and dump trucks we have today."
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80ki7r | Why do engines/motors sometimes have a hard time starting up in colder weather? | My car had a little trouble starting up today in well below freezing weather, but it starts up fine any other time. What is it about the cold weather that engines don’t like? | Engineering | explainlikeimfive | {
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"1. The gasoline vaporizes considerably slower. 2. The oil is less \"warm maple syrup\" and more \"cold jelly.\" 3. The battery is cold and you need a new one or block heater if you're somewhere it is very cold.",
"There can be a number of reasons depending on the car. Firstly if it's a diesel then you need to let the glow plugs in the combustion chamber warm it up. Usually you will get [this symbol]( URL_0 ) on your dashboard to tell you that they're heating up. These are basically little electric heaters that warm up th chamber to allow diesel to combust. The temperature could be low enough that your engine oil is starting to get rather viscous, it won't freeze under most conditions but it will get harder to move which could make the pumps unable to start/require more battery power to get going. This is why it's important to make sure you have the right oil for your car and the temperature range you will be using it in. The batteries are too cold. When temperature drops so does that conductivity in a solution so with your batteries being cold less electricity is going to be produced which could be low enough to not give the car the initial jump it needs to start. You've got water in your fuel and it's frozen or became slushy. This is pretty uncommon but it can happen and will mean fuel can't get to your engine. No fuel, no engine start. If you've got an older car with a carburetor then this can become clogged with moisture and can result in again no fuel going in to the engine."
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80oigu | Why do some lights get brighter when you turn on other electronics instead of dimmer? | Engineering | explainlikeimfive | {
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"text": [
"You probably have a broken or loose neutral connection. A neutral connection in the house regulates the voltage on both 'legs' of a household split-phase electrical system, but a disconnected neutral will cause the per-leg voltage to vary based on load, so if you put a heavy load on one leg the other leg will see much higher voltage than normal."
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80pc7j | What happens when lightning strikes a plane? | Engineering | explainlikeimfive | {
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"text": [
"It flows through the most conductive parts (the aluminum outsides) and then down to the ground. Lightning protection is a mandatory part of the design of aircraft, so nothing bad should happen. It might leave a scorch mark, but maintenance can clean that off.",
"Airplanes are designed to survive lightning strikes pretty well. The two major danger spots are the fuel tanks, and the electrical systems. The electrical systems have locations that melt and break the circuit if the power is too high, thus sparing critical electrical components (like computers) from damage. All the electrical systems in an airplane are redundant. For example, the wires that go from the flight computer to the rudder control motors might have 3 or 4 different and independent circuits that connect these two parts. If one of the circuits is overloaded, devices in the circuit break the current (thus saving the computer and the motor). Then the remaining connections can be used to send signals back and forth from the computer and rudder control motors. Fuel tanks in the wings are not exposed to any lightning sparks – hence why the surrounding metal, structural joints, access doors, vents and fuel filler caps must be able to withstand any burning from a bolt of lightning, which can have temperatures of up to 30,000C. In addition, the aircraft skeleton and skin is made of aluminum which is conductive, so the lightning tends to flow through these components and out of the plane, leaving the electrical systems and fuel systems untouched. In newer composite planes, extra copper components are added to the skin to maintain the faraday cage effect."
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80qdne | Can you scale up the size of a plane? | Of you were to double the size of every part of a plane, would it still fly? If so, is there a limit to this (10x, 100x)? If not, why not? | Engineering | explainlikeimfive | {
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"The math doesn't quite work that way. Some things scale up linearly, others to the second power (squared), others to the third power (cubed). If you merely double the size of everything (linearly), you won't end up with a correct, strong, and efficient design. In addition, some things will violate important physical constraints if doubled in size -- the oil may overheat and ignite, or the turbine blade tips may go supersonic causing destructive shock waves.",
"No. If you make the plane twice as long, and scale up everything else on the plane to match, then the surface area of the plane will quadruple, but the volume (and weight) of the plane will *octuple*. This is because surface area depends on length^(2), while volume depends on length^(3). This scaling behavior is known as the [square-cube law]( URL_0 ). Flying a plane depends on a careful balance between lift and weight. Lift depends on (among other things) surface area. So if you double the plane's size, your weight goes up faster than your lift does. This makes it no longer possible to fly, unless you change the actual design on the plane to increase lift somehow.",
"At some point this (hypothetical) plane would become so heavy that even the to-scale fuel tanks wouldn't be able to contain enough fuel for long-duration sustained flight. It might get off the ground, but it wouldn't be going very far, or high. As it got heavier it wouldn't even be able to get off the ground.",
"No. Because all of the fluid analysis done in the engine, the wings, ... also depends on the fluid properties like the Reynolds number for example. Those cannot be scaled that simply. [Such model similarity is called \"similitude\"]( URL_0 ). So scaling the airplane x10 could require you to scale the density of the air by x1000, which is impossible of course. Of course, small and big planes that exist nowadays are not scaled up piece by piece.",
"That's not how planes are made. The objective is to make them just strong enough to fly at minimum weight, as this maximizes operating efficiency. Even a 10% change in size would require some parts to be redesigned. Some parts could be made stronger out of normal materials at 10% larger scale, perhaps by removing holes that make them lighter. Others would have to be both larger and heavier to make them strong enough. A plane can be made [very large]( URL_0 ) or [super large]( URL_1 ), but the weight tradeoffs mentioned by others restrict these very large planes to being filled with only very low density stuff. You can't fill them with people and have them fly.",
"The short answer is \"no, you can't simply scale up the size of a plane.\" The reason has to do with the interaction between surface area effects and volume effects. Surface area is a square function of length, while volume is a cube function of length; i.e. if you double the length, then the surface area is multiplied by four (2^2) while the volume is multiplied by eight (2^3). This means that when the length is doubled, the volume grows by twice as much as the surface area. Why is this relevant? Because the main thing that helps planes fly is lift provided by the wings, and lift is a function of surface area. The main thing that keeps planes from flying in the weight, and weight is a function of volume. So if you double the length scale of a plane, you're multiplying the surface area (and therefore lift) by four, but multiplying the volume (and therefore weight) by eight. The Lift/Weight ratio is now 1/2 of what it was in the original plane, so it's probably not producing enough lift to fly. Of course, this is all an oversimplification. For one thing, it's not factoring in drag (which is a function of surface area) or the complex shape of a plane, but that's the simple gist of it."
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80sjm3 | if there is internal combustion engines then is there external combustion engines and if so how do they work? | Engineering | explainlikeimfive | {
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"> if there is internal combustion engines then is there external combustion engines and if so how do they work? Yes, an external combustion engine example would be a steam engine. While the firebox may be enclosed it is still considered external combustion because the working fluid is isolated from the exhaust gas."
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811mrm | How do zambonis work? | During a hockey intermission, they flood the rink and use the zamboni. What does it do? Is the water already frozen or does the zamboni put down, freeze, and smooth the water? | Engineering | explainlikeimfive | {
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"text": [
"Cooling tubes embedded in the concrete beneath the ice keep it at a temperature which is somewhat below zero. This is important, as it needs to keep the ice cold enough to freeze the additional water which is laid down during resurfacing. The machine first scrapes the ice to remove shavings and prevent weak spots with any entrained air being frozen into the surface. The ice shavings are scooped up into a holding tank in the machine. Then, fresh water is laid down, and spread via squeegee to ensure consistent and minimal thickness."
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811vls | How could an electric aircraft hope to achieve the thrust of a kerosene jet? | Engineering | explainlikeimfive | {
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"A portion of a turbojet's power does come from the heated gases creating extra exhaust pressure, but a good portion comes from the mouth of the engine being very wide and sucking a lot of air in, which is then compressed by the engine and shot out the back at higher speeds. Theoretically, electrical motors can provide enough power to move the air through similarly to the kerosene engine, and if you look at cars, electrical motors are smaller than engines yet can provide the same power. So the issue wouldn't be with electrical motors instead of the kerosene turbine, it would be with creating and supplying the required watts of electricity TO the electrical motor(s). 90,000 horsepower = 67,000 kilowatts, which is the output of a couple small hydro power plants (dam on a small river).",
"It's tempting to look at a jet engine and think that it's making most of its thrust by burning kerosene to produce a high pressure exhaust. That kind of jet exists, but the jets you see on passenger planes are mostly using kerosene as a way to get the power to spin a big fan at the front of the engine. It doesn't really matter how you get that fan spinning, just that it spins to move lots of air. In this regard an electric engine is very well suited. Electric motors have great power per unit weight. It shouldn't be terribly difficult to design an electric ducted fan that has similar thrust in a similar size and weight class (or at least it shouldn't be much more difficult than designing an aircraft engine already is). The real problem isn't *thrust* (how hard the engine pushes) or *power* (how much energy the engine can impart on the aircraft per unit time), but *energy*. So far electric drivetrains have had a really hard time with storing enough energy to have endurance that is competitive with combustion engines. For things like cars you can just increase the percentage of the vehicle's weight taken up by energy storage since it's such a small percentage anyway. For an aircraft that's less feasible--a 747-8 weighs 485,300 lbs empty and carries a load of up to 431,000 lbs of fuel. Battery technology is getting better, but it's quite a way from being able to compete with kerosene and the like."
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814ycq | Why do windows on the back of a car only go a part of the way down? | Engineering | explainlikeimfive | {
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"text": [
"Some designed that for safety. Some doors are not large enough for the window to totally be enclosed while open.",
"Look at the car from outside. Take into account the size of the window, then imagine it all the way down. It will most likely be in the way of the wheel!"
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815pp6 | Phase, Earth and Neutral. | What is a single phase, three phase etc? What is the neutral wire? Why is the neutral at 0 voltage? Does current flow through the neutral wire? What is the purpose of a neutral link? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"Let's say you have some kind of water powered gizmo, like a ventilator which is driven by water flowing through it instead of electricity. The ventilator has an inlet and and outlet hose attached to it: one for the water to go in, and another to let the water flow out again. Now, where does the water come from? To supply (water) power to the ventilator you could have a bucket of water. A hose attached to a pump draws water from the bucket and pumps it into the ventilator at a certain pressure. The water goes in, drives the ventilator and flows back out again through the outlet hose into the bucket. Now, to prevent dripping in case the ventilator is leaky, you put a funnel with a hose underneath it which lets the leak water flow back to the bucket. This is now sort of like an electric circuit: The inflow hose with the pump is the phase. This is the thing that is supplying the water flow (amps, in electrical terms) and the water pressure (voltage). The neutral is the outflow. It doesn't generate water flow or pressure in itself. So, as long as no inflow (phase) is connected, its basically just a dead hose (wire). The funnel with the hose is kind of like the earth in an electrical system. Its task is to funnel away any excess water (electric charges) that may have escaped the circuit. In the case of electrical appliances, the casing of the appliance could end up carrying a voltage if the circuit leaks i.e. is short circuited to the casing. The amount of work you can do with this kind of water circuit is limited by the performance of the pump (pressure, max flow, etc.) and by the thickness of the hose - too high pressures will rupture the hose. So in order to be able to perform more work, you could hook up multiple inflows to your machine which are all driven by separate pumps. This is basically the principle behind multiple phases. Of course, this analogy is based on direct current. However, it also works for alternating current if you imagine the pump going from suck to blow 50 or 60 times a second, continuously pumping water in and out of the devices. There are probably lots of logical limitations to this analogy but, hey, ELI5..."
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818cev | Why does the pitch of a motor engine increase as I increase the revs? | After all, all I am doing is making more quick ‘bangs’ per second. | Engineering | explainlikeimfive | {
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"text": [
"The pitch of a sound is its frequency. Higher tones have higher frequencies. So, as you rev up the engine, it's creating 'bangs' more frequently, and your ears perceive that as increased pitch."
],
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3
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81ax35 | Where does the energy from the engine go when my (automatic) car is stopped? | This is actually a question my friend asked me, but I had no idea how to answer so here I am. Basically, if my car is stopped, the engine is still running. Presumably, things in the engine are spinning and moving gears around and whatnot. At what point does that energy stop going to the tires (if at all?) Is there some mechanism for discarding the excess energy produced while the car is stopped? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"the disconnect is in the torque converter, 2 fans that face each other, 1 turned by the engine, the other connected to the transmission. they can slip by each other, forcing the fluid through the other fan without it spinning. Ultimately, that energy is converted to heat in the transmission fluid via friction and then expelled through the transmission case/transmission cooler to the atmosphere.",
"With respect to fuel consumption, would an idling manual consume less fuel than an automatic at a stop light? Assuming same make model year vehicle."
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81ep24 | why do incandescent light bulbs have such a large volume compared to the tiny filament? | Engineering | explainlikeimfive | {
"a_id": [
"dv2m2yg"
],
"text": [
"more surface area for cooling itself, less likely to burn you if you touch it. Also, sizing of fixtures was defined decades ago, probably with a healthy dose of manufacturing capabilities driving that decision. we could do better now (without question for LED bulbs) but now we keep that size because we dont want to lose backwards compatibility."
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81glxg | With B-52 bombers reaching 60+ years of service, would they still be effective against a technologically advanced opponent, like Russia or China? | Engineering | explainlikeimfive | {
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"text": [
"there isn't really much difference between the 60 year old b-52 and more modern bombers, except for stealth. bombers generally are not very fast or maneuverable. they rely on operating at high altitudes, remaining undetected, or inadequate anti-air capabilities. the biggest threat to bombers are fighter interceptors. so they usually are escorted if the target has good air defenses.",
"They have been constantly updated since their birth. Military would mainly uses the B2 for open warfare due to its stealth abilities",
"If we otherwise have air superiority, yes. Once we've gained control of the skies and eliminates any anti-aircraft capability, we could be dropping bombs from blimps without a problem. A big part of the US military success is the fact that we can bring in air support almost anywhere at will.",
"B-52 bombers are big, slow, easy targets. Their advantage is that they can carry lots and lots of bombs. B-52 bombers have always been big, slow, easy targets, even when they were first built designed. They were designed and built to operate only when the US had already achieved air superiority. Because of this, their performance relative to enemy fighters, surface-to-air missiles and radar installations is unimportant, and hence why they're still useful today despite their age. Also, while the airframes are still old, the electronics and engines have all since been upgraded to the latest technologies. So if war were to breakout, B-52s would perform the exact same tasks they did back in Vietnam, and the Gulf War. Once air superiority has been achieved over a region, bomb the shit out of it. Enemy fighters would be cleared out by air superiority fighters, and radar and SAM sites would be cleared out by stealth aircraft, thus kicking in the door for the big boys."
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81jhwy | How is it possible that satellites and space stations don't get hit by astroids or other flying objects all the time? | Engineering | explainlikeimfive | {
"a_id": [
"dv3gghe",
"dv3glm5",
"dv3gu8i"
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"text": [
"First off: collisions do occur and have become more and more of a problem as time has gone by. Why isn't it happening all the time? The amount of space they are flying through is HUGE. Imagine a huge freeway that goes in one giant 100 mile loop and then imagine putting 30,000 individual grains of sand on it, the vast majority of them going the same direction, and the vast majority of them [edit:] at roughly the same velocity. The chances of these grains of sand hitting each other are vanishingly small. Well in a very rough (guesswork) way I just described kind of what the orbital neighborhood is like around earth. It's a huge highway and there are about 30,000 grains on sand running on it. They will rarely every connect. [thank you to LoneStarG84 for pointing out the typo]",
"The chances of getting hit by an asteroid are almost nonexistent. But they sometimes are on collision with really small flying objects like debris. In that case they use boosters to move out of the way. In the space station the people move into a escape pod where they could detach and return to earth if something goes wrong.",
"As far as asteroids are concerned, the asteroid belt is nothing like how asteroids are portrayed in Star Wars and similar sci-fi. Space is really really big. Incomprehensibly big. The distance between asteroids is huge. Its not even worth considering the possibility that a probe traveling through the asteroid belt will hit anything."
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81p83c | Why do cars with start/stop start itself again when staying to long in one spot? | I just bought a new car and it has start/stop. Sometimes when I am waiting for the traffic light to go green and it takes long the car starts itself up again from beeing off even though I didn't let go of the brake for 'start up and drive' | Engineering | explainlikeimfive | {
"a_id": [
"dv45xcc",
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"text": [
"While the engine is off, all your AC and radio and lights etc are running off the battery, which isn't really designed to support them. The car monitors the battery charge and when it gets below a certain level, restarts the engine to keep it topped up.",
"It starts again because it needs the engine again. Or. Well. Not the engine. But the electricity the engine produces with the generator that is connected to it. And possibly some of the excess heat it naturally creates with the combustion. There is a lot of stuff in the car that constantly consumes electricity. - the entertainment system - the exterior lights - your phone charger - the climate control fan motor - seat heating The problem is, bluntly put, that if the engine is off for too long, the battery will be so drained that the starter motor won’t be able to restart the engine. And that is kind of dumb. Makes the whole thing useless. So. To assure that the car is able to maintain a reasonable climate inside the car and that it actually is able to restart, it will restart after a while by itself. Because it has to. The time it can stay off differs with temperature. Distance you have driven. How much energy you use in the car. Stuff that directly affect the battery charging status."
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81shcb | How do tree farmers keep up with the global demand for paper? | If you think about all the paper products consumed on a daily basis, e.g. printer paper, lined paper, paper cups, cartons, newspapers, tissue, etc., how is it we have enough trees to keep up with the demand for all of these products? Especially since trees take decades to grow. | Engineering | explainlikeimfive | {
"a_id": [
"dv4tctx",
"dv4syqz"
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"text": [
"Ooo! I can answer this! I studied forestry back in North Carolina, with a focus on sustainable paper milling! 1st: Manufacturing paper is fairly efficient, especially compared to other uses of wood. The wood is pulped and pressed into paper of various uses, which mean a lot of the material doesn’t get wasted (glancing over a lot of stuff, but that’s the basics). 2nd: The trees used for paper don’t need to be very high quality. You don’t need Cherrybark Oak, when loblolly pine would do, that kind of thing. 3rd: Trees used for paper don’t actually take that long to grow. Different companies have proprietary species of trees that will grow fast and big(Weyerhaeuser has a loblolly/longleaf crossbreed that can reach maturity in ~12 years, as opposed to the 100+ years for a pure LongLeaf Pine). The companies spend vast amounts of time and money to research and develop better and better trees all the time, so leaps and bounds are being made all the time. 4th: Even for trees that do take a while to grow, you plant huge batches every years. After a while, you get to the point where you are logging every year.",
"They plant lots and lots more trees. Paper trees are all farmed, like corn but slower."
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81u3zm | why do sedans not have rear window windshield wipers but SUVs do? | Engineering | explainlikeimfive | {
"a_id": [
"dv55qty"
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"text": [
"Usually it's because of how the wind flows past the vehicle. With sedans the back window is at an extreme angle, and as such the dust and grime doesn't build up on it very often, but for SUVs the dust and grime does build up on it quite quickly. As such, they need a wiper to clean it off every so often. And by 'every so often' I mean every 10 minutes when you're driving through road salt, snow, and ice. Sedans don't have this problem."
],
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7
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81vax2 | Why do some places have power lines underground and others above them above ground? | Engineering | explainlikeimfive | {
"a_id": [
"dv5g6jt",
"dv5ft30",
"dv5g885"
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"text": [
"Underground are more expensive, so they only run the lines underground when they get to neighbourhood where people can complain and be listened to.",
"It usually comes down to a cost-benefit calculation. Power lines underground are safer and less likely to be damaged, although actually maintaining them is a little more expensive. Running lines above ground is definitely cheaper. The argument for safety is more compelling in densely populated areas (which may also be more able to pay for the added expense). Some jurisdictions with a high density overall may end up requiring all lines to be run underground.",
"In addition to cost vs safety, some times in the country you'll see an aerial line come down a pole and go buried for a short period of time and go back up the pole, this is usually because some farm equipment is too high to make it under the lines without damage so they bury them in this area. Other times you'll see phone or cable lines do something similar, sometimes it's easier to splice these things and put them in a buried splice cases in what they call a hand hole than it is to mount this sometimes large thing from a pole."
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81vhk8 | Why are potholes so hard to fix and keep under control? | I live in New Orleans and it is just a fact of life down here that our streets are bad, very very bad. How do streets get so bad? And how come we can't keep potholes from forming? It seems as though potholes plague every city, is there not a better option? I mean even some Roman roads are still in great shape. URL_0 | Engineering | explainlikeimfive | {
"a_id": [
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"dv5ixk2",
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"text": [
"Part of the issue is that the filler material never bonds to the original pavement properly and repaving the whole section takes significantly more work.",
"In your case, New Orleans is pretty much rock and then water. There’s a lot of ground water that keeps on loosening groves in the road. That plus cars riding on them cause them to get bigger and bigger. They’re hard to fix because the filler is consistently getting loose as it can’t permanently stay the shape of the pothole. Eventually space gets made, cars ride on the filler and it starts to whittle down. Eventually you have to repave the whole road. It’s frustrating because the government isn’t spending time on how to make roads that aren’t degradable. One day....",
"It's not difficult to keep under control. Some countries have no problem at all with it but they likely made a proper road from the beginning (more expensive). Usually it is simply too expensive to keep patching so they would have to raise taxes, but in some areas people don't want to pay taxes so they have to live with potholes and bad roads."
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820rbo | How do smart phones dissipate the heat created by the processor? | I know that the processing power in a phone is generally much less than a computer. But why do they not require a venting system like computers? How is the heat removed from the system? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"I think some of the answers are great, but missing a major point that I want to add: The chips in smartphones ([SoCs]( URL_0 )) are built from the ground up to turn the energy in the battery into useful work as efficiently as possible. In industry terms, smartphone SoCs have truly insane Performance Per Watt, which is a unit of how much screen brightness, CPU speed, etc can be achieved using only a joule of energy every second. So to answer the question, mainly because they barely produce any heat. Heat is what happens when energy from the battery is lost, and not turned into useful work. This is why the mobile device revolution only happened recently. Chips this efficient are a very new invention.",
"they use passive cooling such as heat pipes (copper tubes) to spread the heat through the phone. that way it can radiate out through the entire surface. but at the same time you dont want too much heat coming out and burning your hand. so a lot of heat ends up staying inside, and the performance is throttled to produce less of it thermals are rarely mentioned in western phone discussion, but in hot places like india it's very important and you can find reviews saying how quickly they overheat, thermal scans showing how hot their surface gets, etc",
"They have heat pipes, if you watch a video of a razer phone being taken apart you can see a big copper pipe",
"Very poorly. Which is why they cannot really perform demanding tasks for a long time. The inner body is usually metal and contacts the screen, so it can absorb a little heat and pass it to the environment, but it cannot run at full performance for long without running hot.",
"Just to add on, the best way to prevent overheating is to throttle the CPU and design the system not to generate heat in the first place. What you may not realize is that the CPU isn't usually the biggest power draw and heat generator in a phone, it's usually the screen itself (which has a large surface area to dissipate it) and the radio (which is designed to be incredibly efficient and generate low heats). But it doesn't always work perfectly."
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822hpv | Why can't we transfer scents the same way we transfer images? | Engineering | explainlikeimfive | {
"a_id": [
"dv6yj4k"
],
"text": [
"Scents are actual physical particles moving in the air. Transferring a scent then requires that you actually move physical things, or that the receiver have the necessary components to manufacture scents when sent instructions. Smell-o-vision is certainly possible. It would just require scent compounds that would need to be replaced as they were used up. Currently there just isn't enough demand for smell-o-vision to support that."
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82b4la | Why is glass wool used as insulation between walls? Why not just air? | Glass is not a very good insulator, and I know that most of the insulation comes from the amount of air and empty space within the wall, with air being a very poor conductor of heat. The answer cannot be structural as glass wool wouldn't give any support. The only thing I can see is perhaps it disrupts convection currents, but would these be a problem at all? | Engineering | explainlikeimfive | {
"a_id": [
"dv8rkch",
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"text": [
"> The only thing I can see is perhaps it disrupts convection currents Correct! The point of fiberglass insulation inside of walls is to prevent convection currents which can transfer heat reasonably effectively. With only tiny air pockets there is no where for convection currents to form so heat can only be passed by conduction through the air and glass and the glass fiber is loosely packed so that it serves as a poor conductor. Packing insulation into a wall will compress it and allow heat to transfer through the fiberglass much easier",
"This is the same reason why a seemingly thin goose down jacket works as well as a heavy, thick jacket. Adding a second sentence because of bots. :)"
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82c87z | How do you talk to a malfunctioning satellite? | Like if there's a hard or software issue, it's kind of hard to do a hard reboot. How do scientists and engineers fix satellites from Earth's surface when they're so far away. If they go offline aren't they sort of permanently out of commission? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"It really depends on the nature of the issue. If a hardware issue props up that's mission critical, the satellite is now space junk, because you can't fix physical components (low earth orbit satellites could be repaired when the shuttle was still in service). If it's a software issue, it's the same basic idea as fixing a software issue on your computer. Ground controllers can analyze telemetry to figure out what's wrong and upload new software, patches, fixes...etc. A lot of satellites, especially probes to other planets, have advanced self-diagnostic capabilities where they can detect when something is wrong and put themselves into \"safe mode\" while ground controllers troubleshoot."
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82ee4t | How does a can of whipped cream work? | Engineering | explainlikeimfive | {
"a_id": [
"dv9fie6"
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"text": [
"Liquid cream is held in the can along with nitrous oxide, under pressure. This pressure keeps the nitrous oxide liquid too. When you press the nozzle, it releases the pressure. This causes the nitrous oxide to boil - turn into a gas - which has the dual effect of both aerating the cream and pushing it out of the can. This is why the can gets cold (as with all aerosols) - when the pressure drops, the temperature drops."
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82ht1q | What exactly IS engine braking, and how do you do it on modern vehicles? | Engineering | explainlikeimfive | {
"a_id": [
"dvam5mz",
"dva8lhi"
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"text": [
"Just to be clear, you might not be asking about what others are talking about. I personally haven’t heard of downshifting as “engine breaking,” but that could be a thing somewhere. Large trucks like semis often have a jake break, aka “engine break.” These are not the same thing as downshifting. They change the way the engine works. I could explain exhaust breaking but I’d just be quoting other pages. I’ll link the wiki on it. URL_0",
"Ever ride a fixed gear bike downhill? You'll get to a speed where your legs can't keep up with the spinning pedals and you have to take your feet off them. In a vehicle or on a bicycle with multiple gears, we need to change up a gear to be able to continue accelerating. What we're doing when we force the engine to brake is we're dropping the gear back where it was, or in the fixed gear example, putting our feet back on the pedals. The rotational force is transferred through the drivetrain (or your ankles & legs) and the back-pressure in the system (leg muscles) gradually slows the rotation and the car decelerates. In a stick/manual, it's easy. Most autos now have a semi-auto shift option to allow you to manipulate the gears manually. Try driving on a (quiet) highway and click it down a gear with your foot off the gas. These semi-auto systems are computer controlled, so they won't allow you to shift down if there's a risk to the drivetrain."
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|
82lgf1 | In a cellular network, is each channel specified for each user? Can there be multiple users on the same channel without any interference? How do telecom companies assign a specific bandwidth to all of its customers? | Engineering | explainlikeimfive | {
"a_id": [
"dvb2g41"
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"text": [
"No, each user doesn't get a dedicated channel. Each phone jumps around a wide range of different channels from moment to moment, in a synchronized dance along with the tower it's talking with. The tower attempts to balance the bandwidth needs of all the phones it's currently handling."
],
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3
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82lm8n | Why are reusable rocket engines cost-effective? The space shuttle (also reusable) cost a ton to maintain. | Engineering | explainlikeimfive | {
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"text": [
"The Space Shuttle required extensive examination and repair of the heat shield tiles after every launch. The Shuttle also consisted of 3 extremely large and complex liquid fuel rocket motors, while a Falcon 9 consists of 9 smaller and less complex Merlin engines; most of the time, not all of the engines are required to acheive the mission objective (launch a payload to orbit), although it may affect the ability to recover the booster. (Although, as we saw last night with the launch of Hispasat, it was too rough in the Atlantic to send out the drone ship, so while the landing procedure was performed, the booster went into the ocean). Were the Shuttle to lose one of its three engines, it would not attain orbit.",
"With a traditional rocket, you have multiple stages and boosters. You drop them all into the ocean and don't get them back. You have to rebuild them every time, and rockets are expensive. With the shuttle, you had three components: the orbiter, the fuel tank, and the Solid Rocket Boosters/SRBs. The fuel tank was the only component that was lost and needed to be rebuilt. The SRBs splashed down in the ocean but were designed to float. That being said, getting dropped from great height and bathing in seawater for the better part of a day aren't very good for rocket equipment. They could kind of get away with it because the SRBs were a simpler design than the liquid fueled engines that act as the main engines on most rockets. The main expense was the orbiter itself. Because it came back through re-entry, the heat shield needed to be meticulously examined for damage to ensure that it remained safe, and many of the heat resistant tiles on the belly needed to be replaced. IIRC the engines also needed a full overhaul after each flight as well. What Space X is doing with the falcon rockets is saving the bottom stages. These have the main liquid engine, which is very complex and expensive. They don't crash into the ocean, so they don't need very much of a refurbishing before they can be reused. The part that goes to space doesn't need to come back and be refurbished like the shuttle.",
"It can be argued that the Shuttle made the wrong part reusable. The massive rockets it sat on were designed to be expendable or salvageable, while the small glider on top with its comparatively tiny rocket engine was reusable. It looked cool but wasn't really cost effective. Reusing the rockets themselves will be expensive but less expensive than building a new rocket for each launch.",
"Reusable rockets are the spiritual successor to the space shuttle. The shape is different and a larger portion is reusable. Turns out changing the shape and making a larger portion reusable can cut your cost hundredfolds.",
"The shuttle itself could bring delicate things (people, equipment, samples) back from space in a controlled landing. A lot of things had to go right to ensure that, and as we know, not all of them went right all the time. The reusable engines are just engines, and aren't dropping from orbital height. They just need to land gently, but they can flip and tumble and withstand higher forces on their way back down. They also don't require as high a factor of safety, because there's not much at stake. TL;DR: Apples and oranges. They're not comparable to shuttles, because they're just reusable versions of what we used to strap to the shuttles."
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82qbx6 | Why are wall outlets only limited to 2 plugs? | Engineering | explainlikeimfive | {
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"The building codes, in the US, require an outlet every 12 feet, to reduce the use of extension cords. Making them all 4 outlet would double the cost and still leave most of them unused. When a location is planned for lots of electronics, like a media space, it's common to see lots. The cost/work to add more is also quite low, as electrical changes go.",
"They're not limited to just 2, that's just the smallest format box you can install. If you put in a wider box, there's no stopping you from filling it up with outlets if you like. I think part of the reason there are usually only two is for convenience - it's better to have a lot of different locations to plug in such that you're not stretching cords across the room, rather than to put all the outlets for that room in one big box.",
"Light switches were common in the U.S before outlets. In the old days if you wanted to plug in a say a vacuum cleaner you unscrewed a light bulb and the vacuum cleaner came with a fitting that would screw in in it's place. Eventually they started putting light bulb sockets on the baseboard just for appliances, and then developed regular outlets that were safer and easier- no risk of sticking a finger in. Two outlets happened to be the number that would fit the same electrical boxes that were already in use for light switches."
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82r15i | Dyson Swarm | I know the idea of a Dyson sphere enclosing a star to obtain all of its energy output but I have heard a couple of variants of the concept but the one that caught my attention is the Dyson swarm. Can someone explain exactly what it's supposed to be compared to the sphere idea and how it's supposed to be more feasible than the sphere? | Engineering | explainlikeimfive | {
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"A Dyson swarm is a huge number of free floating individual satellites occupying the space a Dyson sphere (or ring) would occupy without the connections between them. This is useful because it still captures a lot of the energy and provides a lot of the benefits of a Dyson sphere, but doesn't have near the engineering challenges and has the advantage that it can be put up/expanded piecemeal much more easily (just launch more satellites!) than a normal Dyson structure could.",
"There is a net force concern with building a Dyson sphere. While it's pretty stable once built, a 1/3 built one is hard to make stable. Instead, a Dyson swarm is just a bunch of satellites orbiting the star. You start with 2-3 satellites in each orbit, and layer them to avoid collisions. Then you add more satellites to occupy more of each orbital ring. Presuming the satellites are light, this can be a stable configuration. When you have enough satellites that you block 1/2 of the light from the star, you have a 1/2 star Dyson swarm. There are efficiency losses as you go from 90% upwards, but by the time you get to that, maybe you have the energy to make a full sphere."
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830c9g | How do they decide where and how many stoplights they put at a given intersection? | Engineering | explainlikeimfive | {
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"First you perform a turning movement count of the intersection. These days these are done by putting up a camera that records traffic for a day, and counts the cars. Using the peak traffic volumes, an engineer figures out how many through lanes, left turn lanes, and right turn lanes are required, as well as how long they should be. Also, whether or not pedestrian crosswalks are needed. Once the geometric design of the intersection is complete, the signal is designed. Rule of thumb is to have n+1 signal heads per movement, with n being the number of lanes. So if you have a left turn lane and two through lanes, you'll have two left turn signal heads, and three through signal heads. Most jurisdictions either require the signal heads to be in line with the lane lines, or in line with the center of each lane. All signal heads should be within a 20 degree cone of vision for where the first car in line will be stopped. Sometimes at larger intersections this means you'll need a reverse mast arm from the left side for one of your left turn signal heads. Sometimes you include a nearside signal head on the right side, usually only if you have right turn arrows. To keep the intersection balanced and looking good, if one or two approaches need nearside signal heads, you put them on all approaches. The location of the signal posts is dictated by the location of the pedestrian crosswalks these days. Pedestrian pushbuttons can't be too far from the crosswalk, so unless you want stupid little posts all over the place, you put the mast arms and posts right by the crosswalks. Mast arms get stupid expensive as they get longer, so you design them as short as possible. Also, existing underground and overhead utilities can be a factor as to where you put your signal equipment."
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830fk2 | why does the Hyperloop concept use a vacuum tube rather than just a tube of air moving at the same speed as the pods? | Engineering | explainlikeimfive | {
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"> why does the Hyperloop concept use a vacuum tube rather than just a tube of air moving at the same speed as the pods? Air has friction. Moving air against all that length of pipe will introduce significant losses in efficiency which the entire point of the tube was to avoid. If we didn't care about moving air past that much surface area we would just run the cars on the surface in open air!",
"Air has mass. Moving cars + air takes more energy (=$$) than just moving the cars. Accelerating the air also means that to stop you have to stop all the cars. With no air, the cars can get closer together or farther appart without additional energy penalties."
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8317q5 | How do potholes form in a road, and why is tar used to fill them instead of asphalt? | Engineering | explainlikeimfive | {
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"text": [
"Car wheels are like 500 lb rolling hammers, and truck or bus wheels are even heavier. To say vehicles 'pound' the road is an understatement. Over time, heat, cold and water cause cracks in the road surface. Water and debris make the cracks worse over a few weeks or months. When the crack is large enough that a car's wheel might even fit a little inside, then every wheel that goes over it could take a chunk out. Depending on traffic and weather, this can turn a small crack into a dangerous pothole in as little as a few hours. Asphalt is mixed with rock and uses a tar-like substance to hold the rocks together. It's more work to lay asphalt than just the tar by itself, and tar on its own more easily flattens. Often, repairing a pothole is just temporary until the entire road can be repaved. Again depending on traffic and weather, this can/should be done about once a decade, more or less. (Small infrastructure budgets and small towns notwithstanding.) Potholes don't damage the road all *that* much because the thing that actually does the work of carrying the weight of cars and trucks is actually a thick gravel layer underneath the asphalt: the surface just keeps those rocks dry and protected from things moving them around.",
"The extreme temperature changes from winter to summer cause roads to expand and contract, causing cracks. Then, in the winter water gets into those cracks, freezes and expands which loosens the asphalt. As it continues to break away, potholes form. Often when they are patched, there are seams between the original street and the newly applied asphalt that let the process repeat in the same place year after year and over time you get even bigger potholes Tar isn't used to fill in potholes, but sometimes used to seal cracks to prevent future potholes"
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8342xj | how were huge rope-bridge/ziplines over great distances and depths made, before the use of present technology? | Engineering | explainlikeimfive | {
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"text": [
"The two easiest ways are... 1) have a guy on each side of the chasm. The guy on one side has a bow and arrow or something to fire the rope to the other guy, who pulls the rope to the other end. 2) have a guy at the top of the chasm and another guy on the bottom. Drop the rope, with a bunch of slack, to the bottom guy, who climbs the other side of the chasm. Repeat as necessary."
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839tdr | What is the significance of the falcons that SpaceX continues to launch? | ELI5: It had been all over reddit that SpaceX has done great things for the space industry, specifically being able to reuse and land some early stages of the launch. Can someone explain what this means for the future of space travel, and the significance of this new technology? What is it that they are transportating with all of the falcons they have launched up to this point? And while on the topic, can you also explain the relationship NASA and SpaceX have? I think all of this space travel and exploration is very interesting and I would love to be an informed spectator. Thank you | Engineering | explainlikeimfive | {
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"I can't speak to the NASA space x connection But...spaceX is utilizing reusable boosters. This is a huge deal because prior to space X...the boosters utilized in rocket launches were dumped over the ocean. Hence the location that NASA launches from. This is important because it moves space travel more into the same realm as air travel. A good analog is that spaceX makes planes that can take off and land. Prior to spacex, NASA had planes that could launch, deliver their payload in the air...and then smashed into the ground. That's a lot of money down the drain over and over. So now spaceX just has to, after they get better at designing rockets, worry about fuel and upkeep. Not starting from the ground up over and over.",
"They have deployed civilian satellites, government satellites, military secret payloads, supplies for the space station, and a tesla as a test payload."
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839tr2 | Why do electric cars accelerate so much faster than traditional cars? | I saw that the Tesla semi truck accelerates much faster than the traditional truck (sorry I don't have the number, just saw an animation) and that the roadster set a record for 60mph time. | Engineering | explainlikeimfive | {
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"Electric motors work very differently from 'traditional' petrol engines: their output torque only depends on how much current you feed them, not on RPM or other mechanical quirks. This is a huge advantage when starting from a stop because you don't have to wait for the engine to rev up or switch gears. Plus, it's a lot easier to create a electric motor with a lot of torque than an internal combustion engine (since the main thing you have to worry about is how much current you can pass through it)",
"Electric cars have no gears. They don't need to shift. Electric motors get their power faster than a combustion engine receives/burns fuel. Electric motors can generate maximum torque at 0RPM, gas engines typically have to build up torque."
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83ak7f | Why are buses not designed in a more aerodynamic way? | Engineering | explainlikeimfive | {
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"text": [
"Aerodynamics only matter at high speeds. Drag is proportional to velocity squared. If your travelling at low speeds (like city buses typically do), it's better to optimize for carrying capacity than for aerodynamics. If a bus was designed to be more aerodynamically efficient (rounded front, tapered back), it wouldn't have as large an interior, and thus couldn't carry as much people. Even for a Greyhound bus, which DOES travel at highway speeds, more people is better. The equation for Greyound's income for one bus trip is this: Income = (Ticket Price * Number of Passengers) - Fuel Costs - Operating Costs Making the bus more aerodynamic lowers fuel costs, but also reduces the capacity of the bus. The extra $$$ from additional passengers outweighs the slight fuel savings from a more aerodynamic bus.",
"Previous answers are correct. That said, buses that *do* frequently operate at highway speeds could indeed stand to be more aerodynamic. As with trucks, the large flat back is poor form, but adding a tail cone is a lot of work and cost and it would have to fold away very often."
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83bdgj | What are the differences between the Voyager Spacecraft and other Spacecraft? | Engineering | explainlikeimfive | {
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"text": [
"None, they are old but robustly built. Their capabilities equivalent to today’s satellite probes as a Model T is to a Prius. As it is some of the instruments have stopped functioning and others have been shut off to save power.",
"This is really way too broad of a question to answer. What other spacecraft? They're all different. Are you asking about spacecraft made in the same era or recent ones? Are you asking about comparing the Voyagers to Earth-orbiting satellites to planetary exploration satellites? Or both? We really need more to go on."
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83ffor | My car suggests using 93 gas, what would happen if I regularly used 87/89? | Engineering | explainlikeimfive | {
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"The number is the octane level. This basically tells you how easily the gas can be ignited. The lower the number, the easier it ignites. Now, your gasoline engine works by compressing gas, and then igniting it with a spark plug. The more the gas compresses, the more efficient/performant your engine. Unfortunately, if you compress the fuel too much, then it will ignite on it's own. This is called knocking and is bad for your engine, because then the fuel explodes in a way that wasn't intended. Modern engines are usually smart enough to change engine timing to prevent knocking. Anyway, combining all I said before. Higher octane gas allows more compression, and thus allows greater performance. If you use lower octane gas, then you either risk damaging the engine, or have substandard performance.",
"For many cars, nothing at all, except you saving money. Most newer vehicles have internal sensors that will help make sure you avoid engine knock (which is a bad thing, and something to avoid once you start driving in 11 years :P). Having said that, the answer can get a little more complicated based on your car. Here’s a longer (but still pretty readable) explainer: URL_0 TL; DR: if your car says it REQUIRES premium, I’d listen to it. My Miata required 93, and ran like crap on 87. My VW is the same way. If your car simply *recommends* the expensive stuff, you can probably get away without it (but you’ll want to do your homework for your specific model/year)."
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83j6a7 | How are railroad crossing signs able to go down when there is a moving train near by but when a train is stopped 10 feet away from the tracks the signs aren’t active? | Engineering | explainlikeimfive | {
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"From what I gather, there's circuits that sense the train coming from a ways off, because of the train's conductivity. I'm certain it can sense if it's moving, since the circuit will be opened and closed repeatedly because of the movement. In either case, the stationary train may not be sitting on the detector, and the detector may notice that it's not moving and therefore that it's not a danger. From that point it's just programming.",
"There are three sections to the train circuit. The two approaches and the island. The island is the area that goes across the roadway. The approaches determine the direction of the train. There are two types of crossing settings that I know of. CW- constant warning, and MD- motion detect. When a train enters a crossing circuit, the setting will determine how the crossing behaves. In CW, the gates come down for predetermined amount of time from when a train enters the circuit. In MD, the time it takes before the gates come down depends on the speed of the train. In MD, if a train enters an approach circuit and stops outside of the island circuit, the crossing gates will evntually go up again- the crossing will \"recover\". The gates will come down, or will be \"active\" when movement is detected again. Not sure if a CW crossing will behave this way as well.",
"I used to work for CSX as a conductor. There are sensors along the tracks on either sides, that detect the train coming and after it passes. If the train stops on the circuit for a certain amount of time the gates come up, also after passing off the circuits the gates come up.",
"Track circuits identify where the train is to the interlocking system. There are different kinds. Frequency tracks. Impulse tracks, DC tracks. Basically all of them hold up a relay and when a train is on a track it shorts out the circuit and the relay drops. The crossings are fail safe so you don't prove a train is there. You prove a train isn't there to hold the boom gates open(so if something fails the booms fall down). The crossing control relays are controlled by the track circuit. The approach tracks are determined to be a long enough distance depending on the speed of the line for the lights to flash long enough to warn the driver the booms are coming down. The booms to come down then a and buffer zone for the booms to be down before the train arrives. When the train is stopping just before the crossing there's a signal before the crossing and the boom will actually go down but come back up after the approach track has been occupied long enough to prove the train has stopped before the signal. If the train is departing the signal will not clear until the crossing is proved to be closed. Edited for spelling (still probably missed some)"
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83jt7o | "In Case Of Emergency, Break Glass". Wouldn't the glass go into your hand and wouldn't it cut you? | Engineering | explainlikeimfive | {
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"text": [
"Well no. Most of the time it's either cut in such a way that the pieces will break apart easily, or a small hammer/tool is provided to break the glass safely. Or you could just use any hard object you have handy to break it.",
"These days they often use weakened plastic instead, for just that reason. Usually if it's real glass they provide a little hammer or other tool to help you safely break it.",
"Depending on the emergency, it might be worth taking some glass to your hand in the event there is no other option."
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83r8ey | Why are variable propelling nozzles seemingly only fitted to jets with afterburners? | I've never seen variable nozzles on any other kind of engine. Example below. GIF: URL_0 EDIT: Spelling | Engineering | explainlikeimfive | {
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"text": [
"Most planes are designed to operate effectively at their cruising speed. They're going to take off, climb quickly to altitude, then spend a few hours cruising at a fixed speed and altitude before they descend. Their engines are optimized for this speed and altitude, and most of their thrust comes from the big fan at the front, not the exhaust from the jet. Aircraft with afterburning turbojets generally operate under different conditions. They need to be able to operate effectively at a wide range of speeds(both subsonic and supersonic), work at a wide range of altitudes, have a huge range of fuel rates depending on if they're cruising or hitting the afterburner, and generally need maximum performance over anything else(weight, money, fuel) The variable nozzle lets the engine operate more efficiently over the wide operating range of the aircraft and can result in increased thrust which is important to a fighter plane but not to jetliner. On an afterburning turbojet, all of the thrust comes from the combustion of fuel in the turbine so it is important to optimize the exhaust.",
"You may be confused about the difference between a turbo**jet** engine and a turbo**fan** engine (and also turbo**prop** and turbo**shaft**). All of those are \"jet\" engines, but they provide thrust in different ways. The engine in your gif is from an F15 fighter jet, and it is a turbojet engine. Turbojet engines provide thrust when the burning, expanding exhaust from the burning fuel exits the back of the engine at high speeds, and because of Newton's third law, the plane is propelled forward. The spinning fans in the turbojet engine don't provide any thrust. Instead, they are there to compress the air flowing into the engine. The more compressed the air is, the more efficiently the fuel will burn and the more power you get. On its way through the back of the engine, the exhaust passes through fans that siphon off some of the power to turn the fans at the front that compress the incoming air. The nozzle at the end focuses the exhaust to maximize the thrust coming out. The more focused the exhaust is, the more efficiently the engine runs, but if you try to focus it too much, it slows down the exhaust, which causes a \"traffic jam\" of exhaust, air, and fuel in the engine, which reduces your power and efficiency. Afterburners are a very inefficient way to add a significant amount of power when necessary by dumping additional fuel into the burning exhaust *after* the combustion chamber. It's not efficient because at that point, the air isn't as compressed because it's already expanding and burning, so the fuel doesn't burn as completely. The combustion chamber can only take so much pressure, so you can't just dump as much fuel as you want into it - not to mention that if you add more fuel than you have air to burn it, it won't work anyway. So you dump more fuel into the back just before the exhaust leaves through the nozzle. When you do that, you have to control the flow of air, fuel, and exhaust, *plus* you've just added a ton of extra pressure from burning, expanding exhaust, so you need to adjust the nozzle accordingly. **All** turbojet engines have the capability of using afterburners, provided they have an additional fuel line into that part of the engine, and also provided it can withstand the pressure. Some of them are just designed better than others. Turbo**fan** engines get power by using the exhaust to power a larger fan that pulls air into the *bypass* around the main core of the engine. As exhaust leaves the engine, it creates *inducted flow*, as air around the engine is pulled into the exhaust. The induction fan also helps. The result is that your exhaust ends up going a lot slower, since you're siphoning off some of the speed and power to accelerate additional air around the engine. Your engine goes slower, but it's also more efficient than a turbojet engine, at least at the speeds the turbofan is designed to operate at. I guess you could conceivably add an afterburner, but it wouldn't do you much good. It would just waste fuel. Turbofan engines don't use those same variable nozzles because you're not trying to control the exhaust in the same way. Your power does come from the exhaust, but you really care more about what the inducted air is doing. Many of them can control the airflow through the induction channel, though."
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83t4yk | What exactly is the Unification algorithm and how is it used by compilers to do type inference? | I’m having a hard time wrapping my head around the unification algorithm used in compilers. Apparently, one of its applications is type inference but I don’t understand how it works and why. Searched on wikipedia and stackoverflow but the explanation is quite dense and I’m looking for a layman’s explanation/high level overview. Here’s the unification algorithm I’m talking about [Wikipedia link]( URL_0 ) | Engineering | explainlikeimfive | {
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"Since this is ELI5, **how** it works isn't really important - the important thing is that **some algorithm exists** which can, given a bunch of variables (eg - integers, floats, strings, arrays, etc) and functions/expressions using them, figure out what the appropriate data types for those are. This is important for both ensuring the correctness of code (we had a running joke that \"If an ML program compiled, it was correct\") and providing optimizations during code generation. If there's a code path that could potentially try to add `\"apple\"` and `37` together - it means that something has gone wrong.",
"Let's start from type inference. Type inference is what we call systems that can figure out what sort of values your variables can take in code. For example if you have a function f like `f(x) = x + 1`, then you don't need to say that `x` must be a number. The compiler can figure that out by itself, and will not let you compile if you try to call f with something other than a number (such as by trying to call `f(\"hello world\")`). We say that f is of type `Number - > Number` (that is, a function that takes a number and returns another number) You can even do slightly more complicated things, like a function g: `g(x,y) = x + y`. You could call `g(1,1)` (which produces `2`), or you could call `g(\"hello \", \"world\")` (produces `\"hello world\"`). What you can't do is mix the two things. `g` is of type `(a, a) - > a`: A function that takes two things of type `a`, and returns another thing of type `a`, for some undetermined `a`. when you call `g(1,2)`, then `a` is `Number`. When you call g(\"hello \", \"world\"), `a` is `Text`. So, what about unification? Unification is the process through which we took the information that g is of type `(a, a) - > a`, and _unified_ that information with the information that 1 is of type `Number`, to determine that g in g(1,...) has to be of type `(Number, Number) - > Number`. By the time it examines the 2 in g(1,2), it already knows that it must be a number. (This is somewhat inaccurate, and strictly speaking even figuring out the type of `f` is unification in action. But I think this is a better illustrative example to get the rough idea of what's involved)"
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83zyau | How Do Musical Roads Work? | Engineering | explainlikeimfive | {
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"text": [
"The road has a bunch of bumbs in them, which when driven over produce vibration, just like sound. In order to get different notes, you just need to space out the bumbs to get a different frequency which changes the pitch."
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843jxu | US Interstate Nomenclature | Is there any reason freeways are named what they are named? | Engineering | explainlikeimfive | {
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"Evens are east/west. Multiples of 10 are major freeways. Generally ordered so that the lower numbers are further south and the higher numbers are further north. Examples: I-10 goes from LA to Jacksonville and I-90 goes from Seattle to Boston Odds are north/south. Multiples of 5 are major freeways. Generally ordered so that the lower numbers are further west and the higher numbers are further east. Examples: I-5 goes from San Diego to Seattle and I-95 goes from Miami to Maine. Three digit freeways are auxiliary freeways usually local to specific metropolitan areas. The second and third digits represent the major freeway that it is associated with. The first digit rougly describes the type of auxiliary freeway. If the first digit is even, then it is a loop (usually for bypassing the metro area). If the first digit is odd, it is a spur. Those aren't very strict... in some larger metro areas, they sometimes run out of first numbers. Examples: I-94 goes straight through Minneapolis and Saint Paul, I-494 loops south, I-694 loops north and I-394 is a spur to the west. There are a few other quirks. Sometimes there are E/W or N/S splits. I-35 happens to go through two pairs of \"Twin Cities\": Minneapolis-StPaul and Dallas-FortWorth. So, in both metro areas, I-35 splits into I-35W and I-35E."
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848n8s | Why do aeroplanes/airplanes have headlights? | Engineering | explainlikeimfive | {
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"> Why do aeroplanes/airplanes have headlights? Aircraft commonly land at night when it is dark. Being able to see the ground when coming in for a landing is important because they need to touch down at a reasonable speed. So not to put too fine a point on it, they are so the pilots can see when it is dark. You know, like with regular car headlights.",
"So they can see what's ahead. The lights shine on the runway, making it brighter and easier to see during both take-off and landing. They are called landing lights.",
"Ait traffic controller here. Often lights are attached to retractable landing gear. This helps to identify when landing gear is down at night and during the day. Some lights come on steady, others will flash to make them more visible. The lights also help a pilot see and be seen while taxing. They don't call them headlights though. Taxi lights or landing lights. There are a few other lights on aircraft"
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849z66 | Why are some rocket engines only able to turn on and off a specific number of times? | Engineering | explainlikeimfive | {
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"Assuming you're referring to liquid engines, because solids can only be ignited once: It largely depends on the propellants used and the engine design. Some propellants need an external ignition source that can only be provided on the launch pad, or some other consumable that once it's used, means the engines can't be ignited again. That's largely a design choice determined by what the engine is supposed to do. If it's a first stage or a booster, it only needs to ignite once and the stage is jettisoned after the propellant is used. There's no reason to add extra weight and complexity by provided more ignitions than will ever be used. Take the J-2 for example. That was the engine used on the S-IV B of the Saturn V. It ignited twice - once to finish the burn into an Earth parking orbit, and once again for the burn to the moon. It had 2 and only 2 containers of pressurized gas meant to start the ignition because it only ever needed to light 2 times. Some other engines using what's called hypergolic fuels can be ignited potentially an unlimited amount of times (not all are designed to, but many are). Hypergolic fuels ignite on contact with each other, so there's no need to any other ignition source. As long as there's fuel and some mechanism to combine the fuels in the combustion chamber, they'll always ignite. The only limiting factor there is the rated burn time of the engine itself."
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84eyi6 | Why weren't the sails unfurled all the time? | From what I understand – unless I don't obviously – they make the ship faster, don't they? So why keep them down? | Engineering | explainlikeimfive | {
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"You don't always want to go faster. Wooden sailing ships were not an exact science and many of them weren't perfectly stable or perfectly weight balanced. If there's a real strong tail wind blowing and the seas are rough, you didn't want to be slamming into the choppy waves at full blast at the mercy of the wind. In extreme cases the wind gusts could even tear the sails down, topple the masts, or in the most disastrous scenario of all, capsize the ship. On blustery days the old sailing ships would only have a few sails up for safety and sanity. On really bad days they migh not use any at all and just ride out the currents.",
"Why don't you always floor it when you drive your car? As fast as possible isn't always safe, easy to control, or would take them in the right direction. The amount of sails used, the angle they're at, etc. are how sailing ships were controlled.",
"They don't always make the ship faster, and can even make it slower. Boats have a hull speed ( URL_0 ) which is as fast as they can go without planing. To get a boat on plane so that it is skipping above the water instead of pushing through it takes a huge amount of power, and only the lightest racing sailboats can do it. So, if you are already at hull speed without your all your sails up, there is no reason to put more up, since you can't go any faster. If you do put more sail up, you'll be fighting harder with the rudder to keep course, the more you turn the rudder, the more drag in the water, and it'll slow you down. You'll also be putting more stress on your equipment. Also, the more sails up, the more the boat will lean/heal. A bit of heal is ok, but the more the boat heals it reduces the effective size of the sails, giving you less power. Which is why everybody sits on the high side of the boat, trying to keep it level as possible."
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84lnje | How does an architectural drawing get translated into a physical building? Who decides how many bolts, the type of material, etc? | Engineering | explainlikeimfive | {
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"I am a plumbing designer. The basic answer is that all the pieces go through many hands before a building is built and it depends on what kind of project. A structural engineer will determine what type of skeleton holds a building up, concrete footers, if steel and concrete, what columns and beams hold up the floors, ect. Framers work with them to determine trusses in wood frame buildings. On big enough projects(basically anything commercial and not just single houses), there is a design process for each trade. Engineers most of the time come up with plans for the trades but depending on the quality of engineers(usually cheaper bids taken and therefore corners are cut), the level of detail and similarity to what actually gets built will differ. I work for a company that installs plumbing hvac and fire protection systems. In the office we have project managers, estimators and designers(me). Between us we take plans that architects and engineers produce and turn them into workable and cost effective projects for our trades. Construction companies work on maybe a 5% profit margin so this is important because the plans we receive are generally not set up to be cost effective(knowledge of code and experience in the field is important to know what does and doesn't work and how to save money and be more effective with time). Jobs tend to change while in progress as well so we have to be able to adapt to make a functional building. Estimators bid a job based on rough plans. They determine what type of materials in a bid based on job specs, cost and codes. Designers take those plans after a bid is won and produce shop drawings for the field guys and do material takeoffs as well sometimes to determine what is needed for the project. Project managers take this info and convey it o the field foremen who make it happen on site. Hope this wasn't too wordy. Any other questions related to this are welcome",
"When you’re talking about things that detailed like bolts, it’s usually the building codes that take precedence because the construction company never wants to spend a dime more than they have to. For instance, when putting in a plywood shear wall, you need to nail it to the studs; the architect and/or structural engineers will (based on local building codes) usually put nail size and placement requirements directly on the blueprints (for example 12d @ 4” oc) which means 12 d nails, every 4” on center. In the real world, these are rarely followed “to a t” because things get in the way and we’re dealing with people.....Also, things come up like let’s say the plywood guy is 3 nails short and he’d have to drive to Home Depot to install those last three nails on a sheet of plywood that already has 97 nails in it. Chances are he’s going to say “meh, close enough” and move on. Inspectors aren’t checking every single sheet of plywood and counting every single nail and comparing it to the plans. Fortunately, architects/engineers/building codes apply safety factors into the design to account for everything from variances in building materials to “nailing issues” TLDR: these specs are usually on the blueprints. Source: I’m a structural engineer."
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84lvgg | Why do you have two cables going from one switch to another? | So, I work as a Junior Network Engineer, my colleague is a Senior Network Engineer, every time I ask any questions he says "It's pretty self explanatory" so I really get no help there. I would like to know why one network cable would go from one switch to another. I have two switches at home and would like to know if there is any point in doing this? | Engineering | explainlikeimfive | {
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"If you have one cable going from Switch A to Switch B, then you're simply joining two switches together. So 24 ports + 24 ports = 46 ports (2 used up for the connection). If you have two cables plugged into Switch A, both of which go to Switch B it's one of: redundancy (can lose a cable and the other one still does the job), bonding (twice the bandwidth with two connections), or a VLAN setup where the switch is internally split into segments, plus two cases of joining two switches together.",
"Switches connect Ethernet devices together so you actually have a network. If you have more devices than you have ports on your switch, your options are to either buy a bigger switch or connect the switches together so traffic can flow between all the devices. Many switches are designed with high-speed uplink ports (eg - a 10/100 switch might have one or two gigabit ports) specifically for this purpose.",
"If I understand your question correctly, it's for redundancy. If one fails, you have a backup already plugged in. Then you can trouble shoot and fix any issues without any downtime."
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84ooub | why do large indoor areas use one giant flat fan instead of a couple small fans? | Engineering | explainlikeimfive | {
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"The purpose of those big fans is not to \"blow\" air on the people in the area. They are instead for [destratification]( URL_0 ). When you have a building with a high ceiling you end up with very different temperatures on the floor vs. up at the ceiling. This causes a couple of problems. If your area is something like a gymnasium with bleachers where you have people at different levels vertically in the room, the people on the bottom will be feeling a very different temperature than the ones at the top. Also, if you have a large temperature difference from floor to ceiling, your heating system will be much less efficient and you'll spend a lot more money on electricity or gas to heat the area. Little fans aren't powerful enough to get the air all the way down to the floor.",
"You get more bang for your buck when it comes to surface area: usually the single set of large blades can push more air than several smaller ones. It also simplifies wiring & installation: when you have several fans you need to run several wires and drill several holes. The big guy is one and done."
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84ow73 | How can only one jet engine fly a twin engine airplane for hours after the other has shut down? | Wouldn't the thrust from one engine just propel the plane in a rather large circle? | Engineering | explainlikeimfive | {
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"For large passenger jets, the aircraft is designed to be able to fly with one engine shut down (an “in-flight shutdown, or IFSD). That’s why they’re designed with at least two engines. There’s a concept called ETOPS (Extended Twin Engine Operations, or, more humorously, “Engines Turn Or People Swim”) which means that if a two-engine aircraft has a single engine shut down, statistically-speaking it is a good bet that the other engine will NOT shut down by an independent cause within a certain number of minutes. How many minutes depends on things like engine reliability data, maintenance records, and maintenance crew experience. A twin-engine commercial aircraft is allowed to fly a certain route only if diversion airports are within the range of the aircraft, flying on a single engine, for less time than the ETOPS time limit at all points during the route. Modern twin-engine aircraft are ETOPS certified for quite a large number of minutes, making long flight times after an engine failure (thankfully) possible. Hope this helps. You can always do a search for ETOPS to get more info. Edits: minor grammar",
"All things being equal, yes. But planes have the ability to turn and this can be used to counter the effect of having an engine out.",
"The thrust from one engine is enough to turn the plane. In fact, there was a flight that lost it's hydraulic power and they attempted to land it exclusively using alternating engine power. [United Airlines Flight 232]( URL_0 ). If a plane is operating under only one engine, it will use it's turning mechanisms to counter the imbalanced thrust. As for how long they can fly, every plane has what is called an ETOPS rating. This is the number of minutes they are allowed to be away from a diversion airport. For instance, the Boeing 787 has an ETOPS rating of 330 minutes, meaning it is allowed to be in a place that, if an engine failure occurs, it has 5 and a half hours to reach a diversion airport. However, that doesn't mean you fly to that maximum duration. There will be a specific airport to defer to for any given location on your flight, considering both how far you are from the airport, and how suitable that airport is to handle your passengers. As an example, Cold Bay Alaska is a diversion airport for trans-pacific flights, but airlines also need a plan for how to get the passengers away from there (as well as housing, meals, etc.).",
"Most times, either the engines are close enough together not to make any noticable difference, or you can easily \"trim\" the aircraft to counter the rotation if they are farther apart, like an airliner. Even then, it's never a significant enough amount to throw the aircraft out of control. Aircraft are naturally very balanced, and most are designed so that one engine can power the internal electronics and provide enough thrust to maintain stable flight."
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84y1nn | When a heartbreaking situation like the bridge collapse at Florida International University occurs what kind of liability falls upon the engineers themselves, and are they subject to criminal charges like murder if negligence is proven? | Engineering | explainlikeimfive | {
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"They could absolutely be charged with murder, if they can prove a felony was committed. If say, hypothetically, it turns out that corners were cut on the construction materials or process so the guy in charge could keep the extra money for himself, that's embezzlement, which is a felony. The felony then lead to the death of another person. That makes that death fall under what's called a \"felony murder\" law, which states that if someone dies during the commission of a felony, it's considered 1st Degree murder. Subject to all the same penalties, including the death penalty. Doesn't matter how they died, just that they did. In reality, the company is probably looking at a huge fine, and several wrongful death lawsuits.",
"The standard in those contracts is generally that the designers/engineers can only be held liable up to the amount they were paid for the project. I do not do criminal law. So I have no idea what potential criminal liability exists. Edit: Also depends if the flaw was even in the design. The engineer would have no liability if the contractor was negligent and failed to follow applicable laws and regulations."
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84yewn | what happens to sewage on submarines? | I’ve tried googling this but the answers seem to be conflicting. They’re underwater for months on end, so must produce a lot of shit. What happens to it? Is it stored in giant tanks? Is it ejected into the sea - and if this is true, couldnt enemies follow a shit trail? Is it treated and recycled into drinkable water? Come to think of it, how do they store enough water and fuel for such a long period of time, are showers rationed? | Engineering | explainlikeimfive | {
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"Former submariner here that was on a nuclear powered sub. When we shit, shower, ect. wastewater goes into a \"sanitary tank\", which is basically a septic tank. There are different tanks for black and greywater, but that's besides the point. When the tanks start getting full, we go through an operation that pumps the wastewater into the ocean until the tanks are sufficiently low. We don't recycle any of it because it's easier to make fresh water from sea water than it is to make it from shit water.",
"> What happens to it? Is it stored in giant tanks? Is it ejected into the sea - and if this is true, couldnt enemies follow a shit trail? It is usually ejected into the sea after being stored for a time in a tank. The chances of being detected when doing this (mainly through sound) mean it is generally delayed until some level of safety is assumed. Considering it is done infrequently, the ocean is rather large, and other things poop in the ocean, following a trail of shit isn't a reasonable method of tracking down a submarine. Listening for the waste ejection could be though. > Is it treated and recycled into drinkable water? Some of it probably could be but they are surrounded by a suitable source of water already. They just need to get the salt out of it which requires a lot of electricity. Guess what nuclear submarines have in excess? > Come to think of it, how do they store enough water and fuel for such a long period of time, are showers rationed? The fuel of nuclear submarines is very compact and lasts for a long time. Conventional engine submarines would need to be within snorkel range and would burn fuel out of large tanks.",
"If you're interested in reading how handling sewage on a sub can go wrong, very very wrong... URL_0"
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84zdqn | What’s the difference in having an engine in the front and back of a car? What does that do for torque/power and does it effect handling? | Engineering | explainlikeimfive | {
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"It doesn't change the torque or power at all. What it does change is weight distribution. Distributing the weight evenly over all four wheels generally makes a car handle better. Putting more weight over the driven wheels improves traction, and thus acceleration. How weight is distributed also effects handling characteristics. Rear-engined cars, for example, tend to be a little \"tail-happy\" which means that if you aren't careful losing traction on a turn could result in the back end coming around and you going backwards into a tree.",
"Torque and power are completely dependent upon the engine and how it is built not how it is placed. Most cars are front engined and front wheel drive, this is just simple and avoids having components run the length of the vehicle. It allows for a large cabin space and trunk and puts a lot of weight on the front wheels providing good traction to the drive wheels Very very few cars are rear engined, pretty much only the Porsche 911. A rear engine car puts a lot of weight on the rear wheels giving you good traction for accelerating, but is prone to oversteering in corners since a lot of the weight is in the back. Mid engined cars are significantly more common than rear engine cars. A mid engine car has the engine in front of or over the rear axle. This helps keep the weight distributed a bit better, reduces the tendency to oversteer, but significantly cuts into the cabin space since the engine is in front of the axle. Almost all mid engine cars will have rear or all wheel drive Front engined rear wheel drive is probably the second most common and a hold over from older designs. It has the downside of a hump running down the floor to make room for the drive shaft to drive the rear wheels and limited weight on the rear wheels which can make it tricky to get moving in low traction situations compared to a front engined front wheel drive car. Pickup trucks are generally Front-Rear and are notoriously bad in slippery situations due to the lack of weight on the rear wheel when unloaded"
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84zw8r | How come cars, and other vehicles, need suspension (other than comfort of passengers) | Engineering | explainlikeimfive | {
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"Suspension helps distribute shock throughout the vehicle. If you didn’t have suspension, parts on your car would snap from constantly absorbing the full force of the impact experienced as your wheels hit bumps and potholes",
"Also steering and handling would be difficult without. You should go spend some time on a forklift (which have no suspension). Now imagine trying to drive that at 70.",
"suspension also keeps the tires in contact with the ground. it helps efficiency, traction, cornering. keeps the tires planted. so even a race car that drives on smooth race tracks have suspension (obviously less plush) to keep the tires planted in the slight bumps and deviation of the road when accelerating, decelerating, and making corners.",
"Let’s say their is a pothole. While driving over a pothole the size of a small plate for example, the tire dips in the hole and the suspension pulls it out. On the flip side, if there is a rock, the tire will raise into the body and reset down. Without this a car would constantly be shaking, it would be hard to drive on less than perfect roads, intense rattling could cause mechanical problems, and it would a lot of damage on the axils.",
"Suspension does a lot more than provide a comfortable ride. In fact, I would go as far as to say that ride comfort is a secondary benefit of suspension rather than its primary purpose. The primary purpose of suspension is to keep the surface of each tire in square contact with the road at all times and to distribute the weight of the vehicle over all wheels as best as possible under all conditions. Steering and traction suffer terribly when one or more wheels is not making proper contact with the road surface. Shocks and struts allow each wheel to move up and down largely independently of other wheels, ensuring contact on uneven terrain. Springs, dampers, and shock aborbers reduce the impact energy caused by a several thousand pound vehicle dropping an inch or two or hitting a small pit at high velocity. Without them, all of this impact energy would be transferred to the vehicle itself where it would be absorbed by mechanical and structural components. Whereas dampers dissipate energy by heating up a viscous fluid or forcing a gas through a tiny hole, mechanical and structural components don't dissipate energy, they absorb it through plastic deformation. Without suspension, a vehicle moving over uneven terrain will eventually shake itself apart."
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850884 | How were the first perfect objects manufactured? First straight piece, first perfect circle, first perfect sphere, first perfectly straight sword, etc.? | As the title says. Thank you! | Engineering | explainlikeimfive | {
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"There's no such thing as a \"perfect object.\" In fact, we use a concept called \"significant figures\" to tell you how confident we are about a measurement. For example, if I say something is 100.00 meters long. That actually means that object is 100.00 +/- 0.01 meters long. So it could be 99.99 m to even 100.01 m. A perfect object would have an infinite amount of significant figures. This is completely impossible, absolutely everything we will have some estimation to it. Even with modern technology, your measurements aren't perfect. They are so good it doesn't really matter, but they aren't perfect."
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85apef | What creates the pressure in a residential public water system? | Engineering | explainlikeimfive | {
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"There are two ways. The first, as the Romans did, is to obtain the water from a high source, a lake in the mountains, or a river upstream from the use point. The second is to obtain the water from a lower place, often water is obtained from wells. It is pumped up from the water table. Once on the surface the water is stored in a tank. The tank may also hold a bubble of water under pressure which keeps the pressure fairly constant. A pump will kick on as the pressure drops. The tank may also be on stilts above the ground. Depending on your location you may see water towers above the ground, or you may be like me where the water source comes from a lake in the mountains. It feeds down to the water treatment plant on a hill above my house. After treatment the water flows down to my water pipes. Because of the elevation change pumps are not actually required to supply me with water. In Rome, women looking for work would hang around under the arches of the aqueduct. Their name derives from 'under the arches.' At Ft. Polk when it was a busy fort, certain women looking for work, would hang around under the water tower.",
"This is what water towers do. At night when power is cheap pumps fill the tank. During the day when demand is high gravity provides the pressure"
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85gitt | Why do large trucks have their exhaust pipes on the side of their cabin? | As the title asked, Why are the exhaust pipes of trucks on the sides of the truck, also, why do most of them point upwards? | Engineering | explainlikeimfive | {
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"That's the only place that they can go. They cannot come out of the bottom at the rear like most sedans because that would eject the hot exhaust gasses underneath the trailer, causing all sorts of problems. They cannot come out of the bottom at the side like many pickup trucks because the high volume of hot -- and sometimes sooty -- diesel exhaust would obscure the driver's view of the trailer wheels. This would also be extremely unpleasant for anyone in a vehicle beside the tractor or any pedestrians on a nearby sidewalk. Ejecting the exhaust to the rear of the cab straight up into the air ensures that the driver's view is clear and prevents the exhaust from causing any problems of its own.",
"Their engines are very powerful and so need more than one exhaust pipe. This in turn means there is not really enough space for them under the truck due to the equipment needed to attach the trailer rig, and the extra wheels they have for traction. So they have them go up the side for space, and send it up above the cab so that it makes the air a tiny bit cleaner to breath at the truck stops when they get out. They also tend to have air horns using one or more of those pipes and those need to be high to best spread the sound.",
"you mean like large work trucks? by venting up, you dont spew exhaust on your trailer or someone standing behind the truck while its idling. Also reduces the smell for vehicles driving behind them."
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85ihh3 | How do submarines resurface, or go up in general? | Engineering | explainlikeimfive | {
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"Replying as a serving submariner, so trying to use ELI5 terms: The vast majority of depth changes are done using hydroplanes. Try keeping your hand flat (horizontal) and push it through water in your sink. It should move easily and you won't feel any push up or down. Now do the same with your hand tilted upwards - you should feel a push up on the bottom of your hand. By pushing themselves through the water using various means, and by altering the angle of their hydroplanes, the subs can dive or come shallow as they need. Many submarines have two sets of hydroplanes, one at the forward end and one at the aft (rear) end. The forward set of planes are the ones that primarily change depth, and the aft ones are used to control the boat's 'trim' (think angle of the boat with respect to the ground). You can still control the boat's depth using the aft ones if you had a problem with the forward ones though. Now, what we haven't covered yet is buoyancy. The sub's weight changes as depth changes - the deeper you are, the denser you are as the boat gets squeezed tighter by the additional water above us. So you get this effect where if you're heavier than you should be, you might start going deep. This makes you compress, get more dense, and start going deep quicker etc etc. Not good. To counteract this, the Sub control team try and keep the buoyancy of the boat as close to neutral as possible - if we go a bit shallow, we bring some water into some of the tanks in the submarine, and if we go deeper we pump some water out, keeping us neutral. You could use this effect to surface, but the pumps are pretty slow as they have to fight against sea pressure. It would take some time. If you have an emergency and want to just get to the surface NOW, you can push out lots of water from tanks outside the submarine using high pressure air stored in large bottle groups. This will make us very buoyant and help get us to the surface, but uses up air which we can't get back until we have surfaced. Nothing beats having propulsion and planes to get us to the surface - if we were flooding and super-heavy, pointing up and pushing hard will get us back up there. There are various systems that work in different ways involving these three methods - planes, pumping water and using high pressure air - but without going into (sometimes classified) detail these are the basic principles behind all of them. Edit: clarified a sentence, and thanks for the gold! Now the wife thinks I'm Internet famous! (she does not use reddit despite my continued supply of r/aww pictures)",
"Think of a sub like an airplane, and the water as air. Subs have what are called dive planes, which are essentially wings and can be rotated much like the flaps on an airplane. Unless there is a major emergency, a sub always \"drives\" itself to the surface. The high pressure air is what's called the emergency blow system, and is only used in an emergency. Source: naval shipyard pipe fitter. I'm not an actual submariner but have been on, under and in many of these things.",
"Source: was sub officer Submarines use both buoyancy control (levels of air/ water (and even poop and oil) in various tanks) and their dive planes (wings like an airplane) to control depth. They can use one or both depending the mission/ reason for surfacing. It a complex dance, however, because the fine tuning of the buoyancy is slow to react and changes with water temp and how much the dive planes work depends on how fast the submarine is moving. For periscope operations, you generally try to get the buoyancy neutral, or slightly negative, and control depth with just enough speed for the planes to work. (Slightly negative because it's better to sink the scope than pop up and be seen)",
"Is it still true that submarines can't come to a complete dead stop under water like in the movies? I was told they had to maintain forward movement at all Times.",
"There are two things going on. First, lets talk about the small boats. Small submarines, little research vessels. For example, Alvin. They build the submarine to be \"neutrally buoyant\", that is, it won't sink or float, just stay where you put it in the water column. Use a small tank of air to stay floating, and then they fill that tank. when they fill the tank, the boat sinks. To surface, Alvin has some ballast it will drop, and the boat becomes buoyant again, and comes to the surface. For \"Big submarines\", they're flown, much like airplanes. They rarely sit still, and fly. Going up and down, is done with dive planes, on the front and rear of the ship. That's the \"first\" way that they control their depth. You'll notice that big submarines sit WAY above the water when they're on the surface. This is because they have really big tanks to provide flotation. To go under water, they fill those tanks with water. This is the second way they control their depth. Now those big boats can change their weight quite a bit. People and machinery move around a lot too. Also, the density of water changes, both with temperature, salinity, and somewhat, depth. The big submarines have trim tanks, and ballast systems to keep the boat veritcal (an even keel), and level horizontally (fore and aft). Those trim tanks, also allow the boat to chose to rise, or submerge without moving forward. As long as the boat is moving, most controls are done with the dive planes. Hopefully that's ELI5 enough.",
"They have tanks filled with sea water that makes them heavy enough to go down. So to go up, they blow air into those tanks, forcing the sea water out, making the boat lighter, thus going up. To go down again, they let the sea water back into the tanks. Edit1: They used to have 2 types of air to put into those tanks. When they weren't in a hurry they would use 10 pound blowers to push low pressure air into the tanks. When they needed it done faster, they used 600 pound high pressure air they stored in cylinders. Since they couldn't easily replenish that high pressure air at sea Edit2: Nowadays they do it in reverse: They use the high pressure systems to get most of the way up, then use the low pressure systems for the last part. That's because they can now easily replenish the high pressure air when surfaced",
"Submarines are airplanes for the water. The basic principles are the same but the terminology is different. For example instead of altitude you say depth. The control planes in conjunction with the ballast tanks control depth.",
"U.S. submarines have 2 sets of wings (fairwater planes, stern planes). By propelling with the screw, basic hydrodynamics drive depth (water moves around planes). To assist with depth submarines also have ballasting features to either preferentially sink/rise/float by moving water in and out of multiple tanks inside. In emergencies, a high pressure air system allows rapid rise to the surface (under certain conditions). TLDR; Manta ray wings + propulsion, moving air in/out, moving water in/out allows changes in depth and surfacing/diving. Source: Was submariner",
"Submarines have ballast tanks. The tanks are filled with water to go down. when they want to go up, they flush air into the ballast tanks, making it go up. To control elevation (up and down angle) they have hydroplanes on the aft and bow. They raise and lower the hydroplanes to control the angle of elevation. Using the propeller shaft they can control how quickly the submarine raises and lowers. Additionally, an \"emergency blow\" is when they fill the ballast tanks and go into the highest elevation possible to get up as quickly as possible. That's how subs \"leap\". ( URL_0 ) In the same sense, there is a crash dive, to get to the bottom as quickly as possible. same thing happens in reverse by filling it with water and turning the hydroplanes all the way down. *This is like a ELI3",
"There's actually two answers to two different questions. General submerged operations use a combination of control surfaces (stern planes back aft and fairwater/bow planes up forward) and speed to change depths. Water is pumped on or off to fine adjust the overall weight of the boat, so that it can maintain depth without the need of control surfaces. For submergence and surfacing, there are giant ballast tanks with vents forward and aft. On the surface, the vents are shut, and the tanks are (mostly) empty to provide buoyancy. When submerging, the vents are opened, and the tanks are allowed to fill. This massive change in weight is the rough adjust for buoyancy and allowed the boat to submerge. When resurfacing, air is blown in the tanks displacing the air and making the boat buoyant again. Source: former submariner",
"In order to dive a submarine a set of tanks are free flooded with water. This is what initially gets the sub down. As others have alluded to, the planes can also be used to control depth and these function in a manner similar to that of airplane wings. When under water the buoyancy of the submarine can be affected by density of the surrounding water, so additional tanks can take on water to maintain neutral buoyancy. It is not possible to surface a submarine only by emptying these tanks, but the sub can be brought close to the surface with these tanks. These tanks are typically emptied using pumps. In order to surface the submarine, the free flooding tanks that were filled to dive the submarine are the tanks that get emptied. These tanks have safety features that always maintain a configuration that allows the submarine to surface or remain on the surface. The tanks are emptied by blowing high or low pressure (depending on depth) air into them and forcing the water out leaving the submarine too light to remain underwater.",
"There are many different kinds of submarines. Most use [dive planes]( URL_1 ), which are like fins on fish. If they are tilted down, the submarine goes down. If they are tilted up, the submarine goes up... provided the submarine is going forward at the time. Other submarines use [ballast tanks]( URL_0 ). They fill the tanks with water when they want to sink, and fill them with air when they want to surface. Still other types of submarines (mostly research and maintenance subs and robotic submarines ) have thrusters. These are small propellers that point down when they want to go up, or point up when they want to go down. Some thruster types pivot to push the submarine where they want it to go. Other thruster types are fixed in position and the submarine will have many of these pointing in various directions. Most submarines and submersibles use a combination of these things to control their vertical motion. For example, I was in a tourist submarine in the Caribbean a while back. They used ballast tanks to get the submarine to a point of having a small positive buoyancy and then used thrusters to push us underwater. The point behind that was that if the power failed, the sub would naturally float to the surface. They also had ballast plates, huge chunks of steel, that they could jettison if all else failed.",
"As another former submariner, it's worth mentioning that oftentimes we (as submariners) get a \"lies to children\" level of explanation as to *why* the things we do work. Which is to say an explanation that is technically incorrect but leads to the same assumptions about operating the actual machine that would cause it to be operated correctly in 99.9% of all cases. It isn't really apparent that we've been taught wrong until/unless we try to go into that field in the civilian side. That being said, here's my version of the ELI5 for how this actually works. Submarines, like normal ships, are buoyant. On the outside the ship they have big tanks the fill with water to a point that the submarine is neutrally buoyant. At that point, the submarine *can* sink. In order to submerge the submarine, the \"planes\" (hydroplanes) are angled downward, much like an airplane's wings, such that more water flows over them than under them. This causes the force of water (from the submarine moving forward through it) to submerge the ship. In order to resurface the ship, generally the planes are merely angled the opposite direction, gradually surfacing the ship. It's worth mentioning that this is dependent on the ship having propulsion; in the absence of any other factors, if the ship has no propulsion it doesn't go anywhere (it actually generally sinks because our \"neutral buoyancy\" is a little on the heavy side but that doesn't matter for this discussion). \"Neutral Buoyancy\" is a constant process- at any given moment a senior watchstander is maintaining this as a primary duty by moving water from tank to tank and taking more on or more off based on the current heading and level of the ship. These can change due to a variety of factors, including what depth we intend to maintain, what temperature the water is at, and whether we will be moving any non-water weight in the ship soon (or have already moved it). There is one other method of surfacing, called the \"main ballast tank blowm,\" which results in [the rather more impressive, whale-like surfacing you see in movies.]( URL_0 ) This is not routine. The main ballast tanks are large, partially external (in between the main pressure hull and the exterior hull of the submarine) tanks that contain the bulk of the water used to submerge the ship. These tank levels are maintained by passively flooding or draining the tank by regulating pressure; they have a bottom \"drain\" and a top \"vent\" and as long as the ship is mostly level the tank level remains the same. The emergency blow works by shutting the vent and then injecting several thousand psi of air into the void in short order, which forces the bulk of the water out as well as generating a very large air bubble. The change in volume moves the ship upwards. The emergency blow with whale-like surfacing only happens very close to the surface; its effects are less impressive deeper down. It is, as the name implies, for emergencies (and demonstrations) only. Hope this helps, please send another message if something I said didn't' make sense, with the added caveat that my own understanding is only a *little* better than the base ELI5 level since this wasn't my job. ;)"
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85oebp | On WWII propeller planes, how did the nose guns shoot without damaging the blades? | Engineering | explainlikeimfive | {
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"With a [interrupter gear]( URL_0 ). The technology was developed in WW1. It's a simple mechanism that blocks the gun from firing when the propeller blade is directly in front of gun.",
"Though it was limited to WW1, it's too funny not to mention: earlier solutions to this problem involved reinforcing the propellers with steel deflectors to ricochet bullets in (hopefully) harmless directions, then just shooting, in the hope that most bullets would make it through. This saw success, even though bystanders were killed by ricochets at the first demonstration of this technology. Earlier even than that, pilots would snipe at each other with handguns and rifles. I've even read of a brick being thrown. Earlier even than that, to even fly was so novel that there was a collegial atmosphere in the air. Planes were used for reconnaissance, and pilots would exchange friendly waves. Earlier even than that, we hadn't invented planes.",
"This was more of an unknown in WW1, actually. The guns were synchronized with the propellers, which aren't in some random location, but in a very predictable position based on position of some shafts. Here's a picture of how it works. URL_1 From this article. URL_0",
"when you hold the trigger the gun doesnt fire until a little dingaling on the rotor blade shaft (inbetween two rotors in the empty space) hits a whatchamacallit, but since the blade spins so fast your dont really notice a delay. therefore if the gun is mounted on top of the aircraft at 12:00 it only fires when the rotors are at 9:00 and 3:00. Thats as eli5 as it gets.",
"The answers given below are correct. Interrupter gears were used, and if they ever failed it wasn't a big deal, the pilot could glide to a safe landing (hopefully behind their own lines). Some alternative designs had the pilot or bombardier able to stand up in their seat and fire a gun mounted to the top wing, over the propellor arc, also useful as it could slide back and fire directly upwards, so one could fly underneath an enemy and shoot upwards into the plane above.",
"If the gun was shooting through the same space as the propeller, there was a timing mechanism incorporated between the gun and the propeller so that it only fired during the space between the blades. A much cheaper alternative was to mount the gun above or below the propeller."
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