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c7rjrj | How do they make fireworks spin or spiral rather than go in a straight trajectory? | Engineering | explainlikeimfive | {
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"By making them unbalanced. Imagine balancing the firework on your finger; if it rolls or falls off your finger, or you have to make a lot of adjustments to keep it balanced, then it will tumble (spin or spiral) if you were to replace your finger with a thrust like a little rocket engine."
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c7vcjy | Why do some charging cables charge faster than others? | Engineering | explainlikeimfive | {
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"The wires that carry current to the battery can be different circumferences . Bigger ones carry more current but are more expensive. In all likelihood the company that made the one at the gas station didn't invest much in its design.",
"There's two distinctly different things here: the charger (really an AC/DC converter) and the cable. First, the charger. All chargers will be designed for the same or similar voltages, in the US 110-120 Volts AC in, 4.7V - 5V out. The difference will be the current output, an iPad charger will output a higher current than an iPhone charger. In fact, the iPhone charger will most likely not keep up with the demand of a full sized iPad, at least while in use, and will just slow down the battery drain rather than truly charging it. The iPad charger can be used to charge the iPhone though, and does so much quicker. In short, converters will charge at different rates depending on their current rating. Now on to the cable. On a very basic level, in DC power there are two factors: length and diameter. The shorter the length and the larger the diameter, the less resistance and therefore more efficient current delivery. So if you have a very long, very narrow diameter cable, it's possible that this could have an effect. Average diameter, and 10 feet or less though, I can't see that having an effect. Not to scare you, but if a cable/wire is too thin for a delivered current, it's possible to melt/burn. That said, hopefully consumer charging cables are overbuilt to eliminate that possibility."
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c84zy2 | Why does touching the bulbs from headlights damage them but most other lightbulbs are fine to touch? | If you've ever changed your headlights yourself someone has probably given you the standard spiel about not touching them with your bare hands because the oils on your skin will damage them. But why is this an issue for headlights but not other types of lights? | Engineering | explainlikeimfive | {
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"To sum up, you leave oil from your fingers after touching a halogen bulb. When you turn the bulb on, the spot with the oil will heat up more than the rest of the bulb and can also cause the spot to warp and crack. All of this will cause the bulb to stop working early. This extends to any halogen bulb, car or house.",
"Lighting tech here, a lot of lamps require this kind of treatment. Basically only leds and household lamps have protection.",
"Car headlight bulbs run a lot hotter than most of the other ones we come across on a regular basis. 300 degrees F (about 150 C) isn't unusual. Pretty much any bulb that'll see those temperatures or higher will have that warning."
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c8ly03 | How do prosthetics work? Like the really good arms with the individual moving fingers? | Can you have an arm that's like all cyborg, if you just have a nub/stump left? | Engineering | explainlikeimfive | {
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"Most prosthetics with internal mechanisms work by using the muscles left behind after amputation in the upper limb. Electrodes are placed on them, and contraction of the muscle triggers the mechanism to move joints or extremities in the prosthesis. Others have toggle switches that can be operated by the amputee manually. If you had no remaining muscles, there's no way to currently get a prosthesis to function using brain signals (like regular muscles would). Research is ongoing, but it's still early in development."
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c8mocp | How are logic chips (AND/OR/NAND/etc) actually built to work like their chosen logic gate? | Like with an AND gate I understand how it works in that an input of 1 AND 1 make an output of 1, but how do the logic chips actually perform the function? How would you build a logic gate without using logic chips? | Engineering | explainlikeimfive | {
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"The basic building block of electrical/electronic gates is the transistor. You can think of a transistor as an electrically operated switch. This switch has an input, output and a \"switch\" (also an input). If the switch is \"on\" it connects the input to the output, if the switch is \"off\" the input is disconnected from the output. By differing configurations of this switch (several transistors connected in different circuit patterns), all logical gates can be constructed.",
"URL_0 This video might help you out. Demonstrates the actual construction of a chip.",
"The basic building block is a transistor, which is basically just a little switch with three connections: the input, the output, and the gate. If the gate value is 1, the output value is the same as the input. If the gate value is 0, the output value is 0. An AND gate can be made with two transistors. You connect each of the inputs to the gate of one of the transistors. One transistor has the input connected to a constant 1 source and the output connected to the input of the other transistor. The other transistor's output is the output of the AND gate. As long as both transistors are switched on, the 1 value will flow through and out to the output. If either is switched off, the output will be 0."
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c8tbcz | Why can't we use materials with lower boiling points to generate power? | My understanding is that water is commonly used in coal-fired power plants and the like, and that water has a rather high boiling point. Wouldn't using something with a lower BP like acetone produce the same amount of energy at a lower temperature, making it easier to generate with heat sources like the sun or geothermal? I also understand that according to thermodynamics more energy can't be gotten out of a system than is put into it, but shouldn't the turbines spin just as fast no matter what gas is being passed through them/what temperature that gas is? | Engineering | explainlikeimfive | {
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"Water's volume expands 1600-1700 times when it turns to steam. That increase in volume allows the pressure it creates to be used for mechanical motion. Acetone doesn't expand that much, so it is not as efficient or useful. Acetone is also quite dangerous when heated while water is not.",
"> also understand that according to thermodynamics more energy can't be gotten out of a system than is put into it, but shouldn't the turbines spin just as fast no matter what gas is being passed through them/what temperature that gas is? this is correct, in fact the boiling point of the liquid used is irrelevant for the engine. The efficiency of the engine is largely determined by the temperature difference between the hottest and the coldest point in the cycle, so using another liquid is not going to help there.",
"You are correct that it doesn't matter what is spinning the turbines, as long as they are spinning. The reason that we use water instead of something else comes down to two reasons. First, we've got lots and lots of water, so it's easy to get and super cheap. Second, water is good at making pressure. The turbine spins because the evaporated stuff takes up more space than the liquid stuff, and that creates pressure. Water takes up more space than a lot of other things when it evaporates (or freezes). There are certainly going to be things that are better at making pressure (e.g. Pepsi), but the cost-to-output isn't going to be as good because of that first reason.",
"You actually want the high boiling point. We want the fluid to have certain properties when it comes out of the boiler. and when we have a high boiling point, we can put in more energy. Also, water can store a lot of energy before it actually changes temperature, way more than acetone. The turbine speed and torque is going to be based on super complicated fluid mechanics stuff that I don't know to well. Edit: I made a mistake in what causes the speed/torque",
"They do precisely this at many geothermal plants. So-called \"binary cycle\" plants like Casa Diablo near Mammoth Lakes, CA and the now-famous Puna Geothermal Venture in the area of last year's Kilauea eruption, use a working fluid like butane as it has a lower boiling point and therefore works with lower temperature (100-300 degree F) heat sources. A typical binary-cycle plant pumps hot water (usually brine) from a geothermal field, runs it through a heat exchanger (basically a tank with tubes running through it) that boils the low-temperature working fluid, which then, as a gas, runs a turbine generator. The butane is condensed and recirculated through the process just like with a water-based power plant. The brine is then pumped back into the field to be re-heated, and the cycle repeats. Incidentally, there were a few experimental plants (one in my father's hometown of Hartford, CT if I recall correectly) which used mercury(!) as a working fluid in an attempt to boost efficiency. They didn't take off due to mercury hazards and other issues, and modern combined-cycle (gas turbine generator + steam generator) plants achieve far better efficiency. One could get into enthalpy, superheat, heat rates, supercritical (once-through) cycles, etc., but after all, this is ELI5.",
"Liquid water expands in volume around 1650 times when it boils into water vapor. The expanding gas pushes on the blades of the turbine and is what actually transfers the energy into electricity. The turbine converts the energy stored in the pressure of the gas into mechanical work and electricity; the expanded gas after the turbine will be at a lower pressure. After the turbine, you actually have to cool the low-pressure gas back down into a liquid so you can loop it back to the starting point and repeat the process. The increase in volume between the liquid and the gas phase is really what drives the process, not just that the gas is hot. So, materials which expand a lot during boiling are the best. Luckily, simple water is one of the best in this category.",
"1. Water is used to convert the thermal energy of burning coal into mechanical energy of turning a turbine that then produces electrical energy. So you aren't concerned with the boiling point too much. Instead you want a high density liquid so that you end up with more pressure in your gas. 2. A bigger concern is you want a chemical that isn't very reactive. When you boil it and then condense it back to a liquid you don't want it to break down into other chemicals. Water is pretty low reactive so you can boil it and condense it thousands of times and not end up with anything but water. 3. Safety!! You want a chemical that possess as little safety concern as possible. Acetone for example is extremely flammable and poses a minor health risk. This means if there was a leak in your boiling vessel of acetone, the gas could ignite causing a massive explosion. And if it didn't you would need special chemical suits to protect your maintenance personnel while repairing the leak. Whereas water is not going to have any fire hazard and the only health risk would be temperature."
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c8wgd9 | What are all those crazy sounds an MRI makes? | Just got out of an MRI scan and spent a few or many minutes pondering why there are squirrels in the walls of this machine and who gave them hammers. | Engineering | explainlikeimfive | {
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"The MRI uses a very powerful electro magnet which needs to be switched on and off for the MRI to function - URL_0",
"The noise is actually the shifting of magnets inside the mri, causing gradients over the magnetic field. Minor changes in the magnetic field causes frequency changes of water atoms in the body. When an electical radio pulse is sent, it ‘charges’ a particular water atom in the body. It is that particular water atom with that particular frequency. By changing the gradients (the sound you keep hearing) you can change the water molecule you want to see in your image. The inside of an MRI machine is not at all the same as a CT machine."
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c8wzx5 | How do cities not collapse if there are hollow spaces underneath them like sewer systems and subways? | Engineering | explainlikeimfive | {
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"Many of these subway systems go underneath roadways while the skyscrapers that are encircled by these tunnels are literally anchored into the bedrock below. There are also lots of people who spent many years in school to figure out how much weight can be supported above these tunnels in the ground",
"How do roofs not cave in if there are hollow spaces underneath them like attics and bedrooms?"
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c98o1g | How do the factory pipes and vats that produce something like biscuits or mayo stay bacteria and mold free if it's running 24/7/365? | Engineering | explainlikeimfive | {
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"URL_0 You basically just run chemicals through it and/or spray things. They would either not actually be working 24/7 or would have different assembly lines/stages that get shut down for cleaning periodically.",
"As others have stated, most machines get shut down periodically to be cleaned in place. To add to this, most factories will have multiple machines so that production can continue while one line is down. I was helping wire up unrelated electrical equipment in a bottling plant once. They would shut one line down and flush cleaners through the pipes and nozzles that carried beverages. Meanwhile, workers would spray, hose, and wipe down external surfaces. All the while 9 other lines are bottling, and once this machine is clean and running again, another will be taken offline for the same process.",
"Most of those are not ‘constant run’ but are, in fact, run by batches. And the same system can be used fore different products. Cleaning/disinfecting can be done between batches, a more intense cleaning is done if changing products, In some instances, disinfecting os part of the process.",
"I’m so pleased to hear so many positive experiences about cleanliness. I honestly half expected horror stories about when they finally opened some of these things up.",
"The plant stays in production 24/7, but individual machines do not. Machines/equipment are regularly broken down, cleaned and repaired, then reassembled.",
"Former large scale food producer here. Basically after every cycle you do a clean, which normally includes spraying super hot water and adding different soaps or cleaners. Then you do a microbe check that shows the ppm of any bacteria that may be left.",
"I'm a sanitation coordinator for a factory in WI that makes some damn fine cheese. The answer to your question is that they simply don't run 24/7. All factories, at least once in a 24 hour period, have to shut down production and perform a cleaning of the equipment. You can do C-I-P (clean in place) where you change a couple connections to run acid and caustic washes through the lines, followed by a sanitization step shortly before production begins. At the factory I work at, we run short washes between each pump over and refill of the cheese vats until we meet our quota for the day, and then we do our daily sanitation work. For the smaller pieces, there are C-O-P tanks (clean our of place) where we break connections and clean them away from where they normally are when we are in production mode. At least where I work, we are able to step sanitation because of how long the process takes from pump over to emptying the tables of cheese, so we tend to start filling the vats back up around the time that we are finishing our last table. It takes several hours, so we are able to clean the block-forming towers after the tables are cleaned. Hope this clears things up.",
"they don't run 24/7. often they'll have sterilize passes where steam at 150+ Celsius and a few Bar are passed through the lines to decontaminate.",
"They \"usually\" have third shift that is considered sanitation duty. Between the end of 2nd and a few hours before 1st shift. Every place is different tho, the place I used to work at ran their production this way.",
"Most of these answers talk about CIP, but I read the OP as how are they able to run more than 24 hours straight and thought I’d give some examples of what can keep the bacteria at bay for 24-72 hours, maybe slightly longer. Bottling: citric acid will keep the pipes clean and pathogen free and enable pretty long runs...a day or too; think about the acids in Coke and Pepsi If you keep your liquid at a high enough temperature you can kill many of the pathogens with some product lines: Gatorade and brewed tea for example Bottling water: Ozone in the water basically sanitizes everything the water comes in contact with, allowing multi day runs. Running alcohol through the pipes and processing equipment will keep the pathogens at bay for a few days. Spirits and beer. Also you can essentially run your product at temperatures that provide an environment where pasteurization is occurring to the production process, or you could also set up a production environment if you had big enough equipment to run your product at extreme pressures that provide pascalization of the products and equipment during the production process. This would allow a few days of continuous production depending on the product. None of these processes above kill all bacteria, so eventually you have to get to a point of CIP using heat, chemicals and motion to sterilize and kill the tough bugs.",
"I work in a quality control lab for a beverage producer. Nothing runs truly 24/7. There is always down time to clean and sanitize. Production might want to run 24/7 but it cant. There r rules and protocols put in place. These protocols are figured out by testing and becoming qualified. For example some products can be run 72 hours straight with out a Sanitization process. Some less then 24 hours. These are figured out by doing micro testing. You take samples at different times and test them to see if you can get certain bacteria like for example yeast and mold to grow. If you pass the Company and government qualifications then you become qualifies. It all depends kn what your producing. Hope this was a short easy explanation.",
"An interesting thing to note is that there is a difference between “Ready to Eat” foods and “Ready to cook” foods. The ready to cook foods have lines that are cleaned way less regularly and more quickly. I think it’s something that if people saw for themselves, they would be shocked. All the top rated comments seem to be describing ready to eat foods, where quality is more important.",
"Production worker in a milk plant here. We run product just during the day, and we have wash systems that run a multi-step wash cycle through every silo and set of pipes after use/draining. Plus after each different product ran the lines all get a flush and sanitize",
"On a related question if a pipe have milk or similar liquid flow costantly does it need still cleaning. There is drinking fountains in rural regions of my country. They run all year without stopping and no one ever cleans them. The ground water falls is mossy but pipe itself is clean and water is always drinkable.",
"Most factories will have shutdown periods to clean, repair or upgrade equipment. There may also be secondary systems used while the primary is sanitized. In short, no factory actually run 24/7/365",
"I used to work for the cleaning company ISS. I did cleaning in a section of a factory that processed peas. Every morning at 5, it would be shut down and we would have 4 hours to clean it all. High pressure water, soap it up, wash it, disinfect with some chemicals and wash it",
"Have worked in the food manufacturing industry for 5 years as a mechanic. Facilities that operate under the USDA sanitize every food contact surface once a day as well as everytime a non-food grade item comes in contact with a food contact surface. We also do far more in-depth cleanings frequently and on a rotating basis.",
"I work for chobani, the greek yogurt company. They don't run 24/7, they stop and clean the pipes with a CIP system. Also, it's very rare to open a hatch or really anything that exposes the yogurt to any kind of contaminant. Dairy is a different world since there is so much bacteria and live cultures, the government requires higher standards for production and cleaning .",
"Any factory that produces food like that will have a cleaning protocol. They will eithet shut down for a while to clean at certain times or use product switches to clean. Even a factory that makes the same product will have these switches. Different costumers will have different wrappings or slightly different ingredients requirements. Different brands are often made in the same factory. Thats why cheap brands are often so similar to expensive brands",
"Hi, prior mix room worker at a large ice cream manufacturer, and I have some info to give to you. During production, milk based products are kept in a state of constant movement until it finally reaches it's storage state in large vats, and these vats are kept at a low temperature (just above freezing) until it is packaged/refrigerated/frozen. This inhibits bacterial growth in the product itself while it waits for the packaging stage. As for the pipes it travels through, I know for ice cream that between different fat contents/flavors, we run a water and solvent flush through them that completely clears the insides of the pipes of anything that could possibly grow mold or bacteria, and none of that gets into the product because there are small elbows that redirect the cleaning solution from it's storage tank into drainage holes located throughout the plant. At the \"end\" of the day (meaning at one certain time every day when the production stops and cleaning begins) these same redirects are all hooked up at the same time and flushed through with a much more caustic solution that contains various acids that break down anything the soft wash might have missed. In essence, we keep everything SUPER clean through strict procedure so that your food doesn't make you sick, saavy?",
"Reading most of the answers here makes me worry about a bakery I worked at for a couple months. They are a pretty big company and produce different bread products for many major grocery stores, and even the bread sticks for olive garden, and the sourdough for Jack in the box. The place ran 24/7 and the only time for cleaning the machinery that actually touched the dough and bread was very limited. The dough mixers were cleaned between each batch, and the flat trays went through a brush after they removed the fresh bread, but nothing about that brush was sanitary. It looked like the brush that cleans your car at a car wash, and whenever it wasn't spinning, it looked filthy and if you touched it, you would have to wash your hands because it would be covered in what looked like dark soot. The deeper containers used to bake loafs were the worst though, they got greased before every use, but when they were finished using them, they just put them back on their carts and stored them without cleaning them. They were so greasy and crusty that you had to pry them a part to separate the stacks. The whole conveyer system itself was never disinfected in any way, if anything they would periodically wipe or brush it down. This was only something people did during the short bits of downtime though in order to look busy. It also does not get rid of the layer of dirtiness on it that you can't see untill you run your hand over it. This place though was somehow operating completely legitamite though, and has even won several inspection awards for like 5 years straight. This meant an inspector every year comes through, sees the place running like this, and gives the place an award for it."
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c9asbc | How do they make fireworks so that they explode in particular shapes like stars, instead of out in every direction? | Engineering | explainlikeimfive | {
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"There's a \"How It's Made\" episode on it... super fascinating and you should watch it! Basically they pack them in the cylinder with plastic pieces that help provide a platform for the metals/powder to explode from. Keep it seperated. Then how they position those pieces make the shape..hopefully.",
"Typically the firework is loaded with smaller fireworks (kinda like firecrackers) that are launched out of the main firework. When the main firework explodes, it lights the fuses of these smaller ones and launches them out. They then travel and explode in certain shapes."
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c9b5yw | Why doesn’t water damage a jet engine | Like for example when a plane goes through rain, Wouldnt water mess with combustion? | Engineering | explainlikeimfive | {
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"Rain is almost all air, with very little water. The water does get into the get engine, where the heat boils it to steam which exist with all the other exhaust. Jets are made of materials that can survive this situation, as it's common whenever you fly through a cloud.",
"Engines like a little water, it actually helps them make more power. Too much, though, will cause a flame-out. When air (and water) enters the engine, it gets compressed through a series of rotary disks. The disks are composed of many blades shaped like airfoils that serve to accelerate and move the air as they spin, like a fan with many blades. They get smaller and smaller with each disc or “stage” of compression. In between each rotary disk is a stationary disk or “stator”. The stator vanes serve to accelerate and guide the air onto the next stage of compression. A stage of compression consists of one rotor and one stator. As the air is compressed, it gets hot. Really hot. When there is water in the air, it wants to make steam...but it can’t because it is compressed, hot and moving fast. At the back of the compressor just after the last stage of compression is a diffuser that allows the compressed air to expand and slow before it is mixed with fuel and ignited to make thrust. Here’s where the magic happens. Compressed air with water in it, when mixed with fuel and ignited, *really* expands as the moisture laden air flashes into steam. The steam expansion increases the amount of gas exiting the engine as thrust or being used to turn a turbine wheel. Because about 80% of any air that comes in the intake for a jet engine is used for cooling and flame control in the combustion section, it takes quite a bit of water to cause a flame-out in a jet engine. There are plenty of videos where you can watch engines being tested for water ingestion while they’re mounted on special test stands. It’s quite impressive the amount of water needed to result in a failure."
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c9g7bh | How does a pump work? | Engineering | explainlikeimfive | {
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"It creates pressure which then provides the force to move something. Exactly how it works depends upon what type of pump you are talking about and whether it is water air or something else that is being pumped.",
"There are many types of pumps, but the simplest to understand may be one you're familiar with: [the syringe]( URL_0 ). In essence, liquids and gasses fill volumes when they can (if they have the energy to do so). A pump creates space for the liquid or gas to fill, and it is then able to un-create that space to force the liquid or gas out. The syringe shown only has one opening, but most pumps have a secondary opening to allow one to be used as intake, and one as an exit (exhaust).",
"Some pumps create suction to pull the medium to where it's going, others create pressure behind to push. Some pumps use the syringe model to create the suction or pressure with one-way valves to keep things moving, some spin the medium to create pressure with centrifugal force, some use screw-looking things, some just lift the medium up with a ladle-like thing, etc. TLDR: There are lots of different kinds of pumps."
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c9hfcp | how bidets work/are supposed to be used. how does it cut tp use so much? | Engineering | explainlikeimfive | {
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"Okay, here’s the deal. A bidet attachment sits on your toilet, so after you go, you can directly spray the area with pressurized water. All of this is happening inside the toilet bowl, so there’s no mess to deal with. Compare having to wipe several times with toilet paper and still not being totally clean, vs. using the bidet for roughly 10 seconds, knocking off all the waste, and then only needing a couple of sheets to dry off. My boyfriend and I got one, and we’ve saved so much on TP it’s crazy. What would have lasted us 2 weeks suddenly lasts 2 months. Get into it!"
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c9mqi7 | why do some roller coasters exhaust their lifespan in 8-10 years while others made of the same materials last 40+ at the same parks? | Engineering | explainlikeimfive | {
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"Most short lived coasters tend to have significant g forces that put extra stress on the structure. This causes harder wear on the rails, substructure, and train components. If the coaster has a unique or complex mechanism (tilt coasters, original LIM, launchers, etc) it will most likely have life span issues due to those components failing more and more with age."
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c9nzc0 | How does CVT fluid work? | There is ample info on how CVT transmissions themselves work but I want to know how the FLUID acts as what appears to be the opposite of a lubricant. | Engineering | explainlikeimfive | {
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"It is not the opposite of a lubricant. It is a very high quality lubricant with a bunch of wear reducing additives. The mechanism has geometry that doesn't slip in the direction of applied torque, but it does slip in the direction that changes the \"gear ratio\"."
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c9p2ey | How are skyscrapers resistant to earthquakes? (How) Are they the safest place to be during an earthquake? | Just moved to downtown LA to work in one of the tallest buildings in the city; scared like I’m five with the recent earthquakes. Someone please explain why I shouldn’t be! | Engineering | explainlikeimfive | {
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"The LA building code has significant earthquake safety provisions. You're much safer in an LA skyscraper than you'd be in a NYC building of the same height, if there was a 6.6 magnitude earthquake happened 150 miles from NYC. Large buildings get much more strict inspection when they are being built that lower cost structures. Single story homes are arguably safer, because they are smaller.",
"Skyscrapers are heavily engineered to resist earthquakes. There are many different protection techniques used. One option is a tuned mass damper (think free-hanging giant ball eg Taipei 101). The ball swings in the opposite direction of the quake wave, cancelling some of it out. Another is to isolate the base plate (what the structure is built on). This can be done by putting rubber pads under the base plate. When the earth moves sideways, the rubber absorbs it. There are also other systems like rolling ball bearings and columns, but in general these systems will be found only in skyscrapers because of their high cost.",
"I can't speak for all skyscrapers but I know some are built with foundations that function much like car shocks and have some motion built into the so that they away and flex rather than snap and buckle. Here's a pretty good article on the subject URL_0"
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c9s6ur | What causes a ship's propeller to lose efficiency as its RPMs increase past a certain point? | Why does an engine have to turn at high speed and then be reduced to shaft speed? Why can't they be both low or both high? What all is taken into account when determining "efficiency" of the propeller? Does reducing the RPM of the shafts reduce fuel usage? Or do propellers actually become less effective past a certain speed? | Engineering | explainlikeimfive | {
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"Cavitation causes the prop to loose efficiency. When the blades cut through the water too fast they cause hundreds/thousands of tiny bubbles to form as the water is literally cut, this is cavitation. These bubbles lower the density of the water causing loss of efficiency. They will also over time cause damage to the prop blades. The bubbles act as tiny blasting caps and they erode the metal. The reason the engine needs to run faster and the speed needs to be reduced (and this isn’t always the case on larger engines like in freighters) is for torque. Water is 800x as dense as air, so for the blade to cut through the water at all it needs the torque behind it or it just won’t move.",
"> Why does an engine have to turn at high speed and then be reduced to shaft speed? Why can't they be both low or both high? This depends on the propulsion system design. A small craft like a powerboat might have the shaft speed similar to the engine speed at high power. However, you also need to stop and reverse, which is the main reason for a gearbox on most small craft. Note that these vessels will generally have small propellers designed to run at these high speeds. The largest ships use slow-speed engines directly coupled to the propeller shaft. These engines run from 60-120 RPM as required, and can usually be run backwards for astern propulsion. However, these engines are very big to develop the amount of power required. For many vessels, such as passenger ships, having a higher speed engine that is more compact is more desirable than the moderate gain in fuel economy. > What all is taken into account when determining \"efficiency\" of the propeller? A whole host of factors. Ultimately, the designers will be aiming to achieve a particular service speed, as laid out by the buyer. The power required to reach it depends on the ship's hull form. This also determines the maximum diameter of the propeller according to the design draft of the vessel. From there, it's considering the chosen propulsion system (slow speed, medium speed, diesel-electric etc.) and the RPM available to determine the propeller geometry, such as blade angle and skew. Depending on the vessel needs, peak efficiency might be given up for a wider operating range or other features like controllable pitch, allowing astern thrust without reversing the direction of rotation. > Does reducing the RPM of the shafts reduce fuel usage? Generally, yes. A larger, slower propeller will be more efficient at the same hull speed. > Or do propellers actually become less effective past a certain speed? A given propeller geometry will be most efficient at a particular speed, and develop its maximum power at another particular speed. Kind of how a car engine might have peak power at 5-6000 RPM but for highway cruising you want to be nearer 2000 RPM."
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c9uiqz | How does a ski chairlift slow down for the people getting on/off, but the rest of the chairs in the middle of the ride up don’t feel it? | Engineering | explainlikeimfive | {
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"The chairs disconnect from the main drive cable as they enter the stations at each end. They proceed on a slower drive system in the stations then reconnect to the main cable as they leave. See: URL_0",
"Because on modern chair lifts the chair is actually lifted off the cable and slowed down. The cable continues to move at the same speed but the chair moves slower. After you get on the chair is then accelerated back to cable speed and dropped back on the cable. Since all chairs are slowed down the same in the turn, spacing is kept uniform along the cable.",
"I have experience operating the type that aren't detachable. When getting on the chairlift there is an attendant who stand in the center of where the chair comes around. As the skiers come up the attendant grabs on the side of the chair and pulls back. At the same time they sit down and it's all a smooth transition, or should be. When you exit the chair comes up/down towards a ramp of snow. You stand up as the speed of the chair along with slope of the ramp guides you down. Yes, people do fall down getting both on and off, and yeah it's funny sometimes",
"Well at blue mountain (Poconos) they just stop if for every chair. You don't feel it because it's barely moving anyway"
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c9vsg6 | Why is it preferable for an airplane to have the propeller pulling at the front, instead of pushing from the back? | Engineering | explainlikeimfive | {
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"Generally propeller placement actually comes down to engine placement. You need the center of mass of a plane to be in front of the center of lift otherwise the nose of the plane will constantly be lifting up and trying to stall you out. This generally calls for the big heavy engines to be in front of the wing. There are a bunch of designs that have made it work, from little ones like the [Saab J21 fighter]( URL_0 ) to bigger ones like the [Convair B-36 Peacemaker strategic bomber with 6 engines]( URL_1 ), but it adds a fair bit of complexity to the design, and complexity generally equals weight, and weight is something you're trying really hard to avoid Other downsides for it are that the prop wash actually helps generate more lift on a propeller plane as you're shoving fast air across the wings, and having the prop blow air across the engine helps keep it cool and reduces the risk of engine fires. If your prop is behind then wing then the wing goes through the air before the prop can causes turbulence which reduces the propeller efficiency by a few percent as well which requires a stronger(aka heavier) engine than if it were in front."
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c9xsxs | What is the difference between mixing and mastering a song? | Engineering | explainlikeimfive | {
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"text": [
"The most basic explanation is mixing is making adjustments to the individual tracks/instruments so that they mesh together best (adjusting volume of tracks against each other, adjusting eqs or the panning of certain tracks.) Mastering is adding whatever final touches/effects to the track as whole once the individual tracks have been mixed together. This could be making the track fade out, adjusting the overall eq, or adding reverb to the entire track."
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c9zcm0 | Why do minivans have pop-out third row windows? | Engineering | explainlikeimfive | {
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"I'd imagine it's due to some combination of cost reduction (car companies are *incredibly* stingy lol), improving airflow without causing the annoying buffeting effect inside the vehicle, and the simple fact that the rear wheels of a minivan are right under the third row windows, so those windows couldn't roll down all the way anyway."
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cadkrr | Why do passenger side mirrors in cars make objects seem farther away? What’s the need for it in the first place? | Engineering | explainlikeimfive | {
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"The mirrors are convex shaped, i.e. the middle part bows out. This gives you a larger angle that is reflected so you can see more in the mirror but distorts the size of things, making them smaller in the middle and stretched on the edges. This is similar to a fisheye lens on a camera but to a lesser degree.",
"Set up properly (and most drivers don't set it up properly), the side mirrors (passenger and driver) is supposed to cover most of the blind spots for a driver sitting in the car. A typical car has pillars around the doors which obstruct the driver's vision towards the side rear angles of their car. Most people don't recognize that these blind spots are large enough that bicycles, motorcycles and cars are easily hidden within them at normal driving separation distances. A driver cannot detect things within the blind spot without turning around and bending their body to the side (not a safe maneuver). The side mirrors are supposed to eliminate most (not always all) of the blind spot. They are convex to give better blind spot coverage which also makes objects seem further away."
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cahl27 | Handbrake and brake pedal, how are they different? | Apart from the fact that the handbrake can be locked and the pedal can't. Also please explain which wheel they brake. | Engineering | explainlikeimfive | {
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"The foot brake activates the brakes of all wheels, is usually operated by hydraulic pressure, and is used for typical braking scenarios. The hand brake, or parking brake, is usually cable operated, activates the rear wheels, and as the name implies, is used while the car is parked instead of relying solely on the transmission to keep the car in place(this is very important when parked on an incline). It can also be used for emergencies if the hydraulic brakes fail.",
"The brake pedal actuates hydraulic brake calipers that squeeze the surface of brake disc. In most cases the hand brake pulls a cable to engage brake shoes on the inside of the rear brake hubs.",
"The brake pedal applies all four brakes, but mostly the front if you are traveling forward. The hand brake only applies the rear brakes (mechanically) and will not slow a moving car very quickly. The hand brake is meant to hold your car in place while parked."
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cas2gf | Why do American electric plugs have a "fatter" side? What is the danger of plugging in opposite? | Engineering | explainlikeimfive | {
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"For electrical appliances to be safe it is important that the power comes through a fuse or circuit breaker, into the wall outlet, then the plug must take that same wire to the on/off switch of the appliance. This way, when the appliance is turned off, there is no power coming to the appliance. Put the plug into the wall backwards, and the appliance switch stops the electricity after it has gone through the whole appliance. If any wire is shorted out to the casing of the appliance, it will always be hot, even if the switch is off. That accounts for that fuzzy feeling in some lamps for example. If you touch this \"hot\" appliance and a sink faucet, even with the switch off, you could still get a shock. That is one of the basic reasons we have polarized plugs -- one fat and one thin. When you change a plug on the end of the cord, you have to make sure that the thin prong takes electricity directly to the on/off switch of the appliance -- and when wiring outlets you always have to make sure that the \"hot\" wire from the fuse goes to the \"gold\" screw on the thin side. The wide prong on the plug goes to the silver screws, and the white return wire."
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cay1tq | why do cars take longer to start when in cold weather vs warm weather? | Engineering | explainlikeimfive | {
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"text": [
"Gasoline burns when it's a mixture of gasoline vapor and air. When it's cold it is harder to evaporate it"
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cb6eg8 | - How does pushing air through an instrument make a note? And specifically for brass, why does buzzing work but not regular air flow? | Engineering | explainlikeimfive | {
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"to make a sound you need vibrations for example, when you talk, it is your vocal cords that are vibrating. For a woodwind instrument, like clarinets and saxophones, it is easy to see that a reed is vibrating when you blow through it, making the sound. For brass, you have to vibrate your lips to have that sound travel through the horn. the different pitches comes from different tube length controlled by the valves."
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cbael9 | Generally speaking, how do architects and engineers design multi story buildings knowing that they will withstand the weight burden? | Watching the MLB all star game and the thought occurred to me. How do we know that the upper deck of a stadium will hold the weight without collapsing? | Engineering | explainlikeimfive | {
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"I’ve heard it this way, you build it from the top down. Let’s say the top floor is 100 tons so the next floor down has to support that weight and is 120 tons itself. Now the floor below that has to support 220 tons, etc. Part of the weight of each floor is all the stuff that goes in it; furniture, people, etc and each floor is rated to support X times it’s real life load.",
"The weights of everything are carefully calculated and structures are over designed slightly.",
"Engineers work with worst case scenario and safety factor. So let’s say a stadium can hold 1000 people.. let’s say the average person weighs 175-200 lb. you would take 200x 1000 for your max weight and then you would say I want a safety factor of 15 because people could get hurt if the structure fails. So you end up with 200x 1000 x 15 as you max load. The rest gets more complex dealing with physics and statics.",
"As everyone else has said, engineers calculate the loads and adjust for factors of safety. Architects generally aren't concerned with how to make it work, just how it should look.",
"As an Architect I can answer this question. Some other answers are correct but are only partial. First you establish the intended usage for each level of the building. Think of it this way: An inaccessible roof holds a lot less weight than a level intended for parking. Once you know what it's going to be used for, you find out how much it weighs. Each floor has to support what you put on it (people, furniture, machinery, parked cars) plus its own weight. Each level is supported by columns - each column has to support every level above it. So the columns on the top floor only have to support the weight of the roof over that floor, but the columns on the ground floor have to support the weight of EVERY level above it. Then you also have safety margins, so whatever you build should support a little bit more \"just in case\" to account for variations in load (things like the building being used for a slightly different purpose later in its lifetime or oblivious people putting things they shouldn't) and chemical/mechanical imperfections in the materials used. And then, if you want to be absolutely sure the actual materials hold as much as you calculated, when you're actually building it you take samples of structural materials (such as concrete) and send them to a materials laboratory that will take those samples and put things on them until they break, and send you a report on how much it actually supports. If by any chance the test results are less than what you expected, and it's greater than the safety margins you used, you re-do all the calculations and if it *still* won't hold, you *will* have to take the whole thing down and re-build it because you're criminally liable and it's your responsibility to deliver a building that will hold well for the purpose that was requested. It is, however, not your responsibility if it later breaks because someone decided to use it beyond its capacity without asking a qualified professional first. An example of this is the [Sampoong Department Store collapse in Seoul, Korea, in 1995]( URL_2 ). After construction started, the owner decided to add another floor AND make it a very heavy floor. The Architects refused because it wouldn't hold, so he fired them and finished it anyway. It miraculously held due to the original safety margins, until a few years later when additional work was done that weakened the structure further, dooming it to collapse almost immediately. There are several documentaries [about]( URL_1 ) [this]( URL_4 ) [event]( URL_3 ). On tall enough buildings you also have to account for wind sway, so that it neither bends far enough to crack, nor leans far enough to topple over. On the plus side, if you're ever in a very tall building and it sways because of the winds, you can be sure it won't topple over because it's been accounted for. Except at the [Citicorp Center in New York]( URL_0 ) (1976), where only well after construction was finished and the building was occupied, did the structural engineer realize he did the calculations wrong and the building would collapse in strong wind. They went and did heavy structural reinforcements quietly at night so that nobody knew. Of course, people eventually found out and they were quite displeased with this course of action. The building stands today and now it *should* be safe."
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cbbbqv | Why is it the standard/manual/stick shift car more reliable and gas efficient than automatic transmission cars? | Engineering | explainlikeimfive | {
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"Not entirely true for modern vehicles, but I personally feel you feel more connected with the road and driving in general with standard. Back in da day you could easily get better gas mileage with standard just due to more gears and changing gears early. But nowadays any auto can match or beat standard in fuel economy. The one thing that still stands is reliability, since a standard doesnt need a torque converter, theres less heat in the transmission, and less components to go wrong = lasting basically the life of the vehicle. Other than that, a standard transmission can make any basic biotch 4 cyclinder feel \"ok\". Due to more control on shifting inbetween gears."
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cbbd2t | how do transmissions work in the big rig trucks? | Bc ik some of them can have like 18 plus gears, is it like a regular manual car? Or something different? | Engineering | explainlikeimfive | {
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"same as a car just a lot more shifting. Difference is gearing layout if is a 4 over 4 or 8 high range 8 low range for 16... some manufacturers have different gearboxes"
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cbeagn | What is the difference between an AWD, 4WD, FWD, and RWD? | What are the pros and cons of each drivetrain? | Engineering | explainlikeimfive | {
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"If I remember correctly, AWD is when the vehicle is driven by all wheels and cannot be altered, 4WD is a feature commonly found in trucks or off road vehicles and is driven by all wheels but can be switched from 4WD to RWD or FWD. RWD and FWD is when the vehicle is driven by the rear or front tires respectively.",
"FWD is Front Wheel Drive RWD is Rear Wheel Drive AWD is All Wheel Drive 4WD is 4 Wheel Drive To explain the differences, I need to explain traction/grip. A car gets traction by pressing down on the road. The weight distribution of the car effects how much it presses down. Acceleration effects traction by shifting the weight. Have you noticed how a car 'leans' forward when you brake, and then leans back once you come to a stop? That's because when decelerating, the wheels slow down, and the rest of the car pulls against the wheels. Also keep in mind that traction on the driven wheels is more important. Modern cars can 'intelligently' distribute power and braking; if one wheel starts to slip the car will send power/braking to the other wheels. This is because of the difference between sliding and static friction; what you need to know here is that once you start sliding it's hard to stop, because static (non-sliding) friction is much higher than sliding friction. AWD can more easily maintain traction because it distributes power across all four wheels, so it has more places to 'share' to and from. This is useful for driving in bad weather or when driving 'aggressively', like racing or Autocross. However, AWD requires extra weight and more parasitic losses (power lost due to friction in the drive-train), so they're heavier and less fuel efficient. The extra weight is general well-balanced throughout the car, though, which also aids traction. RWD is the simplest design, since the driven wheels don't need to also be able to turn. Power to the rear wheels is important when accelerating, which is why RWD and AWD are popular for sporty cars; the car 'leans back' when accelerating, which takes pressure off the front tires but puts more weight on the back tires. However, RWD generally doesn't have 'natural' weight on the driven tires, because the engine (a lot of the cars weight) is usually in the front, though the drive-train weight is mostly in the back, helping weight balance somewhat. During winter in snowy areas, you'll often see people in RWD cars and trucks put a few sandbags in the rear to keep weight on the driven wheels. Losing grip in RWD often means oversteer, as well. Oversteer means that, when turning, the front of the car behaves 'normally' while the rear end slides, causing the car to face further into the turn than needed for the turn. Understeer is a car more or less just not turning as much as you wanted to due to lacking grip in the front. Oversteer is dangerous when unexpected because it's more unpredictable and the movement needed to counter it isn't 'natural'. This makes RWD generally the worst in bad weather, with poor natural grip and a bad reaction to losing grip. The simplicity of the design meant RWD was standard up until the 1980s or so. FWD is a balance between AWD and RWD. Only having two driven wheels still has some downsides, but FWD is the lightest of the three (the need to transfer power from the engine in front to the wheels in back adds weight to RWD), understeers when it loses grip (preferable to oversteer), and having the engine above the driven wheels helps maintain grip better than RWD. This makes FWD the cheapest to build and keep fueled in modern cars. 4WD is basically unique to trucks; it lets them turn on a type of AWD that's focused for off-road driving. Off-road driving means you *don't* want the anti-slip 'intelligent' power distribution that AWD uses to share power between front and rear, so 4WD uses a locked differential. This means 4WD will tear up your tires if you use it on hard paved surfaces. If you just need a car as an appliance, get a FWD, unless you live in a snowy and hilly area, in which case AWD might be a better choice. If fuel economy is important. FWD. If you want something 'sporty', AWD or RWD. If you want to do serious off-roading (not just driving on gravel or dirt once in a while), a 4WD truck/SUV is needed."
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cbolak | Why haven’t motorcycles transitioned into being automatic like cars? | Engineering | explainlikeimfive | {
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"There are some bikes with DCT, but not many. Automatic transmissions are a good bit larger and heavier than manuals, and in motorcycles being lighter is almost always the goal. Also, you have much more control and feel when you can feather the clutch. Last point, it’s way more fun to bang through the gears and have total control of where you’re riding in the rev range.",
"The bottom line is that the people who buy motorcycles do not like them. A while ago, Honda made the Hondamatic 750. They are rare and highly sought-after by paraplegics, who can convert them into a motorcycle that can be run with only hand controls. I've seen one where a wheelchair is rolled-up into a custom sidecar, and then the paraplegic starts the Honda and rides away. They were not hugely expensive, and yet they did not sell well. The same question is sometimes asked about 18-wheel trucks with an auto transmission. They DO exist, but the are more expensive and they are expensive to rebuild when they wear out.",
"Because how am I supposed to do clutch up wheelies without a clutch?",
"Multiple reasons the main one being it’s just more fun with a manual transmission. Also I don’t know if I would trust an automatic while I was taking curves. I want to be in complete control in curves because any loss of traction could be life and death.",
"One thing no one mentioned is that having the clutch gives you a few things. Being able to feather the clutch (ie keep it in the friction zone where you can easily ease the power on or off) is much more precise than rolling on or off the throttle as it is instant. This allows you to do things like make u turns in one lane’s width. Additionally it makes it real easy to kill the power to the wheel with your left hand. So with your right you can roll off power, and with your right you can kill the power. So if one control malfunctions you have a back up. And if you happen to be scratching your ass with your right hand at speed you can immediately kill power with your left hand even if you have the cruise set and hit your foot break on the right. What I have seen recently is options that add an automatic clutch but don’t remove the traditional clutch. So you can shift without clutching but can still use the friction zone with the clutch lever.",
"I've seen a lot of other answers that I think hit 95% of the reasons. As someone who drives a manual transmission car, and a manual motorcycle, I can think of one more, which is that manuals on motorcycles are way easier than in a car. Because the transmission in a motorcycle is both a wet clutch (longer engagement zone, lesser risk of burning out), and dog-engaged gears, and motors that idle higher, it is far more forgiving. You're much less likely to stall out a bike by accident. Also, clutches are much lighter and are controlled by your hand with much more control. All this compared to a manual car means I commute on my motorcycle because it's less work lol."
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cbucud | How does the petrol pump know when your car’s tank is full so it stops the flow of petrol? | Engineering | explainlikeimfive | {
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"If I remember correctly, there is a small tube connected to a pump which continually sucks in air. Once the tube is under the surface of the petrol it will start to suck that which triggers a sensor which then stops petrol from flowing through the petrol pump."
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cc28f9 | How does an Anti-Thef system knows if an item's bar code was scanned or not, which makes it goes off(if the thing wasn't scanned)? I feel like there is something more than scanning some lines and *poof* no alarm | Engineering | explainlikeimfive | {
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"From what I know the thing that triggers the alarm gets destroyed by a strong magnetic field that is built into the cash desk.",
"Very few items in a store have a thing in them to set off the alarm. Things like razor blades (very commonly stolen from stores) have a security device that is deactivated by the magnetic bar next to the scanner as they are passed by.",
"Scanning them does nothing. A device at the register is used to deactivate them. \"Security labels house a small receiver within an adhesive sticker that can easily be affixed to products. This receiver remains in constant communication with an EAS antenna, and when that label comes too close, an alarm sounds, alerting staff that an item is passing through the entryway. Labels are deactivated by breaking the circuit in the receiver, so they no longer communicate with the antenna. They can then pass an antenna without sounding an alarm.\" [ URL_0 ]( URL_0 )"
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ccb29h | Why is the driver's seat on a north american boat on the right side? | Engineering | explainlikeimfive | {
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"text": [
"Because the \"right of way\" rules for boats are international, and they are \"yield to other boats on your right\". So sitting on the right gives you a better view to make sure you avoid a collision when it's your responsibility to turn to avoid one.",
"There really isn’t one accepted answer. They have theories but nothing provable. URL_0"
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ccnn90 | How does an alternating current flow? If the electrons are constantly moving equidistantly forward and back, how is it travelling? | Engineering | explainlikeimfive | {
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"text": [
"They do, but they don't really 'flow' in the way you think, the water flowing is a fairly poor analogy because it sets up our minds to think of electrons as if they are H20 molecules, but at the scale we are talking about electrons act nothing like liquid water. [ URL_0 ]( URL_0 ) They 'move' at about 1.2 inches a minute! So precious little of the actual power is done by the 'flow' of electrons. It is probably more accurate to think of the flow of electrons as a gradiant where when voltage applies the free electrons tend to move in the direction the voltage has been applied. Importantly, this is 'free' electrons, not the electrons that actually comprise the atoms of the conductor. The atoms in a metallic structure are packed closely enough that free electrons can gradiate through them, as they move along the medium they bump into the atoms of the metal constantly.",
"Conceptually it is easier to think of current as \"flowing\" and use the analogy of water flowing in pipes to visualize what is going on. But, as you say, there is nothing truly analogous to a \"flow\". Think of throwing a rock into a pond, you see ripples forming and spreading on the surface but there is no actual water flowing in those ripples. It is the wave that is propagating through the pond surface, the individual water molecules are mostly moving up and down. Sound is another example. When you hear a sound, it isn't a flowing of some \"sound particle\" or air molecules from the source to your ear. The sound energy is transmitted via a compression wave in the air with the air molecules mostly vibrating back and forth carrying energy to your ear."
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ccoy2y | how does a cowl around aircraft engine help with efficiency at high speed? | Thanks everyone for the info, learned a lot. | Engineering | explainlikeimfive | {
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"An engine provides power to an aircraft. Almost all of the power goes into doing mechanical work on the aircraft, pushing it forward. This work is exactly to overcome aerodynamic drag. When you go faster through the air, the drag force becomes huge. This is because it goes up with the square of the speed. Twice the speed and you get four times the drag force. But the power goes up with force times velocity. So with another velocity term for power. Twice the speed needs 2x2x2 = 8 times the power from the engine! It's very important to have an aerodynamically streamlined aircraft so that you can have smaller engines, less fuel tanks because it burns less fuel per mile, and more passengers or cargo. So it is more fuel efficient with a cowling. Engines have lots of bulky bits of metal (oil and air pipes, actuators, wiring, a gearbox with startup motor, fuel pump, and electricity generator. It also has big boxes that hold the Engine Control Unit and Environmental Health Monitoring computers, and big flaps that open out called thrust reversers). An engine cowling instead goes around the outside makes the engine more streamlined, so the air can flow smoothly past the outside without causing too much extra drag. It also has heating on its leading edge to prevent ice build-up, and helps protect the inside. It also directs the air smoothly at the inlet as before it meets the fan at the front. It also has clever materials and shapes on the inside to absorb noise, because a noisy aircraft will not be allowed to fly."
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ccp8lw | how does aircraft brake to reduce speed when landed | Engineering | explainlikeimfive | {
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"text": [
"A combination of spoilers, engine reverse thrusters and brakes on the wheels. The spoilers are upper surface flaps that deploy and disturb the airflow on the wing. They kill the lift on the wing to prevent the aircraft from getting enough lift to come up again, and create a lot of drag to help the plane slow down. Engine reverse thrusters work exactly as the name implies. The jet plume from the engine is re-directed forwards, so the thrust acts backwards, thus slowing the plane down. Wheel brakes work more or less the same as they do in a car.",
"The wheel brakes provide the majority of the deceleration, with the thrust reversers and spoilers assisting. We never rely on the reversers assistance, however. All landing calculations are achievable using just the wheel brakes.",
"Some military jets deployed a parachute on landing. Called a brake chute. The brake chute is then jettisoned as the aircraft leaves the runway and some guys will drive over and collect it. The brakes on aircraft are more heavy duty and substantial than cars. The aircraft wheel will have what’s called a brake pack, this is a sandwich made up of multiple brake discs and brake pads in layers (instead of one brake disc on a car) One layer will be stator the other rotor. The brakes get to hundreds of degrees C and some aircraft have cooling fans built into the wheels. It was common to have brake fires. Some aircraft can move nozzles in their engines to deflect the exhaust in another direction towards the front of the aircraft. This is called reverse thrust."
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cctg4d | Since electrons flow from negative to positive, why are cars grounded to the negative terminal on the battery? | Seems like you'd want to be careful with the source of the electrons so the negative terminal would be covered/protected and the positive would be wired to the frame. But it is the opposite. I presume there is a good reason that I do not understand. | Engineering | explainlikeimfive | {
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"It really doesn't matter whether you use positive or negative ground. Some vehicles have had positive ground. The convention of saying that current flows from positive to negative was established by Ben Franklin before we knew about electrons. Conventional current flow is opposite electron flow. Current flows in a loop from source to load and back to source. Interestingly, electron flow is very slow compared to the speed of electricity. It is usually less than 1mm/second while electricity flows at 50% to 90% of the speed of light. Electricity travels as an electromagnetic field. I like to think of it as one charge pushing the others along like a drinking straw full of peas. Add a pea to the end and one will immediately pop out the other end.",
"The direction electrons flow is, in terms of circuitry, irrelevant. Voltage is actually relative, and does not exist without two terminals. For instance, if you have the negative terminal grounded (at zero volts) and the positive terminal is \"live\", then you can think of it as the positive terminal sucking electrons from the grounded terminal. This also means that grounded terminals are almost always safe to touch, no matter if they're \"positive\" or \"negative\" (I use quotations because if it is grounded, it is at zero volts and therefore neither positive nor negative). In q high voltage system, if you're shocked by a positive terminal, it is because the positive terminal sucks electrons out of you, which causes a shock. In this case, you are serving as the negative terminal.",
"Ground is simply defining a given point in the circuit as a 0 reference. It doesn't much (there are some secondary considerations) matter which way it is as long as you're consistent about it. Lots of old vehicles, military vehicles, and old military vehicles do it the other way around and define the positive side of the battery as ground. It's even fairly straightforward to convert a vehicle's grounding."
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cculg3 | How are large-scale, intricate construction sites organized so that every worker knows exactly what and when to do? | Engineering | explainlikeimfive | {
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"text": [
"Generally a single company is hired to do the overall site management. They set the schedule, organize the sub-contractors, manage deliveries of materials, and do everything else it takes to make sure construction proceeds. Each individual sub-contracting company will have their piece of the project schedule and plans provided to them, and then they break that out into individual tasks for their teams of workers. So when you show up to the site with your tools you know what your job is for that day/week, but you don't need to know the entire project plan.",
"It’s like the military. You have crews run by crew foremen managed by site foremen governed by project planners. Consider the offices of any large corporation - how does each individual know what they should be doing? A hierarchy of superiors."
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cd4q5i | What is bandwidth in terms of signal processing? | Engineering | explainlikeimfive | {
"a_id": [
"etrmmsx"
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"text": [
"In very simple, rough terms, the bandwidth is the full range of frequencies that a signal or communication system (like speech, music, etc.) covers. The *frequency* itself is a characteristic of one component of the signal, but the signal might have many different components, and the difference between the upper and lower frequencies is what determines the bandwidth of the signal."
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cd75jc | Why aren't elevators required to have battery backup to at least be able to return to the ground floor after the loss of grid power? | Engineering | explainlikeimfive | {
"a_id": [
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"text": [
"Safety. When there's no power, do you want to hope your battery backup is operational when you remove the brakes, or simply keep people in place? Additionally, economy. How many blackouts last for so long that by the time firefighters come to rescue people, power has not been restored?",
"Im an elevator mechanic. Alot of elevators are old and didnt have the technology. There are elevators that can do this",
"It is mainly not necessary, floors are really close to each other so even if the emergency brakes fire you can normally open the doors and get people out when the car is between floors."
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cda463 | how does a propane refrigerator work? | Went camping this weekend with my father in-law who had a fifth wheel. We got into a debate on which was more efficient; running the generator or letting the fridge run off propane. We both have no idea how a propane refrigerator works and I was hoping someone might be able to explain it to me. | Engineering | explainlikeimfive | {
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"Ammonia is very soluble in cold water, but not very soluble in hot water. So heating the water (by burning the propane) expels ammonia from it at high pressure in one part of the system. Cooling the water back down (by a radiator) while it moves someplace else allows it to reabsorb low-pressure ammonia from a different part of the system. Put them together, you have a compressor which has no moving solid parts. Of course water wants to move rapidly from the high pressure side to the low pressure side, so to prevent that you add hydrogen to the low pressure side to bring up the total pressure (so it is really low pressure only when looking at the partial pressure of ammonia gas) Once you have this odd kind of compressor, it fits into the refrigeration process the same way a mechanical compressor would.",
"Others have explained it. Now I'll add that using a propane powered fridge is always more fuel efficient than running a generator: Propane fridge: Small propane heating unit, uses about as much gas a pilot lights on a stove. Most only burn gas when actively running the cooling cycle Generator: uses (typically) gasoline to drive a engine, which then spins what amounts to an electric motor running backwards. Generator has to run at all times to make sure there is power available for the electric fridge. (which uses another electric motor to drive a compressor to perform cooling) Things are a little better if the generator is powering a battery bank and only comes on when the bank is depleted. But the bottom line is that a generator converts energy through a lot more steps and no energy conversion can ever be 100% ('cause physics) so there is power loss at each step. Plus the generator is going to be running all the time, not just when cooling is needed. & #x200B; As a bonus, a propane fridge is virtually silent compared to the combination of small 4 stroke engine and compressor motor."
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cdo3jr | When a generator rotates it produces electricity, which is flow of electrons. From where is the generator getting unlimited source of electrons? | As far as i know, If you keep rotating the generator shaft using wind or water or any other source, it keeps producing electricity. But from where is it all coming? | Engineering | explainlikeimfive | {
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"text": [
"It has to be a closed circuit so eletrons flow in a circle. Think of it as a bike chain where the pedal is the generator, the other gear is the load (device reciving power like a light bulb) and the bike chain are the electrons.",
"the electrons are already there it just moves them, you can think of electricity like water and the generator is the pump, you crank the pump it moves the water, thats as simple as i can explain it without getting wizardry like magnetic fields involved"
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cdohh5 | What point of time the electricity needed to power the lamp was generated, when turned on? | Considering electricity travels almost at speed of light. When A light is turned on at home. Does it mean at almost the same time, somewhere in a Dam or wind turbine the electricity was generated? | Engineering | explainlikeimfive | {
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"The short answer is, yes. When you are talking about these millisecond changes in the current consumption and corresponding changes in current production they are taken up by flywheels. As you know the electricity grid alternates the current 50 or 60 times a second. An old fashioned three phase motor/generator will try to keep its output shaft synchronized with this alternating current. When you power on the lamp you will be causing a ripple effect throughout the electricity grid reducing the voltage and therefore all these motors/generators will start to slow down and thereby give off their energy which will keep up the voltage in the system. How fast they spin down and how much energy they can deliver into the system depends on how big of a flywheel they are attached to. A hydroelectric, coal, gas and nuclear plants have fairly big turbines that acts as flywheels. The rest of the power plant is just to make sure the turbine is not slowing down. The issue in the future is that solar cells and wind turbines do not have big flywheels synchronized with the electricity grid. So as coal and gas power plants are disappearing there is not enough flywheels in the hydroelectric and nuclear power plants to handle the common power surges you might see on a daily basis."
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ce1p61 | why a diesel engine is so much more powerful than an gas engine. | Engineering | explainlikeimfive | {
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"text": [
"Several reasons - diesel fuel has more stored energy than gasoline. It combusts much sooner than gasoline, so it spends more time pushing pistons. The engines are built to compress fuel/air at a higher ratio; that plus longer stroke length for pistons means that diesel fuel spends more time moving mechanical parts and those mechanical parts are designed to provide more power",
"It isn't. An equal size diesel and petrol engine will produce different horsepower, and the gasoline engine will make more of it. The difference is that a large diesel engine making the same amount of horsepower as a gas engine will make loads more torque and consume less fuel. This is why trucks and tractor trailers use large diesel engines. Torque is useful because it allows towing extremely heavy loads. TL:DR: Horsepower is how fast you hit the wall, torque is how far you push the wall. Apologies for the formatting, I'm on mobile.",
"So, this is partly right. A diesel engine typically produces far more torque (twisting power) than a gasoline engine because diesel is more dense than gasoline (it is literally heavier) so each explosion creates more power. Thing is, the motor doesn't turn very fast. As a result almost all diesel engines have some sort of forced induction, like a turbocharger which help with horespower tremendously. Horsepower is torque times engine RPM times 5252. So if you get all of your torque at 2,000 RPM, you have a lot more power than a gasoline engine at the same RPM but the motor will not be able to rev as high. The turbo helps with that but you will still notice diesels have a lower red-line than gasoline engines."
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ce4yg6 | Why do you never hear the people above you in a hotel room, but in an apartment it sounds like elephants stampeding overhead when neighbors make noise? | Engineering | explainlikeimfive | {
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"text": [
"The hotel probably used concrete between the floors, not wood.",
"Hotels would like to attract business back. They don't lock you into a year of tenancy, and so if you have a bad experience with noise you're less likely to go back. Also hotels have the money to build to a higher standard. You have probably at least 2 rooms for every 1 small apartment in an equivalent apartment building, and every room is full and generating say $130 a night, those same 2 rooms are generating nearly 8 grand a month. A small apartment the size of two hotel rooms even in NYC probably isn't running you anywhere near that."
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ce6rgj | Where do cell phone towers get their power? | Engineering | explainlikeimfive | {
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"text": [
"Towers have backup batteries, they are indeed hooked up to the grid, but in the event of a blackout they have backup power for 3-4 hours.",
"They are connected to the grid, but also have back-up batteries and/or generators to keep them powered for a time even during a power outage."
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ce7s7t | Why are laptop and tablet batteries often split into several smaller cells instead of having only one large cell? | Looking at teardowns of laptops and tablets, their batteries are usually comprised of several identical lithium ion cells placed right next to each other. Examples include the [iPad]( URL_1 ) and the [Surface Pro]( URL_0 ). To me, it seems it would be more cost efficient and more space efficient to have as few cells as possible, and in most cases having one giant cell will not only reduce the complexity of the device but also allow more "battery" to fit into the same space since less insulating material is needed. So why are laptop and tablet batteries made up of separate cells? | Engineering | explainlikeimfive | {
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"text": [
"If you hook multiple cells together in series where you put positive to negative that increases the voltage but keeps the amperage the same. Three 1.5 volt AAA cells connected in series become a 4.5 volt battery with 1000 mAh. Hooking them in parallel puts their amperage together but keeps the voltage the same. Three 1.5 volts AAA cells become a 1.5 volt battery with 3000 mAh. You could make the cell bigger like a AA cell which has 2400 mAh, but smaller cells can be more stable and reliable than larger cells. If a small cell goes bad it's less catastrophic than a large cell going bad. You also have the benefit of being able to produce lots of small cells and help them together in interesting ways. A small 12 volt battery like would go in a garage door opener is 8 button batteries like from a watch hooked end to end. A small 9-volt battery like you would put in a smoke detector is 6 AAAA batteries wired together like would be used in a Microsoft Surface pen."
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ce805b | why is using cruise control more efficient than simply maintaining the speed manually? | Engineering | explainlikeimfive | {
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"text": [
"When you drive without cruise, you tend to speed up and slow down quite a bit, by a few kph. Every time you speed up that couple kph, you waste a small amount of gas. Then there are up and downhills, which both suck up your gas. You tend to speed in both, and on larger uphills might give way more gas than nececsary. Cruise control can detect the speed variation much earlier and adjust correctly.",
"In my experience it's more about convenience than efficiency. I find that in some situations cruise control is actually less efficient, because when the computer realizes that the vehicle isn't travelling fast enough it will push too hard sometimes to regain that speed. Especially on hills.",
"Because the ECU is better at it than you are. There are situations where a good driver can be more efficient than the cruise control but normally the cruise control will fluctuate less than a human. The power required to maintain a certain speed goes up as the square of the speed so the if you go a little above then a little below, you burn more energy for the same average speed than just maintaining that exact speed. To try to give an example: Let's assume something like: 100 km/hr - > 32 kW 90 km/hr - > 27 kW 80 km/hr - > 24 kW Over 1 hr, the car that does 90 km/hr steady burns 27kWhr of fuel. Over the same hour the car that fluctuates between 80 and 100 burns 32+24/2 = 28 kWhr of fuel. The difference here is due to the fact that 80 to 90 is 3kW and 90 to 100 is 5kw.",
"Because the exact amount of engine revolutions needed to maintain that speed is being used. This means that provided you don't suddenly go up hill or have to make sudden changes in direction, the amount of fuel used is constant. When you drive manually you are constantly speeding up and slowing down based on the pressure of your foot on the accelerator. This is less efficient overall."
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cea1gf | Speed cameras | I understand the actual physics behind the measurement of speed of an approaching object I'm just curious as to how the software/hardware works, what happens if 5 speeding cars come at once? How/when does the camera take a photo and of what? | Engineering | explainlikeimfive | {
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"text": [
"I don’t know where you live, but where I live, the most lanes a speed camera will cover, is two (To my knowledge) - On motorways, they have an array of speed cameras; one camera per lane. Also, I believe most speed cameras use number plate recognition, to identify which car is the speeding vehicle. I’m not sure how they’re able to discriminate on two-lane roads, such as dual-carriageways, but when it comes to roads with anymore lanes, they use more than one camera. I live in the UK, so this might not apply to you though. Please let me know if this answers your question"
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cedm15 | How does software control hardware? | Engineering | explainlikeimfive | {
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"text": [
"The pin is wired to a driver transistor inside the chip. That transistor is like a switch, it connects the \"power\" pin to the \"output\" pin when the gate voltage is high. The transistor's gate is connected to one output bit of an output latch that's addressable in the computer's memory or I/O address space. The software writes a value into the latch's location, and the latch turns the appropriate control wires on, potentially including the one that controls the driver transistor."
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ceihl6 | Why do tires need to be hollow and filled with air? Why can't they just be filled in with rubber? | Engineering | explainlikeimfive | {
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"text": [
"Air weighs less than rubber. Rubber costs more than empty space. The ride quality would be worse. But there are some interesting designs for airless tires that may revolutionize how we design tires.",
"Solid tires exist for some applications like military vehicles which you dont want to pop when they are shot. But air is way lighter so it uses less energy to spin them for cars that dont have significantly high chance of being popped every few minutes. They also make airless tires for offroad vehicles where fuel efficiency isnt the biggest concern.",
"Heavy tires make for worse handling and rough rides. This is why no one makes solid rubber tires for cars and trucks. Things like forklifts and tugs will have solid rubber tires since they rarely go over 8mph, and are at a high risk of running over sharp objects. Having aired tires also makes the tires more versatile. People will often partially deflate tires to get better traction on snow, or they can also inflate them higher to get better fuel economy on roads. There are some experimental airless tires that maintain the lightweight properties of aired tires, but they still have yet to prove their worth. The biggest hurdle is making them both last as long and perform well on highways as aired tires/",
"It's difficult to get a (rubbery solid) material that will distribute the pressure in a similar way to how a pressurized gas does it. Solids and liquids are usually not compressible; if you push down on a water bed the water has to go somewhere else, it has inertia and it doesn't compress the way an air mattress does. If you jump on an air mattress, it will distribute your impact force more efficiently than a water bed (jumping into water from a certain height is lethal when you hit the water). So the response of the tire to impacts with sharp rocks or other shapes that will cause a spike of pressure to a point of the tire is different for solids and liquids vs. pressurized gas. It's also much cheaper and lighter weight to inflate with air than it is to have some sort of permanent rubber. There are designs for [airless tires]( URL_0 ), using a series of springs / elastic rubber materials, rather than a solid mass. Reason you don't see them everywhere is probably because they're expensive right now."
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|
cemp9i | How antennas receive signals | What makes a long thin piece of metal better at picking up radio waves, than for example, the metal car it's attached to? | Engineering | explainlikeimfive | {
"a_id": [
"eu3r0h1"
],
"text": [
"The metal car does also \"receive\" the signal, and you could in theory detect the signal on it, but because its geometry and electromagnetic properties aren't tuned to the types of electromagnetic radiation that makes up the signal, it will be very difficult to get the useful information from the received signal as most of it will have been lost through attenuation or non-existent as the metal chassis wasn't being affected by the signal precisely enough to pick it up in the first place, among other challenges."
],
"score": [
3
],
"text_urls": [
[]
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} | [
"url"
] | [
"url"
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cenj8u | why do lamps take two clicks to turn on and off? | Engineering | explainlikeimfive | {
"a_id": [
"eu3rvei"
],
"text": [
"Most likely you are dealing with a 3 way lamp switch. Off-low-high-off I haven't seen one in a long time, but there used to be light bulbs with 2 filaments in them, and the switch would light one for low, then both for high. If there is just a regular one filament bulb in it, you still have to click through both low and high, but the bulb won't change brightness."
],
"score": [
3
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} | [
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|
cet2y2 | Why do diesel trucks often keep their engines running while refueling despite it being illegal for gas cars to do so? | Engineering | explainlikeimfive | {
"a_id": [
"eu4otuq"
],
"text": [
"Gas vapors will ignite if you provide a source of ignition, diesel won't. Diesel fuel is basically a very light oil that only combusts when a vapor of it is mixed with air then compressed until it explodes. Diesel is so stable it's not as dangerous to work with as gasoline."
],
"score": [
22
],
"text_urls": [
[]
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} | [
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] | [
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|
cetzrq | What happens with out of duty nuclear reactors? | Engineering | explainlikeimfive | {
"a_id": [
"eu4wzpm"
],
"text": [
"The fuel is removed from the reactor and everything that can be decontaminated is removed. All the buildings and structures outside the containment vessel are torn down or taken apart And the core of the reactor is moved to a safe place, encased in concrete and buried."
],
"score": [
5
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"text_urls": [
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} | [
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|
cewidh | How can an LCD panel independently control every pixel without having one signal wire for every sub-pixel? | Taking apart an LCD screen, the connections between the panel itself and the controller driving it, while much more numerous than the digital connection delivering the data, still is "too few" for the number of discrete pixels that the display has to drive. For a 4K panel, how is it that there isn't a giant tangle of tiny wires going from the controller board to every single sub-pixel on the display? I understand that LCD pixels are arranged in a matrix, with one wire for each row and one for each column, but even then how do you independently control each pixel without also affecting the other pixels? | Engineering | explainlikeimfive | {
"a_id": [
"eu5gk03"
],
"text": [
"Lets keep it simple and use a 3x3 grid of LCD cells. In front of the cells, there are 3 horizontal electrical conductors, behind the cells there are 3 vertical conductors. 1v 2v 3v [ ] [ ] [ ] 1h [ ] [ ] [ ] 2h [ ] [ ] [ ] 3h In order to actually change what a cell is displaying, you need to pass a current through it. For the top left cell, you'd send a signal on 1v & 1h. Middle cell? 2v & 2h, etc. Passing a current along *only* a horizontal conductor is not going to have any affect on that row without a signal on the vertical conductors as well. So in our example, we would need basically 6 wire connections to control 9 cells individually. As you scale it up, the disparity between number of connections and individual cells increasing exponentially. so for 640x480, you have 640+480 = 1,120 connections for 640 * 480 = 307,200 cells."
],
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9
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} | [
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cex6pc | Why is it a big challenge to get people back on the Moon again? | Engineering | explainlikeimfive | {
"a_id": [
"eu5js36"
],
"text": [
"Money is the biggest issue. It wasn't cheap to get to the moon before (in fact it was wildly expensive), and it still isn't going to be cheap now. Technologically, its not that insane a task, we have the knowledge, expertise, and means to do it and build the items... however its gonna take a TON of money to do it, especially if you want to do it fast."
],
"score": [
5
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} | [
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|
ceytrv | Why powerful prop planes (Such as the P51-D) can't break the sound barrier? | Engineering | explainlikeimfive | {
"a_id": [
"eu5wm51",
"eu693yk",
"eu5xxln"
],
"text": [
"I assume that by \"break the sound barrier\", you're referring to the plane as a whole. But the propeller is moving much faster than the rest of the plane. So long before the plane itself approaches the sound barrier, the propeller blades are already past it. Even worse, the tips of the blades are moving faster than the part further in, so that means part of the blade is moving faster than the speed of sound and part of the blade is moving slower than it. This causes *horrible* shockwaves that can destroy the propeller.",
"It's due to the phenomenon of [drag divergence]( URL_2 ). When an aircraft exceeds the critical Mach number, part of the airflow becomes supersonic, and this leads to the formation of shock waves. These shock waves disturb the airflow over the wing, leading to a very rapid increase in drag as the aircraft approaches the speed of sound. The plane simply does not have enough power to accelerate past this point, and what's worse, the plane has a [strong tendency to nose dive]( URL_0 ) because the shockwave alters the pressure distribution on the wing. This phenomenon resulted in the crash of a prototype [P-38 Lightning]( URL_3 ), one of the first aircraft to encounter the \"sound barrier\". To allow a plane to fly at supersonic speeds, the airframe needs to be specifically designed to address the difficulties associated with supersonic flight. This is why supersonic aircraft usually have highly swept triangular wings - they are inefficient for subsonic flight, but better suited for supersonic speeds. They also have a fuselage design that adheres to the [area rule]( URL_4 ), and a [supercritical airfoil]( URL_1 ) which help to limit the increase in drag when accelerating past the speed of sound. And they have plenty of thrust, usually supplied by jet engines.",
"Generally speaking, prop driven aircraft are not aerodynamically efficient enough to travel at supersonic speeds. The prop adds a tremendous amount of drag at high speeds. That is not to say that it cannot be done, there is just no reason to do it. & #x200B; There was one exception, the XF-88B which was a hybrid jet/turboprop which could break the sound barrier in a dive. & #x200B; \\-the bootlicker"
],
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"https://en.wikipedia.org/wiki/Mach_tuck",
"https://en.wikipedia.org/wiki/Supercritical_airfoil",
"https://en.wikipedia.org/wiki/Drag-divergence_Mach_number",
"https://en.wikipedia.org/wiki/Lockheed_P-38_Lightning#High-speed_compressibility_problems",
"https://en.wikipedia.org/wiki/Area_rule"
],
[]
]
} | [
"url"
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|
cfc0ub | Do trains take any special precautions when crossing roads that only have a yield sign and no arms to block the road? | Engineering | explainlikeimfive | {
"a_id": [
"eu8u8y7",
"eu8v407"
],
"text": [
"Sound their horn and obey any speed limit signs. There's very little else they can do. It takes a long time and distance to stop, more than any visual sign of danger could give them.",
"It’s not up to the train to take precautions. It’s up to the car drivers to take precautions."
],
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23,
9
],
"text_urls": [
[],
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|
cfg5cv | why are the tines on corn cob holders asymmetrical? | Engineering | explainlikeimfive | {
"a_id": [
"euauwr9"
],
"text": [
"It takes a lot more force to first pierce the cob than it does to slide the tine in further once it's in. If the tines were the same length, it would take a lot more force to push them both in at the same time."
],
"score": [
3
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|
cfm4o3 | Why are honeycombs hexagonal? Why not triangular, circular, etc.? | Engineering | explainlikeimfive | {
"a_id": [
"euawdnn",
"euau2xf"
],
"text": [
"All cells start out circular when the bee draws it out with her wax. Once it has been joined with an adjacent circular cell, the wax is heated and it melts into a flattened shape, becoming hexagonal. [Study done on this subject]( URL_0 ) The cells on the open edges of honeycomb are rounded. I'm a beekeeper and a while back, I wondered about this too.",
"The hexagonal lattice is the most energy efficient shape for a 2D tessellation. The most efficient shape for a contained area is a circle (least possible perimeter for a given area), but circles do not tesselate without gaps. You can see this phenomenon in soap bubbles. As you nest them together, they pull themselves into hexagonal shapes. Anyway, bees evolved over millions of years to be efficient at what they do. If you look at the base of a honeycomb cell, it is actually prismatic, forming part of the base of three cells on the opposite side. We use honeycomb cores in some composite materials for the same reason. The greatest stiffness is achieved with the lightest possible weight."
],
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35,
22
],
"text_urls": [
[
"https://www.livescience.com/38242-why-honeybee-honeycombs-are-perfect.html"
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[]
]
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|
cfnuc5 | During the Apollo missions, what is the purpose of the "beep" you hear regularly in the background? | At first I thought it was maybe something marking the passing of time, but the beep seems to happen in an irregular pattern. It also doesn't seem to correspond to when people start talking, so what could it be? | Engineering | explainlikeimfive | {
"a_id": [
"eubbr62"
],
"text": [
"They are called Quindar tones and they served to turn the transmitter on and off such as with push-to-talk. The issue is that to keep track of the astronauts from a rotating Earth required a worldwide network and frequent switching between transmitters and receivers. The tones mediated all that, so they should in general follow when people are talking."
],
"score": [
27
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"text_urls": [
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} | [
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] | [
"url"
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cfpr1p | What's the hissing sounds made by semi trucks when they stop? | Sounds like air being released, but I can't figure out why or how they release the air | Engineering | explainlikeimfive | {
"a_id": [
"eubld5l",
"euboezk"
],
"text": [
"The brakes are air assisted. The hissing sound is when the driver lets go of the brake pedal",
"On your car the brake lines aren’t pressurised while you’re driving. When you press the brake pedal it puts pressure into the brake lines, causing the brake pads to squeeze. The problem is if your brake lines leak you loose brake fluid and loose pressure in the lines so your breaks won’t as well. On heavy vehicles (Trucks) the brakes are spring loaded to be always on. Pressurised air is used in the brake lines to counteract the springs, turning off the brakes so the truck can move. When a truck comes to a stop and uses the brakes, the air pressure in the brake lines is released (that’s the noise you hear) and the brakes are applied. This is a more expensive/complex system to install but if brake lines leak slowly the truck is unaffected. If brake lines fail completely the brakes are fully applied to stop the vehicle moving."
],
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12,
5
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[],
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cfpssq | Why does brake fluid need to be changed even though it never really "runs out"? | Engineering | explainlikeimfive | {
"a_id": [
"eubmt4g",
"eubn6ks"
],
"text": [
"Brake fluid absorbs water. When water gets in to it and the brake line pressurizes, the brake brake fluid can get hot enough to turn the water to steam. Steam can compress more than the brake fluid, it can cause your brakes to fail when water is in the brake fluid. It usually happens when you brake really hard.",
"Brake fluid tends to absorb moisture from the air. Having water in your brake fluid is bad for 2 reasons; 1. Under heavy braking the brake fluid can get hot enough for the water to boil which reduces the effectiveness of the brakes dramatically. 2. Having a bunch of water in the lines will cause the brake lines to rust from the inside out which can lead to a sudden failure of the brake system. This tendency to absorb water means it's also generally a good idea to dispose of a partially used bottle of brake fluid rather than saving the rest for later."
],
"score": [
16,
10
],
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} | [
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|
cfqmwj | Why do cargo planes such as the C130 and the A400M use propellers instead of jet engines? Wouldn't jet engines be more powerful and allow more weight to be carried over a larger distance? | Engineering | explainlikeimfive | {
"a_id": [
"euby1d2",
"eubsqkf"
],
"text": [
"They do use turboprop engines which are turbine engines that turn a propeller. The turboprop is more efficient at lower speeds and altitudes than the turbojet.",
"Jet engines are pretty fragile. If they suck a rock up it can damage the turbine blades and take the engine out. Propellers don't suck things into them and the engines are much higher up the wing which keeps them protected from debris. These planes are able to land on short and sketchy dirt air strips that would cripple a jet plane. Not moving as much as a jet plane isn't a problem because they're not used for their total lift capacity, they're used because they can move enough and land on an airstrip made with just a bulldozer"
],
"score": [
32,
10
],
"text_urls": [
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} | [
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|
cfsdu4 | Why have supersonic flights such as the Concorde been abolished completely? | Engineering | explainlikeimfive | {
"a_id": [
"euc55wa",
"euc58gy",
"euc6fss"
],
"text": [
"Supersonic flight atill happens regularly. The concorde was the only commercial supersonic aircraft in use, and it was not a moneymaker. That huge accident made them retire the plane, and commercial supersonic service went with it.",
"It is not economical. The fuel requirements for flying that fast on a jet engine is much higher than that of flying subsonic at the speeds they do. [Here is a video that explains it in more detail]( URL_0 )",
"The commercial window just closed on that option. To keep flying it, the planes need updating. (expensive to develop, few buyers) Therefore ticket prices would have to be high. The advent of better and better small (private/semiprivate) jet aircraft closed the window. Ultimately if you were willing to spend more than 20K for your flights regularly, you will probably save time and convenience by using a slower much more comfortable private jet. Avoid airport hassles, get more point to point travel (rather than switching flights). So the end issue is, not enough guaranteed full paying customers (any flight less than nearly full would be a loss maker) for it to be commercially viable to an airline."
],
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"https://youtu.be/n1QEj09Pe6k"
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|
cfunxk | Why do all of the contraptions and hoses stay connected to a rocket right up until liftoff? What systems are they servicing? | Engineering | explainlikeimfive | {
"a_id": [
"eucobeq",
"eucpts0"
],
"text": [
"Liquid-fueled rockets usually use liquid oxygen as an oxidizer. Liquid oxygen is cold as heck and will boil away relatively quickly if left in the fuel tanks for a while. To compensate for this, rockets are usually only fueled up momenta before takeoff, hence all of the hoses. Rockets are also often held up by huge struts or launch towers, as they cannot simply sit on the engine bells. These towers cannot let the rocket go until it is capable of holding itself up with its own thrust.",
"They're called umbilicals. They're used to fill up the propellant tanks and provide cooling as well as power and data/telemetry/communications. You want ground power and cooling supplied to the spacecraft for as long as possible so it doesn't use its onboard supply, and many rockets use cryogenic fuels or oxidizers which are super cold, and will boil off if filled too soon. You'll usually need at least some of these things supplied to each stage of the rocket as well as possibly the payload itself, so there are usually multiple umbilicals at different heights. Then there are some that are simply structural and are used to hold the rocket upright and on the pad until the engines are ignited and spool up to full thrust."
],
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10,
4
],
"text_urls": [
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} | [
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|
cg0oab | How can planes fly for 12 hours straight without running out of fuel? | Engineering | explainlikeimfive | {
"a_id": [
"eudslm3"
],
"text": [
"The simple explanation is that they contain a lot of fuel and it can be a significant percentage of the mass of the aircraft at take off. A example is a Airbus A380 that have a maximum takeoff weight of 575 tonnes and it can carry 254 tonnes of fuel, that is 44% fuel by mass The range is 14800km and the cruise speed is 903km/h That result in a flight time of 14900/903 =16hours 30 minutes. The fuel used would be 254/16.5= 15.4 tonnes per hour. The calculation assume that there is not reserve and fuel use is constant and that is not the case because the aircraft get lighter when furl is used but it is a simple estimation to get in the ballpark"
],
"score": [
3
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"text_urls": [
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} | [
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|
cgb7hf | What makes a helicopter go forward? I know the main rotor generates lift and the side facing one on the tail keeps it from spinning out, but what allows it to move forward/tilt different directions? | Engineering | explainlikeimfive | {
"a_id": [
"eufsync",
"euftdz5"
],
"text": [
"The angle of the blades of the main rotor can change mid-revolution, so that they can be more flat at the front, and at a higher angle at the back, creating more lift in the back and tilting the entire helicopter forward. Helicopter pilots also have another control available to them: the collective, which changes the average pitch of the rotors, allowing them to control lift.",
"There are a few different ways this works. The most common (despite what others say) is NOT the whole rotor tilting forwards. This is actually very hard to do (not impossible). The mechanism is quite complex. [Each rotor blade is on a pivot.]( URL_0 ) As it rotates around on the central shaft, each blade will pitch up and down at different points in the rotation. This has the effect of providing more lift in a certain direction, which can make the whole helicopter move in any direction required. [This video]( URL_1 ) is from a camera mounted on a main rotor assembly of a helicopter in flight."
],
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8,
3
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[],
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"https://i.imgur.com/vYYQud0.jpg",
"https://www.youtube.com/watch?v=Pu48f7s5Ru8"
]
]
} | [
"url"
] | [
"url"
] |
|
cgetlr | What’s the difference between highside and lowside in electrical functions? | Engineering | explainlikeimfive | {
"a_id": [
"eugvkk2"
],
"text": [
"In DC systems, high side is connected to the positive side of the supply, low side to the negative. Typically, the negative side of a single-side supply will be the common ground, which gives high-side and low-side switches distinct advantages. A high-side switch will disconnect the positive supply, leaving the subsystem connected only to ground, a neutral reference potential. On the other hand, a low-side switch will disconnect ground, so a system powered by one supply can be disconnected by one from another (as long as they share grounds), which makes it easier to switch loads that require a higher voltage than the logic can handle. Which is why most RGB LED strips are common-anode."
],
"score": [
3
],
"text_urls": [
[]
]
} | [
"url"
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|
cghdkk | How is a space rocket's weight supported while on the launch pad? | Engineering | explainlikeimfive | {
"a_id": [
"euh7sn2"
],
"text": [
"Any rocket can support it's own weight many times over by design. The reason is that on the pad, only gravity is affecting the rocket, but once launched, the acceleration will typically be 10 to 20 times bigger than gravity. So if a rocket can handle the launch, than it can handle being on the pad without support."
],
"score": [
5
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"text_urls": [
[]
]
} | [
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|
cgkj9s | Why doesnt the fire of a light lighter not go back in the lighter? | Engineering | explainlikeimfive | {
"a_id": [
"euhzbgi"
],
"text": [
"To ignite, fire needs a mix of oxygen and fuel. The lighter works because the fuel is sprayed and mixed into the air, giving the right combination to keep the flame lit. However, the tube connecting to tank of the lighter to the outlet doesn't have oxygen and thus won't combust."
],
"score": [
3
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"text_urls": [
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} | [
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|
cgmng9 | Why do heatwaves cause massive power outages? | Engineering | explainlikeimfive | {
"a_id": [
"euij4a0",
"euij7t0",
"euije62"
],
"text": [
"Air conditioning draws a lot of power. The grid stability is challenged, and once one part starts to fail e.g. brownouts, all the loads kicking back on at once when re-connected causes bigger disturbances. This is mitigated by having more capacity within the grid, and the ability to load follow (add electricity as needed on demand.)",
"The biggest issue is air conditioning. Air conditioners actually use a lot of power. When you have thousands of people turning on air conditioners in a relatively short time span, it can overdraw from the power grid and cause major issues.",
"It’s hot, everyone cranks their AC to 12. That’s a massive draw on the power grid, often more than it can sustain. Even the houses where the people set their thermostats to 80 when they leave have the AC units cycle on constantly because it’s just that damn hot. And as another poster said, equipment can fail at high temperatures. The power grid is always run at point where the loss of one big circuit can cause a chain reaction of outages."
],
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3,
3
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|
cgnshz | why is turbulence no big deal for an aircraft the vast majority of the time, even though it feels like a big deal inside of the plane? | Engineering | explainlikeimfive | {
"a_id": [
"euiv3ub",
"euiucxl"
],
"text": [
"Planes are designed with a substantial safety factor, where its wings can hold some multiple of its normal weight. Normally, we experience one gee of acceleration due to gravity, so planes are designed to withstand 5,6,7 gees in order to cope with turbulence and other unexpected phenomena. Not only this, but we as humans don't mind acceleration itself (anything between 0.5 and 1.5 gees feels okay), but sudden changes in acceleration startle and scare us. This is why many car drivers apply the brakes slowly, instead of suddenly; it makes the ride feel smoother even if the peak acceleration is higher. Planes, meanwhile, don't really mind sudden changes in acceleration - it makes no difference to the plane what direction it is being thrown, so long as it isn't being thrown too hard.",
"Because the plane is built to tolerate much stronger shaking than the human body's comfort level. Basically, no wind less than flying into a tornado should break the plane up."
],
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7,
6
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|
cgqc5r | If I buy a 5 kwh solar system and the efficency of the panel is 20% do I only get 1khw? | Engineering | explainlikeimfive | {
"a_id": [
"eujq2m1"
],
"text": [
"A 5kW panel will generate up to 5kW of electricity. Due to the sun rising and setting and the weather, you’ll only average about a seventh of that, even if the panels are ideally positioned. So 5kW times 24 hours divided by 7 gives around 17kWh per day, on average. In the summer it could be more than double and in winter less than half; much depends on the climate where you live, orientation of the panels, and any shading from things like trees and houses."
],
"score": [
5
],
"text_urls": [
[]
]
} | [
"url"
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|
cgyimr | Why do generators and electricity in general make a buzzing sound? | Engineering | explainlikeimfive | {
"a_id": [
"eumlirz",
"eummj6u"
],
"text": [
"Alternating current is used for most electrical transmission, and it's also in the walls in your house. The North American standard is 60 Hz, which means that the current alternates 60 times per second. When you hear a hum - the mains hum - it's either caused by magnetic fields from the wires causing the metal of the generator to vibrate, or it's caused by the wires in your speakers picking up the electric current in nearby power wires.",
"In addition to the 60hz sounds, if you are on an aircraft you may notice a higher pitched whine, particularly when the captain or flight attendant is speaking to the cabin. This is because aircraft use 400 Hertz a/c power. This allows smaller and lighter transformers to be used."
],
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12,
7
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"text_urls": [
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} | [
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|
ch4o08 | why Americans are more used to air wents on a floor for heating their homes and Europeans are using radiators and floor heating? | English is not my native language, I hope I expressed myself clearly enough. | Engineering | explainlikeimfive | {
"a_id": [
"eup745w",
"eupa9hc"
],
"text": [
"We do also have floor heaters and radiators if it’s an older home, but since have moved to forced air systems. Cooled and heated air can move through the same vents to get to the rooms from the unit. Since the units are usually in the basement imagine the system being a trunk of a tree, and the vents are the tips of the branches coming off of it.",
"American buildings do use floor heat some, but air conditioning is much more common in the US which makes forced air more practical."
],
"score": [
9,
3
],
"text_urls": [
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} | [
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chl1rg | Why does mankind not build the same way as they would in space? | Engineering | explainlikeimfive | {
"a_id": [
"euummfy"
],
"text": [
"We could, and there are a few examples of where we have, but the main reason is the same one driving pretty much all decisions that are made - cost. It is much, much, much cheaper to build traditional style buildings and just pump resources into and through them and out the other side than it is to build the kinds of buildings you are talking about. At the moment, there is just no real incentive to, although there is plenty of reason. Ultimately, we’ve had the tech to address sustainability issues for decades, but the political and economic will hasn’t been there to actually develop and implement it. Going somewhere like Mars forces us to do so."
],
"score": [
25
],
"text_urls": [
[]
]
} | [
"url"
] | [
"url"
] |
|
chtd9t | Gas pipelines rarely explode during earthquakes, why is that? | Engineering | explainlikeimfive | {
"a_id": [
"euxiv7h",
"euyge67"
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"text": [
"Mostly because it needs a lot of oxygen and a spark to ignite it. As long as one of these isn't around nothing can happen.",
"One reason is that breaks or cracks in the gas pipes can be quite rare. Piping will bend with the earths movement to a certain degree. To prove this, take say a 20-ft length of 1\" inch steel pipe laying on the ground. Pick up one end you will see the bending down to the other end that is still laying one the ground. Key words with the above are \"can be\" and \"certain degree\". Lots depend on the type of earth, how strong the quakes, etc. I been in two major quakes over 6-point Richter scale in a large city. Don't recall any gas explosions per se, but yes there were some gas line fires. And as a steam boiler guy, we were going under buildings looking for not only steam line leaks, but also doing drop pressure testings on gas lines for leaks just a couple days after. Pretty freaky-deaky when aftershocks were still hitting us."
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cibsw4 | how WhatsApp prevents spammers from using the app | I've never got a single spam call or a text message on WhatsApp. How are they so successful in fighting spammers? | Engineering | explainlikeimfive | {
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"Their main tactic is really just making it difficult to sign up for a WhatsApp account. To sign up for WhatsApp, you need to put in a real cellphone number. They have a way of telling a real phone number from a \"virtual\" one (using VoIP), and block any attempts to sign up with one. By doing this the barrier to entry is high enough that scammers don't bother with WhatsApp and just use platforms that are easier to access."
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ciez30 | How do stabilisers work? | Engineering | explainlikeimfive | {
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"There is a small sensor built into the device that detects movement - similar to the one on your phone that detects tilt. The system is then setup so that when the sensor detects some movement, the motors offset it by moving in the opposite way. Obviously there is a limit to this, as the motors can only move so much in any one direction, to counteract this the system is designed to detect different types of movement and react accordingly - so short, sharp movements can be counteracted completely, while long movements (such as panning a camera) can be detected and dealt with appropriately - such as countering vertical movement but not horizontal, or just smoothing out the bumps in the movement. If you were to drop a stabilised device, it can often go as far as to detect it had been dropped, and park all of the moving components in a safe way so add to limit damage."
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cikzjg | Why does a car engine make a 'dripping sound' when you turn it off? | You know, this 'ting, ting, ting' | Engineering | explainlikeimfive | {
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"Metal expands and contracts with temperature. That's just hot parts cooling off, they make that sound as they settle back to their cold size and position.",
"It's the sound of metal parts in the engine cooling and contracting."
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cingov | what stops lighters from exploding when you use them. | Engineering | explainlikeimfive | {
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"For combustion, you need oxygen. There is no oxygen inside a lighter. When the butane inside a lighter is released, it reacts with oxygen in the air to make fire. Because there's a much higher pressure inside the lighter than outside it, oxygen can't get in while the trigger is flipped \"on\" since butane gas wants to get out into the lower pressure and oxygen doesn't want to go into the higher pressure zone. Even if oxygen wanted to get into the higher pressure zone, it's being consumed too quickly to get in.",
"I literally came in this subreddit to ask this question and you just posted it, wow, amazing"
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cis3qm | Why do drones usually have 4 rotors - has this been proven to be the best (most efficient or stable) design, or is it possible that a different number would be better? | Engineering | explainlikeimfive | {
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"An even number of rotors makes it easy to prevent unwanted spinning. Each rotor spinning clockwise has a twin rotor spinning counterclockwise, which allows the drone body to sit still without rotating. Now, in order to tilt forward, backward, left, and right, the drone needs three or more blades. Four is the smallest even number greater than three, so it is usually chosen. There are, however, drones with more rotors. I've seen six and eight rotor drones, and there are probably even bigger ones. Usually, for small and simple systems, four works best. Edit, since you lot keep asking it: HELICOPTERS USE COMPLEX ANGLE-SWITCHING ROTOR BLADES AND VERTICAL TAIL ROTORS THAT AVOID THE ISSUE",
"Am I allowed to post a follow-up? Why don't modern manned helicopters use 4 rotors? Cost, design efficiency, don't fix what isn't broken? Do we know of any modern manned designs that take inspiration from UAVs or are there are plans for up-scaled and manned quadcopters?",
"Ooh, ooh! I know a really good answer to this one! First of all, when people tell you \"an even number of rotors is good for preventing unwanted spinning\" etc, etc, this is, frankly, only a half truth. The bottom line of the matter is that the mathematics involved in keeping a multicopter in flight is hella complicated, and the problem of handling an odd number of propellers is trivial by comparison to the problem of reacting successfully to random fluctuations, slight differences in rotor behavior, and discrepancies between control and response. In fact, even the act of a quad-copter turning involves all the math necessary to overcome an odd number of rotors. Want proof? Here you go: [Pentacopter]( URL_0 ) It's uncommon, but it is possible and has been done. ***That being stated,*** it is very much true that the ***amount*** of correction needed is significantly reduced if you have an even number of rotors. # The real reason you want at least four rotors is actually straight out of linear algebra! There is a concept called ***Degrees of Freedom.*** In a single sentence, it basically says that if you want to be able to control N aspects of your kinematic state, you need N independently controllable force/torque appliers. The reason this comes from linear algebra is that you can think of the total torque and force at any given moment on your multicopter as a vector in 6D space. To be able to achieve any torque/force combination, you need to have force appliers that, when combined in different ratios, let you access the entire 6d space. If you only have three rotors, you are restricted to a 3D slice of the 6D space. To make this less math, and more practical, think of it this way: Each additional rotor gives you, essentially, control over one additional aspect of your multicopter's kinematic state. A quadcopter has four rotors, and hence (with typical configurations) has control over the following aspects: Acceleration Forward/Back, Acceleration Left/Right, Acceleration Up/Down, and Yaw. That's four. With four rotors, once you pick those four, you have no control over the remaining two torques: Pitch and Roll. That is why quadcopters ***always*** tilt the at the same rate for any given side-to-side acceleration. They don't have a choice in the matter. Now, you can see the problem with only three rotors. You can control your position in 3D space, but to do so you must sacrifice control over your yaw torque. The exact math works out that to maintain stable position, a tricopter would have to sit there spinning violently. (***This is where the notion that an even number is required for torque balancing comes from -- but in reality, you just need a number greater than 3*** That makes them \"unusable\" for most purposes. Now, this seems odd, because there do exist a few tricopters out there. But, if you [look closer]( URL_1 ) these copters have a hidden servo which achieves a fourth Degree Of Freedom by tilting one of the rotors. # TL;DR It's not true that you need an even number of rotors. It is only true that you need at least 4, because you need 4 degrees of freedom to be able to move around and turn with stability.",
"It's the smallest number that makes control really easy, because you can change direction forward, backward, left, and right, simply by changing the amount of power each rotor gets.",
"Lots of great answers here about \"fixed pitch\" blades vs a traditional helicopter rotor head design. With small electric motors you have instant access to 100% of the tourqe that electric motors supply. Being that there is no spool up or spool down time, a fixed pitch blade is the lightest design for electric motors.",
"You need an even number. If you don't have the same number of motors going clockwize as there are ones going anticlockwize, or the drone will just end up spinning. And with two, you could only tilt forward or back, but not side to side. But when you need a fairly large drone, you can ad pairs of rotors for extra lift. Professional camera drones are often hexacopters, with 6 rotors. You could do 8 rotors or more, but once things get bigger than a certain level, you can be better off ditching the electric motors and fixed blades, and building a real helicopter.",
"4 rotors give you control with fixed rotors and no servos. 3 rotors and you need 1 servo. 5+ rotors give you redundancy and increased max lift at the cost of efficiency. An octocopter can lose motors and not fall put of the sky but is less efficient.",
"There's an economical side to it too. This is regarding general drones on the market, not military or high spec. 4 rotors is typical because it's cheaper. Its the lowest and easiest quantity to work with, for the reasons others have said (stability, maneuverability etc). Generally speaking, 4 rotors is the most common because it's decent stability and control without too much manufacturing or cost involved. It's also lightweight which makes them more agile (ideal for drone racer), and saves on manufacturing costs. If you go up to 6 or 8 rotors, the stability would be a lot better. But it also draws more power, meaning less flight time and higher expense. If it was designed PROPERLY with an 8 rotor, it could still be efficient and have a decent flight time. Each rotor would only need to produce a small amount of thrust, so each rotor would operate more efficiently, therefore resulting in a more efficient system. Buuuuut the world runs on money so the cheaper option is always taken for generic rotors. The whole system would have to be custom, each individual rotor and motor. Highly doubt the ones you find on the shelves are custom designed. Economically speaking it's cheaper to take 4 off-the-shelf rotors to make the zipzip go zoomzoom. I hope this makes sense I'm really high and my aerospace engineer boyfriend confuses me. Also this is his personal educated views on it as he does not work in the specific market (his work is on jet engines/turbine engines, not drones).",
"It was quite interesting following multi rotor experiments in the R/C helicopter forums back in the day. A R/C helicopter generally has a 3 (every 120°) or 4 (every 90°) servo setup to control the swash plate. The swashplate controls the pitch of the blade as it makes 1 full rotation. For example to induce forward flight the swashplate will induce more pitch as the blade travels to the rear and less at the front of the circumference of its travel resulting in the helicopter pitching forward. Mixing inputs to all the servo's from the control sticks makes it possible to move in any direction. The tail rotor (rudder) pitch is controlled by another servo and is pretty much impossible to control without aid. You have to compensate the torque from the main rotor as well as giving the right amount of output to change the direction you wish the helicopter to face. A gyroscope is used to keep the tail in control and keeps the nose pointing in its current direction when there is no input from the pilot. People started adding gyroscopes to the servo's controlling the swashplate in an endeavour to create a stable easier to fly helicopter with success. Then replacing the swashplate servo's with motors that drive an individual rotor leads us to the familiar drone look. It can be looked at like a flying swashplate which removes all the extra complicated mechanisms required to make a helicopter.",
"There's an interesting way at arriving at 4 rotors that ignores virtually all of the physics of how they fly: looking at degrees of freedom. Degrees of freedom is a way of considering how something moves. Take a car, for example. It can go forward and backwards and it can turn left and right. It can't (normally) roll on its side or pull its nose up. It can't go up or down (not like hills, but flying). It can't slide sideways. Since cars can move in two ways we say they have two degrees of freedom. In order to control that they need two control inputs. For a car those inputs are the steering wheel and the combined effect of throttle and brake. Note that *any* attempt at giving car-like motion to a vehicle will need two control inputs. If you only have one then that kind of control is impossible. If you have more then you can't actually get a meaningful effect out of some of your control (we see that with throttle and brake on cars, where holding both pedals gives the same effect as pressing neither—at least in terms of motion). A tank, for example, has the ability to go forward and backwards and to turn left and right, but its two control inputs are the speed of the left track and the speed of the right track. With this means of analysis in our back pocket we can look at drones. We want the ability for them to fly up and down. They should be ale to turn left and right. Then we look at moving around left/right and forward/backwards. We don't actually need to have those two degrees of freedom because we can instead make them able to tilt nose down which makes any upward thrust also push the drone forward. This means our last two degrees of freedom are tilting nose up/down and rolling left/right. Note that full-sized helicopters need the same four degrees of freedom. With these four degrees of freedom we need to start finding our control inputs to allow for that motion. Helicopters get their control inputs by having the ability to alter the pitch on the blades rapidly, as they move in a circle. That makes for a very complicated mechanism. On a drone the goal is for the control input to be speeding up and slowing down the motors, so we need to arrange as many rotors as we have degrees of freedom: four. Not just any arrangement would work here—stacking four rotors on top of one another doesn't actually give four meaningfully different control inputs—but luckily there's a simple arrangement that does work. We arrange all four rotors on a plane and make them alternate the direction they spin. To go up/down you increase/decrease all of their speeds. To turn left/right you increase the speed of the clockwise-spinning rotors and decrease the CCW rotors, or vice versa. To roll left/right or tilt nose-up/down you increase the speed on the side that's going up and decrease the speed on the side that's going down. It winds up being a pretty simple control approach to design and operate at high frequency. You can make a drone with more rotors easily, though you generally want to stick to even numbers so that half of the rotors can spin one direction and the other half spin the other direction. 6- and 8-rotor drones are common for drones that need to carry a heavier payload. You can make a drone with 3 rotors but then you need to have another control input. I've seen this done by adding the ability to tilt one of the three rotors in its entirety. You can also make a drone with only two rotors, but then you need two additional control inputs. That's a helicopter. 4-rotor designs are common because that's the minimum number of rotors (and, more importantly to cost, motors) needed to give the control you want without needing some extra method for enacting control on the drone.",
"URL_0 This guy experiments with different types of rc vehicles if you were interested",
"These drones have a total of 4 moving parts which makes them mechanically extremely simple machines.",
"Six is best for resilience. If a motor or rotor fails, its partner on the other end can also be stopped.",
"A search of the first 200 comments didn't touch on it so I'll just bring up the reason that real helicopters don't use the same configuration as drones, weight. Ideally 4 rotors is a great setup but as an item gets bigger it's mass increases exponentially, specifically the [inverse square law]( URL_0 ). As a result, scaling everything makes multiple rotors insufficient to lift a full size helo. Small electric motors are good enough for a drone, but batteries are super heavy so a full electric manned helicopter is just not feasible with today's technology. Add in the weight of the extra rotor and gearboxes/motors and it's easy to see why single main rotors are the standard with just a few specialized designs opting for 2 rotors. Even drones suffer from this, larger ones often have 6 or 8 rotors but each time you add another rotor you get fewer and fewer gains for the same motor being added; as size increases mass just increases faster. Eventually you hit the point where the complexity of making a single main rotor work far outweighs the downsides of adding a half dozen more rotors.",
"As the other posts have shown, this is quite a complicated subject. First, let’s look at a modern helicopter. They have a single rotor on top that provides their lift and thrust, with a small rotor on the rear for stabilization. This second rotor is required because of Newton’s 3rd Law of Motion, “Every action has an equal and opposite reaction.” Similar to how, floating in a pool, if you push your arms to the left, your body will spin to the right, this means that once the helicopter is off the ground (and therefore has no friction holding it straight), the rotation of the upper rotor causes the body of the helicopter to rotate in the opposite direction. The rear rotor on most helicopters is designed to prevent this rotation, and also to speed up or slow down to force the body to rotate in either direction. Most retail drone designs use a multiple of two rotors spinning in opposite directions, because the spins offset each other and prevent rotation of the drone body during stationary flight. To fly forward, most helicopters have their upper rotors on an actuated joint (it can be angled forward or backwards, usually using pneumatics or hydraulics), and when angled forward the upward thrust goes up at an angle, pulling the body of the helicopter forward. These types of joints are rather complicated, and drone makers don’t end up needing them, because they have a different way to angle their rotors. Instead, they (typically) use four rotors. This is the simplest way to address the complications of 3D flight. With four rotors arranged in a square, and each rotor traveling the opposite direction of its neighbor (so the rotor on opposite corners spin the same direction), the drone will not rotate in the air normally. To rotate the drone clockwise, you spin up the counter-clockwise rotors, because then the clockwise rotors won’t completely offset the rotation; and vice versa. For lateral movement (forwards, backwards, and side to side), you spin up the motors opposite the direction you want. To go forward, the rear 2 rotors spin up, and the added upward thrust tilts the back of the drone up, which angles all the rotors so that they’re slightly forward. This creates the same forward angle as the helicopter, without the need for the extra joints, and the thrust then pulls the drone forward. The reason 4 rotors are used is because it’s the fewest rotors to be effective without complicated math. With 2 rotors, you can only rotate and go forwards and backwards; no side to side. With any odd number of rotors (like 3), you need to carefully balance the total force of all clockwise motors against the counter-clockwise ones to prevent unwanted rotation, and that mostly prevents using the same type of motor for each rotor. And more than 4 is functional, but more rotors = more power. That’s why the ones you see at the store have 4. There are hobbyists who use more rotors, however, and it can offer advantages. More rotors means more fine control of directional travel, for example. It’s not hard to take a 4 rotor drone and make it fly at an angle (forward+right), but a 6 or 8 rotor drone will do it more smoothly. Also, with more rotors, if one motor goes out, you can have the onboard computer shut off the opposite rotor to stabilize, and you still have at least 4 rotors for 3D movement. This is particularly handy if you are drone racing or doing aerial acrobatics with an expensive drone, so at least you can get it down relatively safely for repairs. TL;DR - 4 rotors is just the cheapest way to build retail drones, so it’s most common. Depending on the drone’s purpose, and how important stability is when a motor fails, more rotors can be used, but it will increase power consumption and require a more complex computer, making it inefficient for beginning hobbyists who just want to fly a cool drone around."
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cit44b | How do fans without blades work? | Like the blade less Dyson type of fans | Engineering | explainlikeimfive | {
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ciy7p9 | How was Space X able to build better rockets than NASA having less budget and experience? | How come space X, blue origin and other private space companies are capable of building better rockets than NASA, when NASA has +50 years of experience in the industry and a massive budget? | Engineering | explainlikeimfive | {
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"I think the other people have missed the critical element here. The critical element was the shift from disposable to reusable rockets. Now, to their great credit, NASA did make that move in the 70s with the space shuttle, but it was a bit too early, and the whole shuttle project was a bit of a logistical and political shitshow. By the time SpaceX came along, technology had advanced considerably. Here's a great talk by Raffaello D’Andrea explaining it [ URL_0 ]( URL_0 ) In short, between the Space Shuttle and the Falcon 9, feedback control got good enough, and cheap enough, that the idea of landing an otherwise traditional rocket vertically was very feasible. By this point, the Shuttle was seen as a failure - technologically impressive, but economically unviable, and increasingly outdated. So NASAs focus was off of reusability and onto cheap disposability. In that environment, dumping money into R & D is a bit pointless if you have designs from the 60s that are reliable and well understood. They'll only fly once, and the cost of space flight is really in the minimization of failures. So, advances in engines and control systems largely stopped. SpaceX saw things differently. Believing they could take what looked like a conventional rocket and land and refly it, meant they could change the economic model. Launches were rare because the cost of disposing a rocket each time was high, and because the cadence was low - there was no incentive to build out production lines that could spit out a rocket every two weeks. But if you could reuse the rocket, even just one more time, you'd need half as many of them. Refly it 4 times, and you need to build only 20% as many. So, SpaceX went for a commodity strategy - build one really really good motor to serve first and second stage, rely on modern control electronics to regulate 9 motors operating together in the first stage, and engineer for reusability, and get your reliability that way, and dramatically cut costs to fly the rocket and use that money to pay for the new R & D. Digital feedback control wasn't the only thing that had dramatically advanced since the 70s, so had manufacturing techniques and materials engineering. So SpaceX could build simpler rockets that performed better than was possible in the 70s. NASA didn't want these rockets, believing in their tried and true approach, but private companies did want them and with time SpaceX won them over. Bezos saw the same opportunities, as did others. Existing companies didn't see the opportunity because their value was in their tried and true methods, their decades of engineering experience at this, their detailed knowledge of how these old system worked, which made them reliable. This is also a story of why established companies rarely pivot their business model to adapt to changes, and why startups and other new entrants are key to advancing industries. They can take these risks, they can invest in the new technology and not invest in the legacy technology. Had SpaceX failed, we wouldn't even be talking about them - so there's some survivors bias baked in here too.",
"They haven't really. 1) They utilized all of the science that NASA learned thus they \"had\" the same experience level as NASA. 2) NASA has never had a massive budget. Even during the Space Race their budget was relatively small. Companies like Space X's budgets are comparable in size. 3) NASA stopped designing new Rockets for a time when they were operating the shuttle. When they retired the shuttle they started designing new Rockets again and will be constructing them for the upcoming missions.",
"Aside from using known published resources from NASA and likely the old Soviet era, technology has changed dramatically since the 60s. They’re able to run many simulations in a short time period that would have been impossible for NASA back in the day. There’s likely less politics involved too. Computer technology I’d guess been the key difference that has allowed them to do things like reusable components. I’d assume material science has had a big change since the 60s too. In all fairness, NASA built the shuttle which was a pretty damn impressive bit of tech. Not without it’s issues but it was cutting edge.",
"To build a new rocket, lots of design and testing is needed. Problems will be found, so how quickly these are found and fixed decides how quickly a new rocket can be completed. Because of restrictions due to it being run by the state, Nasa can't adapt anywhere near as quick as spacex; for example, when spacex realised carbon fibre wasn't the way to go for the BFR, they fired all the workers working on carbon fibre designs. This was necessary, but would not be possible in Nasa. The fact that they have to be acting in the best interests of the country means that designs from Nasa are dictated by created jobs, and less by the actual advantages of the design, leading to many issues that are delaying many programs. They also have major direction changes with each election, meaning that they work towards one goal for 5 years, then a new leadership tells them they want a different project doing, so they cancel the first one, and end up getting nowhere.",
"NASA is heavily regulated, risk avoiding, and making decisions by committee. SpaceX has been able to build on all of NASA's experience, as well as take advantage of more modern underlying technology. The other big innovator in this field is Elektron, in my opinion. I think the elephant in the room is nothing to do with NASA, but rather ULA. Why is SpaceX able to build such a better business model than ULA, who has the years of experience and the resources?",
"A point I do not see mentioned yet is that SpaceX does not need to please any governors in different states, so they can have their production wherever they want, and basically SpaceX can do whatever they want in that regard. In comparison, Nasa has widespread production and facilities, and is a much older organization. If Nasa were to be completely wiped and restarted, I doubt that they would not be able to do similar things to what SpaceX has done.",
"SpaceX has a few advantages, but I'd say the core thing here comes down to two main factors. First, big NASA rocket projects (and by this I mean something like the Shuttle or SLS), which are not exactly made by NASA but are designed and built by them in cooperation with oldspace companies like Boeing and Rockwell, have to answer to a large number of competing interests. The companies of course want to make a profit and NASA wants their science, but also congressmen want some of the contracts for building these rockets to go to their states, and sometimes the military wants specific capabilities, and sometimes the system needs to be designed to use a specific set of preexisting resources or expertise, etc. And since all this stuff is subject to funding bills, etc, it can't be easily or cheaply changed. SpaceX, in contrast, is more or less answerable only to themselves (except when fulfilling a specific contract). If they need to switch an approach or try a new thing or whatever, they can just decide to do it. This gives them a lot more flexibility and helps them do things for a lower price. The second is the drive of necessity/lack of money/lack of being established. Oldspace companies (who are a better comparison here than NASA, which doesn't really build Falcon style rockets) have been launching rockets for ages and were dominant in the market. Sure, their rockets were expensive, but they got the job done effectively and all their customers were willing to pay the price and no one was undercutting them. Spending a lot of research money to make it possible to build rockets for cheaper and maybe even reuse them just didn't make sense to them. Why take the risk and expense when life was already good? But SpaceX didn't have that option. If they wanted to survive at all, they had to be able to figure out a way to undercut the competition and win their contracts. That means taking risks and working hard to make rockets for cheap, using more cost efficient manufacturing. That was the first revolution of SpaceX, and more important to their past success than reusable landings (though that will be more important going forward). There have been other new rocket companies that tried this and failed..it is a risky approach, but SpaceX made it work. Worth noting that this is not limited to the rocket industry, many industries have had established players get overturned by new upstarts who were better because only a better company could break into the market in the first place. It's not even limited to industry for that matter. A factor I _don't_ think is as important: technology...the stuff SpaceX is doing is impressive but not due to any particular unique technological breakthrough on their part. It was within the capacity of NASA or ULA or Boeing or whoever to do the same thing if they chose to put in the engineering work. They couldn't have done it in the 60's (and SpaceX probably couldn't have existed at all back then) but they could have done it when SpaceX was doing it."
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cj2kf7 | Why is the QWERTY keyboard's arrangements the way it is? | Engineering | explainlikeimfive | {
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"Typewriters are complex mechanical contraptions, so having commonly used keys next to each other makes them jam easily if you're using an alphabetically ordered keyboard. QWERTY has the keys more or less evenly spread so that typewriters could write at high speeds without jamming, and once the convention set it became nearly impossible to sell alternatives. Dvorak is one relatively popular (and supposedly faster) layout, but since learning different layouts takes a lot of effort, QWERTY is the norm."
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cjbgvi | Is there a correlation between a bird's weight and its wingspan? Is there a formula or pattern that is followed? | Similarly if you had a bird the size of an elephant how large would its wing span have to be to fly? | Engineering | explainlikeimfive | {
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"What you're looking for is called \"wing loading\" and tells you the ratio between the mass of the object and the area of the wings. Birds have wing loadings between 1 and 20 kg/m^2 . If you assume 20 then even a small 2500 kg elephant needs 125 m^2 of wing area or 2 8x8 meter wings. That's a lot of wing, more than an A320!",
"It's not that a formula is followed, but there are formulas that can be used to determine if a bird can fly or not, based on wingspan, flight mechanics, and other factors. The largest bird ever capable of flight (known to man) is the Argentavis Magnificens (Magnificent Silver Bird) had a wingspan of up to 20 feet, a length of up to 4, a height of over 6 and a half feet, and a weight of 157 pounds. Another contender would be the Pelagornis Sandersi (named after discoverer Albert Sanders), which was lighter but had a wingspan of up to 24 feet."
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cjbh1x | Why are some soldiers wearing pixelated camouflage? | Engineering | explainlikeimfive | {
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"Under certain conditions the pixelated/digital camo does a better job at breaking up the lines/outline/silhouette of the soldier. The point of camouflage is not only to match the background colour of the soldier's environment, but also to break up any easily identifiable lines. If camo was colour matched to the surroundings but still in big stripes, the soldier would still stand out pretty good (depending on lighting conditions etc.) Old camo schemes were big hand designed \"splotches\". Good, but smaller more random splotches break up the pattern of the soldier, load carrying vest, gun/ammo etc. better. The newer digital patterns are even better.",
"URL_0 here you can see how good it can be",
"So in the early 2000’s, the Marines began moving over from BDU’s (the old green/black woodland pattern) to something called MARPAT. MARPAT was a pixelated pattern designed to blend more easily into the background and to distinguish Marines from other services. It came in two colors, though the desert-tan version has seen more use than the green version you typically see in-garrison. Like other Marine uniforms, everyone loved it and wanted their own version. The different service chiefs put out a call from vendors to design something specific to each branch, and after several years of expensive designs and testing, they got their wish. The Army came out with the ACU pattern which was a sort of digital-grey pattern, the Air Force came out with ABU, and the Navy went with the Type I blue pattern uniform before dropping that and going to the NWU tan/green uniform. Here’s the problem that came with everyone coming up with their own service-specific digital pattern uniforms: it cost a lot of money, and they mostly sucked at what they were supposed to do. The Army and Air Force uniforms blended in with absolutely nothing, and would quickly fade to a light-grey after too much use out in the field. The Navy Type I uniform was great at blending in with the ocean....which is exactly what you don’t want to do if you get tossed overboard from a boat. Also, they had a tendency to be explosively flammable. So after a decade and a half of these uniforms, the Army and Air Force are now consolidating into the OCP pattern, which is a chunky tan/green/black camouflage pattern similar to the original BDU’s, but cut like the Army ACUs, The Navy, as I mentioned, is going with the NWU, and the Marines are doing their own thing and sticking with MARPAT. TLDR: The Marines made a functional and cool uniform, service chiefs from other branches got jealous and made crappy knockoffs.",
"The human eye and brain is very good at detecting the shape or silhouette of an object, the color and movement. Camo tries to break up the outline and color of what a human should look like. If you look at a forest or a bush. The shadows of the leaves and the sunlight breaking through create small patches of light and dark, similar to the pixelated dots on a camo shirt. And from a distance have those pixels not all blend together into a gray outline URL_1 Compared to URL_0 By seeing the outline, aiming is easier.",
"It's counter-intuitive, but the pixelated camo is better at making you disappear against a background than the more \"organic\" patterns.",
"[Uniform History]( URL_0 ) This guy has a great YouTube channel for Uniforms. And has a good video on Canadian CADPAT. Which started the usage.",
"So why do people in the navy wear blue camo? What are they trying to blend into? The water if they fall overboard?"
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cjc2gn | How do GPS satellites and receivers compute our position without synchronous clocks between them? | So I recently read that GPS satellites use atomic clocks and all the GPS satellites have synchronous time, but the receivers on the ground use a quartz clock which is not so accurate compared to the atomic clocks onboard the satelites. We know that the transmitted signal travels at the speed of light, so if we find out the delay between the time when the signal was transmitted and the time the signal was received we can calculate distance, and by using 4 satelites we can pinpoint our location. But this is all assuming that both the transmitter and receiver have the same exact time, but one uses a atomic clock and the other a local quartz clock. So how do they achieve sub-meter accuracy with GPS? | Engineering | explainlikeimfive | {
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"If you assume that the receiver have the exact same time as the transmitter then you would only need 3 satellites as you would be able to know the exact distance to each satellite alone just based on the difference in time. However the receiver does not have an accurate clock. But it is accurate enough to measure the time between the signals from two satellites. So it does not know its exact distance to a single satellite but it knows how much closer it is to one satellite then the other. This is why you need 4 satellites instead of 3. In practice 4 satellites is too few to get an accurate position and you need far more in different directions.",
"The receiver has \"atomic time\" by constantly resetting itself based on the satellite signals it receives. It receives 4-5 signals all of which can *only* overlap at a single point in 3D space. The receiver, knowing it's inaccurate, also knows that 4+ spheres can only overlap at a single point when given the \"correct\" (atomic) time. From there it can do some fancy math to figure what the correct time *must* be for all of those spheres to overlap at a single point, any other time must be wrong so it sets itself to the calculated \"correct\" time. It then uses that correct time to perform the appropriate calculations against all the signals it received to figure out its location in relation to the satellites. So essentially, because of how spheres relate in 3D space, the receiver is able to calculate the atomic time itself by calculating exactly where the spheres overlap (only the atomic time will cause all the signals to overlap at a single point)."
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cjctax | On boats & ships, how do propeller shafts remain watertight despite the friction caused by the propeller movement? | Engineering | explainlikeimfive | {
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"The boats I'm familiar with, sailboats around 30 - 40 ft, it's not water tight. You should get about 1 drip every 30 seconds. You need a bit of water coming through to act as cooling. Then you have a bilge pump to pump the water back out. The shaft goes through what is called a stuffing box. ( URL_0 ). You have a fiber like flax, hemp or some type of rope and treat it with wax or oil and then pack it around the propellor shaft. You want it tight enough that water doesn't come through too fast, but loose enough th shaft can still spin and few drops of water keeps everything cool.",
"Here are some longer answers and a few drawings: URL_0 Basically, it is not 100% watertight, but with proper materials stabilizing the propeller tube and proper lubrication, the remaining amount of water coming through is small enough to be pumped out easily.",
"Exact same method as your car's engine/transmission/rear axle/etc... There's a rubber and steel gasket that seals against the spinning driveshaft and the surrounding housing. In cars, a bit of air sneaks past the seal, goes through the oil and is burped through a breather at the top of the gearbox. In boats, water sneaks past the seal and is pumped out on a regular basis.",
"How does this work on submarines, do they just use the same stuffing boxes but engineered to the much higher pressures, or?",
"Quick external source: URL_0 Granted, that isn't a maritime application, but the concept is the same. The short of it is that you have packing (somewhat flexible material) that is compressed around the shaft so that it maintains the seal. That said, such seals do not necessarily need to be *perfectly* water-tight. They mostly just need to restrict flow enough that the bilge pumps can comfortably keep up.",
"So, a little off topic, but you might also be interested in labyrinth seals: [ URL_0 ]( URL_1 )",
"I used to be a mechanic on a submarine, the shaft seals leak. There is a catch-basin just under the shaft hull-penetration. A pump cycled on a regular basis to clear it out. The tank it pumps into has to occasionally be pumped overboard, preferably when we are reasonably near the surface. The Sturgeon class I was on had seven seals around the shaft to slow the leakage. The deeper we are, the faster it leaks."
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cjdbsm | Why are baseball bats and cricket bats shaped the ways that they are? | Engineering | explainlikeimfive | {
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"The cricket bat shape has evolved to be optimal for striking the ball when coming to you as it does from a bowler, and holding it as batsmen and women do. There is a \"sweet spot\" on a bat, more or less opposite the widest part (front to back) - hit a ball square on there and it will fly.... when you watch top batsmen, it's amazing how far and fast the ball travels when they barely seem to hit it with any force; just stroke at it and, catch it right, it will go.... I presume the same goes for baseball bats."
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cjegmx | Will it be possible in the near future to create laptops that are able to withstand severe heat? | As a computer science student, I have to keep my laptop with me most of the time, and probably will still have to keep it around all the time when I get a job. This is especially annoying in the summer because I want to be able to take my computer out with me without it getting cooked in the summer heat. Are engineers possibly coming up on solutions (in the near future) that would allow laptop components to not break down at these extreme temperatures? What are the bottlenecks in creating this type of technology? | Engineering | explainlikeimfive | {
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"The problem is one of \"small.\" In order to make the laptops as flat and light as possible, everything gets jammed together. Airflow suffers, and components run hotter. Compound this with components getting faster, which causes then to run hotter, too. Bigger, bulkier laptops, or laptops without the highest-end components don't have these problems because they don't generate the heat on a confined space. Laptops exist with the highest-end components, and they don't overheat, but they're an inch thick and weigh a couple pounds. A better solution is to adjust your expectations of what a laptop is for, use one with less power for when you're away from the desk, and use a high-end desktop for the heavy work. Or offload all of those computer things you need to do to the cloud, leaving just displays and input for you laptop's chores. If you want a desktop in a tiny package, hear is the price you pay. Well, money, too. *Heat is the price you pay... Also paying the price for not proofreading...",
"I dont k ow where you are but I used \"ruggedized\" panasonis toughbooks in the desert years ago for work in temperaturs well over 100 f. They weren't the fastest by any means but they worked."
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cjlrt0 | Does using internet on a flight really affect the plane’s instruments? | Engineering | explainlikeimfive | {
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"The short answer is that it's possible, so don't. The Mythbusters did an episode on this. What they found was older electronics had more of an effect than the newers ones. Also that most modern airliners have shielding on the instrumentation to help protect them against this sort of thing. But on smaller and older planes there was a significant affect on the instrumentation. Airliners are tightly regulated for your safety, so when they say turn your phone off do it! Better safe in the air than not."
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cjp9d4 | What is a probability density? | Would anyone please explain what a probability density is and what is its use? Thank you all very much in advance & #x200B; Edit: Hi everyone. a huge thank you to all of you that generously helped and provided thorough explanation. I really and I mean it, really enjoyed reading all of your answers, and each one of them make it more clear for me. it was as if all answers complemented each other :) I sincerely thank you all and wish you the best. Thank you all very much:) | Engineering | explainlikeimfive | {
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"You can divide random variables into two types: discrete and continuous. Discrete random variables have precise values that they can be. For example, the roll of a 6 sided dice can be any integer value from 1 to 6, so you'll never get 2.5 for instance. Continuous random variables can have infinite possible values that they can be. Some things that can be considered continuous variables are height, weight, time... If you want to know \"What's the probability that a person weighs exactly 100 kilograms\" then the probability is zero, because they can weigh any value from 0 to (lets say) 1000 kilograms. There's infinitely many values in that range. Now, if you can define an interval instead of a single value, then it makes sense talking about probabilities with continuous variables. Using the same example, if a person was 1kg off, maybe it's close enough for your purposes. So if you consider everyone weighting between 99kg and 101kg, now you can look up at your probability density function and multiply (or integrate) it in this interval. The probability density function has higher values for more likely events. So still using the last example, the probability of finding someone between 99kg and 101 kg is much greater than finding someone between 399kg and 401kg, even though both intervals are the same size",
"The way I understand it, it's a volume or system that contains varying probabilities for (I'm guessing you're talking about electrons) something to occur at any given time. Let's take a basketball as an example. You are bouncing a basketball up and down. Your epretty good at it and you don't drop the ball, or anything. You're just standing there, bouncing it up and down for half an hour. The ball spends more time on the ground and touching your hand than it does flying between them. Now, take all the single bounces, and record the ball's position every, say, tenth of a second. You get a bunch of points close to your hand and the ground, and fewer points in between, because the ball spends less time travelling between the ground and your hand, that it does when it stops and changes direction. What you have there is a *probability gradient.* This means that if you were to consider that system (someone bouncing a basketball up and down), you could try to predict where you'd most be likely to find the ball at any given time. Since the ball spends more time touching your hand and the ground, the probability density at those points is greater than in the space in between.",
"Probability density refers to how likely it is that a random measurement falls into a particular range of values. For completeness, let's get some things straight about probability. In general, we talk about probability as a decimal number between 0 and 1. If the probability of something is 0, then it will never happen, and if it's 1, then it's guaranteed to happen every time you measure something. Most people are familiar with probability in the discrete sense (where you can write down all the possible outcomes). Things like die rolls, coin flips, and decks of cards are easier to understand. The probability of rolling a 2 on a die is 1/6, because there are 6 possible values, and they're all equally likely. The probability of rolling a number between 2 and 5 (inclusive) is 4/6, because that range contains 4 of the 6 possible values. When we start talking about probability for continuous values, we have to think a bit differently. If you're familiar with continuous functions, then you know that there are infinitely many values contained inside any range you choose. Consider the probability of the temperature outside being 30°. 30° is one possible value, but there are infinitely many values, so the probability of the temperature being EXACTLY 30° is 1/∞ ≈ 0. So, for continuous variables, you can't know the probability of a single value. To get around this, we say \"ok, what is the probability that the temperature is between 25° and 35°\"? To find that out, you need to know the probability density function. A probability density function (PDF for short) tells you the fractional probability for a very small range of values. If you're familiar with calculus, then the PDF for P(x) is the total probability that the measurement will be between x and x+dx. If you don't know calculus, that's cool, it's not required information. Just think of the PDF as similar to regular density. An object that is very dense has more mass in a small area, and vice versa. Sometimes you have an object that has varying density - let's imagine a ruler that is most dense on the 0 side and least dense on the 12 side. It makes sense that there is more mass between 1-2 on the ruler than there is between 10-11, right? PDF's work the same way. They tell you whether there's more likelihood between 1-2 than between 10-11."
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cjzy2s | why jellyfish tanks are always circular? | Engineering | explainlikeimfive | {
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"As you can see when you look at them, jellyfish don't have brains. As such, they are absurdly prone to getting stuck in corners where they die and decompose and get gross. Circular tank means no corners means no gross jellyfish. Now you know.",
"The design is called a kreisel tank and it's set up to produce a circular waterflow in the tank with the water coming into the tank blowing directly across the outflow grid, preventing anything from getting stuck on it and tending to keep the animals caught in a circular current in the middle of the tank. The goal is to provide jellyfish and other planktonic animals an environment as similar to the open ocean as possible...nothing to bump into and get stuck on, no corners to get caught on"
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ck2c5k | How do they balance planes that have uneven numbers of seats on either side of the aisle, such as 2-1 or 3-2? | I think the title pretty much explains it. But this is something I wonder about whenever I fly on a plane with such a seat configuration. Thanks in advance. | Engineering | explainlikeimfive | {
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"Left Right balance is pretty much a non-issue. The people are all very close to the center of mass so it doesn't create much torque even if wildly imbalanced. Adjusting the trim flap a tiny bit on the end of the wing 10+ meters away results in significantly more torque. Only front back balancing really matters because people in the nose and tail are far from the center of mass so if everyone is in the tail the plane will constantly want to pitch up"
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ckux08 | How did we as humans work out time difference between cities and countries before GMT (and later UTC) were invented/used? | Engineering | explainlikeimfive | {
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"Each city had their own time based on solar noon. You set your watch by the sun. It didn't really matter what time it was anywhere else. It wasn't until trains were invented that time needed to be synchronized.",
"They didn't, Before the popularization of trains, there was no need to know the different between local times time-zones apart. If travelling from London to new york took weeks by boat, If was likely that you might notice each day getting longer/shorter by an hour a day but it didn't effect your punctuality. When trains became a popular, you can travel across time zones in a few hours, so if you needed to be at a meeting at 6 PM and you have a 6 hour travel time, you need to get on the train by 11AM if you lose an hour or you can wait till 1PM if you gain an hour.",
"We really didn't. You really didn't need to know what \"time\" it was in London if you were in Madrid or Istanbul or New York - even the fastest communication between cities took _days or weeks_. Time differences only really became meaningful once we had fast methods of communication (like the telegraph) and transportation (like the locomotive)."
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cl5xnt | why do most lamps/light fixtures require you to click twice when twisting the knob in order for them to turn on/off? | Not sure if this makes sense or if other people have noticed this but when you reach to turn on a lamp or whatever you have to usually twist the knob twice and hear it click twice to get the bulb to turn on or off. Why? Why can’t you just turn it once? | Engineering | explainlikeimfive | {
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"The are usually designed to work with 3-way bulbs to give varying levels of brightness. But if you don’t put a 3-way bulb in the lamp, it’ll just stay same brightness through those turns.",
"That's because the lamps you are talking about are designed for two way or three way lamps. Each click ~~sends a different voltage level to the lamp~~, activates a different set of contacts on the lamp. They usually go from dimmest to brightest. When you put a regular bulb in, it only works on the highest setting."
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Subsets and Splits