data/engineering.jsonl

{"id": "engineering_33737", "domain": "engineering", "question_title": "Why do color TVs make black and white snow?", "question_body": "If you let a color TV display something from an unplugged display port, it displays snow, right? Randomly varying black or white pixels on the whole screen. My guess was that the television was decoding the noise from the unplugged wires as a video signal. Assuming this, it makes sense that B&W TVs make B&W snow (they would decode the noise as being a B&W video signal). But then color TVs should make colored snow , right? Shouldn't they decode the noise as being a color video signal?", "question_score": 93, "question_tags": ["electrical-engineering", "signal-processing"], "choices": {"A": "One of the problems that plagued older rechargeable batteries (e.g. Nickel Cadmium ($\\text{NiCad}$) and Nickel Metal Hydride ($\\text{NiMH}$)) was the memory effect . The memory effect occurs when a rechargeable battery is not fully discharged. It then \"forgets\" that it has a greater capacity than it thinks it has, and so in the future it discharges less. A good example is a water bottle. Initially, water bottles have a certain capacity for water. Let's say that I drink most of the water in a water bottle during one usage. If the memory effect affected water bottles, I would not be...", "B": "If the receiver does not detect the sub-carrier for the \"colour burst\" signal which is transmitted during the horizontal blanking period the receiver switches on the \"colour-killer\" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise. Figure 1. The colorburst signal is transmitted on the \"back porch\" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately...", "C": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...", "D": "It depends on how steep the hill is. On a slight hill, the energy added by gravity is still not enough to overcome rolling friction and air resistance, so the car still needs power to maintain speed. On a steeper hill, the two may balance out, so no power is used, and no power is generated. On a hill that's steep enough to require braking to control the speed, the car recovers energy. It's called regenerative braking. If the car is going too fast, applying the brakes turns the motor into a generator and charges the battery."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/33737/why-do-color-tvs-make-black-and-white-snow"}
{"id": "engineering_1926", "domain": "engineering", "question_title": "Why is kVA not the same as kW?", "question_body": "I thought my electric car charging unit uses 6.6 kW of power. However, I found the label and it actually says 6.6 kVA. When I saw this I thought something along the lines of... Well, $ P=VI $, therefore kVA must be the same thing as kW... strange, I wonder why it's not labelled in kW. So a quick Google search later, and I found this page , which has a converter that tells me 6.6 kVA is actually just 5.28 kW. The Wikipedia page for watts confirmed what I thought, that a watt is a volt times an ampere. So what part of all this am I missing, that explains why kVA and kW are not the same?", "question_score": 59, "question_tags": ["electrical-engineering"], "choices": {"A": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...", "B": "If I model this as a simply supported beam having load at mid span [...] I suspect that this is where your analysis went awry. First off, you should always model bridges with distributed loads, not a single concentrated load at midspan. The most significant load on a bridge will almost always be its own self-weight; load-trains are heavy but, well, so are bridges. Secondly, I assume you're thinking of the bridge like this: Indeed, we can see here that the bending moment is greater at midspan. However, that's not the bridge we're looking at, it's missing the cantilevers! So...", "C": "I will expand on DKNguyen answer, because to my knowledge also the two reasons are: reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation) change the joint tightening characteristics (see joint diagram). reduce contact stresses on surfaces. The basic idea is that since contact stress is defined as: $$\\sigma = \\frac{F}{A}$$ Obviously the larger the area the less the stress. Also, like Nuclear Hoagie pointed out the area changes with a square law of the diameter. UPDATE 2 - Calculation for M6 bolt (thanks to BenC) *The question gathered enough interest for me to carry out a...", "D": "On this processor, the register that holds the conversion result is 16 bits wide. A right-justified result means that bits [( N -1):0] (where N is the number of bits of precision) of the register contain the ADC value and the most-significant bits of the register are set to zero. A left-justified result means that bits [15:(16- N )] of the register hold the result, and bits [(15- N ):0] are set to zero. For example, if your actual conversion result is 0x123, it would be read as 0x0123 if the register was right-justified, and as 0x1230 if it was..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/1926/why-is-kva-not-the-same-as-kw"}
{"id": "engineering_5570", "domain": "engineering", "question_title": "How was Volkswagen able to trick the lab pollution test?", "question_body": "The recent Volkswagen pollution test cheating scandal has shocked many people with its widespread extent as well as how it was hidden for so many years. Last year, the International Council on Clean Transportation (ICCT) in Washington DC contracted scientists from the Center for Alternative Fuels Engines and Emissions at West Virginia University in Morgantown to test emissions from three light-duty diesel vehicles under more-realistic conditions than are possible in the lab. To do so, the scientists fitted cars with a portable emissions measurement system to gather a continuous stream of data over a variety of US road types. The tests found that the levels of NOx emitted by a Volkswagen Jetta were 15–35 times greater than dictated by the US standard (31 milligrams per kilometre), depending on road and driving conditions. How was this cheating program carried out, and why were the lab tests unable to detect the presence of this cheating program?", "question_score": 52, "question_tags": ["automotive-engineering", "product-testing", "air-quality", "emissions"], "choices": {"A": "After the 2014 ICCT report revealed that these light-duty passenger diesel vehicles were emitting too much NOx and US regulators confronted VW about the results, VW did some testing and proposed a voluntary software recall to recalibrate the various emissions control devices on the affected vehicles. From the California Air Resources Board's (CARB's) in-use compliance letter to Volkwagen AG, 2015-09-18 , following that recall: To have a more controlled evaluation of the high NOx observed over the road, CARB developed a special dynamometer cycle which consisted of driving the Phase 2 portion of the FTP repeatedly. This special cycle revealed...", "B": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "C": "From the point of view of the driver of a car, impacting another car is about as bad as crashing against an ideal wall (a wall with zero deformation whatsoever). If there were a plane reflection between the two cars, then vs. Car would be exactly equal to vs. Wall (the contact points between both cars would all be on the same plane, due to reflection, so each car could be considered a wall for the other). But this plane reflection does not exist: What we have instead is a 2-fold rotational reflection . Let's say the left part of...", "D": "Solid particle settling time in air depends mainly on the size of the particle. Different forces become significant depending on what size range you're talking about, so it's hard to give an answer that's both concise and accurate. I'll do my best to synthesize the important points rather than parrot a reference; that said, where practical applications in the field of air quality are concerned, the text I recommend is Air Pollution Control by Cooper & Alley . In particular, I'm going to pull many of the details for this answer from Section 3.3: Particulate Behavior in Fluids. Gravitational Settling..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/5570/how-was-volkswagen-able-to-trick-the-lab-pollution-test"}
{"id": "engineering_2534", "domain": "engineering", "question_title": "My customer wants to use my products to do something unsafe. What is my ethical obligation?", "question_body": "We sell products that attach to a motor drive's DC bus. We also formerly sold diode kits that let you hook one product up to multiple drives. We stopped selling those diode kits because they were unreliable with modern hardware, and we had better solutions. My customer tells me he wants to keep using the old diode kits, because he can have his repair techs disconnect power to one drive and have it replaced, while the others are still powered on. I maintain that this is an unsafe practice, because diodes are not safety-rated devices. (See question here .) But my customer is rather insistent. What is my ethical obligation in this case?", "question_score": 48, "question_tags": ["electrical-engineering", "ethics", "sales", "safety"], "choices": {"A": "As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\\arctan\\left(\\frac{0.15}{15}\\right)=0.57^\\circ$ or 1%. the middle ,...", "B": "Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book \" Wind Energy - The Facts \". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...", "C": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "D": "I'm going to set aside the potential legal liability aspects of your question. In part because you didn't ask about them, but also because liability will vary based upon jurisdiction. Obviously, consult an attorney familiar with the relevant law. Ethically, I think you've done everything that you're obligated to do. You have contacted the customer and notified them of an unsafe situation or configuration of the product. And you have (strongly) advised them that they need to discontinue using the older product in the unsafe configuration. If the customer remains insistent about using the product in an unsafe manner, you..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2534/my-customer-wants-to-use-my-products-to-do-something-unsafe-what-is-my-ethical"}
{"id": "engineering_7080", "domain": "engineering", "question_title": "Is it possible for a bicycle chainwheel to have a fractional number of teeth?", "question_body": "Background In the world of bicycle motocross, also known as BMX racing , gearing is a hotly-debated topic. Since the bikes are all single-speed, gear ratio is a fixed number defined as chainwheel / cog (front gear divided by rear gear). Altering your gear ratio is understood as an immediately-noticeable tradeoff between acceleration and top-end speed. Here is a series of common gear ratios: ╔════════════╦═════╦════════╗ ║ Chainwheel ║ Cog ║ Ratio ║ ╠════════════╬═════╬════════╣ ║ 43 ║ 16 ║ 2.6875 ║ ║ 41 ║ 15 ║ 2.7333 ║ ║ 44 ║ 16 ║ 2.75 ║ ╚════════════╩═════╩════════╝ In 2012, a company called Rennen Design Group created a supposed breakthrough innovation called \"decimal gearing\" . The claim is that through manipulations of tooth profile and ring diameter, in-between gear ratios can be created - for example: ╔════════════╦═════╦════════╗ ║ Chainwheel ║ Cog ║ Ratio ║ ╠════════════╬═════╬════════╣ ║ 43 ║ 16 ║ 2.6875 ║ ║ 45.7 ║ 17 ║ 2.6882 ║ ║ 37.7 ║ 14 ║ 2.6929 ║ ║ 43.1 ║ 16 ║ 2.6938 ║ ║ 41 ║ 15 ║ 2.7333 ║ ║ 41.1 ║ 15 ║ 2.74 ║ ║ 52.2 ║ 19 ║ 2.7473 ║ ║ 44 ║ 16 ║ 2.75 ║ ║ 44.2 ║ 16 ║ 2.7625 ║ ╚════════════╩═════╩════════╝ Note: Table is not exhaustive. For example - a 44.2 tooth gear actually only has 44 teeth, but the tooth spacing, tooth profile, and chainwheel diameter is supposed to have been manipulated to create a larger gear. In the world of BMX racing, the existence of in-between gear ratios like this is a Really Big Deal. Since the man behind Rennen has a Master's from MIT - and since most BMXers would rather hit jumps than do math or measure things - nobody has really ever checked up on whether or not this is valid. Some questions were asked a long time ago in the dusty corners of a BMX forum, but the testing methods didn't properly control for all variables and the thread descended into a bunch of name-calling and ad-hominem attacks. The Actual Question Is this physically possible? I understand \"gear ratio\" to be defined as: For a given gear ratio x / y , one rotation of the gear with x teeth will result in x / y rotations of the gear with y teeth. For a gear ratio of 44/16, one full rotation of the 44 tooth gear (chainwheel) should result in 2.75 rotations of the 16 tooth gear (cog). So for a \"decimal ratio\" of 44.2/16, one full rotation of the 44.2 tooth gear (which again - only has 44 teeth) is supposed to result in 2.7625 rotations of the 16 tooth gear. My biggest reservation is the fact that a chain-driven drivetrain is a TIMED DRIVETRAIN. No matter how big or small you make the teeth on the chainwheel, if they fit the chain, they're only going to push as many links through per rotation as the chainwheel has teeth. For a true 44.2 tooth chainwheel, one would expect that 442 links get pushed through over 10 full rotations of the chainwheel - but that's not the case. Only 440 links will ever get pushed through to the cog because only 44 links get pushed through per full rotation of the chainwheel. I actually spent my whole afternoon yesterday taking video and counting links and measuring. But I'm not a scientist. My high school didn't even offer a physics course. I'm just a racer that trains really hard and knows how to do basic math. If this were a belt-driven system, I would completely understand how a manipulation of the chainwheel diameter would change the effective ratio - but it's not. It's a timed drivetrain, limited by the physical dimensions of the chain. I have several hundred dollars and months of training and metrics invested in these stupid chainwheels. If someone could confirm or deny my theories, I would really appreciate it. I just want some closure. Here's a photo of the teeth from a 41 tooth chainwheel on top of the teeth from a 41.2 tooth chainwheel - both are Rennen gears: Here's a 41t on top of a 41.2t: Here's the 41.2t on top of the 41t, from behind:", "question_score": 47, "question_tags": ["mechanical-engineering", "gears", "measurements"], "choices": {"A": "My first thought is that it might be intended to be a wing nut driver of some sort, but those are usually hollow cylinders with slots for the wings. Ah ... sure enough, it's described as such in this Ebay ad :", "B": "I suspect that the answer to this is that, ultimately the gear ratio comes from the ratio of diameters of the gears rather than the number of teeth, although in most circumstances practicality dictates that they are proportional. Say you have a 10 tooth cog and a 40 tooth chainwheel. It's fairly simple to imagine that you could remove every other tooth from the 40 tooth wheel while keeping the diameter the same and maintain exactly the same gear ratio. Similarly you could have a completely gearless wheel (putting aside issues of slippage) driving a chain which drove a geared...", "C": "Short answer : make it thicker. Long answer : The moment of inertia affects the beam's ability to resist flexing. Use one of the many, free, online moment of inertia calculators (like this one ) to see how increasing the height of the beam will have an exponential effect on increasing the stiffness of the beam. And this site helps provide a pictorial view of the load(s) upon a beam depending upon differing configurations, such as where the supports are and where the load is applied. It also provides a calculator to determine the forces involved. Wikipedia has a decent...", "D": "Some ideas: Wheel Load Distribution : The load is greater on the rear wheels providing the power; more force on the front ones bring no benefit and would provide less traction. Better manoeuvrability from having a shorter wheelbase. Better Ground Clearance in some conditions, especially for bumps and or up a increasing slope for instance. Better Driving : The front wheels now turn around a point closer to the C.G. than with the rear wheels. Not good with vehicle dynamics but this appears better than the rear end 'trailing' behind. Structural : As some people have pointed out, it's better..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/7080/is-it-possible-for-a-bicycle-chainwheel-to-have-a-fractional-number-of-teeth"}
{"id": "engineering_19758", "domain": "engineering", "question_title": "Transmitting power over long distances what is better AC or DC?", "question_body": "I found this answer to a related question. The part of the answer that's confusing me is: Transmitting DC power over a long distance is inefficient. Thus AC supply is a far more efficient to transmit power. According to Siemens it's quite the opposite : Whenever power has to be transmitted over long distances, DC transmission is the most economical solution compared to high-voltage AC. Also, from Wikipedia HVDC transmission losses are quoted as less than 3% per 1,000 km, which are 30 to 40% less than with AC lines, at the same voltage levels. Is the posted answer correct? - - EDIT - - Chris H made a very important observation (see his comment below): The context of the post I mentioned was of low voltage whereas I was blindly thinking of high voltage. Indeed I learnt loads by the answers and comments. Thanks.", "question_score": 43, "question_tags": ["power-transmission"], "choices": {"A": "I'm going to set aside the potential legal liability aspects of your question. In part because you didn't ask about them, but also because liability will vary based upon jurisdiction. Obviously, consult an attorney familiar with the relevant law. Ethically, I think you've done everything that you're obligated to do. You have contacted the customer and notified them of an unsafe situation or configuration of the product. And you have (strongly) advised them that they need to discontinue using the older product in the unsafe configuration. If the customer remains insistent about using the product in an unsafe manner, you...", "B": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "C": "Electricity production optimization is a very complex subject. It is also affected by many parameters , which I will try to outline below. TL;DR: Demand is constantly monitored and supply is constantly adjusted TL;DR 2: Lshaver's excellent post is a suggested reading after reading this, because it expands and explains what happens at timescales ranging from the micro-second to minutes timescales. Changes in energy demand during the day First of all, the energy demand during the day, week, month of year can change quite a lot. Below is a typical day in New England. Fluctuation in power energy consumption a...", "D": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/19758/transmitting-power-over-long-distances-what-is-better-ac-or-dc"}
{"id": "engineering_569", "domain": "engineering", "question_title": "What is the purpose of these diversions in a natural gas line?", "question_body": "The campus where I work has a long covered walkway (~.5 mile) which has several labeled pipes running under the roof (chilled water, fuel oil, air...). All of the pipes run dead straight except for the natural gas lines, which have little loops spaced about every 250ft, as seen in the attached image (the lowermost, yellow line. There's another natural gas line hidden above all the others, which also does the same thing.) The line isn't branching at these points, and there doesn't seem to be any need to divert the pipe in order to support it. I've looked at some building codes to see if I could find a reason (or even a requirement) to insert these. Any ideas as to what these are? It's driving me batty!", "question_score": 39, "question_tags": ["mechanical-engineering", "building-physics", "pipelines"], "choices": {"A": "I'm going to set aside the potential legal liability aspects of your question. In part because you didn't ask about them, but also because liability will vary based upon jurisdiction. Obviously, consult an attorney familiar with the relevant law. Ethically, I think you've done everything that you're obligated to do. You have contacted the customer and notified them of an unsafe situation or configuration of the product. And you have (strongly) advised them that they need to discontinue using the older product in the unsafe configuration. If the customer remains insistent about using the product in an unsafe manner, you...", "B": "After referring to some good online resources such this , I know why we shouldn't transport it laying down. Compressor is filled with oil which is critical to its operation. In the normal upright position gravity keeps the oil in the compressor. When we lay the refrigerator flat, some of the oil can leave the compressor and go into the cooling lines. The oil is a thick viscous fluid and can clog the cooling lines thus hampering the refrigerator's ability to cool. Lack of oil in the compressor can also damage the compressor. If we must lay the fridge down,...", "C": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "D": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/569/what-is-the-purpose-of-these-diversions-in-a-natural-gas-line"}
{"id": "engineering_32639", "domain": "engineering", "question_title": "What is the purpose of these “partially filled in” locomotive wheels?", "question_body": "I recently visited a railway museum with a lot of Soviet-era locomotives. The picture below shows the wheels on a locomotive from the 1930s or 1940s. Some of the wheels are thicker on one side (as it it were a partially filled cup). What is the purpose of that? (Now that I am looking at the picture again I wonder if they are balancing the weight of the coupling rods in some way.)", "question_score": 39, "question_tags": ["rail", "wheels"], "choices": {"A": "Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train .", "B": "There are a few main reasons why suspension bridges aren't used for railroads. The main reason is that suspension bridges are typically used where very long spans are needed. Trains are very heavy, especially when compared to lanes of highway traffic. This means that long spans require very strong support structures, which in the case of suspension bridges are cables and towers. The second reason goes along with the first; trains cause high dynamic loads as they move along the rail. This can increase the vertical loads by 30%. Third is that trains don't really have suspensions, especially freight trains....", "C": "Pantograph and third rail are pretty much it. Engineering principles: both have a conducting surface on the train (moving) in contact with the stationary rail/wire, in both cases you need a material that's resilient and conductive. The contact strip is a wear material . Differences: overhead catenary wires are flexible and will move around when a train drives underneath them. At high speeds, the pantograph can set up waves in the catenary wire, if these waves aren't damped sufficiently, the pantograph will start bouncing. third rail is limited to low voltages to reduce the risk of arcing between the rails....", "D": "Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/32639/what-is-the-purpose-of-these-partially-filled-in-locomotive-wheels"}
{"id": "engineering_63", "domain": "engineering", "question_title": "What are the pros and cons of a traffic circle versus a traffic light intersection?", "question_body": "The debate of traffic circles (also called roundabouts or rotaries) versus traffic light intersections has been in progress for a while. Those in favor of traffic circles say that , among other things, that they are safer than traffic light intersections. This claim has been scientifically proven. On the other hand, traffic light intersections are more space-inefficient. Even Mythbusters has joined the fun, testing the efficiency (which is one of the main arguments both sides seem to concern themselves with) of each method. For comparison, here's a quick picture of a traffic circle: And of a four-way traffic light intersection: So, what are the pros and cons of a traffic circle versus a traffic light intersection?", "question_score": 36, "question_tags": ["traffic-light", "highway-engineering"], "choices": {"A": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "B": "I think you are talking about roundabouts, not traffic circles. It is baffling to those of us in the UK that Americans think roundabouts are a new idea. In the UK we have so many variants, from mini-roundabouts all the way up to full motorway junctions (a giant roundabout above or below the motorway). So do roundabouts take up more space? Not necessarily, this is a mini roundabout: It's nothing more than a slightly domed area of paint on the road, no lights are necessary, you can actually drive straight over the top of it rather than around it, its...", "C": "As others stated before, induction loops are the primary - most reliable method: the coils (usually just several loops of wire) embedded in the road; fed given frequency from a generator, in presence of metal the frequency of the LC circuit changes and the sensor circuitry detects the change of frequency, producing a presence signal. In some cases these may fail to detect bicycles, but they are by far most common as they aren't affected by weather (or more precisely, the detection circuit tunes in to slow changes of frequency caused by weather) and are immune to accidental false positives....", "D": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/63/what-are-the-pros-and-cons-of-a-traffic-circle-versus-a-traffic-light-intersecti"}
{"id": "engineering_443", "domain": "engineering", "question_title": "How fast does solid waste fall in vertical drain pipes?", "question_body": "Some building are really tall, if you flush the toilet and the contents go into a pipe and straight down, there could be a lot of energy, potentially enough to cause harm to the sewer pipe at the end of the fall. I know that in my home, the pipe goes straight down and then there is just a 90 degree bend. According to Wikipedia the calculations for Terminal velocity have a lot of variables, but in essence things that are falling get to top speed quickly. When solids are falling straight down a drain pipe in a high rise building, how fast do they fall? What is the potential for the things that are falling to damage the pipes and how is this addressed when engineering the structure?", "question_score": 35, "question_tags": ["structural-engineering", "piping"], "choices": {"A": "Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with...", "B": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...", "C": "I'm going to set aside the potential legal liability aspects of your question. In part because you didn't ask about them, but also because liability will vary based upon jurisdiction. Obviously, consult an attorney familiar with the relevant law. Ethically, I think you've done everything that you're obligated to do. You have contacted the customer and notified them of an unsafe situation or configuration of the product. And you have (strongly) advised them that they need to discontinue using the older product in the unsafe configuration. If the customer remains insistent about using the product in an unsafe manner, you...", "D": "Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/443/how-fast-does-solid-waste-fall-in-vertical-drain-pipes"}
{"id": "engineering_42931", "domain": "engineering", "question_title": "Why are railroad tank cars bent in the middle?", "question_body": "I recently noticed that the cylindrical shape of a railroad tank car is not completely straight but has a bend in the middle. The entire tank is a bit lower there. What's the reason for this bend? This is also visible on the drawing of a DOT 117 tank car on Wikipedia (the red nearly-horizontal lines were added by myself). I first thought it had something to do with pressure containment, but the model 117 is apparently used for non-pressured goods only.", "question_score": 35, "question_tags": ["mechanical-engineering", "design", "rail"], "choices": {"A": "As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\\arctan\\left(\\frac{0.15}{15}\\right)=0.57^\\circ$ or 1%. the middle ,...", "B": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "C": "From the point of view of the driver of a car, impacting another car is about as bad as crashing against an ideal wall (a wall with zero deformation whatsoever). If there were a plane reflection between the two cars, then vs. Car would be exactly equal to vs. Wall (the contact points between both cars would all be on the same plane, due to reflection, so each car could be considered a wall for the other). But this plane reflection does not exist: What we have instead is a 2-fold rotational reflection . Let's say the left part of...", "D": "The turbulence model can make a big difference in your simulation . There are many turbulence models around. It becomes a tough job to select one out of them. There is no perfect turbulence model. It all depends on several parameters like Reynold's number, whether the flow is separated, pressure gradients, boundary layer thikness and so on. In this answer, brief information about a few popular models is given along with pros and cons and potential applications. However, interested users can see this excellent NASA website and references therein to know more about turbulence modeling. A) ONE EQUATION MODEL: 1...."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/42931/why-are-railroad-tank-cars-bent-in-the-middle"}
{"id": "engineering_1977", "domain": "engineering", "question_title": "Why dig out and then fill in before building a large structure?", "question_body": "I work in the middle of London, in an area full of large office blocks. Across the road from my office they have started construction of a large building (10 stories plus). Over the last few weeks, diggers have dug a large (and vertical walled) hole. Lorries have taken the resulting dirt and old concrete away, leaving a very neat hole. In the last day or so, the lorries have returned with new dirt (or the old dirt crushed) and diggers have been putting it back in the hole (and compacting it). Why put the dirt back? Surely leaving the hole deeper would allow for deeper basement (or digging it shallower would be cheaper)? I'm not a structural engineer, so this is all lost on me, but I'm fascinated.", "question_score": 32, "question_tags": ["structural-engineering", "geotechnical-engineering", "building-design", "foundations"], "choices": {"A": "As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\\arctan\\left(\\frac{0.15}{15}\\right)=0.57^\\circ$ or 1%. the middle ,...", "B": "This is to make sure they know what the foundation is made of. For all they knew there may have been an old tunnel underneath that would have collapsed when the new building is put on top. London is built on top of an old marsh, this type of soil is very prone to sinking and uneven settling, digging down and reinforcing the foundation alleviates that. It also ensures the foundation is uniform under the building to avoid a new tower of Pisa. Given the age of the city it may have been to scour the land for potential archaeological...", "C": "From the point of view of the driver of a car, impacting another car is about as bad as crashing against an ideal wall (a wall with zero deformation whatsoever). If there were a plane reflection between the two cars, then vs. Car would be exactly equal to vs. Wall (the contact points between both cars would all be on the same plane, due to reflection, so each car could be considered a wall for the other). But this plane reflection does not exist: What we have instead is a 2-fold rotational reflection . Let's say the left part of...", "D": "Electricity production optimization is a very complex subject. It is also affected by many parameters , which I will try to outline below. TL;DR: Demand is constantly monitored and supply is constantly adjusted TL;DR 2: Lshaver's excellent post is a suggested reading after reading this, because it expands and explains what happens at timescales ranging from the micro-second to minutes timescales. Changes in energy demand during the day First of all, the energy demand during the day, week, month of year can change quite a lot. Below is a typical day in New England. Fluctuation in power energy consumption a..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/1977/why-dig-out-and-then-fill-in-before-building-a-large-structure"}
{"id": "engineering_49207", "domain": "engineering", "question_title": "What is the lowest point below sealevel that we have built where a human can go?", "question_body": "According to google searches, the Jinping Underground Laboratories are the \"deepest\" building or buildings constructed, reaching 7900 feet (2400 metre) below the surface.... However, the surface in question is a mountain. While that does classify as underground, it highlights a flaw in the question of the deepest underground building. I can't seem to find the lowest building in the world though, or the deepest in relation to depth within the earth's crust. What is the lowest point below sea level that we have built where a human can go? I imagine this is likely another laboratory. But where would a building of this description be?", "question_score": 31, "question_tags": ["civil-engineering", "architecture"], "choices": {"A": "Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book \" Wind Energy - The Facts \". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...", "B": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...", "C": "The Kidd Mine in Ontario, Canada: per the Wikipedia article , it is \"the deepest accessible non-marine point on Earth\" at \"2,733 metres (8,967 ft) below sea level\". I found this from the Wikipedia article on Extremes on Earth , which differentiates between depth from the surface and depth below sea level, and also between an actual mine vs. a bore hole.", "D": "Theoretically pontoon bridges with rope anchors keeping them to the bottom would work against wind and flow, overcoming the problem jhabbot mentioned in his answer (same as train length limit - stretching force). In practice these come with more problems of their own. They drift on water surface and as result, rise and fall with water waves. The larger the body of water they span, the higher the waves; at certain point in stormy weather the bridge would just launch the vehicles into the air. The anchoring isn't exactly simple if it's to withstand such forces. You could just as..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/49207/what-is-the-lowest-point-below-sealevel-that-we-have-built-where-a-human-can-go"}
{"id": "engineering_270", "domain": "engineering", "question_title": "Why do hydraulic systems use special fluid - what's wrong with water?", "question_body": "As a hydraulics layman thinking about hydraulic systems, it seems that the important factor is to have a liquid that doesn't compress much or at all. Doesn't water meet this requirement, and what other properties should the liquid have (if any) that water doesn't?", "question_score": 29, "question_tags": ["mechanical-engineering", "hydraulics"], "choices": {"A": "Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....", "B": "Train Brakes The common brakes on trains are air brakes . As the name implies, these work off of air pressure. The braking power isn't controlled in the way that you would immediately think of though. They do not work like car brakes where the harder you press on the brake pedal, the harder the pressure goes through the lines to the brake cylinders. They work the opposite. The less pressure in the line, the more braking force is applied. Fail-safe Rail brakes are designed to be fail-safe . That is, when a failure occurs, the safe operation happens. In...", "C": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "D": "OP injection molding tag is correct. OBall uses injection molding and plastic welding. The OBall is the invention of David E. Silverglate. Toy Ball Apparatus with Reduced Part Count Reduced image from Kids II . It consists of four identical, flat, injection molded, pentagon and hexagon shapes with circular (or elipitical) holes, which are shaped and plastically welded into spheres. Pentagon and hexagon edges are the same size and individual connected circles are only connected along one edge. The four shapes are clearly shown in colors above and from the patent. Solid lines on each part are hard connections, while..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/270/why-do-hydraulic-systems-use-special-fluid-whats-wrong-with-water"}
{"id": "engineering_3099", "domain": "engineering", "question_title": "Which is Worse: Car vs. Car or Car vs. Wall?", "question_body": "So I got myself questioning what could be worse for the driver... a collision of two identical cars at equal speed (frontal crash) or the same car with the same speed crashing through a wall? The first case I see it would double the impact, but also it will absorb the energy into the other car structure, otherwise, in a solid and rigid wall, all the energy would come back to the vehicle. Which situation is worse for the passengers?", "question_score": 29, "question_tags": ["automotive-engineering", "safety"], "choices": {"A": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...", "B": "After the 2014 ICCT report revealed that these light-duty passenger diesel vehicles were emitting too much NOx and US regulators confronted VW about the results, VW did some testing and proposed a voluntary software recall to recalibrate the various emissions control devices on the affected vehicles. From the California Air Resources Board's (CARB's) in-use compliance letter to Volkwagen AG, 2015-09-18 , following that recall: To have a more controlled evaluation of the high NOx observed over the road, CARB developed a special dynamometer cycle which consisted of driving the Phase 2 portion of the FTP repeatedly. This special cycle revealed...", "C": "From the point of view of the driver of a car, impacting another car is about as bad as crashing against an ideal wall (a wall with zero deformation whatsoever). If there were a plane reflection between the two cars, then vs. Car would be exactly equal to vs. Wall (the contact points between both cars would all be on the same plane, due to reflection, so each car could be considered a wall for the other). But this plane reflection does not exist: What we have instead is a 2-fold rotational reflection . Let's say the left part of...", "D": "Nothing is rigid, the raceways and the ball in a ball-bearing are no exception. The contact area deflects and accommodates the ball in a small contact surface, not a point. Also, the balls take the load in groups. The digrams are from SKF ball-bearings. '"}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/3099/which-is-worse-car-vs-car-or-car-vs-wall"}
{"id": "engineering_2826", "domain": "engineering", "question_title": "Understanding required torque for a motor lifting a weight", "question_body": "This is a continuation of me trying to understand torque and stepper motors in my other question . I'm trying to understand the torque a motor would be required to generate to lift a small weight, and the formulas involved. The first part of my question is to verify if I am calculating this correctly: Let's say I have a 450 g mass (roughly one pound) then the force of gravity pulling it down is: $\\begin{align} F &= ma \\\\ &= 0.450 \\:\\mathrm{kg} * 9.8 \\:\\mathrm{m}/\\mathrm{s}^2 \\\\ &= 4.41 \\:\\mathrm{N} \\\\ \\end{align}$ If I have a stepper motor with a spindle for my string that pulls up my motor with a radius of 5 cm. I think my torque needed would be: $\\begin{align} T &= Fr \\\\ &= F * 0.05 \\\\ &= 0.22 \\:\\mathrm{Nm} \\\\ \\end{align}$ So now if I want to move that mass I need to find a stepper motor that can output more than 0.22 Nm of torque, right? The follow-on to my question is that if I want to see how fast I can move it then I need to look at a Torque speed curve, right? My confusion is this: do I have to ensure that I'm moving slow enough to get the torque I need, or does that curve say if you need this torque you won't be able to go above this speed because the motor won't let you?", "question_score": 27, "question_tags": ["mechanical-engineering", "motors", "torque", "stepper-motor"], "choices": {"A": "You have the right concept, but slipped a decimal point. 5 cm = 0.05 m. The gravitational force on your 450 g mass is 4.4 N as you say, so the torque just to keep up with gravity is (4.4 N)(0.05 m) = 0.22 Nm. However, that is the absolute minimum torque just to keep the system in steady state. It leaves nothing for actually accellerating the mass and for overcoming the inevitable friction. To get the real torque required, you have to specify how fast you want to be able to accellerate this mass upwards. For example, let's say...", "B": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only...", "C": "The turbulence model can make a big difference in your simulation . There are many turbulence models around. It becomes a tough job to select one out of them. There is no perfect turbulence model. It all depends on several parameters like Reynold's number, whether the flow is separated, pressure gradients, boundary layer thikness and so on. In this answer, brief information about a few popular models is given along with pros and cons and potential applications. However, interested users can see this excellent NASA website and references therein to know more about turbulence modeling. A) ONE EQUATION MODEL: 1....", "D": "If the receiver does not detect the sub-carrier for the \"colour burst\" signal which is transmitted during the horizontal blanking period the receiver switches on the \"colour-killer\" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise. Figure 1. The colorburst signal is transmitted on the \"back porch\" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2826/understanding-required-torque-for-a-motor-lifting-a-weight"}
{"id": "engineering_28254", "domain": "engineering", "question_title": "Why use steam instead of just hot air?", "question_body": "As I understand, a steam machine needs a pressurised gas to work. This can be compressed air but it used to be steam . Energy was provided to the steam engine by heating up water. I don't understand why it used water as the pressurised gas could have been generated only by heating air in a closed tank. Is the design of heating air in a tank instead of heating water to generate steam feasible? If so, why do steam engine work with steam instead of hot air?", "question_score": 27, "question_tags": ["steam"], "choices": {"A": "I would say that using a warm pressurized gas is not very feasible. Ratchet freak already mentioned how you can get much more volume out of heating water into steam than just heating up air until it's warmer. This touches on, but doesn't completely address an important factor about steam as power. Converting to steam includes a phase change from liquid to gas. This phase change actually acts as an additional storage of energy. You can draw this energy out of the steam later in the system (through a heat exchanger for example), converting it back into a liquid, which...", "B": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only...", "C": "Nothing is rigid, the raceways and the ball in a ball-bearing are no exception. The contact area deflects and accommodates the ball in a small contact surface, not a point. Also, the balls take the load in groups. The digrams are from SKF ball-bearings. '", "D": "Electricity production optimization is a very complex subject. It is also affected by many parameters , which I will try to outline below. TL;DR: Demand is constantly monitored and supply is constantly adjusted TL;DR 2: Lshaver's excellent post is a suggested reading after reading this, because it expands and explains what happens at timescales ranging from the micro-second to minutes timescales. Changes in energy demand during the day First of all, the energy demand during the day, week, month of year can change quite a lot. Below is a typical day in New England. Fluctuation in power energy consumption a..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/28254/why-use-steam-instead-of-just-hot-air"}
{"id": "engineering_38991", "domain": "engineering", "question_title": "Why are engine blocks so robust apart from containing high pressure?", "question_body": "Lately, I've been pondering why some engine blocks are so bulky, I always thought intuitively it was because they had to last a long time whilst containing thousands of combustion cycles but the more, I look into the reality of engine design that understanding doesn't always hold up to the design of an engine. It seems cylinder walls have a set thickness which makes pretty good sense, then there is a water-jacket chamber for coolant which again, makes sense but then there is an outer shell of material that is sometimes even thicker than the cylinder walls themselves. This is where I become confused (An example image below) Surely, this outer shell doesn't need to be as thick as it is. In my mind all it's doing is containing a pressurized coolant which can't amount to more pressure than the combustion of the engine itself. So why is it sometimes as thick as the cylinder wall?", "question_score": 27, "question_tags": ["mechanical-engineering", "structural-engineering", "structural-analysis", "automotive-engineering", "vibration"], "choices": {"A": "If you were going to turn left 90 degrees, without turning the wheels, then you wind up dragging the wheels sideways while you turn. 16 seconds into this video shows exactly what I'm talking about . So every time you try to back out of your driveway, or a parking spot, or turn into a parking spot, or turn anywhere for any reason, you're going to lay down rubber because the tires are rotating quickly while remaining nearly stationary as you turn. Again, look at the wheel slip in the video I linked. You'd be hard pressed to find \"four...", "B": "You want to let air into the pipe when you switch the pump off, without letting water out. Here's a few ways to do this. Make a pinhole in the highest point of the pipe. You will lose a bit of water this way, but if it is above the tank, the water will drip back in (provided it doesn't spray too far.) You could even put it just under the tank lid. Install a tee and riser at the highest point of the pipe. this will need to be high enough to avoid the pump pressure pumping water out...", "C": "Actually, handlebars and steering wheels are less similar than you might think. When a two-wheeled vehicle is moving fast enough to balance, the front wheel is never turned more than a few degrees. The primary mechanism for steering is leaning the vehicle, not turning the front wheel. For example, to turn right, you actually tug briefly on the left side of the handlebar. This causes the wheels to track to the left of the center of mass, which in turn causes the bike to lean to the right. This lean is what causes the direction to change, while maintaining balance...", "D": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/38991/why-are-engine-blocks-so-robust-apart-from-containing-high-pressure"}
{"id": "engineering_1785", "domain": "engineering", "question_title": "Why doesn't a lightning strike destroy the lightning rod?", "question_body": "Lightning strikes have been known to cause massive amounts of damage . The stats on a lightning bolt are: current levels sometimes in excess of 400 kA, temperatures to 50,000 degrees F., and speeds approaching one third the speed of light These are massive numbers, but lightning protection systems are designed to draw the lightning away from the building or structure that they are protecting. Lightning protection systems can be simply thought of as lightning rods connected to the ground via cabling (downconductor). The NOAA specification for lightning protection requires that lightning rods be at least 0.5in (13mm) in diameter. The downconductor is a similar size copper cable ( 4/0 AWG or 12mm ). The allowable amperage for this type of wire is only about 250A for constant current. I realize that this is more of a heat limit rather than a instantaneous current capacity limit. From this paper on lightning protection (page 28): Positive feedback on the operation of a lightning protection system is seldom documented and most often not even noticed. Only in some rare cases can it be documented that a lightning protection system has been struck if it works properly and there is no damage. There is sometimes evidence at the strike termination point which can be noted during a careful inspection, but it is seldom cost effective for the owner of a lightning protection system to obtain the expertise necessary to conduct such a careful inspection. How can a seemingly small 0.5in (13mm) piece of metal handle a lightning strike with little or no visible damage much less without being completely destroyed?", "question_score": 25, "question_tags": ["electrical-engineering"], "choices": {"A": "As it happens, I just recently went through that calculation myself for a different site. Given the following facts from a quick web search, it isn't difficult to work out the numbers. The maximum efficiency of a (large) windmill is about 40%. The density of air is 1.225 kg/m 3 You need about 50 mW (10 mA at 5V) to light up an LED First, we'll need about 50 mW / 0.40 = 125 mW of air power flowing through the windmill to create the electricity we need (ignoring other factors such as the actual efficiency of a small windmill...", "B": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "C": "If I model this as a simply supported beam having load at mid span [...] I suspect that this is where your analysis went awry. First off, you should always model bridges with distributed loads, not a single concentrated load at midspan. The most significant load on a bridge will almost always be its own self-weight; load-trains are heavy but, well, so are bridges. Secondly, I assume you're thinking of the bridge like this: Indeed, we can see here that the bending moment is greater at midspan. However, that's not the bridge we're looking at, it's missing the cantilevers! So...", "D": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/1785/why-doesnt-a-lightning-strike-destroy-the-lightning-rod"}
{"id": "engineering_2363", "domain": "engineering", "question_title": "Why are rear wheels not placed at the extreme rear of a bus?", "question_body": "In some cars, I have noticed the rear wheels are located at the extreme rear of the vehicle. However, I have noticed that the rear wheels of buses are always located about 1/4th of the way forward from the rear. What is the reason for this?", "question_score": 25, "question_tags": ["mechanical-engineering", "automotive-engineering"], "choices": {"A": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "B": "To slightly generalize I'll reform the question slightly. A ridged 2-D body (car) has a line $l$ that moves with it. The car can be linearly transformed as long as the instantaneous center of rotation lies along $l$ at least distance $R$ away from a point $c$ that also moves with the car. In this case point $c$ lies in the center of the rear axle and $l$ lies on the rear axle. Now imagine the car's domain is limited to a quarter plane with edges $A$ and $B$. It initially is placed against $A$, far from $B$ with $l$...", "C": "Some ideas: Wheel Load Distribution : The load is greater on the rear wheels providing the power; more force on the front ones bring no benefit and would provide less traction. Better manoeuvrability from having a shorter wheelbase. Better Ground Clearance in some conditions, especially for bumps and or up a increasing slope for instance. Better Driving : The front wheels now turn around a point closer to the C.G. than with the rear wheels. Not good with vehicle dynamics but this appears better than the rear end 'trailing' behind. Structural : As some people have pointed out, it's better...", "D": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2363/why-are-rear-wheels-not-placed-at-the-extreme-rear-of-a-bus"}
{"id": "engineering_7536", "domain": "engineering", "question_title": "Why would you launch a large ship by dropping it sideways?", "question_body": "I'm referring to the process shown in this video: https://youtu.be/Quyr5R1Rbfw?t=20 Or this image from Wikipedia: In it, a large warship is launched into the water by essentially dropping it sideways down some ramps and off of a pier. The ship rolls hard to one side, and then oscillates back to the other, making the process seem like a fairly risky one. For instance, if it rolls back towards the pier too aggressively it might strike the structure and cause damage both to the pier and the shiny new warship. Or if it rolls too far on the initial drop, the ship might capsize. So my question is, what are the advantages of launching a large ship sideways like this, as opposed to, say, dropping or gently lowering it vertically into the water?", "question_score": 25, "question_tags": ["mechanical-engineering", "structural-engineering", "safety", "naval-engineering", "ships"], "choices": {"A": "Some ideas: Wheel Load Distribution : The load is greater on the rear wheels providing the power; more force on the front ones bring no benefit and would provide less traction. Better manoeuvrability from having a shorter wheelbase. Better Ground Clearance in some conditions, especially for bumps and or up a increasing slope for instance. Better Driving : The front wheels now turn around a point closer to the C.G. than with the rear wheels. Not good with vehicle dynamics but this appears better than the rear end 'trailing' behind. Structural : As some people have pointed out, it's better...", "B": "I think you are talking about roundabouts, not traffic circles. It is baffling to those of us in the UK that Americans think roundabouts are a new idea. In the UK we have so many variants, from mini-roundabouts all the way up to full motorway junctions (a giant roundabout above or below the motorway). So do roundabouts take up more space? Not necessarily, this is a mini roundabout: It's nothing more than a slightly domed area of paint on the road, no lights are necessary, you can actually drive straight over the top of it rather than around it, its...", "C": "I suspect that the answer to this is that, ultimately the gear ratio comes from the ratio of diameters of the gears rather than the number of teeth, although in most circumstances practicality dictates that they are proportional. Say you have a 10 tooth cog and a 40 tooth chainwheel. It's fairly simple to imagine that you could remove every other tooth from the 40 tooth wheel while keeping the diameter the same and maintain exactly the same gear ratio. Similarly you could have a completely gearless wheel (putting aside issues of slippage) driving a chain which drove a geared...", "D": "A specific reason for doing this is simply when there isn't enough room to do a bow or stern first launch. This is often the case when a ship or boat is built in a yard on a river or canal either because the hull is especially long or the channel it is being launched into is narrow. There is also the consideration that a sideways launch can be done from any quay that will take the weight but a bow first launch requires a specially constructed slipway. There is also the consideration that if you go in sideways the..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/7536/why-would-you-launch-a-large-ship-by-dropping-it-sideways"}
{"id": "engineering_8619", "domain": "engineering", "question_title": "How to stop water flow in a siphon?", "question_body": "I have assembled a small DIY drip-irrigation system for my terrace garden. Please have a look at the attached image. I switch on a small pump to start the drip-irrigation system and then switch it off. But even after that, the water keeps flowing through the system and stops only when I physically lift the pump out of water. How can I stop this water flow without requiring any physical action? Please note that I plan to automate the switch-on/off of the pump using a timer so that it functions without requiring my physical presence. ======================================================================== I tried the suggestion of \"Making a pinhole in the highest point of the pipe.\" It worked like a charm and solved the issue I was facing. Completely loved it, more so, because it does not involve procuring new stuff. I also like the option of using a solenoid-valve, but have not tried it yet. Will use it when I install such a system again where the tank is at a considerably higher place than the plants. Thank you everyone who took the effort to write an answer, and that too with detailed explanations.", "question_score": 25, "question_tags": ["fluid-mechanics", "siphon"], "choices": {"A": "As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\\arctan\\left(\\frac{0.15}{15}\\right)=0.57^\\circ$ or 1%. the middle ,...", "B": "You want to let air into the pipe when you switch the pump off, without letting water out. Here's a few ways to do this. Make a pinhole in the highest point of the pipe. You will lose a bit of water this way, but if it is above the tank, the water will drip back in (provided it doesn't spray too far.) You could even put it just under the tank lid. Install a tee and riser at the highest point of the pipe. this will need to be high enough to avoid the pump pressure pumping water out...", "C": "Electricity production optimization is a very complex subject. It is also affected by many parameters , which I will try to outline below. TL;DR: Demand is constantly monitored and supply is constantly adjusted TL;DR 2: Lshaver's excellent post is a suggested reading after reading this, because it expands and explains what happens at timescales ranging from the micro-second to minutes timescales. Changes in energy demand during the day First of all, the energy demand during the day, week, month of year can change quite a lot. Below is a typical day in New England. Fluctuation in power energy consumption a...", "D": "I will expand on DKNguyen answer, because to my knowledge also the two reasons are: reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation) change the joint tightening characteristics (see joint diagram). reduce contact stresses on surfaces. The basic idea is that since contact stress is defined as: $$\\sigma = \\frac{F}{A}$$ Obviously the larger the area the less the stress. Also, like Nuclear Hoagie pointed out the area changes with a square law of the diameter. UPDATE 2 - Calculation for M6 bolt (thanks to BenC) *The question gathered enough interest for me to carry out a..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/8619/how-to-stop-water-flow-in-a-siphon"}
{"id": "engineering_48864", "domain": "engineering", "question_title": "How do ball bearings not crack from a load concentrated on an infinitesimally small point?", "question_body": "How do ball bearings last? Most bearings in a standard hub (bottom of the image) are not actually taking a load. Only the single ball that happens to be at a particular location while spinning takes 100% of the load. If the bearings were cylindrical (top of the image) supporting a rolling plate, it would be slightly better, but not by much. Regardless of the load (in a bike hub), and even if it's just cyclist+bicycle, that the area is infinitesimally small would suggest that the stress skyrockets towards infinity. And that's before even talking of the higher loads in a car's bearings. How do ball bearings (in a bike hub) not crack from a load concentrated on an infinitesimally small point? This is a sequel question to a question I just asked on bicycles.SE about how much grease I should use after I overhaul bike hubs—whether I should put so little to keep the wheel spinning freely with minimal drag, or put so much that water ingress would be unlikely (the red areas in the image).", "question_score": 25, "question_tags": ["stresses", "bearings"], "choices": {"A": "Nothing is rigid, the raceways and the ball in a ball-bearing are no exception. The contact area deflects and accommodates the ball in a small contact surface, not a point. Also, the balls take the load in groups. The digrams are from SKF ball-bearings. '", "B": "Some ideas: Wheel Load Distribution : The load is greater on the rear wheels providing the power; more force on the front ones bring no benefit and would provide less traction. Better manoeuvrability from having a shorter wheelbase. Better Ground Clearance in some conditions, especially for bumps and or up a increasing slope for instance. Better Driving : The front wheels now turn around a point closer to the C.G. than with the rear wheels. Not good with vehicle dynamics but this appears better than the rear end 'trailing' behind. Structural : As some people have pointed out, it's better...", "C": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...", "D": "I'm going to set aside the potential legal liability aspects of your question. In part because you didn't ask about them, but also because liability will vary based upon jurisdiction. Obviously, consult an attorney familiar with the relevant law. Ethically, I think you've done everything that you're obligated to do. You have contacted the customer and notified them of an unsafe situation or configuration of the product. And you have (strongly) advised them that they need to discontinue using the older product in the unsafe configuration. If the customer remains insistent about using the product in an unsafe manner, you..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/48864/how-do-ball-bearings-not-crack-from-a-load-concentrated-on-an-infinitesimally-sm"}
{"id": "engineering_3450", "domain": "engineering", "question_title": "What is the purpose of these "steps" in rivers?", "question_body": "I have seen a number of structures within rivers, which resemble steps, and which allow the water to cascade down them instead of flowing naturally down the course of the river. Example 1: River Avon in Bath Example 2: River Seine in Paris (from Les Miserables (2012)) What is the purpose of these structures?", "question_score": 24, "question_tags": ["civil-engineering", "hydrology", "flow-control"], "choices": {"A": "The two photos in the post show the same structure: Pulteney Weir , downstream of Pulteney Bridge on the River Avon in Bath. The shot of the \"Seine\" in Les Miserables was filmed on location in Bath. Pulteney Weir was designed by architect Neville Conder , and built between 1968 and 1972. It's one component of the Bath Flood Prevention Scheme, which was carried out after the disastrous flood of December 1960 . The Bath in Time website has photos of the old weir (which descended in a single step) and of the new weir under construction ( 1968 ,...", "B": "It is a trench shield. It gets placed in a trench after the trench is dug to prevent workers from being hurt or killed in the event of a trench collapse. This picture from GMC trench shield shows a partially collapsed trench with a shield installed that would protect the workers installing the blue brute pipe.", "C": "If I model this as a simply supported beam having load at mid span [...] I suspect that this is where your analysis went awry. First off, you should always model bridges with distributed loads, not a single concentrated load at midspan. The most significant load on a bridge will almost always be its own self-weight; load-trains are heavy but, well, so are bridges. Secondly, I assume you're thinking of the bridge like this: Indeed, we can see here that the bending moment is greater at midspan. However, that's not the bridge we're looking at, it's missing the cantilevers! So...", "D": "Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/3450/what-is-the-purpose-of-these-steps-in-rivers"}
{"id": "engineering_44701", "domain": "engineering", "question_title": "Why exactly does a washer help distribute the stress around a bolt?", "question_body": "Usually the reason for having a washer under a bolt head is stated to be that it helps to evenly distribute the stress to the clamped material surface. But why is this? I would understand if the washer was significantly larger than the bolt head. Then there would understandably be more surface area. But for a bolt like in the picture, the washer is only slightly larger than the bolt head, so why would it make much difference? EDIT: Thank you everyone for good discussion and answers! Didn't expect this to spark such a discussion.", "question_score": 24, "question_tags": ["mechanical-engineering", "civil-engineering", "stresses", "fasteners", "machine-elements"], "choices": {"A": "Car wheels have holes mostly due to weight and cost considerations. Each hole is a chunk of material that you aren't wasting and weighing down the wheel with. As another bonus, the holes help with cooling the brakes by allowing airflow between the inside and outside. The shape and size of the holes are calculated to have a minimal impact on the structural integrity of the wheel.", "B": "I would say that using a warm pressurized gas is not very feasible. Ratchet freak already mentioned how you can get much more volume out of heating water into steam than just heating up air until it's warmer. This touches on, but doesn't completely address an important factor about steam as power. Converting to steam includes a phase change from liquid to gas. This phase change actually acts as an additional storage of energy. You can draw this energy out of the steam later in the system (through a heat exchanger for example), converting it back into a liquid, which...", "C": "I will expand on DKNguyen answer, because to my knowledge also the two reasons are: reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation) change the joint tightening characteristics (see joint diagram). reduce contact stresses on surfaces. The basic idea is that since contact stress is defined as: $$\\sigma = \\frac{F}{A}$$ Obviously the larger the area the less the stress. Also, like Nuclear Hoagie pointed out the area changes with a square law of the diameter. UPDATE 2 - Calculation for M6 bolt (thanks to BenC) *The question gathered enough interest for me to carry out a...", "D": "Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book \" Wind Energy - The Facts \". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/44701/why-exactly-does-a-washer-help-distribute-the-stress-around-a-bolt"}
{"id": "engineering_42598", "domain": "engineering", "question_title": "Why not put servers in a refrigerator?", "question_body": "Recently I was thinking about home improvement, and one idea that is haunting me is that I could use some kind of refrigerator for gaming consoles and laptops. The obvious benefit is that it maintains a constant low temperature. Also, the doors usually can be closed firmly and it won't let dust come inside, so there isn't any need to disassemble and clean hardware. That is a good thing in long term. Despite it makes airflow less intensive, according to my understanding it should not be a problem, as all that is needed for hardware is cooled air, not new air. And whenever I get what I consider a good idea, I usually stop for a minute and ask myself a question \"why doesn't this exist yet?\". So there are definitely big-scale expensive enterprise servers which could possibly use the same approach to reduce maintenance effort, but that is not the case. So the disadvantages I can think of is that refrigerators are just not designed to cool a source of constant heat for a long period of time and (probably) will break quickly. Another problem that came to mind is that refrigerator boxes tend to generate condensate and it might be harmful for hardware. Am I correct with my assumptions, or is it not that crucial and are there in fact similar solutions?", "question_score": 23, "question_tags": ["airflow", "cooling", "computer-engineering"], "choices": {"A": "Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train .", "B": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...", "C": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...", "D": "We do. It's just an up-sized (i.e. more powerful) version of a refrigerator known as an air conditioning unit. Essentially all server rooms and most spaces where PCs are located (speaking for the U.S., at least) are air conditioned. Server rooms almost universally have dedicated HVAC systems and they will indeed be designed to keep the room at a more-or-less constant temperature (and typically humidity, too.) Other answers mention water blocks, heat sinks, fans, etc., but those are mostly just used to move the heat from the CPUs and other such hot components to the ambient air in the server..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/42598/why-not-put-servers-in-a-refrigerator"}
{"id": "engineering_531", "domain": "engineering", "question_title": "What is a reason that handlebars fit to a motorcycle and steering wheel fit to a car?", "question_body": "What is the reason that handlebars are installed on motorcycles and steering wheels are installed on cars? Notice that the way to use both handlebars and steering wheels are quite similar, but a steering wheel allows for much more rotation than a handle bar when you control a vehicle. Please give me a reason why a massive vehicle should use a steering wheel and a light weight vehicle should use a handlebar? The reason maybe involve scientific reasons, safety reasons or designing reasons.", "question_score": 22, "question_tags": ["mechanical-engineering"], "choices": {"A": "The turbulence model can make a big difference in your simulation . There are many turbulence models around. It becomes a tough job to select one out of them. There is no perfect turbulence model. It all depends on several parameters like Reynold's number, whether the flow is separated, pressure gradients, boundary layer thikness and so on. In this answer, brief information about a few popular models is given along with pros and cons and potential applications. However, interested users can see this excellent NASA website and references therein to know more about turbulence modeling. A) ONE EQUATION MODEL: 1....", "B": "Actually, handlebars and steering wheels are less similar than you might think. When a two-wheeled vehicle is moving fast enough to balance, the front wheel is never turned more than a few degrees. The primary mechanism for steering is leaning the vehicle, not turning the front wheel. For example, to turn right, you actually tug briefly on the left side of the handlebar. This causes the wheels to track to the left of the center of mass, which in turn causes the bike to lean to the right. This lean is what causes the direction to change, while maintaining balance...", "C": "Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with...", "D": "My first thought is that it might be intended to be a wing nut driver of some sort, but those are usually hollow cylinders with slots for the wings. Ah ... sure enough, it's described as such in this Ebay ad :"}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/531/what-is-a-reason-that-handlebars-fit-to-a-motorcycle-and-steering-wheel-fit-to-a"}
{"id": "engineering_2873", "domain": "engineering", "question_title": "Why do car wheels have holes?", "question_body": "If you have a look at the car's wheels, you'll notice that they have holes which can be of different forms (mostly circular or rectangular). Why do they have such holes? Doesn't that reduce the stiffness of the wheels?", "question_score": 22, "question_tags": ["mechanical-engineering", "automotive-engineering", "wheels"], "choices": {"A": "The turbulence model can make a big difference in your simulation . There are many turbulence models around. It becomes a tough job to select one out of them. There is no perfect turbulence model. It all depends on several parameters like Reynold's number, whether the flow is separated, pressure gradients, boundary layer thikness and so on. In this answer, brief information about a few popular models is given along with pros and cons and potential applications. However, interested users can see this excellent NASA website and references therein to know more about turbulence modeling. A) ONE EQUATION MODEL: 1....", "B": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...", "C": "Car wheels have holes mostly due to weight and cost considerations. Each hole is a chunk of material that you aren't wasting and weighing down the wheel with. As another bonus, the holes help with cooling the brakes by allowing airflow between the inside and outside. The shape and size of the holes are calculated to have a minimal impact on the structural integrity of the wheel.", "D": "As with all good things, it depends. If you can assume that your supports are totally stiff and that the loading on the shelf will be approximately uniform, then you basically have the following structure: A rectangular cross-section (such as a plank) will behave equally under positive or negative bending moment, so your objective should be to balance both. To do so, you want your main span to be $2\\sqrt2 \\approx 2.83$ times the cantilevers. This is found by calculating the cantilever required to offset half of the bending moment due to a uniform load along a simply supported beam:..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2873/why-do-car-wheels-have-holes"}
{"id": "engineering_15798", "domain": "engineering", "question_title": "What's wrong with transporting a refrigerator on its side?", "question_body": "I tried to deliver a refrigerator and the customer saw that the refrigerator was lying down in the truck instead of standing up. He refused to accept it, claiming that the compressor would be damaged by having it on its side. I tried to explain that that made no sense, but his friend came to me and also said the same thing. Is transporting a refrigerator on its side a problem? If so, why?", "question_score": 22, "question_tags": ["refrigeration"], "choices": {"A": "After the 2014 ICCT report revealed that these light-duty passenger diesel vehicles were emitting too much NOx and US regulators confronted VW about the results, VW did some testing and proposed a voluntary software recall to recalibrate the various emissions control devices on the affected vehicles. From the California Air Resources Board's (CARB's) in-use compliance letter to Volkwagen AG, 2015-09-18 , following that recall: To have a more controlled evaluation of the high NOx observed over the road, CARB developed a special dynamometer cycle which consisted of driving the Phase 2 portion of the FTP repeatedly. This special cycle revealed...", "B": "After referring to some good online resources such this , I know why we shouldn't transport it laying down. Compressor is filled with oil which is critical to its operation. In the normal upright position gravity keeps the oil in the compressor. When we lay the refrigerator flat, some of the oil can leave the compressor and go into the cooling lines. The oil is a thick viscous fluid and can clog the cooling lines thus hampering the refrigerator's ability to cool. Lack of oil in the compressor can also damage the compressor. If we must lay the fridge down,...", "C": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...", "D": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/15798/whats-wrong-with-transporting-a-refrigerator-on-its-side"}
{"id": "engineering_38920", "domain": "engineering", "question_title": "How does turning off electric appliances save energy", "question_body": "We all know if we use less electricity we save energy. But the energy we're getting is result of burning (not necessarily the literal meaning) the fuel. Even if we don't use that energy that fuel is gone forever. (Except saving money on bills) My questions Am I wrong in above statement? If yes then how does actually electricity production work so that we're able to save it? Do power plant (or associate agencies) constantly monitor demand and reduce the production of electricity on real time thus saving fuel (hence energy)?", "question_score": 22, "question_tags": ["power", "energy", "power-engineering", "fuel-economy"], "choices": {"A": "Electricity production optimization is a very complex subject. It is also affected by many parameters , which I will try to outline below. TL;DR: Demand is constantly monitored and supply is constantly adjusted TL;DR 2: Lshaver's excellent post is a suggested reading after reading this, because it expands and explains what happens at timescales ranging from the micro-second to minutes timescales. Changes in energy demand during the day First of all, the energy demand during the day, week, month of year can change quite a lot. Below is a typical day in New England. Fluctuation in power energy consumption a...", "B": "I will expand on DKNguyen answer, because to my knowledge also the two reasons are: reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation) change the joint tightening characteristics (see joint diagram). reduce contact stresses on surfaces. The basic idea is that since contact stress is defined as: $$\\sigma = \\frac{F}{A}$$ Obviously the larger the area the less the stress. Also, like Nuclear Hoagie pointed out the area changes with a square law of the diameter. UPDATE 2 - Calculation for M6 bolt (thanks to BenC) *The question gathered enough interest for me to carry out a...", "C": "After the 2014 ICCT report revealed that these light-duty passenger diesel vehicles were emitting too much NOx and US regulators confronted VW about the results, VW did some testing and proposed a voluntary software recall to recalibrate the various emissions control devices on the affected vehicles. From the California Air Resources Board's (CARB's) in-use compliance letter to Volkwagen AG, 2015-09-18 , following that recall: To have a more controlled evaluation of the high NOx observed over the road, CARB developed a special dynamometer cycle which consisted of driving the Phase 2 portion of the FTP repeatedly. This special cycle revealed...", "D": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/38920/how-does-turning-off-electric-appliances-save-energy"}
{"id": "engineering_50946", "domain": "engineering", "question_title": "Why do wooden gate designs recommend a brace under compression instead of tension?", "question_body": "Looking at guides for building wooden frame gates I almost always find the following design recommendation: The idea is that the cross brace will distribute load from the top outside corner into the bottom of the hinge bracket. I understand this logic and am sure it works but wouldn't it be preferred to use a tension brace rather than a compression brace in this application? For example virtually every recommendation for a wire rope support shows the opposite: In this case the wire rope supports the far end under tension, but is there a reason a 2x4 couldn't also be used under tension rather than compression? Is the former design more effective for some reason? I realize that using the 2x4 for tension would require stronger joinery but it's certainly possible to secure it more than well enough to take the load. Additionally a common theme here seems to be that the load on the gate is too trivial to really care either way but I strongly disagree with that statement. Wooden gates almost never hold up to time, and we aren't talking decades here...most wooden gates will sag within 1-2 years. Wood is certainly a troublesome material but plenty of critical structures have been made from wood with proper engineering and have held up much longer than this. So maybe we should consider complicating things more? It's certainly cheaper and easier to use more complicated joinery/a 99 cent steel bracket and 2x4s than it is to fabricate an all steel gate frame. Are 2x4s just undersized to support a load like this or is it possible the bracing can make a difference?", "question_score": 22, "question_tags": ["structural-engineering"], "choices": {"A": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...", "B": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only...", "C": "The reason is simple: the load applied on the gate is pretty trivial, and the brace (be it a 2x4 or a wire) is mostly added for rigidity. Indeed, you could put that 2x4 in tension and it would work just as well (since the load is usually trivial, a 2x4 is probably 10x stronger than necessary in both tension and compression). However, as you mentioned, that would require more complex joinery, so... why bother? Obviously, when using a wire, it must be in tension, so you don't even have the choice. But with a 2x4, you can choose, so...", "D": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/50946/why-do-wooden-gate-designs-recommend-a-brace-under-compression-instead-of-tensio"}
{"id": "engineering_68", "domain": "engineering", "question_title": "How does width and thickness affect the stiffness of steel plate?", "question_body": "I have a 2 mm thick steel plate which is 300 mm long and 30 mm wide, supported at either end. It supports a weight-bearing wheel that can roll along the plate. It currently supports the maximum weight that I expect it to support when the wheel is in the middle, but it flexes a little bit too much. Would making it wider help to support the weight and increase its stiffness, or do I need to make it thicker? Also is there a way to calculate how the stiffness will change with the thickness (or width if that would affect it)?", "question_score": 21, "question_tags": ["mechanical-engineering", "steel", "stiffness"], "choices": {"A": "Short answer : make it thicker. Long answer : The moment of inertia affects the beam's ability to resist flexing. Use one of the many, free, online moment of inertia calculators (like this one ) to see how increasing the height of the beam will have an exponential effect on increasing the stiffness of the beam. And this site helps provide a pictorial view of the load(s) upon a beam depending upon differing configurations, such as where the supports are and where the load is applied. It also provides a calculator to determine the forces involved. Wikipedia has a decent...", "B": "The difference between the two equations The cavitation number is the ratio of the static pressure difference to the dynamic pressure difference. So, if you want to use the first equation, you would need to take the pressure using a Pitot tube to measure the total pressure, whereas if you want to use the second equation you will need to measure the freestream velocity, but I would recommend measuring it upstream rather than downstream because of possible effects of acceleration and boundary layer growth. Also, your $V$ should be $V_{in}$ such that it corresponds to the same location where $p_{in}$...", "C": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...", "D": "I suspect that the answer to this is that, ultimately the gear ratio comes from the ratio of diameters of the gears rather than the number of teeth, although in most circumstances practicality dictates that they are proportional. Say you have a 10 tooth cog and a 40 tooth chainwheel. It's fairly simple to imagine that you could remove every other tooth from the 40 tooth wheel while keeping the diameter the same and maintain exactly the same gear ratio. Similarly you could have a completely gearless wheel (putting aside issues of slippage) driving a chain which drove a geared..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/68/how-does-width-and-thickness-affect-the-stiffness-of-steel-plate"}
{"id": "engineering_207", "domain": "engineering", "question_title": "How do engineers really use numerical simulation?", "question_body": "Disclaimer I'm an applied mathematician by training, not an engineer. My work research primarily focuses on creating new \"methods\" to solve different PDE's related to solid deformation (elasticity) and fluid mechanics. In this sense, i know how to solve a pde problem computationally. From my perspective, engineers use my work as \"tools\" to accomplish their work. However, due to my lack of education/experience in engineering, i admit i'm actually rather clueless on how numerical solutions to pde's are really used in an engineers actual practice. The primary source of my confusion is the following: I've been told that engineers never (or should never) conduct numerical simulations (e.g. finite element analysis, CFD, etc...) without knowing or having a good idea ahead of time what the simulation \"should\" look like. This helps engineers discriminate realistic results from questionable ones. However, i argue that if the engineer already knows what is supposed to happen in the simulation, then what's the point of simulation in the first place??? I've always assumed that simulations are needed for predictive purposes, which assumes ignorance of what is to come. That is, I think of a simulation as a stand-alone tool to predict the future when you don't know what to expect . What i'm looking for is a broader perspective into how/when/why engineers use numerical simulations like CFD and Finite Element Analysis, especially if good engineering practice dictates that you should already know what to expect when you're simulating?", "question_score": 21, "question_tags": ["modeling"], "choices": {"A": "Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with...", "B": "I have written mostly about CFD in this answer, however same points should also work for FEA or other simulation techniques. CFD is mostly used for design optimization and parametric study of the design. Following are a few examples showing how engineers use simulations Selection of a design : Read: A conceptual study of airfoil performance enhancement using CFD. This thesis shows use of CFD for selecting the best design out of a number of candidate designs. Engineers often go for simulations to select 'the one' out of many . Shape optimization using CFD : This paper gives an example...", "C": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "D": "Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region...."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/207/how-do-engineers-really-use-numerical-simulation"}
{"id": "engineering_435", "domain": "engineering", "question_title": "What makes suspension bridges unsuitable for railways?", "question_body": "I recall reading in an old issue of Model Railroader an article about railroad bridges. In it, the author mentioned that you shouldn't have a model suspension bridge for a railroad track on your layout because no such arrangement would be prototypical. In his own words, \"suspension bridges and trains don't mix.\" I realize that the author probably wasn't an engineer (at least in the sense of this site), but his remark did get me thinking. I am somewhat of a railroad buff, and I can't think of a suspension bridge for trains - the closest thing that comes to mind are the trolley tracks that were on the Brooklyn Bridge * . I understand that there wouldn't be many suspension bridges for trains, since other bridge types would be better suited (cheaper, easier to build, etc.) but why are they so rare? * Bear in mind, though, I live in the US, and am pretty unfamiliar with foreign railroad infrastructure.", "question_score": 21, "question_tags": ["civil-engineering", "bridges", "rail"], "choices": {"A": "There are a few main reasons why suspension bridges aren't used for railroads. The main reason is that suspension bridges are typically used where very long spans are needed. Trains are very heavy, especially when compared to lanes of highway traffic. This means that long spans require very strong support structures, which in the case of suspension bridges are cables and towers. The second reason goes along with the first; trains cause high dynamic loads as they move along the rail. This can increase the vertical loads by 30%. Third is that trains don't really have suspensions, especially freight trains....", "B": "Theoretically pontoon bridges with rope anchors keeping them to the bottom would work against wind and flow, overcoming the problem jhabbot mentioned in his answer (same as train length limit - stretching force). In practice these come with more problems of their own. They drift on water surface and as result, rise and fall with water waves. The larger the body of water they span, the higher the waves; at certain point in stormy weather the bridge would just launch the vehicles into the air. The anchoring isn't exactly simple if it's to withstand such forces. You could just as...", "C": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "D": "BS5950-1:2000 Clause 1.3.23 defines an H-section as having \" an overall depth not greater than 1.2 times its overall width \", and Clause 1.3.25 defines an I section as having \" an overall depth greater than 1.2 times its overall width \". Note that at exactly a ratio of 1.2, it would be an H section not an I section."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/435/what-makes-suspension-bridges-unsuitable-for-railways"}
{"id": "engineering_4209", "domain": "engineering", "question_title": "Why do cars turn by turning the front wheel?", "question_body": "Why do we design cars to turn by turning the front wheel? Wouldn't it better to keep all wheels straight and turn by rotating the wheels on one side of a car more then the wheels on the opposite side of the car, like tank tracks do? That way you can turn even if you are standing still. Why don't cars do this?", "question_score": 21, "question_tags": ["automotive-engineering", "wheels"], "choices": {"A": "Note: This is specific to gasoline based engines. Assuming your car was produced after the ~1990's it is equipped with a Deceleration Fuel Cut-Off (DFCO) system which cuts the fuel flow to the engine when the gas pedal is not depressed and the car is in gear. In this case it is more fuel efficient to stay in gear because you are letting gravity run the engine instead of fuel. In addition, it saves wear on your brakes because the partial-vacuum created in the engine leads to engine-braking . In many states in the US it is actually required that...", "B": "If you were going to turn left 90 degrees, without turning the wheels, then you wind up dragging the wheels sideways while you turn. 16 seconds into this video shows exactly what I'm talking about . So every time you try to back out of your driveway, or a parking spot, or turn into a parking spot, or turn anywhere for any reason, you're going to lay down rubber because the tires are rotating quickly while remaining nearly stationary as you turn. Again, look at the wheel slip in the video I linked. You'd be hard pressed to find \"four...", "C": "Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste...", "D": "After the 2014 ICCT report revealed that these light-duty passenger diesel vehicles were emitting too much NOx and US regulators confronted VW about the results, VW did some testing and proposed a voluntary software recall to recalibrate the various emissions control devices on the affected vehicles. From the California Air Resources Board's (CARB's) in-use compliance letter to Volkwagen AG, 2015-09-18 , following that recall: To have a more controlled evaluation of the high NOx observed over the road, CARB developed a special dynamometer cycle which consisted of driving the Phase 2 portion of the FTP repeatedly. This special cycle revealed..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/4209/why-do-cars-turn-by-turning-the-front-wheel"}
{"id": "engineering_6435", "domain": "engineering", "question_title": "Why does the Channel Tunnel enter the ground around 10 km from the coast?", "question_body": "I know that digging tunnels is always much more costly than building ways or train above ground. Why doesn't the Channel Tunnel start around the coastline? Why does it have an around 10 km long portion under land on the British side?", "question_score": 21, "question_tags": ["civil-engineering", "geotechnical-engineering", "rail", "tunnels"], "choices": {"A": "From the point of view of the driver of a car, impacting another car is about as bad as crashing against an ideal wall (a wall with zero deformation whatsoever). If there were a plane reflection between the two cars, then vs. Car would be exactly equal to vs. Wall (the contact points between both cars would all be on the same plane, due to reflection, so each car could be considered a wall for the other). But this plane reflection does not exist: What we have instead is a 2-fold rotational reflection . Let's say the left part of...", "B": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "C": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "D": "If I model this as a simply supported beam having load at mid span [...] I suspect that this is where your analysis went awry. First off, you should always model bridges with distributed loads, not a single concentrated load at midspan. The most significant load on a bridge will almost always be its own self-weight; load-trains are heavy but, well, so are bridges. Secondly, I assume you're thinking of the bridge like this: Indeed, we can see here that the bending moment is greater at midspan. However, that's not the bridge we're looking at, it's missing the cantilevers! So..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/6435/why-does-the-channel-tunnel-enter-the-ground-around-10-km-from-the-coast"}
{"id": "engineering_10335", "domain": "engineering", "question_title": "Why would a train automatically derail if a signal is passed at danger?", "question_body": "In a recent incident in London Network Rail said an empty train had travelled past a red signal, which resulted in an automatic derailment. No one was injured. [link] The derailment has caused quite a bit of damage, and a lot of travel disruption along this track. My reading of the National Rail statement is that the derailment was a feature of the system, a response to the signal being passed at danger. While I'm sure it caused less damage than a train collision, it still seems dangerous and expensive. Things like train stops , to trigger the brakes exist, or one could imagine diverting the train into a sand trap. Why weren't options like these used instead of derailing?", "question_score": 21, "question_tags": ["safety", "rail"], "choices": {"A": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...", "B": "Firstly, the incident happened as the train was leaving a siding passing a shunt signal. These provide less authorisation than a normal signal does, even when they are not at danger (the train can permit as far as the line is clear or the next signal , there is no guarantee the track ahead is clear). Now, in the UK, there are four train protection systems: AWS, TPWS, ATP, and ETCS. However, these are all primarily designed for trains on normal running lines, rather than those on sidings. I'll examine each of these in turn insofar as they protect signals:...", "C": "If the receiver does not detect the sub-carrier for the \"colour burst\" signal which is transmitted during the horizontal blanking period the receiver switches on the \"colour-killer\" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise. Figure 1. The colorburst signal is transmitted on the \"back porch\" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately...", "D": "After the 2014 ICCT report revealed that these light-duty passenger diesel vehicles were emitting too much NOx and US regulators confronted VW about the results, VW did some testing and proposed a voluntary software recall to recalibrate the various emissions control devices on the affected vehicles. From the California Air Resources Board's (CARB's) in-use compliance letter to Volkwagen AG, 2015-09-18 , following that recall: To have a more controlled evaluation of the high NOx observed over the road, CARB developed a special dynamometer cycle which consisted of driving the Phase 2 portion of the FTP repeatedly. This special cycle revealed..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/10335/why-would-a-train-automatically-derail-if-a-signal-is-passed-at-danger"}
{"id": "engineering_1706", "domain": "engineering", "question_title": "Does a roadway bridge experience more load when vehicles are parked or when they are moving?", "question_body": "Bridges are designed for the loads that come from the vehicles that are expected to cross them. This includes the weight the vehicle and any dynamic loads that may be introduced from movement of the vehicle. Dynamic loads may be from \"bouncing\" or from hitting a joint or pothole. Initially it would seem obvious that more load is applied to a bridge while the vehicles are in motion (weight of vehicle plus dynamic load). The dynamic loads are proportional to the travelling speed of the vehicles, but as vehicles go faster, they typically are spaced farther apart. When vehicles are stopped, they typically are much more closely spaced than when they are moving. Can there be a situation where more load is on a bridge because of closely spaced parked vehicles than from moving vehicles that are farther apart? Are these two situations covered in bridge design?", "question_score": 20, "question_tags": ["civil-engineering", "bridges", "structural-engineering"], "choices": {"A": "Electricity production optimization is a very complex subject. It is also affected by many parameters , which I will try to outline below. TL;DR: Demand is constantly monitored and supply is constantly adjusted TL;DR 2: Lshaver's excellent post is a suggested reading after reading this, because it expands and explains what happens at timescales ranging from the micro-second to minutes timescales. Changes in energy demand during the day First of all, the energy demand during the day, week, month of year can change quite a lot. Below is a typical day in New England. Fluctuation in power energy consumption a...", "B": "This is to make sure they know what the foundation is made of. For all they knew there may have been an old tunnel underneath that would have collapsed when the new building is put on top. London is built on top of an old marsh, this type of soil is very prone to sinking and uneven settling, digging down and reinforcing the foundation alleviates that. It also ensures the foundation is uniform under the building to avoid a new tower of Pisa. Given the age of the city it may have been to scour the land for potential archaeological...", "C": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "D": "Parked vehicles vs moving vehicles Closely spaced parked (or slow moving) vehicles are definitely more onerous, as stated on page 89, Appendix 2.A, Clause 2.A.1 of the South African bridge design code TMH7 : It is generally accepted and can readily be shown that except in the very small span range, the worst loading condition occurs under congested (bumper to bumper) conditions caused by a traffic blockage and that the dispersion of traffic at speed caused by increased vehicular inter-spacing, more than off-sets the effects of impact. However, this is only true by inspection for an unlimited number of vehicles..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/1706/does-a-roadway-bridge-experience-more-load-when-vehicles-are-parked-or-when-they"}
{"id": "engineering_7245", "domain": "engineering", "question_title": "What material is used to hold molten iron in a furnace?", "question_body": "When iron is melted, I guess it has to be transported and contained. I think the container in which it is has to be able to withstand higher temperatures than what you want to melt. According to this webiste , \"Iron, Wrought\" has a melting temperature of 1482 - 1593 °C. There are a couple of other metals which have higher melting points (e.g. Wolfram (tungsten) with over 3400 °C), but all I can think of are much more expensive. So what material is the oven / \"bottle\" / \"basin\" (or however you call it) made of? (Side question: Iron has been melted for quite a while now. I guess this has changed over the years. Of which materials was it before?)", "question_score": 20, "question_tags": ["metallurgy"], "choices": {"A": "Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste...", "B": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...", "C": "Summary Crucibles are lined with refractory materials. Steel processing makes use of graphite or a combination of chromite and magnesite for direct contact with the melt. Cast iron processing often uses engineered clays, also known as alumina-magnesia-silica mixtures. Graphite is harder to form than clay-type refractories. To be suitable as a refractory, a material must meet a number of property requirements to be both economical and safe. Refractory Materials As you noted, iron has a high-end melting point of about 1,540 °C on the far left side of the $\\textrm{Fe-C}$ phase diagram below, in the form of pure iron. There...", "D": "This will at least depend on the: Rate of Cooling Magnetic field strength Exact composition The magnetic field will alter the microstructure as you can read in, for example, Yudong Zhang, Nathalie Gey, Changshu He, Xiang Zhao, Liang Zuo, Claude Esling, High temperature tempering behaviors in a structural steel under high magnetic field, Acta Materialia, Volume 52, Issue 12, 12 July 2004, Pages 3467-3474, ISSN 1359-6454, http://dx.doi.org/10.1016/j.actamat.2004.03.044 . G.M. Ludtka, R.A. Jaramillo, R.A. Kisner, D.M. Nicholson, J.B. Wilgen, G. Mackiewicz-Ludtka, P.N. Kalu, In situ evidence of enhanced transformation kinetics in a medium carbon steel due to a high magnetic field,..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/7245/what-material-is-used-to-hold-molten-iron-in-a-furnace"}
{"id": "engineering_7394", "domain": "engineering", "question_title": "Why does it take so long to restart a nuclear power plant?", "question_body": "I have heard a couple of times that an operating nuclear power plant which was shut down (non-emergency; e.g. for a regular check) needs over 24 hours (up to 72 hours?) to get up running again. Why does it take that long?", "question_score": 20, "question_tags": ["power", "nuclear-technology", "electrical-grid"], "choices": {"A": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "B": "As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\\arctan\\left(\\frac{0.15}{15}\\right)=0.57^\\circ$ or 1%. the middle ,...", "C": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...", "D": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/7394/why-does-it-take-so-long-to-restart-a-nuclear-power-plant"}
{"id": "engineering_12453", "domain": "engineering", "question_title": "Windmills in empty fields. Why no trees?", "question_body": "I have noticed that windmills are generally built in empty fields with no trees around, and I've been wondering why... A windmill is generally way taller than trees and I can imagine that trees don't actually affect the flow (see picture). But is this the reason why there is nothing around or is there something else that makes them waste so much space around? And is the velocity profile from the first figure a realistic one? If I search for the theory of external convection, the velocity profile looks like this for flow over a plateau: In the second case, wouldn't the velocity be higher if there is an obstacle? Or is it because the obstacle is a porous medium that damps the flow? And lastly, is the velocity profile at earth level looking like that? Is there really an origin to it, or is it fully turbulent all the way?", "question_score": 20, "question_tags": ["mechanical-engineering", "fluid-mechanics", "thermodynamics", "heat-transfer"], "choices": {"A": "Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book \" Wind Energy - The Facts \". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...", "B": "After the 2014 ICCT report revealed that these light-duty passenger diesel vehicles were emitting too much NOx and US regulators confronted VW about the results, VW did some testing and proposed a voluntary software recall to recalibrate the various emissions control devices on the affected vehicles. From the California Air Resources Board's (CARB's) in-use compliance letter to Volkwagen AG, 2015-09-18 , following that recall: To have a more controlled evaluation of the high NOx observed over the road, CARB developed a special dynamometer cycle which consisted of driving the Phase 2 portion of the FTP repeatedly. This special cycle revealed...", "C": "For long freight trains and those that will be climbing to stations at higher altitudes, an extra or two locomotives are attached to the front. I've always wondered why. As usual there are multiple issues. Most important is landscape. If more power than a single locomotive can provide is not needed for the whole trip, but only a short stretch, like climbing or crossing a mountain range, then it would be a huge waste of resources to attach them the whole trip. For example, take a 10 hour trip time, where all, except for a 1 hour stretch, can be...", "D": "Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train ."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/12453/windmills-in-empty-fields-why-no-trees"}
{"id": "engineering_32376", "domain": "engineering", "question_title": "What are the engineering principles for a train to get electricity from the railway", "question_body": "How many general methods are there for transferring electricity from the railway to a train? I could see that some trains are connected by a pantograph and some have a third rail. Are there any other methods? What is the general engineering principle behind each? What are the basic differences (pros and cons) of each method? I was looking for these questions but could not find a good review. If anyone has a reference or could answer them kindly do. Also, is it possible to build a small model of such a system and scale it up? I am not talking about a commercial model train. I am aiming at self-designing the same engineering principals and demonstrating them on a scalable small model. If so, I would be happy to have a nice tutorial reference. I am focusing on Electric multiple units (EMU) .", "question_score": 20, "question_tags": ["rail"], "choices": {"A": "Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste...", "B": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...", "C": "Pantograph and third rail are pretty much it. Engineering principles: both have a conducting surface on the train (moving) in contact with the stationary rail/wire, in both cases you need a material that's resilient and conductive. The contact strip is a wear material . Differences: overhead catenary wires are flexible and will move around when a train drives underneath them. At high speeds, the pantograph can set up waves in the catenary wire, if these waves aren't damped sufficiently, the pantograph will start bouncing. third rail is limited to low voltages to reduce the risk of arcing between the rails....", "D": "If the receiver does not detect the sub-carrier for the \"colour burst\" signal which is transmitted during the horizontal blanking period the receiver switches on the \"colour-killer\" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise. Figure 1. The colorburst signal is transmitted on the \"back porch\" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/32376/what-are-the-engineering-principles-for-a-train-to-get-electricity-from-the-rail"}
{"id": "engineering_214", "domain": "engineering", "question_title": "Can we change steel properties by application of magnetic field while quenching?", "question_body": "Austenite is non magnetic while $\\alpha$-ferrite and pearlite are magnetic . ( Magnetic properties of pearlite vary as a function of carbon content ) If a strong magnetic field is applied in a particular direction while the steel is being quenched (rather, austenite is being quenched!), would the grain structure change? Is it possible to get superior grain structure and hence tougher steel by application of cyclic magnetic field? My speculation is that, at the eutectoid point while quenching the steel, as low carbon zone of pearlite has more permeability, that zone should align itself to the strong magnetic field by pushing the carbon in orthogonal direction, so grain boundaries should take a different shape. Will it actually happen?", "question_score": 19, "question_tags": ["steel", "process-engineering", "materials", "metallurgy", "magnets"], "choices": {"A": "This will at least depend on the: Rate of Cooling Magnetic field strength Exact composition The magnetic field will alter the microstructure as you can read in, for example, Yudong Zhang, Nathalie Gey, Changshu He, Xiang Zhao, Liang Zuo, Claude Esling, High temperature tempering behaviors in a structural steel under high magnetic field, Acta Materialia, Volume 52, Issue 12, 12 July 2004, Pages 3467-3474, ISSN 1359-6454, http://dx.doi.org/10.1016/j.actamat.2004.03.044 . G.M. Ludtka, R.A. Jaramillo, R.A. Kisner, D.M. Nicholson, J.B. Wilgen, G. Mackiewicz-Ludtka, P.N. Kalu, In situ evidence of enhanced transformation kinetics in a medium carbon steel due to a high magnetic field,...", "B": "I'm going to set aside the potential legal liability aspects of your question. In part because you didn't ask about them, but also because liability will vary based upon jurisdiction. Obviously, consult an attorney familiar with the relevant law. Ethically, I think you've done everything that you're obligated to do. You have contacted the customer and notified them of an unsafe situation or configuration of the product. And you have (strongly) advised them that they need to discontinue using the older product in the unsafe configuration. If the customer remains insistent about using the product in an unsafe manner, you...", "C": "This is an admittedly North American response. MGT In the US, how much traffic goes over a given track in a year is measured in Million Gross Tons (MGT) e.g. 1 MGT = 2 000 000 000 lbs [spaces instead of commas to be world-friendly]. This is a measure of the total weight of cargo and vehicles but not necessarily the number of individual trains. Rail Life The life of a typical railroad rail is between 1300 MGT and 380 MGT depending on if the rail is on a straight (tangent) track or in a curve. There is more friction...", "D": "Short answer : make it thicker. Long answer : The moment of inertia affects the beam's ability to resist flexing. Use one of the many, free, online moment of inertia calculators (like this one ) to see how increasing the height of the beam will have an exponential effect on increasing the stiffness of the beam. And this site helps provide a pictorial view of the load(s) upon a beam depending upon differing configurations, such as where the supports are and where the load is applied. It also provides a calculator to determine the forces involved. Wikipedia has a decent..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/214/can-we-change-steel-properties-by-application-of-magnetic-field-while-quenching"}
{"id": "engineering_392", "domain": "engineering", "question_title": "How does a traffic light sense the proximity of vehicles?", "question_body": "Some traffic lights don't operate periodically but instead detect when a car is close by and then turns green. I have heard that they use a magnetic sensor embedded in the road to sense cars as they come near. Is this correct? Do they use other means as well?", "question_score": 19, "question_tags": ["electrical-engineering", "civil-engineering", "traffic-light"], "choices": {"A": "I can understand your concern. This is going to be difficult for two reasons: Finding out the agency that is responsible for the maintenance may be next to impossible for a layperson. Note that I said \"maintenance\", because this might be a different entity than the owner due to agreements. Due to the number of complaints that the public agencies receive, it is very likely that your comment will get ignored. This often has nothing to do with whether or not your concern has merit. Public vs. Private Public agencies will be the easiest to contact. In a city, calling...", "B": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...", "C": "As others stated before, induction loops are the primary - most reliable method: the coils (usually just several loops of wire) embedded in the road; fed given frequency from a generator, in presence of metal the frequency of the LC circuit changes and the sensor circuitry detects the change of frequency, producing a presence signal. In some cases these may fail to detect bicycles, but they are by far most common as they aren't affected by weather (or more precisely, the detection circuit tunes in to slow changes of frequency caused by weather) and are immune to accidental false positives....", "D": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/392/how-does-a-traffic-light-sense-the-proximity-of-vehicles"}
{"id": "engineering_2295", "domain": "engineering", "question_title": "How many train passes can railway tracks endure?", "question_body": "I know that the rubber on car and truck tires wear, and the road concrete wears out. I wondered: While steel is hard and elastic, it still causes friction (interaction between molecules) and therefore abrasion. Let's say we have on average 20-30 wagons with four axles with a full load of 50-60 tons on each wagon and a traction vehicle. How many train passes can a railway rail endure before it must be replaced? What is the equivalent time frame in which this number of passes is achieved on average?", "question_score": 19, "question_tags": ["civil-engineering", "materials", "rail", "transportation"], "choices": {"A": "This is an admittedly North American response. MGT In the US, how much traffic goes over a given track in a year is measured in Million Gross Tons (MGT) e.g. 1 MGT = 2 000 000 000 lbs [spaces instead of commas to be world-friendly]. This is a measure of the total weight of cargo and vehicles but not necessarily the number of individual trains. Rail Life The life of a typical railroad rail is between 1300 MGT and 380 MGT depending on if the rail is on a straight (tangent) track or in a curve. There is more friction...", "B": "Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with...", "C": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...", "D": "It is a trench shield. It gets placed in a trench after the trench is dug to prevent workers from being hurt or killed in the event of a trench collapse. This picture from GMC trench shield shows a partially collapsed trench with a shield installed that would protect the workers installing the blue brute pipe."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2295/how-many-train-passes-can-railway-tracks-endure"}
{"id": "engineering_12984", "domain": "engineering", "question_title": "How should I position two shelf supports for the best distribution of load?", "question_body": "I'd like to mount a shelf on the wall. I have two shelf supports to do so, like this: Relative to the shelf, where do I place the (blue) supports to achieve the best distribution of load?", "question_score": 19, "question_tags": ["mechanical-engineering", "statics"], "choices": {"A": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...", "B": "Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste...", "C": "As with all good things, it depends. If you can assume that your supports are totally stiff and that the loading on the shelf will be approximately uniform, then you basically have the following structure: A rectangular cross-section (such as a plank) will behave equally under positive or negative bending moment, so your objective should be to balance both. To do so, you want your main span to be $2\\sqrt2 \\approx 2.83$ times the cantilevers. This is found by calculating the cantilever required to offset half of the bending moment due to a uniform load along a simply supported beam:...", "D": "The torsion constant $J_T$ relates the angle of twist to applied torque via the equation: $$ \\phi = \\frac{TL}{J_T G} $$ where $T$ is the applied torque, $L$ is the length of the member, $G$ is modulus of elasticity in shear, and $J_T$ is the torsional constant. The polar moment of inertia on the other hand, is a measure of the resistance of a cross section to torsion with invariant cross section and no significant warping . The case of a circular rod under torsion is special because of circular symmetry, which means that it does not warp and it's..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/12984/how-should-i-position-two-shelf-supports-for-the-best-distribution-of-load"}
{"id": "engineering_22848", "domain": "engineering", "question_title": "Why are truss bridges the way they are?", "question_body": "Just by taking a train ride across my home city I can see truss bridges like the one in above picture everywhere. There are numerous variations, but the most common design seems to be this. But why are they built specifically this way? I can intuitively kind-of see why such a design probably is strong, but is there any kind of in-depth reason? I would be interested to know the answer as much from the physics side of things as possible. Googling didn't help much; I could find information on the different variations and many examples, but none really covered what is it about this design specifically that makes it so popular.", "question_score": 19, "question_tags": ["structural-engineering", "civil-engineering"], "choices": {"A": "Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train .", "B": "That looks like a Pratt truss . These trusses have diagonals which go from the outer-top nodes to the inner-bottom nodes (i.e. they connect to the top chord on the node furthest from the center of the span, and to the bottom chord on the node closest to the center). This design means that the diagonals are under tension and the verticals are under compression. Another famous design is the Allan truss , which is the exact opposite: the diagonals go from the inner-top nodes to the outer-bottom nodes, which means that the diagonals are under compression and the verticals...", "C": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "D": "Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/22848/why-are-truss-bridges-the-way-they-are"}
{"id": "engineering_28610", "domain": "engineering", "question_title": "What is the purpose of building foundations?", "question_body": "According to many sources, one of the purposes of building's foundations is \"To distribute the weight of the structure over large area so as to avoid over-loading of the soil beneath.\" (Wikipedia) On the Wikipedia page on foundations, there is the following picture: So the foundation seems to consist of some type of \"legs\", with a larger surface area on the bottom. But if the point is to distribute the weight of the building over an area, why not put the bottom of the building in contact with ground? Isn't the bottom of the building itself a much larger contact area than the area the foundation can provide? In the picture, the bottom of the building does seem to rest at ground, so why do we require the foundation to have these large surface area rectangles at the bottom if the whole weight of the building is already resting on ground? Here is another picture from the same page: This seems to be some sort of lodge or a cabin, with stones used as foundation. Here it is evident that the building itself is raised above ground with the stones used as a contact to ground. But now the small stones suffer all the weight of the structure. Wouldn't they be under quite a heavy load over small contact area and possibly sink into the ground? Why not simply have the building's floor, a much larger contact area, rest on the ground?", "question_score": 19, "question_tags": ["structural-engineering", "foundations"], "choices": {"A": "That looks like a Pratt truss . These trusses have diagonals which go from the outer-top nodes to the inner-bottom nodes (i.e. they connect to the top chord on the node furthest from the center of the span, and to the bottom chord on the node closest to the center). This design means that the diagonals are under tension and the verticals are under compression. Another famous design is the Allan truss , which is the exact opposite: the diagonals go from the inner-top nodes to the outer-bottom nodes, which means that the diagonals are under compression and the verticals...", "B": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...", "C": "Down, Not Out Building foundations don't always have to spread the load out to a larger area. Sometimes the load only needs to be transferred to a stronger (harder) layer. This layer may be deep in the earth and have a softer layer on top of it. Layers Say you want to build a building in an area that has a lot of soft clay at the surface. This clay will not support much of anything (especially when it is wet). Underneath this layer is hard bedrock. You now have two choices: Add a basement to your building so that...", "D": "Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/28610/what-is-the-purpose-of-building-foundations"}
{"id": "engineering_14", "domain": "engineering", "question_title": "How are passive houses made in very hot regions (like Saudi Arabia)?", "question_body": "I think, here is the main problem the difference between the internal and the external temperature. For example, in Saudi Arabia, in 50 C, a passive house needed probably much sophisticated planning as in Paris. Compared to the traditional cooling systems, in the second case is enough only to get a cooling system with bigger power. I think, they are much more scalable. Is it anyways possible?", "question_score": 18, "question_tags": ["civil-engineering", "architecture", "cooling"], "choices": {"A": "If I model this as a simply supported beam having load at mid span [...] I suspect that this is where your analysis went awry. First off, you should always model bridges with distributed loads, not a single concentrated load at midspan. The most significant load on a bridge will almost always be its own self-weight; load-trains are heavy but, well, so are bridges. Secondly, I assume you're thinking of the bridge like this: Indeed, we can see here that the bending moment is greater at midspan. However, that's not the bridge we're looking at, it's missing the cantilevers! So...", "B": "If the receiver does not detect the sub-carrier for the \"colour burst\" signal which is transmitted during the horizontal blanking period the receiver switches on the \"colour-killer\" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise. Figure 1. The colorburst signal is transmitted on the \"back porch\" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately...", "C": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...", "D": "A yahkchal is an example of a type of passively cooled building in Iran They utilise a combination of passive evaporative cooling and thick thermally insulating walls in order to keep the interior temperatures low enough. First, wind is directed into underground aquifers known as qanat . They are then cooled due to the low humidity desert air causing water to evaporate. The cooled air then flows through the interior of the yakhchal, cooling the interior. The thick insulating walls (filled with earth and various insulating materials such as straw and feathers) help to insulate the cool interior from the..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/14/how-are-passive-houses-made-in-very-hot-regions-like-saudi-arabia"}
{"id": "engineering_137", "domain": "engineering", "question_title": "How does measurement uncertainty combine with tolerances?", "question_body": "Given a tolerance within which your workpiece should be manufactured, say some length should be $10\\pm1$mm. If you determine that your uncertainty in measuring this length is $0.2$mm (at 95%). How should a measurement of $9.1$mm be treated? Clearly there is a significant probability that this value will actually be outside of tolerance. Do you need to decrease you tolerance range based on the uncertainty in your measurement?", "question_score": 18, "question_tags": ["tolerance", "measurements", "statistics"], "choices": {"A": "You need to ensure that even in the worst case scenario, you still meet your measurement spec of $10 \\pm 1\\text{mm}$. If your tolerance is $0.2\\text{mm}$ of your measurement, then a measurement of $11\\text{mm}$, while may look like it meets spec, it doesn't because it could be $11.1\\text{mm}$. So the worst case that still meets your spec is a measurement of $10.9\\text{mm}$, because then with a max tolerance of $0.2\\text{mm}$, you still meet $11\\text{mm}$. With a $0.2\\text{mm}$ tolerance, your $10 \\pm 1\\text{mm}$ spec becomes $10 \\pm 0.9\\text{mm}$. How should a measurement of $9.9\\text{mm}$ be treated? So revised spec is between...", "B": "Answering the question: What are possible types of low cost sensors I can use? There are several types of sensors that can provide millimeter level accuracy. \"Low cost\" is a very relative term, so you'll need to do some shopping around based on your specific budget. Optical sensors- Included here are those of the type you listed, though it's a very cheap sensor meant more to provide a \"yeah something is in front of me, about yay-far-away\". There are also laser sensor systems which can provide millimeter level accuracy. Ultrasonic Sensors- Most ultrasonic sensors have relatively low accuracy; in the...", "C": "Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book \" Wind Energy - The Facts \". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...", "D": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/137/how-does-measurement-uncertainty-combine-with-tolerances"}
{"id": "engineering_6230", "domain": "engineering", "question_title": "Can wifi signal reception be improved by opening a door?", "question_body": "A wifi user is in a different room than the router. The computer is having a hard time connecting and receiving the wifi signal. Can the wifi signal from the router to the computer be improved by opening a door to the room where the computer is?", "question_score": 18, "question_tags": ["signal", "wifi"], "choices": {"A": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "B": "As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\\arctan\\left(\\frac{0.15}{15}\\right)=0.57^\\circ$ or 1%. the middle ,...", "C": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "D": "Can the wifi signal from the router to the computer be improved by opening a door to the room where the computer is? Maybe, but probably not to a noticeable degree. All structures, including doors, impede the wireless signal from the router by some amount. Generally, the amount of impedance added by the door is a negligible amount and would not be sufficient to noticeably improve the quality of the signal. That said, differing types of door construction have differing impacts on the signal. A hollow core, wooden door won't impede the signal all that much at 4 dB. A..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/6230/can-wifi-signal-reception-be-improved-by-opening-a-door"}
{"id": "engineering_22072", "domain": "engineering", "question_title": "Fracture patterns in tempered glass", "question_body": "While browsing a local news site, I stumbled upon this picture of a broken bus door. The glass was fractured due to a rock impact. As far as i know there are no heating elements on the window section in the picture. Now, what interests me is the apparent periodicity and structure of the fracture pattern. Are periodic fractures something commonly encountered in tempered glass? Is the mechanism that generates such fractures understood? Is it a consequence of the manufacturing process? Perhaps the way the glass is affixed to the door? Thanks in advance", "question_score": 18, "question_tags": ["glass"], "choices": {"A": "It's an artifact of production - in particular, the construction of the conveyor belt of the machine performing the quenching process. The pattern can be observed through a polarizing filter in the undamaged glass: source and it's a result of the structure of the conveyor belt through which the surface of the glass is cooled to generate the stress that gives it the special properties: source The contact area of the glass with the conveyor is small enough that it doesn't negatively impact the process - all throughout the surface the stress is introduced in sufficient amount, but the amount...", "B": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...", "C": "Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book \" Wind Energy - The Facts \". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...", "D": "Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/22072/fracture-patterns-in-tempered-glass"}
{"id": "engineering_35788", "domain": "engineering", "question_title": "Why do oil tankers heat crude oil?", "question_body": "About all those oil tankers off the coast of California … | Grist The giant ships burn fuel to keep lights on, power equipment, and heat the large volumes of crude oil resting in their tanks. I'm assuming that crude oil can't be heated in steel frac tanks , places other than salt caverns , and salt caverns . While underground caverns may not seem like the best place to store an emergency oil supply, they're actually very secure. For one thing, since they're 2,000 to 4,000 feet (610 to 1,219 meters) underground, the extreme pressure prevents cracks from forming and leading to leaks [source: DOE ]. Also, the natural temperature difference between the top and bottom of each cavern encourages the oil to circulate, which maintains its quality.", "question_score": 18, "question_tags": ["petroleum-engineering"], "choices": {"A": "As Solar Mike's answer says, crude oil is viscous - too viscous to easily pump. Crude oil has a \"pour point:\" the lowest temperature where it will flow under gravity. Heating the crude oil keeps it above the pour point, so it can be pumped. With the large volume of oil in a tanker, it makes more sense to keep it fluid, rather than letting it cool down and then heat it (very slowly) back up). I found one article (PDF) that recommends tankers keep the oil at least 10°C above the pour point to promote circulation within the tank,...", "B": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...", "C": "Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train .", "D": "Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book \" Wind Energy - The Facts \". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/35788/why-do-oil-tankers-heat-crude-oil"}
{"id": "engineering_46922", "domain": "engineering", "question_title": "Why do heavy trucks use air brakes?", "question_body": "I was having a discussion today, which led to the question why do heavy trucks still use air brakes? To my knowledge, it has been used for at least 40 years (I remember that as a kid), and apparently (I was told today but I haven't gotten around to verify it) they are still widely used. From what I remember, one of the things I was cautioned was that if you repeatedly pressed on the air brakes, then after a while the air buffer would empty and it would take time to fill up (thus losing braking capacity). Anyway, I wanted to know what are the benefits and disadvantages of air brakes compared to other technologies, e.g. hydraulic lines, or electrical system, (even KERS systems for more modern electrical vehicles). UPDATE : From the answers I understand that the main issue is reliability and \"technical debt\". I want to push a bit further and understand, what's stopping air brakes from being used in other vehicles. E.g. is it cost, performance, inability to accompany AC/DC drum rhythm?", "question_score": 18, "question_tags": ["mechanical-engineering", "automotive-engineering", "braking"], "choices": {"A": "The turbulence model can make a big difference in your simulation . There are many turbulence models around. It becomes a tough job to select one out of them. There is no perfect turbulence model. It all depends on several parameters like Reynold's number, whether the flow is separated, pressure gradients, boundary layer thikness and so on. In this answer, brief information about a few popular models is given along with pros and cons and potential applications. However, interested users can see this excellent NASA website and references therein to know more about turbulence modeling. A) ONE EQUATION MODEL: 1....", "B": "Short answer : make it thicker. Long answer : The moment of inertia affects the beam's ability to resist flexing. Use one of the many, free, online moment of inertia calculators (like this one ) to see how increasing the height of the beam will have an exponential effect on increasing the stiffness of the beam. And this site helps provide a pictorial view of the load(s) upon a beam depending upon differing configurations, such as where the supports are and where the load is applied. It also provides a calculator to determine the forces involved. Wikipedia has a decent...", "C": "Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with...", "D": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/46922/why-do-heavy-trucks-use-air-brakes"}
{"id": "engineering_48467", "domain": "engineering", "question_title": "How did mechanical engineers work before SolidWorks?", "question_body": "I am studying a bunch of stuff related to mechanical engineering and am considering an eventual switch to the field after taking the necessary classes. I'm older, and I took drafting classes in high school and part of college. I'm familiar with the standard drafting procedure with paper, as well as AutoCAD , and these days SolidWorks . It's a shame they don't teach paper drafting much anymore! But I am left with a question I don't have the answer to, nor the resources to answer it. These days it seems most mechanical engineers (at least the ones I know) are really performing the role of both the drafter and the engineer. Prior to 1995 when SolidWorks came about, what was the duty of the mechanical engineer? If we go even further back before CAD was invented, what duties did a mechanical engineer perform?", "question_score": 18, "question_tags": ["cad", "drafting", "software", "engineering-history"], "choices": {"A": "Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....", "B": "Meet Vladimir Shukhov , a Russian architect who first developed hyperboloid structures. He was born in 1853, died in 1939, and created over 200 hyperboloid structures in the intervening years. He was the reason hyperboloids gained the popularity that they did. His first design, the first hyperbolic structure ever, was the Shukhov Tower in Polibino , pictured here : Another tower also bears the name of Shukhov, and it achieved great fame, too. Shukhov also built the Adziogol Lighthouse . In total, Shukhov designed and built 200 hyperboloid structures. He died in 1939, which could one reason for the decline...", "C": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...", "D": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/48467/how-did-mechanical-engineers-work-before-solidworks"}
{"id": "engineering_3324", "domain": "engineering", "question_title": "Why is it more reliable to use the land/pit transition in a CD-ROM?", "question_body": "I was reading about how CD-ROMs work and I came to this: Although it might seem simplest to use a pit to record a 0 and a land to record a 1, it is more reliable to use a pit/land or land/pit transition for a 1 and its absence as a 0,so this scheme is used. Now, I'd like to know why?", "question_score": 17, "question_tags": ["optics", "computer", "computer-engineering"], "choices": {"A": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...", "B": "Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train .", "C": "I'm going to set aside the potential legal liability aspects of your question. In part because you didn't ask about them, but also because liability will vary based upon jurisdiction. Obviously, consult an attorney familiar with the relevant law. Ethically, I think you've done everything that you're obligated to do. You have contacted the customer and notified them of an unsafe situation or configuration of the product. And you have (strongly) advised them that they need to discontinue using the older product in the unsafe configuration. If the customer remains insistent about using the product in an unsafe manner, you...", "D": "To elaborate on ratchet freak's answer; CD-ROMs work by sensing the intensity of the light reflected from the CD as it is spinning. More light being reflected stands for a 1 and less light being reflected stands for a 0 (or vice versa). One way to encode the information would be to have highly reflective surfaces for the 1's and dark patches for the 0's. Printing dark patches at the size of the laser spot size is actually used in writable CDs , but the technique suffers from degradation over time. Instead commercial CDs rely on the property of interference..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/3324/why-is-it-more-reliable-to-use-the-land-pit-transition-in-a-cd-rom"}
{"id": "engineering_5325", "domain": "engineering", "question_title": "If aircraft are pressurised, why do our ears pop during liftoff and landing?", "question_body": "Something hit me during my last flight: our ears feel that the pressure around us changes quickly during liftoff and landing, they hurt more and more until we make them pop. However, the cabin must maintain a level of oxygen higher than outside the aircraft because it isn't dense enough at ~10km to breathe normally. Does that mean that the aircraft merely takes air from the outside, heats it up (it's around -40°C outside at 10km) and adds oxygen to it before blowing it inside? How else?", "question_score": 17, "question_tags": ["pressure", "aircraft-design"], "choices": {"A": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "B": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...", "C": "Because the cabin isn't pressurized to sea level pressure instead it to about 8k ft equivalent. (while the plane is 4.5 times higher) This means there is less differential pressure than if the cabin was pressurized to sea level pressure. But it's still within the limit of what passengers feel comfortable with. This answer on aviation.SE contains a plot of cabin pressure over time during a flight: Source The composition of air doesn't change with altitude just the pressure. This means that you only need to compress the air before blowing it in.", "D": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/5325/if-aircraft-are-pressurised-why-do-our-ears-pop-during-liftoff-and-landing"}
{"id": "engineering_17118", "domain": "engineering", "question_title": "Does an electric vehicle going downhill recover energy?", "question_body": "I have to model the behavior of an electric car. For this, I use these equations and I can observe by \"playing\" with parameters that, when going downhill at constant speed, the car has a negative consumption (i.e. recovers energy). I was wondering if this is realistic?", "question_score": 17, "question_tags": ["mechanical-engineering", "electric-vehicles", "regenerative-braking"], "choices": {"A": "Train Brakes The common brakes on trains are air brakes . As the name implies, these work off of air pressure. The braking power isn't controlled in the way that you would immediately think of though. They do not work like car brakes where the harder you press on the brake pedal, the harder the pressure goes through the lines to the brake cylinders. They work the opposite. The less pressure in the line, the more braking force is applied. Fail-safe Rail brakes are designed to be fail-safe . That is, when a failure occurs, the safe operation happens. In...", "B": "If I model this as a simply supported beam having load at mid span [...] I suspect that this is where your analysis went awry. First off, you should always model bridges with distributed loads, not a single concentrated load at midspan. The most significant load on a bridge will almost always be its own self-weight; load-trains are heavy but, well, so are bridges. Secondly, I assume you're thinking of the bridge like this: Indeed, we can see here that the bending moment is greater at midspan. However, that's not the bridge we're looking at, it's missing the cantilevers! So...", "C": "It depends on how steep the hill is. On a slight hill, the energy added by gravity is still not enough to overcome rolling friction and air resistance, so the car still needs power to maintain speed. On a steeper hill, the two may balance out, so no power is used, and no power is generated. On a hill that's steep enough to require braking to control the speed, the car recovers energy. It's called regenerative braking. If the car is going too fast, applying the brakes turns the motor into a generator and charges the battery.", "D": "You have the right concept, but slipped a decimal point. 5 cm = 0.05 m. The gravitational force on your 450 g mass is 4.4 N as you say, so the torque just to keep up with gravity is (4.4 N)(0.05 m) = 0.22 Nm. However, that is the absolute minimum torque just to keep the system in steady state. It leaves nothing for actually accellerating the mass and for overcoming the inevitable friction. To get the real torque required, you have to specify how fast you want to be able to accellerate this mass upwards. For example, let's say..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/17118/does-an-electric-vehicle-going-downhill-recover-energy"}
{"id": "engineering_18806", "domain": "engineering", "question_title": "What limits the speed of a car?", "question_body": "From an engineering perspective, what limits the maximum speed you can reach with a regular car? I understand that some of the faster cars are for safety reasons limited to not run faster than say 250 km/h, but that's not my question. I can think of several reasons, but not sure which of these is relevant: Is the limit set by some part (which?) breaking if I increase the rpm, as suggested by red marks on rpm meters? Or is it rather that you cannot get in fuel fast enough to keep increasing the rpm? Or is it that friction/drag increases as you speed up and the engine cannot overcome this as it can only generate a maximum amount of force/torque? If yes, what does this amount of torque/force depend on?", "question_score": 17, "question_tags": ["mechanical-engineering", "car"], "choices": {"A": "Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....", "B": "It depends on how steep the hill is. On a slight hill, the energy added by gravity is still not enough to overcome rolling friction and air resistance, so the car still needs power to maintain speed. On a steeper hill, the two may balance out, so no power is used, and no power is generated. On a hill that's steep enough to require braking to control the speed, the car recovers energy. It's called regenerative braking. If the car is going too fast, applying the brakes turns the motor into a generator and charges the battery.", "C": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...", "D": "There are a few simple reasons why the speed of a vehicle (road conditions notwithstanding) may be limited: Gearing -- Production vehicles with conventional transmissions have a limited number of gears. For most modern cars, this is usually 5 or 6, whereas older vehicles may have as few as 2 or 3. If the gear ratio of the highest gear is too low (\"lower\" gears are expressed as larger numerical ratios), it's entirely possible that the engine will redline before air resistance becomes a factor at all. This ties into your first point about the red zone on the tachometer...."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/18806/what-limits-the-speed-of-a-car"}
{"id": "engineering_51347", "domain": "engineering", "question_title": "Why is this bridge thickest above the support pillars instead of the mid-span where the bending stress is highest?", "question_body": "Why is a bridge designed like this? The depth of the section at pillars is more than the depth at middle. If I model this as a simply supported beam having load at mid span then the bending moment will be maximized at the middle and the area is also less at the middle. So, this will lead to higher bending stress. So, why is it designed like that?", "question_score": 17, "question_tags": ["mechanical-engineering", "structural-engineering", "civil-engineering", "beam", "bridges"], "choices": {"A": "A specific reason for doing this is simply when there isn't enough room to do a bow or stern first launch. This is often the case when a ship or boat is built in a yard on a river or canal either because the hull is especially long or the channel it is being launched into is narrow. There is also the consideration that a sideways launch can be done from any quay that will take the weight but a bow first launch requires a specially constructed slipway. There is also the consideration that if you go in sideways the...", "B": "If I model this as a simply supported beam having load at mid span [...] I suspect that this is where your analysis went awry. First off, you should always model bridges with distributed loads, not a single concentrated load at midspan. The most significant load on a bridge will almost always be its own self-weight; load-trains are heavy but, well, so are bridges. Secondly, I assume you're thinking of the bridge like this: Indeed, we can see here that the bending moment is greater at midspan. However, that's not the bridge we're looking at, it's missing the cantilevers! So...", "C": "Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....", "D": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/51347/why-is-this-bridge-thickest-above-the-support-pillars-instead-of-the-mid-span-wh"}
{"id": "engineering_193", "domain": "engineering", "question_title": "What is the most efficient means of warming a building with a high ceiling?", "question_body": "Consider a large auditorium, a church, or some other very large, essentially one roomed building with a high ceiling. Suppose that the building has many entrances which enable cold air in/hot air out and traffic in and out of the building is unavoidably large. I would imagine that any attempts to control the temperature of such a large building would be very inefficient in terms of energy and cost, particularly because warm air rises. Assuming that it's very cold outside and we're interested primarily in keeping the building warm at the ground level so that it is comfortable for humans to work and interact, what is the best method to keep such a large, essentially one roomed building with a high ceiling warm when outdoor cold air exposure is frequent and unavoidable? When I say \"best,\" I'm interested in balancing energy, maintenance, and monetary costs over the life of the building.", "question_score": 16, "question_tags": ["civil-engineering", "building-physics", "energy", "hvac"], "choices": {"A": "There are a few main reasons why suspension bridges aren't used for railroads. The main reason is that suspension bridges are typically used where very long spans are needed. Trains are very heavy, especially when compared to lanes of highway traffic. This means that long spans require very strong support structures, which in the case of suspension bridges are cables and towers. The second reason goes along with the first; trains cause high dynamic loads as they move along the rail. This can increase the vertical loads by 30%. Third is that trains don't really have suspensions, especially freight trains....", "B": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...", "C": "Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....", "D": "Your question sort of has two parts: How to supply heat, and how to keep it in. Large open rooms with a high ceilings are most efficiently warmed with radiant ceiling heat. Warm air rises, which renders forced-air systems inefficient because the pumped heat ends up at the ceiling and the coldest part of the room is near the floor where you actually want the heat. Radiant floor systems are limited to about 87F because they are in contact with occupants, and so their peak output may not be enough to keep the space comfortable. They also lose more heat..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/193/what-is-the-most-efficient-means-of-warming-a-building-with-a-high-ceiling"}
{"id": "engineering_2859", "domain": "engineering", "question_title": "While going downhill, does my car consume less fuel when in neutral or in gear?", "question_body": "I once meditated on the fact that a car, in downhill, would consume more fuel when in neutral to keep the engine on, rather than in gear. To keep the engine on when in gear, in fact, you mostly need the gravity and enough thrust to keep it rotating, and therefore no fuel (just motor oil). This is what I'd expect cars to do but I'm probably missing something. How's actually the story? And if it's not like this, would/could my idea work somehow? (note: patent pending :P)", "question_score": 16, "question_tags": ["automotive-engineering", "fuel-economy"], "choices": {"A": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only...", "B": "Note: This is specific to gasoline based engines. Assuming your car was produced after the ~1990's it is equipped with a Deceleration Fuel Cut-Off (DFCO) system which cuts the fuel flow to the engine when the gas pedal is not depressed and the car is in gear. In this case it is more fuel efficient to stay in gear because you are letting gravity run the engine instead of fuel. In addition, it saves wear on your brakes because the partial-vacuum created in the engine leads to engine-braking . In many states in the US it is actually required that...", "C": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "D": "Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region...."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2859/while-going-downhill-does-my-car-consume-less-fuel-when-in-neutral-or-in-gear"}
{"id": "engineering_464", "domain": "engineering", "question_title": "Why are hyperboloid towers not popular anymore?", "question_body": "Hyperboloid towers were very popular in the end of 19th and the first half of 20th centuries - water towers, powerline anchor towers, sometimes tall radio towers were built using this design. The claimed advantage is using less steel compared to other designs for the same strength. They are very rarely used nowadays (to such extent that old hyperboloid towers are treated as cultural heritage objects and protected by the state in some countries). Why did they lose popularity? Is there any inherent defect it the design? Is steel not expensive anymore?", "question_score": 15, "question_tags": ["steel", "structures", "engineering-history"], "choices": {"A": "If the receiver does not detect the sub-carrier for the \"colour burst\" signal which is transmitted during the horizontal blanking period the receiver switches on the \"colour-killer\" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise. Figure 1. The colorburst signal is transmitted on the \"back porch\" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately...", "B": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...", "C": "Nothing is rigid, the raceways and the ball in a ball-bearing are no exception. The contact area deflects and accommodates the ball in a small contact surface, not a point. Also, the balls take the load in groups. The digrams are from SKF ball-bearings. '", "D": "Meet Vladimir Shukhov , a Russian architect who first developed hyperboloid structures. He was born in 1853, died in 1939, and created over 200 hyperboloid structures in the intervening years. He was the reason hyperboloids gained the popularity that they did. His first design, the first hyperbolic structure ever, was the Shukhov Tower in Polibino , pictured here : Another tower also bears the name of Shukhov, and it achieved great fame, too. Shukhov also built the Adziogol Lighthouse . In total, Shukhov designed and built 200 hyperboloid structures. He died in 1939, which could one reason for the decline..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/464/why-are-hyperboloid-towers-not-popular-anymore"}
{"id": "engineering_1864", "domain": "engineering", "question_title": "How should the public raise questions about unsafe structures in the United States?", "question_body": "In a program on NPR that I was listening to, there was a bit about a bridge that from the description sounded to a layman as unsound and is still in use. The program described it as an old wooden railway (and I'm aware that has its own set of challenges) bridge with rotting timbers. In the United Sates, if a member of the public sees a bridge (railway, tramway, car/truck, foot, bike, etc...) that is of questionable soundness, what is the process for him or her to determine who us responsible for it, and if they (the responsible parties) should look at it and have it evaluated? Is there a particular agency that is responsible for regulating bridges and ascertaining their safety?", "question_score": 15, "question_tags": ["civil-engineering", "bridges", "regulations"], "choices": {"A": "I can understand your concern. This is going to be difficult for two reasons: Finding out the agency that is responsible for the maintenance may be next to impossible for a layperson. Note that I said \"maintenance\", because this might be a different entity than the owner due to agreements. Due to the number of complaints that the public agencies receive, it is very likely that your comment will get ignored. This often has nothing to do with whether or not your concern has merit. Public vs. Private Public agencies will be the easiest to contact. In a city, calling...", "B": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "C": "Nothing is rigid, the raceways and the ball in a ball-bearing are no exception. The contact area deflects and accommodates the ball in a small contact surface, not a point. Also, the balls take the load in groups. The digrams are from SKF ball-bearings. '", "D": "As others stated before, induction loops are the primary - most reliable method: the coils (usually just several loops of wire) embedded in the road; fed given frequency from a generator, in presence of metal the frequency of the LC circuit changes and the sensor circuitry detects the change of frequency, producing a presence signal. In some cases these may fail to detect bicycles, but they are by far most common as they aren't affected by weather (or more precisely, the detection circuit tunes in to slow changes of frequency caused by weather) and are immune to accidental false positives...."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/1864/how-should-the-public-raise-questions-about-unsafe-structures-in-the-united-stat"}
{"id": "engineering_1908", "domain": "engineering", "question_title": "Purpose of spheres and fins on submarine propeller", "question_body": "On a recently launched Russian diesel-electric submarine, the rear propeller has two distinct features. You can see spheres at the base of every propeller blade: Also, the trailing edge of the shaft has four adjoining fins in line with the axis of rotation: Most importantly, what is the purpose and function of the spheres, and what's beneath the veil in the first image? I believe that it may differ from the second image. It's made of a different material, so possibly a sacrificial element. Is it to reduce noise? Is it to smooth flow or prevent cavitation? Notice that in the first image the spheres are at the leading edge and in the second image the spheres are at the trailing edge. There is a related youtube video .", "question_score": 15, "question_tags": ["mechanical-engineering", "fluid-mechanics", "propulsion", "marine-engineering", "naval-engineering"], "choices": {"A": "From the point of view of the driver of a car, impacting another car is about as bad as crashing against an ideal wall (a wall with zero deformation whatsoever). If there were a plane reflection between the two cars, then vs. Car would be exactly equal to vs. Wall (the contact points between both cars would all be on the same plane, due to reflection, so each car could be considered a wall for the other). But this plane reflection does not exist: What we have instead is a 2-fold rotational reflection . Let's say the left part of...", "B": "I will expand on DKNguyen answer, because to my knowledge also the two reasons are: reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation) change the joint tightening characteristics (see joint diagram). reduce contact stresses on surfaces. The basic idea is that since contact stress is defined as: $$\\sigma = \\frac{F}{A}$$ Obviously the larger the area the less the stress. Also, like Nuclear Hoagie pointed out the area changes with a square law of the diameter. UPDATE 2 - Calculation for M6 bolt (thanks to BenC) *The question gathered enough interest for me to carry out a...", "C": "In the video you've linked to, the spheres are seen on both the leading and trailing edges of the propeller: I expect they are intentional - there are a number of ways to attach a propeller without having to disturb that surface, or using flush caps. Cavitation is caused by a drop in pressure. This would be seen on the trailing edges of the propeller, and is worse on the outer edge of the propeller which is moving faster through water than the inner area. I doubt these spheres have any effect on cavitation at all. It is either a...", "D": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/1908/purpose-of-spheres-and-fins-on-submarine-propeller"}
{"id": "engineering_2589", "domain": "engineering", "question_title": "How do I size metal plates to get the correct dimensions after folding?", "question_body": "I am designing a metal plate that will be laser-cut (or machine-cut) and then folded. I want to know how to size the pre-folded plate in order to get the right dimensions after folding. My actual part is not exactly like this (I simplified it for ease of drawing) but it shows what I want to achieve. In this case it's a 2mm aluminium plate, the red arrows show the inner dimensions after folding that I want to specify and achieve. The holes must also line up and the window should be correctly placed. Intuitively I would expect some compression along the inner part of the folds and stretching on the outer parts - ideally along the centre of the plate - but I don't know if this is what will happen. Assuming the red arrows are 100mm each, should the plate be 300mm? I'm guessing not, so how do I calculate the radius of curvature that will be achieved and if I need to add (or remove) material at the folds in order to achieve my required dimensions?", "question_score": 15, "question_tags": ["machining", "metal-folding"], "choices": {"A": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "B": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...", "C": "Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train .", "D": "Your assumption is right! A 300mm long plate with two foldings won't do! This is because you need to take into account the bend allowance and the bend compensation! But why is so? Here is a diagram of what's going on: When you bend a material, part of it will extend (the external part of the bend), while another part will retract (the internal part). The line (in the thickness of the plate) where the dimension doesn't change is called the Neutral line. The neutral line is usually located between a third and a half of the material thickness (from..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2589/how-do-i-size-metal-plates-to-get-the-correct-dimensions-after-folding"}
{"id": "engineering_2695", "domain": "engineering", "question_title": "Can I use a ultrasound sensor to measure water level?", "question_body": "If I use an ultrasonic sensor will it detect the water level? I was thinking about a product to read water level on water boxes (common in Brazil). I researched about instrumentation for this measure, and I think that an ultrasonic sensor is the best option. Will the water correctly reflect ultrasound and not change the normal measurements against a solid obstacle?", "question_score": 15, "question_tags": ["electrical-engineering", "measurements", "sensors", "control-engineering"], "choices": {"A": "Technology used in depth finder, in marine application is mostly like is the best to measure the water from the top. To measure water level from the bottom of tank one could use a piezo electric ceramic transducer combined with an Analog Front End (AFE) and a micro-controller to measure water level. The diagram below best explains the configuration. You can use a piezo electric ceramic transducer from Steminc, TDC1000 AFE from Texas Instrument and a MSP430 micro-controller also from Texas Instrument. There might be other configurations, but currently I am only aware of this configuration. Piezo Electric Transducer Analog...", "B": "From the point of view of the driver of a car, impacting another car is about as bad as crashing against an ideal wall (a wall with zero deformation whatsoever). If there were a plane reflection between the two cars, then vs. Car would be exactly equal to vs. Wall (the contact points between both cars would all be on the same plane, due to reflection, so each car could be considered a wall for the other). But this plane reflection does not exist: What we have instead is a 2-fold rotational reflection . Let's say the left part of...", "C": "It depends on how steep the hill is. On a slight hill, the energy added by gravity is still not enough to overcome rolling friction and air resistance, so the car still needs power to maintain speed. On a steeper hill, the two may balance out, so no power is used, and no power is generated. On a hill that's steep enough to require braking to control the speed, the car recovers energy. It's called regenerative braking. If the car is going too fast, applying the brakes turns the motor into a generator and charges the battery.", "D": "As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\\arctan\\left(\\frac{0.15}{15}\\right)=0.57^\\circ$ or 1%. the middle ,..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2695/can-i-use-a-ultrasound-sensor-to-measure-water-level"}
{"id": "engineering_6020", "domain": "engineering", "question_title": "Why do we even use engineering stress?", "question_body": "Surprisingly this hasn't been asked before, so I must be missing something simple. We use engineering stress and engineering strain in this eq. Stress = (Young's modulus) × (strain). This eq. is used in analysis of bending beams, twisting shafts and in buckling. So the final equation of bending $(\\frac{M}{I} = \\frac{\\sigma}{y})$ and torsion $(\\frac{T}{I} = \\frac{\\tau}{r})$ will give us value of engineering stress but not the value of stress. Why are we considering engineering stress instead of true stress while we know it will not give correct value of stress? Some things I read are: Difficult to measure. Not that much of a difference and we can just apply a Factor of Safety. \"We don't consider materials to change their cross-sectional area after loading, since we design to have no plastic deformation the elastic region is most important, therefore what happens after the proportional limit is not important\" Firstly, 1 and 2 are not real reasons for me. Number 3 seems plausible since we always design in the elastic region, but is this it? Does engineering strain even give valid information after the proportional limit?", "question_score": 15, "question_tags": ["materials", "structural-engineering", "stresses"], "choices": {"A": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "B": "Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book \" Wind Energy - The Facts \". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...", "C": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "D": "We use engineering strain even though it is not the \"correct\" value because in most cases, specifically in the elastic regime, engineering strain differs negligibly from true strain. For linear elastic, Hookean materials, it is generally the case strain at the elastic limit is very small. Even the strongest steels, for example, have an upper limit when cold worked of about $\\sigma_{\\textrm{el}}=1\\times 10^{9}\\ \\textrm{Pa}$. The modulus of steel is approximately $E=200\\times 10^{9}\\ \\textrm{Pa}$. Thus $\\varepsilon_{\\textrm{el}}=0.005=0.5\\%$ for the strongest steels. So at the onset of plastic deformation, engineering strain is $0.5\\%$. Many useful elastic materials have much lower engineering strain at..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/6020/why-do-we-even-use-engineering-stress"}
{"id": "engineering_33530", "domain": "engineering", "question_title": "What is this large steel structure used in excavation?", "question_body": "I keep seeing these large rectangular structures on job sites they put in the ground. I'm guessing something for strengthening the surface structures?", "question_score": 15, "question_tags": ["civil-engineering"], "choices": {"A": "Parked vehicles vs moving vehicles Closely spaced parked (or slow moving) vehicles are definitely more onerous, as stated on page 89, Appendix 2.A, Clause 2.A.1 of the South African bridge design code TMH7 : It is generally accepted and can readily be shown that except in the very small span range, the worst loading condition occurs under congested (bumper to bumper) conditions caused by a traffic blockage and that the dispersion of traffic at speed caused by increased vehicular inter-spacing, more than off-sets the effects of impact. However, this is only true by inspection for an unlimited number of vehicles...", "B": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...", "C": "It is a trench shield. It gets placed in a trench after the trench is dug to prevent workers from being hurt or killed in the event of a trench collapse. This picture from GMC trench shield shows a partially collapsed trench with a shield installed that would protect the workers installing the blue brute pipe.", "D": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/33530/what-is-this-large-steel-structure-used-in-excavation"}
{"id": "engineering_43112", "domain": "engineering", "question_title": "What unit of weight is that?", "question_body": "Recently I was weighting stuff with my dad and as I was playing with one of the old 2 kg weights I felt with my fingers number \"5\". After cleaning and upon further inspection it turned out the writing on the weight says \"5Φ\". We checked and it indeed weights 2 kg as my dad remembered. It was done with a simple balance scale by comparing with another weight of similar age that is properly marked as \"2 kg\". There are no other marks on that weight. On the opposite side is a bulge but it's an irregular blob - I think it's remainder of sprue. So, question that arised in my head is - what does that Φ mean? Is it some rare unit of mass? Some sort of preferred series or standard for weights? I was looking for answers and found nothing. I ruled out pounds that was first thing that came to my mind because the difference was to big (5 pounds is approx. 2.26 kg) - though there are some signs of wear and corrosion it's in rather good shape and I don't believe it weighted 0.26 kg more originally. Also, considering I'm from Poland and the weight is really old it probably was made during the soviet era (maybe even in USSR) - so that phi could be actually cyrilic fe - but that also gave me no results. As a last resort I decided to ask here - maybe some member of that community will know the answer. Below is a photo of the weight in question.", "question_score": 15, "question_tags": ["measurements", "unit"], "choices": {"A": "As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\\arctan\\left(\\frac{0.15}{15}\\right)=0.57^\\circ$ or 1%. the middle ,...", "B": "You are probably right (it being of Russian origin). The weight seems to be 5 funt. 1 funt is the Russian equivalent of a pound. It is written as Фунт, funt, and around 1900 it was the basic unit of weight measurement in Russia (so it survived in the USSR days), but now its obsolete. 1 Φ is about 409.5 grams.", "C": "Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with...", "D": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/43112/what-unit-of-weight-is-that"}
{"id": "engineering_150", "domain": "engineering", "question_title": "Do solar cells age? Why, and how?", "question_body": "As I heard, the life expectancy of photovoltaic cells is normally some decades, as so. This is very long, regarding their high cost, it has significant (negative) effect on the total costs. Why do they age? I can see the only possibility which could damage their atomic structure, and this is the very high energetic ultraviolet (or even soft röntgen) photon spectrum of the sun (which is only a small part of its power). And, I think, these photons could be maybe easily filtered out by a transparent plastic layer over the silicon. Thus, why do the solar cells age, and what happens in them on the atomic level?", "question_score": 14, "question_tags": ["photovoltaics"], "choices": {"A": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...", "B": "The National Renewable Energy Lab (NREL) estimates something like 0.4%-1% degradation per year for solar panels. I don't know what the chemical action is here (the electrical result being an increase in series resistance of the PV cells), but the long-term degradation of the panels is correlated with UV exposure. Although damaging to the PV material, the UV rays do contain energy that we want to collect, so you won't be able to filter it out completely without defeating the purpose of the panel. Other causes for degradation are mostly weather-related, but in the form of mechanical stresses to the...", "C": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...", "D": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/150/do-solar-cells-age-why-and-how"}
{"id": "engineering_165", "domain": "engineering", "question_title": "Are pontoon bridges being considered to extend bridge span?", "question_body": "Pontoon bridges differ from traditional bridges in that they are supported not by structures anchored to the floor of the body being spanned but by floating pontoons that are connected by a more rigid structure that supports a roadway. They're often used by militaries to provide a temporary crossing point, but they're also used for permanent civilian crossings . I would assume that they make it easier to cross larger bodies of water because there is less structure to be secured below the surface. In areas of deep water, support structures can become unfeasibly large. These larger spans could, though, make the pontoon bridges susceptible to damage from strong winds and currents. Are there plans to use pontoon bridges to cross long distances?", "question_score": 14, "question_tags": ["civil-engineering", "bridges"], "choices": {"A": "It is a trench shield. It gets placed in a trench after the trench is dug to prevent workers from being hurt or killed in the event of a trench collapse. This picture from GMC trench shield shows a partially collapsed trench with a shield installed that would protect the workers installing the blue brute pipe.", "B": "Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book \" Wind Energy - The Facts \". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...", "C": "That looks like a Pratt truss . These trusses have diagonals which go from the outer-top nodes to the inner-bottom nodes (i.e. they connect to the top chord on the node furthest from the center of the span, and to the bottom chord on the node closest to the center). This design means that the diagonals are under tension and the verticals are under compression. Another famous design is the Allan truss , which is the exact opposite: the diagonals go from the inner-top nodes to the outer-bottom nodes, which means that the diagonals are under compression and the verticals...", "D": "Theoretically pontoon bridges with rope anchors keeping them to the bottom would work against wind and flow, overcoming the problem jhabbot mentioned in his answer (same as train length limit - stretching force). In practice these come with more problems of their own. They drift on water surface and as result, rise and fall with water waves. The larger the body of water they span, the higher the waves; at certain point in stormy weather the bridge would just launch the vehicles into the air. The anchoring isn't exactly simple if it's to withstand such forces. You could just as..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/165/are-pontoon-bridges-being-considered-to-extend-bridge-span"}
{"id": "engineering_1903", "domain": "engineering", "question_title": "What happens when you put the wrong type of fuel in an internal combustion engine?", "question_body": "What happens to the working of petrol engine when it is emptied and filled with diesel or to a diesel engine emptied and filled with petrol? Will the engine be able to operate and, if not, why not?", "question_score": 14, "question_tags": ["mechanical-engineering", "automotive-engineering", "combustion"], "choices": {"A": "If you were going to turn left 90 degrees, without turning the wheels, then you wind up dragging the wheels sideways while you turn. 16 seconds into this video shows exactly what I'm talking about . So every time you try to back out of your driveway, or a parking spot, or turn into a parking spot, or turn anywhere for any reason, you're going to lay down rubber because the tires are rotating quickly while remaining nearly stationary as you turn. Again, look at the wheel slip in the video I linked. You'd be hard pressed to find \"four...", "B": "Putting diesel fuel in a gasoline engine is just about impossible. Diesel fuel nozzles are larger than gasoline nozzles, and modern gas caps are too small for diesel nozzles to fit into. However, if you managed to get it in there, the diesel fuel is too heavy and evaporates too slowly for the spark plugs to ignite it effectively. One source says that it won't start at all, another source says that it probably won't start, but there's a chance it will, it'll just run horribly and end up as a smokey disaster. The converse is possible, since the gasoline...", "C": "OP injection molding tag is correct. OBall uses injection molding and plastic welding. The OBall is the invention of David E. Silverglate. Toy Ball Apparatus with Reduced Part Count Reduced image from Kids II . It consists of four identical, flat, injection molded, pentagon and hexagon shapes with circular (or elipitical) holes, which are shaped and plastically welded into spheres. Pentagon and hexagon edges are the same size and individual connected circles are only connected along one edge. The four shapes are clearly shown in colors above and from the patent. Solid lines on each part are hard connections, while...", "D": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/1903/what-happens-when-you-put-the-wrong-type-of-fuel-in-an-internal-combustion-engin"}
{"id": "engineering_2133", "domain": "engineering", "question_title": "If the emergency brake in a train is broken, do the passenger car brakes still work?", "question_body": "Scenario: Suppose the emergency brake button in the cab of a train is not working. Would the emergency brake cords / buttons in the passenger cars still work? This question is inspired by this question on the SciFi StackExchange , asking why the passengers in the train fight scene from the movie SpiderMan 2 did not simply pull the emergency brakes. In that particular scene, a R46 City Subway Car was rendered \"unstoppable\" by ripping out the speed control lever in the car, which also happened to disable the emergency brake button (see 16 seconds into the video). With movies, we suspend our disbelief in order to enjoy the story that is presented. But the above SciFi question got me to thinking about how emergency brake systems are designed for trains. Given trains significant mass and momentum when moving, it seems like there would be multiple, redundant safety systems to provide braking capability for the train. My Question: Is there a common safety design used for the braking system of trains? Does that design account for portions of the system failing and allowing other portions to compensate for the failed components? (ie. would the passenger car emergency brakes still work?)", "question_score": 14, "question_tags": ["mechanical-engineering", "rail"], "choices": {"A": "For long freight trains and those that will be climbing to stations at higher altitudes, an extra or two locomotives are attached to the front. I've always wondered why. As usual there are multiple issues. Most important is landscape. If more power than a single locomotive can provide is not needed for the whole trip, but only a short stretch, like climbing or crossing a mountain range, then it would be a huge waste of resources to attach them the whole trip. For example, take a 10 hour trip time, where all, except for a 1 hour stretch, can be...", "B": "Nothing is rigid, the raceways and the ball in a ball-bearing are no exception. The contact area deflects and accommodates the ball in a small contact surface, not a point. Also, the balls take the load in groups. The digrams are from SKF ball-bearings. '", "C": "Electricity production optimization is a very complex subject. It is also affected by many parameters , which I will try to outline below. TL;DR: Demand is constantly monitored and supply is constantly adjusted TL;DR 2: Lshaver's excellent post is a suggested reading after reading this, because it expands and explains what happens at timescales ranging from the micro-second to minutes timescales. Changes in energy demand during the day First of all, the energy demand during the day, week, month of year can change quite a lot. Below is a typical day in New England. Fluctuation in power energy consumption a...", "D": "Train Brakes The common brakes on trains are air brakes . As the name implies, these work off of air pressure. The braking power isn't controlled in the way that you would immediately think of though. They do not work like car brakes where the harder you press on the brake pedal, the harder the pressure goes through the lines to the brake cylinders. They work the opposite. The less pressure in the line, the more braking force is applied. Fail-safe Rail brakes are designed to be fail-safe . That is, when a failure occurs, the safe operation happens. In..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2133/if-the-emergency-brake-in-a-train-is-broken-do-the-passenger-car-brakes-still-w"}
{"id": "engineering_2478", "domain": "engineering", "question_title": "Is stainless steel plating possible?", "question_body": "Is there a method for plating steel with stainless steel? If so, is it chemical, electrical, or electrochemical? I did a quick search on the internet but was unable to find a service. I'm interested in applying a food safe finish to something that would otherwise be cost prohibitive to make out of solid stainless steel.", "question_score": 14, "question_tags": ["materials", "steel"], "choices": {"A": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only...", "B": "I will expand on DKNguyen answer, because to my knowledge also the two reasons are: reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation) change the joint tightening characteristics (see joint diagram). reduce contact stresses on surfaces. The basic idea is that since contact stress is defined as: $$\\sigma = \\frac{F}{A}$$ Obviously the larger the area the less the stress. Also, like Nuclear Hoagie pointed out the area changes with a square law of the diameter. UPDATE 2 - Calculation for M6 bolt (thanks to BenC) *The question gathered enough interest for me to carry out a...", "C": "Electricity production optimization is a very complex subject. It is also affected by many parameters , which I will try to outline below. TL;DR: Demand is constantly monitored and supply is constantly adjusted TL;DR 2: Lshaver's excellent post is a suggested reading after reading this, because it expands and explains what happens at timescales ranging from the micro-second to minutes timescales. Changes in energy demand during the day First of all, the energy demand during the day, week, month of year can change quite a lot. Below is a typical day in New England. Fluctuation in power energy consumption a...", "D": "Keeping things simple, steel is an alloy of iron and carbon, whereas stainless steel is essentially an alloy of iron, carbon and chromium or iron, carbon chromium and nickel. All forms of steel, whether they be ordinary iron and carbon alloy or stainless steel are made from a melt in furnaces. Because of this stainless steel cannot be plated to ordinary steel by chemical means either. Stainless steel can be welded to ordinary steel but a TIG welder is required, but this wouldn't suit your purposes. Hot dipping is unlikely to be an option due to the melting temperatures of..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2478/is-stainless-steel-plating-possible"}
{"id": "engineering_2991", "domain": "engineering", "question_title": "Why are lasers used for concentrated light applications instead of incoherent light sources?", "question_body": "Some laser applications consist simply of concentrating light into a small spot. Two example applications are laser welding and cutting. In these cases a CO 2 laser is often used which needs a regulated power supply, a water cooling system, and a supply of CO 2 gas. Why do these applications use a laser instead of a simpler (i.e. incoherent) light source such as an AC powered arc lamp?", "question_score": 14, "question_tags": ["optics", "lasers"], "choices": {"A": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...", "B": "Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste...", "C": "There are many reasons why highly monochromatic light, such as that emitted by a laser, is useful for delivering a large amount of power to a small spot. First of all, incoherent light sources such as a lamp are extended sources which means that they are emitting light from a piece of material which takes up a finite amount of space. When focusing this light to a point, the focal spot is limited by the size of your source multiplied by the magnification of your imaging system. This may sound like a small effect, but if you want to focus...", "D": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2991/why-are-lasers-used-for-concentrated-light-applications-instead-of-incoherent-li"}
{"id": "engineering_7344", "domain": "engineering", "question_title": "Why is Mach 0.3 the threshold separating compressible and incompressible flow?", "question_body": "I've read that Mach 0.3 is pretty much the upper limit for treating air as an incompressible fluid. The sources I've read seem to treat this as a given, without proof or justification. Why is this the limit? Is there a mathematical justification for this? Also, does this limit only apply to air? If not, then what does the limit depend on?", "question_score": 14, "question_tags": ["fluid-mechanics"], "choices": {"A": "Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....", "B": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...", "C": "Wikipedia gives the reason for Mach 0.3 as due to the fact that this achieves ~5% change in density. I found a NASA page that describes (analytically!) the relationship. I cited the source, but I'll reproduce the work here for posterity, in the event their links change. Start with conservation of momentum: $$ (\\rho V) dV = -dp \\\\ $$ where $\\rho$ is the fluid density, $V$ is the velocity, and $p$ is the pressure. for isentropic flow: $$ \\frac{dp}{p} = \\gamma \\frac{d\\rho}{\\rho} \\\\ dp = \\left( \\frac{\\gamma p}{\\rho} \\right) d\\rho \\\\ $$ where $\\gamma$ is the specific heat ratio....", "D": "In the video you've linked to, the spheres are seen on both the leading and trailing edges of the propeller: I expect they are intentional - there are a number of ways to attach a propeller without having to disturb that surface, or using flush caps. Cavitation is caused by a drop in pressure. This would be seen on the trailing edges of the propeller, and is worse on the outer edge of the propeller which is moving faster through water than the inner area. I doubt these spheres have any effect on cavitation at all. It is either a..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/7344/why-is-mach-0-3-the-threshold-separating-compressible-and-incompressible-flow"}
{"id": "engineering_22725", "domain": "engineering", "question_title": "Why does column buckling occur when the load is parallel to the column?", "question_body": "I'm studying Euler's work on structural engineering from a book out of curiosity and it is mentioned that he developed a mathematical theory describing the buckling of columns under a parallel load (the weight-force of the load is directed down along the column). The theory is covered quickly without much motivation. But this got me thinking; why does a column \"buckle\" in the first place? If the load presses the column down, why does the column even start deflecting sideways? I know this happens in real life since this fact is easily confirmeable with household objects, but theoretically, why do objects start deflecting sideways instead of just compressing under loads? This might be something obvious and maybe I'm just overthinking but I find this curious nonetheless.", "question_score": 14, "question_tags": ["structural-engineering", "civil-engineering"], "choices": {"A": "If the receiver does not detect the sub-carrier for the \"colour burst\" signal which is transmitted during the horizontal blanking period the receiver switches on the \"colour-killer\" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise. Figure 1. The colorburst signal is transmitted on the \"back porch\" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately...", "B": "Nothing is rigid, the raceways and the ball in a ball-bearing are no exception. The contact area deflects and accommodates the ball in a small contact surface, not a point. Also, the balls take the load in groups. The digrams are from SKF ball-bearings. '", "C": "A yahkchal is an example of a type of passively cooled building in Iran They utilise a combination of passive evaporative cooling and thick thermally insulating walls in order to keep the interior temperatures low enough. First, wind is directed into underground aquifers known as qanat . They are then cooled due to the low humidity desert air causing water to evaporate. The cooled air then flows through the interior of the yakhchal, cooling the interior. The thick insulating walls (filled with earth and various insulating materials such as straw and feathers) help to insulate the cool interior from the...", "D": "Euler buckling occurs because the world isn't perfect. So that theory assumes that there is an initial infinitesimal deviation along the column (assuming the column is in fact not perfectly vertical*). This deviation causes a bending moment along the beam, which increases the deviation, which increases the bending moment, which increases the deviation... For loads lower than the Euler load, this vicious cycle eventually stabilizes and the beam doesn't buckle. For the Euler load and above, the cycle never stabilizes and the deflection goes to infinity. Obviously the real world has initial deviations and other problems which are much higher..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/22725/why-does-column-buckling-occur-when-the-load-is-parallel-to-the-column"}
{"id": "engineering_49405", "domain": "engineering", "question_title": "Why are multiple locomotives attached only to the front for larger trains?", "question_body": "For long freight trains and those that will be climbing to stations at higher altitudes, an extra or two locomotives are attached to the front. I've always wondered why. For argument's sake, if there are 30 bogeys each weighing 10 tons, the three engines are pulling a combined 300 tons. Each must be applying the exact same pull else if one is pulling harder than it is effectively taking on all the work with the other two idling. Plus, the load on the first coupling is 300 tons, on the second 290, and so on. On the other hand, if locomotives are placed after every 10 bogeys, then each is pulling 100 tons only.", "question_score": 14, "question_tags": ["mechanical-engineering", "rail"], "choices": {"A": "For long freight trains and those that will be climbing to stations at higher altitudes, an extra or two locomotives are attached to the front. I've always wondered why. As usual there are multiple issues. Most important is landscape. If more power than a single locomotive can provide is not needed for the whole trip, but only a short stretch, like climbing or crossing a mountain range, then it would be a huge waste of resources to attach them the whole trip. For example, take a 10 hour trip time, where all, except for a 1 hour stretch, can be...", "B": "There are a few simple reasons why the speed of a vehicle (road conditions notwithstanding) may be limited: Gearing -- Production vehicles with conventional transmissions have a limited number of gears. For most modern cars, this is usually 5 or 6, whereas older vehicles may have as few as 2 or 3. If the gear ratio of the highest gear is too low (\"lower\" gears are expressed as larger numerical ratios), it's entirely possible that the engine will redline before air resistance becomes a factor at all. This ties into your first point about the red zone on the tachometer....", "C": "As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\\arctan\\left(\\frac{0.15}{15}\\right)=0.57^\\circ$ or 1%. the middle ,...", "D": "If I model this as a simply supported beam having load at mid span [...] I suspect that this is where your analysis went awry. First off, you should always model bridges with distributed loads, not a single concentrated load at midspan. The most significant load on a bridge will almost always be its own self-weight; load-trains are heavy but, well, so are bridges. Secondly, I assume you're thinking of the bridge like this: Indeed, we can see here that the bending moment is greater at midspan. However, that's not the bridge we're looking at, it's missing the cantilevers! So..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/49405/why-are-multiple-locomotives-attached-only-to-the-front-for-larger-trains"}
{"id": "engineering_34", "domain": "engineering", "question_title": "Do I need to worry about galvanic corrosion when inserting a steel bar through aluminium?", "question_body": "My design uses a round steel bar inserted through a hole in an aluminium component with a tight fit (no movement). The operating environment is dry, 20-40°C. Do I need to worry about galvanic corrosion, and if so how long will this last before starting to corrode? Also, what storage conditions should I avoid that would accelerate the corrosion?", "question_score": 13, "question_tags": ["steel", "corrosion", "materials"], "choices": {"A": "Short answer : make it thicker. Long answer : The moment of inertia affects the beam's ability to resist flexing. Use one of the many, free, online moment of inertia calculators (like this one ) to see how increasing the height of the beam will have an exponential effect on increasing the stiffness of the beam. And this site helps provide a pictorial view of the load(s) upon a beam depending upon differing configurations, such as where the supports are and where the load is applied. It also provides a calculator to determine the forces involved. Wikipedia has a decent...", "B": "BS5950-1:2000 Clause 1.3.23 defines an H-section as having \" an overall depth not greater than 1.2 times its overall width \", and Clause 1.3.25 defines an I section as having \" an overall depth greater than 1.2 times its overall width \". Note that at exactly a ratio of 1.2, it would be an H section not an I section.", "C": "Very probably, no. Galvanic corrosion required the existence of a reactive and conductive medium, which you don't have. If there is some (for example, at least moist air, or similar) best you can do to paint both of the metals, if you can. Generally, if none of the materials would corrode in an environment, galvanic corrosion is a non-issue (in that environment).", "D": "We use engineering strain even though it is not the \"correct\" value because in most cases, specifically in the elastic regime, engineering strain differs negligibly from true strain. For linear elastic, Hookean materials, it is generally the case strain at the elastic limit is very small. Even the strongest steels, for example, have an upper limit when cold worked of about $\\sigma_{\\textrm{el}}=1\\times 10^{9}\\ \\textrm{Pa}$. The modulus of steel is approximately $E=200\\times 10^{9}\\ \\textrm{Pa}$. Thus $\\varepsilon_{\\textrm{el}}=0.005=0.5\\%$ for the strongest steels. So at the onset of plastic deformation, engineering strain is $0.5\\%$. Many useful elastic materials have much lower engineering strain at..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/34/do-i-need-to-worry-about-galvanic-corrosion-when-inserting-a-steel-bar-through-a"}
{"id": "engineering_89", "domain": "engineering", "question_title": "Additional jacking force to overcome stiction/dry friction", "question_body": "It is common to lift bridges to replace bearings, etc. In an ideal world the lifting capacity required of the jacks would be the self-weight of the bridge divided by the number of jacks (+ allowances for wind/snow, etc.). From my (limited) experience, however, bridges begin to 'stick' to their bearings, and an additional allowance for over-coming this has to be provided. Does anyone have any guidance about how to determine this figure?", "question_score": 13, "question_tags": ["civil-engineering", "structures", "bridges", "maintenance", "friction"], "choices": {"A": "So there's an incorrect assumption underlying your question. In an ideal world the lifting capacity required of the jacks would be the self-weight of the bridge divided by the number of jacks (+ allowances for wind/snow, etc.). And the assumption there is that the lifting capacity is equivalent only to the weight of the bridge. The problem is that if anything goes wrong, you're likely to see a catastrophic failure of some sort which could lead to irreparable damage. Real world lifts don't operate in that \"ideal\" manner, and instead rely upon a safety factor in order to make sure...", "B": "That looks like a Pratt truss . These trusses have diagonals which go from the outer-top nodes to the inner-bottom nodes (i.e. they connect to the top chord on the node furthest from the center of the span, and to the bottom chord on the node closest to the center). This design means that the diagonals are under tension and the verticals are under compression. Another famous design is the Allan truss , which is the exact opposite: the diagonals go from the inner-top nodes to the outer-bottom nodes, which means that the diagonals are under compression and the verticals...", "C": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "D": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/89/additional-jacking-force-to-overcome-stiction-dry-friction"}
{"id": "engineering_162", "domain": "engineering", "question_title": "How to correctly size the blades for a small educational wind turbine?", "question_body": "I want to design a small wind turbine that can be easily carried out of the classroom into the sports field in relatively low-wind conditions (5-15km/h) that can power a small ultra-bright 5V LED - just enough to show that it works. How do I calculate the power I can get from different blade diameters in these wind conditions, and the power needed to drive a small DC motor/generator enough to light the LED?", "question_score": 13, "question_tags": ["electrical-engineering", "renewable-energy", "power-engineering", "wind-power"], "choices": {"A": "As it happens, I just recently went through that calculation myself for a different site. Given the following facts from a quick web search, it isn't difficult to work out the numbers. The maximum efficiency of a (large) windmill is about 40%. The density of air is 1.225 kg/m 3 You need about 50 mW (10 mA at 5V) to light up an LED First, we'll need about 50 mW / 0.40 = 125 mW of air power flowing through the windmill to create the electricity we need (ignoring other factors such as the actual efficiency of a small windmill...", "B": "Cross-interference between aircraft is a high unlikely event because all commercial aircraft designs have to pass DO-160 environmental testing requirements. Among the DO-160 testing specification is EMI/EMC Testing. These tests include Radiated Emission, Interference and Immunity Testing. Part of these tests are to answer \" Two aircraft are very close together \" and \" The other aircraft accidentally receives the signal because it is so close. \" Below is a picture of an anechoic chamber used to test aircraft. Fly-by-wire allows the aircraft control system to access and command monitoring and control systems. The monitoring and control systems have a...", "C": "Some ideas: Wheel Load Distribution : The load is greater on the rear wheels providing the power; more force on the front ones bring no benefit and would provide less traction. Better manoeuvrability from having a shorter wheelbase. Better Ground Clearance in some conditions, especially for bumps and or up a increasing slope for instance. Better Driving : The front wheels now turn around a point closer to the C.G. than with the rear wheels. Not good with vehicle dynamics but this appears better than the rear end 'trailing' behind. Structural : As some people have pointed out, it's better...", "D": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/162/how-to-correctly-size-the-blades-for-a-small-educational-wind-turbine"}
{"id": "engineering_179", "domain": "engineering", "question_title": "Derivation for bridge natural frequency estimate in Eurocodes", "question_body": "The Eurocodes gives the following equation for estimating a \"simply supported bridge subject to bending only\"*: $$n_0 = \\frac{17.75}{\\sqrt{\\delta_0}}$$ Where $n_0$ is the natural frequency in hertz $\\delta_0$ is the deflection at mid-span under permanent actions in mm The equation is seemly plucked from thin air, and there is no explanation as to where the constant 17.75 comes from. As an engineer I'm loath to use a formula I don't understand, but more than that it would be helpful to learn the fundamentals behind it so that I can see if it can be altered to work with other support conditions. Can anyone provide a derivation / fundamental origin to this relationship? *Full reference is: EN 1991-2:2003 6.4.4 [Note 8] (Equation 6.3), if that helps.", "question_score": 13, "question_tags": ["bridges", "dynamics", "eurocodes"], "choices": {"A": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "B": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...", "C": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "D": "If we simplify the whole bridge into 2D thin beam with a constant section size, no internal damping and subject only to small vertical deflections, then the natural frequency is determined by simple harmonic motion: $$ n_0 = \\frac{1}{2 \\pi} \\sqrt{ \\frac{ k } { m } } $$ Where $ n_0 $ is the natural frequency, $ k $ is the ratio between restorative force and deflection (the equivalent 'spring stiffness') and $m$ is the mass per unit length of the beam. In a beam the restorative force is the internal shear caused by the deflected shape. As the..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/179/derivation-for-bridge-natural-frequency-estimate-in-eurocodes"}
{"id": "engineering_226", "domain": "engineering", "question_title": "How different are dc motor designs from dc generator designs?", "question_body": "Some dc motors can be used as generators as well by applying mechanical torque to the output shaft to induce a current. However, even if a dc motor can do this, I imagine they were not designed for this purpose and thus perform less efficiently when used as a generator rather than as a motor. In my admittedly naive understanding, dc generators and dc motors are essentially the same machinery, but with inputs and outputs reversed. This leads me to believe that some other design considerations are used to make one direction more efficient than the other. How differently are dc generators and dc motors designed to make one direction of input/output more efficient than the other? What can one do electrically or mechanically to improve the efficiency in either direction? In particular, I'm interested in converting a dc motor into a generator and want to know how I can improve its efficiency in converting mechanical energy into electrical energy.", "question_score": 13, "question_tags": ["mechanical-engineering", "electrical-engineering"], "choices": {"A": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...", "B": "From the point of view of the driver of a car, impacting another car is about as bad as crashing against an ideal wall (a wall with zero deformation whatsoever). If there were a plane reflection between the two cars, then vs. Car would be exactly equal to vs. Wall (the contact points between both cars would all be on the same plane, due to reflection, so each car could be considered a wall for the other). But this plane reflection does not exist: What we have instead is a 2-fold rotational reflection . Let's say the left part of...", "C": "You want to let air into the pipe when you switch the pump off, without letting water out. Here's a few ways to do this. Make a pinhole in the highest point of the pipe. You will lose a bit of water this way, but if it is above the tank, the water will drip back in (provided it doesn't spray too far.) You could even put it just under the tank lid. Install a tee and riser at the highest point of the pipe. this will need to be high enough to avoid the pump pressure pumping water out...", "D": "In Ye olden days DC generators were brushed commutated devices. They had a one or more stator windings and an armature winding. Field wound DC generators as well as motors were commonly connected in one of three methods: Series, Shunt and Compound. Without getting into details, each had its own set of strengths and weaknesses. But you only have to remember these two things: the voltage of a DC motor is dependent on its input shaft speed. Current is a function of torque. More voltage means more RPM's and more amps means more newton-meters (or foot-pounds). So with all that,..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/226/how-different-are-dc-motor-designs-from-dc-generator-designs"}
{"id": "engineering_336", "domain": "engineering", "question_title": "Which turbulence models are suitable for CFD analysis on a streamlined vehicle body?", "question_body": "Many commercial and open-source CFD codes implement several closure methods for the non-linear convective acceleration term of the Reynolds-averaged Navier-Stokes (RANS) equations. Common methods (also known as turbulence models ) include Spalart–Allmaras (S–A) k–ε (k–epsilon) k–ω (k–omega) SST (Menter’s Shear Stress Transport) Reynolds stress equation model Which of these are suitable for CFD simulation of a streamlined vehicle body? The purpose of the simulations is to guide the refinement of the body shape to minimize aerodynamic drag forces. An exemplary answer would briefly outline the advantages and disadvantages of each method for this simulation application. Potentially useful details: The vehicle is a small one-person vehicle with approximate dimensions L = 2.5 m, W = 0.7 m, and H = 0.5 m. It will be travelling at speeds ranging from 0 m/s to approximately 12 m/s. All three wheels are enclosed by the body envelope, and the vehicle has an approximate ground clearance of 15 cm except near the wheels, where the body shell extends down to within 1 cm of the road surface. Normally aerodynamic forces at these speeds are very nearly negligible, but assume that this vehicle is being designed to compete in a \"Super Mileage\" competition on a smooth track, is very light-weight, and uses low friction drivetrain components throughout, so the aerodynamic forces have a significant effect on the achievable fuel consumption.", "question_score": 13, "question_tags": ["mechanical-engineering", "fluid-mechanics", "fluid-dynamics", "simulation", "modeling"], "choices": {"A": "The turbulence model can make a big difference in your simulation . There are many turbulence models around. It becomes a tough job to select one out of them. There is no perfect turbulence model. It all depends on several parameters like Reynold's number, whether the flow is separated, pressure gradients, boundary layer thikness and so on. In this answer, brief information about a few popular models is given along with pros and cons and potential applications. However, interested users can see this excellent NASA website and references therein to know more about turbulence modeling. A) ONE EQUATION MODEL: 1....", "B": "In the video you've linked to, the spheres are seen on both the leading and trailing edges of the propeller: I expect they are intentional - there are a number of ways to attach a propeller without having to disturb that surface, or using flush caps. Cavitation is caused by a drop in pressure. This would be seen on the trailing edges of the propeller, and is worse on the outer edge of the propeller which is moving faster through water than the inner area. I doubt these spheres have any effect on cavitation at all. It is either a...", "C": "I have written mostly about CFD in this answer, however same points should also work for FEA or other simulation techniques. CFD is mostly used for design optimization and parametric study of the design. Following are a few examples showing how engineers use simulations Selection of a design : Read: A conceptual study of airfoil performance enhancement using CFD. This thesis shows use of CFD for selecting the best design out of a number of candidate designs. Engineers often go for simulations to select 'the one' out of many . Shape optimization using CFD : This paper gives an example...", "D": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/336/which-turbulence-models-are-suitable-for-cfd-analysis-on-a-streamlined-vehicle-b"}
{"id": "engineering_486", "domain": "engineering", "question_title": "Measuring longer distances with high accuracy", "question_body": "I am involved right now in the fabrication of steel structures on the order of 5-50 feet long. Presently, we measure these structures with garden variety commercial tape measures. Most of the time, we work to a tolerance of about +/-1/16\" and don't have any problems. Recently, we're trying to make some items to a very high tolerance (at least +/- 1/32\", approaching 1/64\" or .016\") This tolerance is due to visual not mechanical criteria, but it is still very important to our management. My question is, how can we reliably measure these sorts of distances with that level of precision? I'm prepared to order some NIST traceable tape measures, but it's not clear to me if they'll really improve the situation. Are there other technologies or techniques that could practically be applied in a fabrication setting? Are any surveying tools accurate enough to solve the problem? Cost is obviously a factor, but we're more concerned with repeatability and robustness than price. I realize this tolerance will sound ridiculous to most, but I imagine that some other industries may have to perform similarly, perhaps large engines for ships or power plant components?", "question_score": 13, "question_tags": ["measurements", "tolerance", "manufacturing-engineering", "surveying"], "choices": {"A": "Answering the question: What are possible types of low cost sensors I can use? There are several types of sensors that can provide millimeter level accuracy. \"Low cost\" is a very relative term, so you'll need to do some shopping around based on your specific budget. Optical sensors- Included here are those of the type you listed, though it's a very cheap sensor meant more to provide a \"yeah something is in front of me, about yay-far-away\". There are also laser sensor systems which can provide millimeter level accuracy. Ultrasonic Sensors- Most ultrasonic sensors have relatively low accuracy; in the...", "B": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...", "C": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only...", "D": "To get that sort of accuracy over that scale is not trivial and probably won't be cheap. For smaller size up to a few meters a portable CMM would be an option ( here's an example ). These have accuracy on the order of 10 $\\mu$m and are used for things like high end/F1 car manufacture. However, CMM type instruments wouldn't be useful for anything larger than a few meters as it is limited by the arm length and must be fixed in place when measuring to get a sensible result. For larger pieces the best performing option would be..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/486/measuring-longer-distances-with-high-accuracy"}
{"id": "engineering_557", "domain": "engineering", "question_title": "How does a multimeter protect itself from high voltages?", "question_body": "I have used a cheap multimeter to measure voltages in simple DC circuits, but I have seen pictures of them plugged straight into the mains and used to measure various home-built generators. Why doesn't the higher voltage fry the multimeter, and also in theory could a small cheapo multimeter be safely used to measure very high voltages? If you get the setting wrong on the dial, does this matter? I'm not planning to plug one in, nor would I recommend anyone who doesn't know what they're doing to do this either, I'm just wondering how it works.", "question_score": 13, "question_tags": ["electrical-engineering", "measurements"], "choices": {"A": "WARNING : Note that some cheap meters are not suitable for use with 230 VAC AC mains. Some meters may have AC voltage ranges able to conceptually measure to well above AC mains voltage BUT have internal componentry not certified, suitable or safe at eg 230 VAC. Use of such meters to measure such voltages is akin to a safer than usual game of \"Russian Roulette\" which still may end in death. _________________ Failures may occur to power dissipation in components or to voltage breakdown even when power dissipation is within bounds. Voltage ranges are usually less stressed than most...", "B": "Your equation is partly correct. You've calculated the energy per photon ($\\hbar \\nu$), but you've neglected the number of photons. That's why the units don't match (power is energy per unit time, while you've only got energy for each photon). The ideal power (energy per unit time) depends on the area of the solar panel, $A_p$, the number of photons striking it per unit time ($\\Phi$) and the energy of each photon, $E$, such that $W_{Ideal} =A_p \\cdot \\Phi \\cdot E$. A lens or mirror can focus light (a flux of photons) onto a small area. Under really ideal conditions,...", "C": "After the 2014 ICCT report revealed that these light-duty passenger diesel vehicles were emitting too much NOx and US regulators confronted VW about the results, VW did some testing and proposed a voluntary software recall to recalibrate the various emissions control devices on the affected vehicles. From the California Air Resources Board's (CARB's) in-use compliance letter to Volkwagen AG, 2015-09-18 , following that recall: To have a more controlled evaluation of the high NOx observed over the road, CARB developed a special dynamometer cycle which consisted of driving the Phase 2 portion of the FTP repeatedly. This special cycle revealed...", "D": "From the point of view of the driver of a car, impacting another car is about as bad as crashing against an ideal wall (a wall with zero deformation whatsoever). If there were a plane reflection between the two cars, then vs. Car would be exactly equal to vs. Wall (the contact points between both cars would all be on the same plane, due to reflection, so each car could be considered a wall for the other). But this plane reflection does not exist: What we have instead is a 2-fold rotational reflection . Let's say the left part of..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/557/how-does-a-multimeter-protect-itself-from-high-voltages"}
{"id": "engineering_617", "domain": "engineering", "question_title": "Estimating whether the flow through a valve or nozzle cavitates", "question_body": "My understanding is that cavitation occurs in the flow of a liquid when the static pressure drops below the vapor pressure, even intermittently. So even if the time-averaged static pressure (what you might measure) is above the vapor pressure, the pressure fluctuations from turbulence or other unsteadiness could be large enough to cause cavitation locally. So comparing the time-averaged static pressure against the vapor pressure isn't enough; you need to add some extra cushion to account for the pressure fluctuations. (This is my interpretation, not having read too deeply into this.) So, in various books, websites, and journal articles I have seen two different types of dimensionless numbers for estimating whether the flow through a valve or nozzle cavitates. They are generally called the cavitation index or cavitation number. They take one of two forms: $$\\sigma = \\frac{p_\\text{in} - p_\\text{vapor}}{p_\\text{in} - p_\\text{out}}$$ or $$\\sigma = \\frac{p_\\text{in} - p_\\text{vapor}}{\\tfrac{1}{2} \\rho V^2}$$ where $p_\\text{in}$ is the inlet pressure, $p_\\text{out}$ is the outlet pressure, $p_\\text{vapor}$ is the vapor pressure, $\\rho$ is the liquid density, and $V$ is some characteristic velocity of the flow (say, in the nozzle case, the velocity at the outlet). Some forms of this number are inversions of the numbers above, but these aren't that different. What is the difference between these parameters? Based on energy conservation you can relate the pressure drop to the flow rate, but typically there is an empirical coefficient added in to account for non-idealities. Is there something else I am missing? Is one form preferred over the other? Best I can tell whether to use one or the other depends on what sort of data you have (so, for flow over a turbine blade, the velocity form is preferred), but I've seen both even for nozzles. Where can I get accurate data to predict cavitation based on these numbers? I've tried using some data on atomizer nozzles from various journal articles but generally they use different forms of the cavitation number. Some of the data suggests the flow through the nozzle will cavitate at the pressures I want, but other data for similar nozzles suggests it won't. I'm not sure what the source of the inconsistency is. My understanding could be faulty, the cavitation number model could be too simplistic, the data could be inaccurate, etc.", "question_score": 13, "question_tags": ["mechanical-engineering", "fluid-dynamics", "multiphase-flow"], "choices": {"A": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...", "B": "There are a few simple reasons why the speed of a vehicle (road conditions notwithstanding) may be limited: Gearing -- Production vehicles with conventional transmissions have a limited number of gears. For most modern cars, this is usually 5 or 6, whereas older vehicles may have as few as 2 or 3. If the gear ratio of the highest gear is too low (\"lower\" gears are expressed as larger numerical ratios), it's entirely possible that the engine will redline before air resistance becomes a factor at all. This ties into your first point about the red zone on the tachometer....", "C": "In the video you've linked to, the spheres are seen on both the leading and trailing edges of the propeller: I expect they are intentional - there are a number of ways to attach a propeller without having to disturb that surface, or using flush caps. Cavitation is caused by a drop in pressure. This would be seen on the trailing edges of the propeller, and is worse on the outer edge of the propeller which is moving faster through water than the inner area. I doubt these spheres have any effect on cavitation at all. It is either a...", "D": "The difference between the two equations The cavitation number is the ratio of the static pressure difference to the dynamic pressure difference. So, if you want to use the first equation, you would need to take the pressure using a Pitot tube to measure the total pressure, whereas if you want to use the second equation you will need to measure the freestream velocity, but I would recommend measuring it upstream rather than downstream because of possible effects of acceleration and boundary layer growth. Also, your $V$ should be $V_{in}$ such that it corresponds to the same location where $p_{in}$..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/617/estimating-whether-the-flow-through-a-valve-or-nozzle-cavitates"}
{"id": "engineering_1880", "domain": "engineering", "question_title": "I have inherited a product with inconsistent designs sharing the same part number. What do I do?", "question_body": "I am now in charge of a product line my company has been shipping for a decade. One of the previous product engineers was... shall we say, less than conscientious about sustainability and proper documentation. We have shipped hundreds of units, of multiple design variants, under the exact same part number . The manual presently reflects only one variant, meaning many users can't use it. And we've had multiple instances where a user has tried to reorder a unit by part number, only to find that what we ship them does not match what they already had. Obviously, this is terrible. One does not change the specs or user interface of a product without also changing the part number. We will avoid such things in the future. But my question is about the past. We have, on paper, documentation indicating what design variant each serial number corresponds to. My thinking is to create a spreadsheet, and name each variant retroactively, so we can at least support users that call in or place reorders. We would then create proper manual(s) so that the user can, based on their serial number, understand the operation of the units they have. But I'm just making up that solution. It occurs to me that there may be formal, industry-standard methods of dealing with such things. Is there a procedurally-correct way to handle my existing install base?", "question_score": 13, "question_tags": ["project-management", "documentation", "product-support"], "choices": {"A": "I don't think there is any formal standard way out of this mess. The good news is that at least tracked the changes by serial number. Inventing different models after fact seems pointless because the customers won't know what models they have. From the customer view, these are all one model but vary by serial number. Document it that way. You could create a different manual for different serial number ranges if the changes are large, or have occasional sections in the manual that only apply to particular serial number ranges. Make sure that the customer can easily find the...", "B": "As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\\arctan\\left(\\frac{0.15}{15}\\right)=0.57^\\circ$ or 1%. the middle ,...", "C": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...", "D": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/1880/i-have-inherited-a-product-with-inconsistent-designs-sharing-the-same-part-numbe"}
{"id": "engineering_1958", "domain": "engineering", "question_title": "What makes the material travel through Archimedes' Screw?", "question_body": "...or in other words, why doesn't the material just stick to the screw, rotating in place, without progressing along its length? In the simplest case, the answer is obvious: gravity. If it's a granular or liquid material and the screw is tilted to a side, it will just roll/slide/flow along the blade, to remain on the bottom side of the tube. But then, Wikipedia says: A variant of the Archimedes' screw can also be found in some injection molding machines, die casting machines and extrusion of plastics, which employ a screw of decreasing pitch to compress and melt the material. Finally, it is also used in a specific type of positive displacement air compressor: the rotary-screw air compressor. On a much larger scale, Archimedes' screws of decreasing pitch are used for the compaction of waste material. In such case the forces - pressure, shearing, viscosity, adhesion, all would outweigh gravity, and probably in some cases friction against the tube walls. For example, in injection molding, what is there to keep the half-molten plastic from forming a clumping mass, sticking to the screw and keeping rotating in place without any progress? When the pressure increases, which force prevents the material from backing up into area of lower pressure?", "question_score": 13, "question_tags": ["fluid-mechanics"], "choices": {"A": "The turbulence model can make a big difference in your simulation . There are many turbulence models around. It becomes a tough job to select one out of them. There is no perfect turbulence model. It all depends on several parameters like Reynold's number, whether the flow is separated, pressure gradients, boundary layer thikness and so on. In this answer, brief information about a few popular models is given along with pros and cons and potential applications. However, interested users can see this excellent NASA website and references therein to know more about turbulence modeling. A) ONE EQUATION MODEL: 1....", "B": "Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book \" Wind Energy - The Facts \". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...", "C": "There's actually two kinds of screw conveying: Water lifting with screws happens just as in your gif, in the lower part of an inclined pipe we have water and the screw pushes 'packets' of water upwards. The water can't flow back because there's no path, the blades are not fully submerged. This mode is in principle also possible with solids. source: Wasser Wissen Now, screw conveyors for solids or the screws in extruders and meat grinders or certain dewatering presses work differently: Here it is the friction between the medium and the walls that prevents the material from corotating with...", "D": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/1958/what-makes-the-material-travel-through-archimedes-screw"}
{"id": "engineering_1998", "domain": "engineering", "question_title": "How does pressure treatment affect the mechanical properties of lumber?", "question_body": "Pressure treated lumber is specified for many exterior applications because of its resistance to insect damage and fungal rot. But how does it compare to untreated wood, mechanically? For example, consider a rim joist supporting the ground floor of a residential structure with a pier and post foundation. If the joist has been damaged by rot in a location that is impractical to fully protect from exposure, I might be tempted to replace the joist with a pressure treated member of the same nominal dimension (in addition to proper flashing) for added protection in that location. Since this is an existing structure, by far the easiest approach is to use a member with the same dimension to replace the rotted joist. However, this relies on the new member meeting the same load-bearing requirements as the old member. Building codes should provide enough wiggle room that in this particular example, there's not much of a safety concern for the homeowner. After all, the rotted joist had not failed, and it would definitely have less strength than the member was originally rated for. In practice, treated and untreated members may be manufactured from different wood species with different mechanical properties to begin with; for the purposes of this question, assume the species is constant. Does pressure treatment result in a member with more or less strength in tension, compression or torsion? Does it affect the durability of the wood* in ways not related to rot or insect damage? * Not the fasteners; that's a different issue that's pretty well-covered online. See this page from Simpson , for example.", "question_score": 13, "question_tags": ["structural-engineering", "wood"], "choices": {"A": "After the 2014 ICCT report revealed that these light-duty passenger diesel vehicles were emitting too much NOx and US regulators confronted VW about the results, VW did some testing and proposed a voluntary software recall to recalibrate the various emissions control devices on the affected vehicles. From the California Air Resources Board's (CARB's) in-use compliance letter to Volkwagen AG, 2015-09-18 , following that recall: To have a more controlled evaluation of the high NOx observed over the road, CARB developed a special dynamometer cycle which consisted of driving the Phase 2 portion of the FTP repeatedly. This special cycle revealed...", "B": "Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book \" Wind Energy - The Facts \". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...", "C": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...", "D": "Pressure treatment does have a small, but documented effect on the strength of the member, particularly if it is 'incised' (has slots or holes cut into it as part of the pressure treating process.) If you're working to American codes, according to the American Wood Council , pressure treated wood is limited to a maximum duration factor of 1.6. This wouldn't matter for your example of fixing a house, because the duration factor would already be much smaller. Effectively, this just restricts the use of pressure treated lumber for resisting impact loads. More importantly, if it is incised which is..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/1998/how-does-pressure-treatment-affect-the-mechanical-properties-of-lumber"}
{"id": "engineering_2017", "domain": "engineering", "question_title": "What is a Spiral Curve, and How is it Different from a Normal Curve?", "question_body": "I've heard the term spiral curve used to describe a section of highway that is more aesthetically pleasing to the driver's eye. However, I believe I've driven on enough road to say that I can't definitively tell the difference between any given curve other than how \"sharp\" it is. Could anyone explain how one can determine if a section of curve on a highway is classified as a \"spiral\" curve, and are there other advantages besides \"making it look good\"?", "question_score": 13, "question_tags": ["civil-engineering", "highway-engineering", "surveying", "rail"], "choices": {"A": "Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....", "B": "As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\\arctan\\left(\\frac{0.15}{15}\\right)=0.57^\\circ$ or 1%. the middle ,...", "C": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...", "D": "Spiral A spiral curve is a geometric feature that can be added on to a regular circular curve. The spiral provides a gradual transition from moving in a straight line to moving in a curve around a point (or vise-verse). The use of a spiral is about making the road or track follow the same form that the vehicle naturally takes. In a car, you don't go directly from going straight to fully turning. There is a transition area where you slowly turn the steering wheel. Lateral acceleration is slowly increased as the spiral is entered, or it is slowly..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2017/what-is-a-spiral-curve-and-how-is-it-different-from-a-normal-curve"}
{"id": "engineering_2125", "domain": "engineering", "question_title": "How is a mechanical system designed and tested without building a physical model?", "question_body": "I am new to mechanical engineering, although I have a scientific background (postgrad in Mathematics), and I (mostly) code for a living. I have an idea about creating a mechanical device; I envisage it will entail gears, linkages and actuators. I have a rough idea where things will fit but I would like to be able to test and tweak the design in software before building the actual device. As a point of clarification, when I say \"test,\" I mean view via animation, for example, whether two items will collide when in motion, or if there is sufficient leeway between them as they move past each other. This allows me to do the design and simulation testing of the parts, before finalising the design and then building the physical system from the design. The stages are: Build the 3D design in software Run simulation to see if it \"works,\" if not fix design and iterate Build physical system from design that \"works\" I have figured out that the system consists of three subsystems working together. So, I would like to design and test each sub-component before integrating them into the complete system. My question then is this: Is this how design is done in the real world? What are the pros and cons of the scheme I have planned? I am intending to use FreeCAD to do the design and testing.", "question_score": 13, "question_tags": ["mechanical-engineering", "systems-design", "computer-aided-design"], "choices": {"A": "In the video you've linked to, the spheres are seen on both the leading and trailing edges of the propeller: I expect they are intentional - there are a number of ways to attach a propeller without having to disturb that surface, or using flush caps. Cavitation is caused by a drop in pressure. This would be seen on the trailing edges of the propeller, and is worse on the outer edge of the propeller which is moving faster through water than the inner area. I doubt these spheres have any effect on cavitation at all. It is either a...", "B": "Answer: Yes , this is exactly how it is done in the real world. What you have described is what I do in my job to check systems in CAD. Since you indicated you would like me to step through my design process, I have detailed it below. Note that most of this does not involve CAD. CAD is invaluable, but only if you are prepared to get out the pencil and paper first. Also note that this is just my design process, it is by no means the only way to go about things. Preparation When I begin any...", "C": "Nothing is rigid, the raceways and the ball in a ball-bearing are no exception. The contact area deflects and accommodates the ball in a small contact surface, not a point. Also, the balls take the load in groups. The digrams are from SKF ball-bearings. '", "D": "As others stated before, induction loops are the primary - most reliable method: the coils (usually just several loops of wire) embedded in the road; fed given frequency from a generator, in presence of metal the frequency of the LC circuit changes and the sensor circuitry detects the change of frequency, producing a presence signal. In some cases these may fail to detect bicycles, but they are by far most common as they aren't affected by weather (or more precisely, the detection circuit tunes in to slow changes of frequency caused by weather) and are immune to accidental false positives...."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2125/how-is-a-mechanical-system-designed-and-tested-without-building-a-physical-model"}
{"id": "engineering_2612", "domain": "engineering", "question_title": "Distance sensors with accuracy of 1 mm?", "question_body": "I am making a device for measurements. I would like to measure the distance within accuracy of 1 mm. Range could be 2 cm to 15 cm . I looked at Proximity Sensors but the readings displayed by these sensors are not steady. I wish to measure the thickness of the plate (carbon steel). The two sensors will be mounted on a structure. The sensors will give me the distance of the surface from the sensor. Then I will calculate the thickness of the plate. What are possible types of low cost sensors I can use?", "question_score": 13, "question_tags": ["mechanical-engineering", "measurements", "sensors", "tolerance"], "choices": {"A": "Answering the question: What are possible types of low cost sensors I can use? There are several types of sensors that can provide millimeter level accuracy. \"Low cost\" is a very relative term, so you'll need to do some shopping around based on your specific budget. Optical sensors- Included here are those of the type you listed, though it's a very cheap sensor meant more to provide a \"yeah something is in front of me, about yay-far-away\". There are also laser sensor systems which can provide millimeter level accuracy. Ultrasonic Sensors- Most ultrasonic sensors have relatively low accuracy; in the...", "B": "My first thought is that it might be intended to be a wing nut driver of some sort, but those are usually hollow cylinders with slots for the wings. Ah ... sure enough, it's described as such in this Ebay ad :", "C": "Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....", "D": "In Ye olden days DC generators were brushed commutated devices. They had a one or more stator windings and an armature winding. Field wound DC generators as well as motors were commonly connected in one of three methods: Series, Shunt and Compound. Without getting into details, each had its own set of strengths and weaknesses. But you only have to remember these two things: the voltage of a DC motor is dependent on its input shaft speed. Current is a function of torque. More voltage means more RPM's and more amps means more newton-meters (or foot-pounds). So with all that,..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2612/distance-sensors-with-accuracy-of-1-mm"}
{"id": "engineering_3547", "domain": "engineering", "question_title": "How much clearance does a car need when turning a corner?", "question_body": "I am contemplating buying a new car. However, the approach to the underground garage in my apartment has a 90 degree frustrating turn. Given the dimensions of the approach and car, what is the max turn circle for the car to fit the garage and turn? given Ackerman steering and the overhanging front part of the car I believe you can use Pythagoreas' theorem to get R min and R max. delta R should be less than the shortest path in the pathway, ie 2.5m. unfortunately the result does not seem plausible. feedback would be greatly appreciated.", "question_score": 13, "question_tags": ["automotive-engineering"], "choices": {"A": "Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste...", "B": "Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train .", "C": "It depends on how steep the hill is. On a slight hill, the energy added by gravity is still not enough to overcome rolling friction and air resistance, so the car still needs power to maintain speed. On a steeper hill, the two may balance out, so no power is used, and no power is generated. On a hill that's steep enough to require braking to control the speed, the car recovers energy. It's called regenerative braking. If the car is going too fast, applying the brakes turns the motor into a generator and charges the battery.", "D": "To slightly generalize I'll reform the question slightly. A ridged 2-D body (car) has a line $l$ that moves with it. The car can be linearly transformed as long as the instantaneous center of rotation lies along $l$ at least distance $R$ away from a point $c$ that also moves with the car. In this case point $c$ lies in the center of the rear axle and $l$ lies on the rear axle. Now imagine the car's domain is limited to a quarter plane with edges $A$ and $B$. It initially is placed against $A$, far from $B$ with $l$..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/3547/how-much-clearance-does-a-car-need-when-turning-a-corner"}
{"id": "engineering_3986", "domain": "engineering", "question_title": "How do you ensure physical network interfaces always get the same interface name across reboots on an embedded Linux system?", "question_body": "For an embedded Linux system, if I have two or more network interfaces, how do I ensure that they always get the same interface names every boot In other words, I want, for example, eth0 to always map to one physical Ethernet port, eth1 to the next, etc. My Linux \"distribution\" is home-grown, and I use devtmpfs for populating /dev. I use busybox for init (and most everything else), along with custom init scripts for system startup and shutdown. I do not need hotplug facilities of mdev or udev -- I'm referring to \"fixed\" Ethernet ports.", "question_score": 13, "question_tags": ["embedded-systems"], "choices": {"A": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only...", "B": "This works for me with Linux 3.9.0 on an x86_64 architecture. #!/bin/sh # This assumes the interfaces come up with default names of eth*. # The interface names may not be correct at this point, however. # This is just a way to get the PCI addresses of all the active # interfaces. PCIADDRLIST= for dir in /sys/class/net/eth* ; do [ -e $dir/device ] && { PCIADDRLIST=\"`readlink -f $dir/device` ${PCIADDRLIST}\" } done # Now assign the interface names from an ordered list that maps # to the PCI addresses of each interface. # IFNAMES could come from some config file....", "C": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...", "D": "I will expand on DKNguyen answer, because to my knowledge also the two reasons are: reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation) change the joint tightening characteristics (see joint diagram). reduce contact stresses on surfaces. The basic idea is that since contact stress is defined as: $$\\sigma = \\frac{F}{A}$$ Obviously the larger the area the less the stress. Also, like Nuclear Hoagie pointed out the area changes with a square law of the diameter. UPDATE 2 - Calculation for M6 bolt (thanks to BenC) *The question gathered enough interest for me to carry out a..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/3986/how-do-you-ensure-physical-network-interfaces-always-get-the-same-interface-name"}
{"id": "engineering_3959", "domain": "engineering", "question_title": "What are left justified and right justified ADC results?", "question_body": "The TI MSP430F20XX series has a 12-bit internal ADC output, which is right-justified. What is the difference between a left-justified output and a right-justified output? What are their pros and cons?", "question_score": 13, "question_tags": ["electrical-engineering", "embedded-systems"], "choices": {"A": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...", "B": "On this processor, the register that holds the conversion result is 16 bits wide. A right-justified result means that bits [( N -1):0] (where N is the number of bits of precision) of the register contain the ADC value and the most-significant bits of the register are set to zero. A left-justified result means that bits [15:(16- N )] of the register hold the result, and bits [(15- N ):0] are set to zero. For example, if your actual conversion result is 0x123, it would be read as 0x0123 if the register was right-justified, and as 0x1230 if it was...", "C": "At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...", "D": "The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/3959/what-are-left-justified-and-right-justified-adc-results"}
{"id": "engineering_6257", "domain": "engineering", "question_title": "From an engineering standpoint, what is the purpose of the indent in a coffee lid?", "question_body": "I've see the same lids every day but I never really thought about their construction. There is an indent in a \"Solo Travel Lid\" for coffee cups that is just above the hole that you drink the coffee from. You can see the crescent shaped indent in the image. What is it for? Does it somehow increase fluid flow? Is it just a good spot for your upper lip? If so, why is there not a matching one for the nose? I tried googling this but I got no answer.", "question_score": 13, "question_tags": ["design", "applied-mechanics"], "choices": {"A": "If the receiver does not detect the sub-carrier for the \"colour burst\" signal which is transmitted during the horizontal blanking period the receiver switches on the \"colour-killer\" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise. Figure 1. The colorburst signal is transmitted on the \"back porch\" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately...", "B": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...", "C": "I'm going to set aside the potential legal liability aspects of your question. In part because you didn't ask about them, but also because liability will vary based upon jurisdiction. Obviously, consult an attorney familiar with the relevant law. Ethically, I think you've done everything that you're obligated to do. You have contacted the customer and notified them of an unsafe situation or configuration of the product. And you have (strongly) advised them that they need to discontinue using the older product in the unsafe configuration. If the customer remains insistent about using the product in an unsafe manner, you...", "D": "From the \"Solo Traveler\" patent : A more specific object of the present invention is to provide a lid having an opening formed therethrough to enable drinking, and having a recess formed in the lid adjacent the opening to accommodate the upper lip of one drinking from the cup This is also illustrated in the patent diagrams. I came across this interesting article on the history, use, design, and styles of disposable lids. The Solo Traveler is prominently featured for its excellent aesthetics and design . The article reiterates that: the Solo Traveler lid was designed to accommodate the nose..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/6257/from-an-engineering-standpoint-what-is-the-purpose-of-the-indent-in-a-coffee-li"}
{"id": "engineering_8054", "domain": "engineering", "question_title": "What is this Y-shaped screwdriver bit, and what is its purpose?", "question_body": "I found this strange looking bit in a bit set of uncommon bits, including Security Torx, hexalobular, tri-wing, spanner head, 12-point flange, Torq-set and others. It's the only one that I could not identify, what is it for?", "question_score": 13, "question_tags": ["mechanical-engineering", "fasteners"], "choices": {"A": "This is to make sure they know what the foundation is made of. For all they knew there may have been an old tunnel underneath that would have collapsed when the new building is put on top. London is built on top of an old marsh, this type of soil is very prone to sinking and uneven settling, digging down and reinforcing the foundation alleviates that. It also ensures the foundation is uniform under the building to avoid a new tower of Pisa. Given the age of the city it may have been to scour the land for potential archaeological...", "B": "The turbulence model can make a big difference in your simulation . There are many turbulence models around. It becomes a tough job to select one out of them. There is no perfect turbulence model. It all depends on several parameters like Reynold's number, whether the flow is separated, pressure gradients, boundary layer thikness and so on. In this answer, brief information about a few popular models is given along with pros and cons and potential applications. However, interested users can see this excellent NASA website and references therein to know more about turbulence modeling. A) ONE EQUATION MODEL: 1....", "C": "My first thought is that it might be intended to be a wing nut driver of some sort, but those are usually hollow cylinders with slots for the wings. Ah ... sure enough, it's described as such in this Ebay ad :", "D": "OP injection molding tag is correct. OBall uses injection molding and plastic welding. The OBall is the invention of David E. Silverglate. Toy Ball Apparatus with Reduced Part Count Reduced image from Kids II . It consists of four identical, flat, injection molded, pentagon and hexagon shapes with circular (or elipitical) holes, which are shaped and plastically welded into spheres. Pentagon and hexagon edges are the same size and individual connected circles are only connected along one edge. The four shapes are clearly shown in colors above and from the patent. Solid lines on each part are hard connections, while..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/8054/what-is-this-y-shaped-screwdriver-bit-and-what-is-its-purpose"}
{"id": "engineering_8064", "domain": "engineering", "question_title": "What is the difference between the Polar Moment of Inertia, $ I_P $ and the torsional constant, $ J_T $ of a cross section?", "question_body": "This question is so fundamentally basic that I am almost embarrassed to ask but it came up at work the other day and and nearly no one in the office could give me a good answer. I was calculating the shear stress in a member using the equation, $\\frac{Tr}{J_T}$ and noticed, that for a shaft with a circular cross section, $J_T = I_P$. Both $I_P$ and $J_T$ are used to describe an object's ability to resist torsion. $I_P$ is defined as, $ \\int_{A} \\rho^2 dA $ where $\\rho$ = the radial distance to the axis about which $I_P$ is being calculated. But $J_T$ has no exact analytical equations and is calculated largely with approximate equations that no reference I looked at really elaborated on. So my question is, what is the difference between the Polar Moment of Inertia, $ I_P $, and the torsional constant, $ J_T $? Not only mathematically, but practically. What physical or geometric property is each a representation of? Why is $J_T$ so hard to calculate?", "question_score": 13, "question_tags": ["mechanical-engineering", "structural-engineering", "applied-mechanics", "stresses"], "choices": {"A": "This is to make sure they know what the foundation is made of. For all they knew there may have been an old tunnel underneath that would have collapsed when the new building is put on top. London is built on top of an old marsh, this type of soil is very prone to sinking and uneven settling, digging down and reinforcing the foundation alleviates that. It also ensures the foundation is uniform under the building to avoid a new tower of Pisa. Given the age of the city it may have been to scour the land for potential archaeological...", "B": "The torsion constant $J_T$ relates the angle of twist to applied torque via the equation: $$ \\phi = \\frac{TL}{J_T G} $$ where $T$ is the applied torque, $L$ is the length of the member, $G$ is modulus of elasticity in shear, and $J_T$ is the torsional constant. The polar moment of inertia on the other hand, is a measure of the resistance of a cross section to torsion with invariant cross section and no significant warping . The case of a circular rod under torsion is special because of circular symmetry, which means that it does not warp and it's...", "C": "As with all good things, it depends. If you can assume that your supports are totally stiff and that the loading on the shelf will be approximately uniform, then you basically have the following structure: A rectangular cross-section (such as a plank) will behave equally under positive or negative bending moment, so your objective should be to balance both. To do so, you want your main span to be $2\\sqrt2 \\approx 2.83$ times the cantilevers. This is found by calculating the cantilever required to offset half of the bending moment due to a uniform load along a simply supported beam:...", "D": "Some ideas: Wheel Load Distribution : The load is greater on the rear wheels providing the power; more force on the front ones bring no benefit and would provide less traction. Better manoeuvrability from having a shorter wheelbase. Better Ground Clearance in some conditions, especially for bumps and or up a increasing slope for instance. Better Driving : The front wheels now turn around a point closer to the C.G. than with the rear wheels. Not good with vehicle dynamics but this appears better than the rear end 'trailing' behind. Structural : As some people have pointed out, it's better..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/8064/what-is-the-difference-between-the-polar-moment-of-inertia-i-p-and-the-tors"}
{"id": "engineering_45701", "domain": "engineering", "question_title": "How was this toy made?", "question_body": "This is a toy for infants. My son was playing with it, and I started wondering how you could manufacture this. I can't think of any reasonably cheap way it could be done. I believe it's made of plastic and not silicone. It's flexible, but not elastic. My first thought would be injection molding, but I can't see any way to get parts of the mold in and out of the sphere. My wife suggested that perhaps the mold had no interior surface and was rotated like they do with chocolate, but I don't think that's the case, because if you look carefully you'll see that the inside surface is rounded and not flat. Anyway I'm stumped. How do you think they did it?", "question_score": 13, "question_tags": ["injection-molding"], "choices": {"A": "OP injection molding tag is correct. OBall uses injection molding and plastic welding. The OBall is the invention of David E. Silverglate. Toy Ball Apparatus with Reduced Part Count Reduced image from Kids II . It consists of four identical, flat, injection molded, pentagon and hexagon shapes with circular (or elipitical) holes, which are shaped and plastically welded into spheres. Pentagon and hexagon edges are the same size and individual connected circles are only connected along one edge. The four shapes are clearly shown in colors above and from the patent. Solid lines on each part are hard connections, while...", "B": "After the 2014 ICCT report revealed that these light-duty passenger diesel vehicles were emitting too much NOx and US regulators confronted VW about the results, VW did some testing and proposed a voluntary software recall to recalibrate the various emissions control devices on the affected vehicles. From the California Air Resources Board's (CARB's) in-use compliance letter to Volkwagen AG, 2015-09-18 , following that recall: To have a more controlled evaluation of the high NOx observed over the road, CARB developed a special dynamometer cycle which consisted of driving the Phase 2 portion of the FTP repeatedly. This special cycle revealed...", "C": "Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....", "D": "I'm going to set aside the potential legal liability aspects of your question. In part because you didn't ask about them, but also because liability will vary based upon jurisdiction. Obviously, consult an attorney familiar with the relevant law. Ethically, I think you've done everything that you're obligated to do. You have contacted the customer and notified them of an unsafe situation or configuration of the product. And you have (strongly) advised them that they need to discontinue using the older product in the unsafe configuration. If the customer remains insistent about using the product in an unsafe manner, you..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/45701/how-was-this-toy-made"}
{"id": "engineering_47905", "domain": "engineering", "question_title": "How can I tell if I got counterfeit aluminum?", "question_body": "I recently received an order of extruded aluminum strip 3/32\" thick that supposed to be 6061 hardened to specification ASTM B221. However, when we tried to bend it, it broke and the interior looked like cast zinc coated with aluminum. Is 6061 supposed to look that way and be unable to bend 90-degrees? How can I verify that I have solid aluminum and not some fake alloy with zinc in it?", "question_score": 13, "question_tags": ["metallurgy"], "choices": {"A": "I figured out a simple test. Since the density of 6061 is 2.5 g/cm3 and the density of zinc is over 7 g/cm3, all I had to do was measure the density of the material. This showed that indeed it was aluminum. So, its brittleness is probably just because it was hardened.", "B": "The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. \"Too hot\" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...", "C": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "D": "Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/47905/how-can-i-tell-if-i-got-counterfeit-aluminum"}
{"id": "engineering_6", "domain": "engineering", "question_title": "What is the process of transferring an engineering license from the United States to Germany?", "question_body": "The United States have different rules about exactly how one obtains an engineering license, but the general process is the FE exam, a few years of work, and the PE exam. Suppose one then wishes to practice engineering in Germany. What are the legal requirements for doing so?", "question_score": 12, "question_tags": ["licensure", "international"], "choices": {"A": "Unlike within the United States and the PE license , it doesn't appear that it's possible to receive an EUR ING license by direct comity . EU Engineering licensing is handled by FEANI , and on their EUR ING page they state: Application is open only to individuals if they are members of an engineering association represented in FEANI through a National Member But it may be possible to join one of the German FEANI members and see if they can support a reciprocity or comity process. 1 1 There are other member organization in other countries, but your question...", "B": "Electricity production optimization is a very complex subject. It is also affected by many parameters , which I will try to outline below. TL;DR: Demand is constantly monitored and supply is constantly adjusted TL;DR 2: Lshaver's excellent post is a suggested reading after reading this, because it expands and explains what happens at timescales ranging from the micro-second to minutes timescales. Changes in energy demand during the day First of all, the energy demand during the day, week, month of year can change quite a lot. Below is a typical day in New England. Fluctuation in power energy consumption a...", "C": "As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\\arctan\\left(\\frac{0.15}{15}\\right)=0.57^\\circ$ or 1%. the middle ,...", "D": "After the 2014 ICCT report revealed that these light-duty passenger diesel vehicles were emitting too much NOx and US regulators confronted VW about the results, VW did some testing and proposed a voluntary software recall to recalibrate the various emissions control devices on the affected vehicles. From the California Air Resources Board's (CARB's) in-use compliance letter to Volkwagen AG, 2015-09-18 , following that recall: To have a more controlled evaluation of the high NOx observed over the road, CARB developed a special dynamometer cycle which consisted of driving the Phase 2 portion of the FTP repeatedly. This special cycle revealed..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/6/what-is-the-process-of-transferring-an-engineering-license-from-the-united-state"}
{"id": "engineering_121", "domain": "engineering", "question_title": "Determining Drill Speed", "question_body": "How are spindle speeds calculated for a drill bit? I've seen dozens of charts that highlight the rpm that should be used for specific drill bit types, bit diameter, and material. However, what if my chart doesn't have the particular type of material or bit that I am using? I'd also like to have some intuition to know if the chart looks right or wrong. Upon some quick research, it appears that the \"cutting speed\" is what is ultimately needed for a particular material. I assume the cutting speed for each material must be looked up? Is there a standard or \"go-to\" place to find these? Then, information about the drill bit can be used to determine spindle speed. Again, what if I'm using a big hole saw or circle cutter, and it's not listed? How do I model the bit to use the cutting speed to determine rpm (for a given material of course)? I'd also like to know to calculate feed speeds for a drill or mill, but there are presumably more variables. It is probably better answered in another question.", "question_score": 12, "question_tags": ["machining", "mechanical-engineering"], "choices": {"A": "For long freight trains and those that will be climbing to stations at higher altitudes, an extra or two locomotives are attached to the front. I've always wondered why. As usual there are multiple issues. Most important is landscape. If more power than a single locomotive can provide is not needed for the whole trip, but only a short stretch, like climbing or crossing a mountain range, then it would be a huge waste of resources to attach them the whole trip. For example, take a 10 hour trip time, where all, except for a 1 hour stretch, can be...", "B": "You are correct that the cutting speed of the material is what determines the rpm for your drill-bit. This actually makes the calculation very simple. $$ \\text{Spindle speed (RPM)} = \\frac{\\text{Cutting speed}}{\\text{Circumference}} = \\frac{\\text{Cutting speed}}{π \\cdot \\text{Diameter}} $$ The thing you need to be careful of is the units of cutting speed and diameter. For example: Metric: If your cutting speed is in $m/min$ and your diameter is in $mm$ then you need to multiply your cutting speed by 1000 so that it is in $mm/min$ Imperial: If your cutting speed is in $ft/min$ and your diameter is in $inches$...", "C": "Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with...", "D": "When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/121/determining-drill-speed"}
{"id": "engineering_211", "domain": "engineering", "question_title": "What causes rechargeable batteries to age? What can be done to extend life of these batteries?", "question_body": "These days most of modern electronics use rechargeable batteries as a power source. Also, these days most modern rechargeable batteries are Lithium Ion or Lithium Polymer based. Like any other devices, over time these rechargeable batteries lose the ability to recharge, retain and discharge energy thus users have to replace the devices or rechargeable batteries. It is my understanding that the rise in battery internal resistance is the primary cause the rechargeable battery aging. Is this accurate? If so what can be done to lower or eliminate the internal resistance in rechargeable batteries. If my understanding is inaccurate what is the cause for rechargeable battery aging? If the causes for battery aging are understood, how can electronic engineers design charging and discharging circuits to extend the rechargeable battery life? References: Battery University All About Batteries, Part 1: Introduction All About Batteries, Part 2: Specifications & Terminology All About Batteries, Part 7: Lithium Thionyl Chloride", "question_score": 12, "question_tags": ["energy", "electrical-engineering", "energy-storage", "chemical-engineering", "battery"], "choices": {"A": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "B": "It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only...", "C": "I will expand on DKNguyen answer, because to my knowledge also the two reasons are: reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation) change the joint tightening characteristics (see joint diagram). reduce contact stresses on surfaces. The basic idea is that since contact stress is defined as: $$\\sigma = \\frac{F}{A}$$ Obviously the larger the area the less the stress. Also, like Nuclear Hoagie pointed out the area changes with a square law of the diameter. UPDATE 2 - Calculation for M6 bolt (thanks to BenC) *The question gathered enough interest for me to carry out a...", "D": "One of the problems that plagued older rechargeable batteries (e.g. Nickel Cadmium ($\\text{NiCad}$) and Nickel Metal Hydride ($\\text{NiMH}$)) was the memory effect . The memory effect occurs when a rechargeable battery is not fully discharged. It then \"forgets\" that it has a greater capacity than it thinks it has, and so in the future it discharges less. A good example is a water bottle. Initially, water bottles have a certain capacity for water. Let's say that I drink most of the water in a water bottle during one usage. If the memory effect affected water bottles, I would not be..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/211/what-causes-rechargeable-batteries-to-age-what-can-be-done-to-extend-life-of-th"}
{"id": "engineering_389", "domain": "engineering", "question_title": "Thermoelectric Technology to Harvest Energy from Internal Combustions Engines", "question_body": "Background: In an automobile, only 1 / 3 of the potential energy in the fuel is converted into mechanical energy and significant portion of the energy is lost as heat. There have been previous attempts to recuperate this lost energy. In the early 1990's, Porsche developed automotive thermoelectric generators (ATEG) which didn't go past prototyping stage. Currently, Porsche Motorsports is testing a thermal energy harvesting system in their LeMans series Race car. In addition to Porsche's research, GM is in collaboration with Future Tech, LLC. to explore the idea of using themoelectric technology to harvest energy from internal combustion engines. Other automotive manufactures, such as BMW, are also exploring this technology. Currently the power usage in a Small car is approximately 150 W Full size truck is approximately 500 W If this technology can successfully be implemented, then components such as the radiator, water pump, and alternator could effectively have reduced workload or removed from the system, thus reducing the load to the internal combustion engine. Question: With the growing interest in green technology, are there technology barriers beside efficiency that are preventing the implementation of energy harvesting from internal combustion engines using thermoelectric technology? References: Which one must be used matched output voltage or open circuit voltage? Benefits of Thermoelectric Technology for the Automobile The Promise and Problems of Thermoelectric Generators Modeling of an Automotive ThermoElectricGenerator (ATEG) Thermoelectrics to replace car alternators and improve MPG Thermo-Electric Generator in Turbocharged Diesel Engine Kettering University researchers are working with General Dynamics to convert the unused heat energy of their propulsion systems to useful and clean energy Porsche 919 Hybrid LeMans Racer Goes After The Two Thirds of Gasoline’s Energy That’s Wasted As Heat Germans trying to replace Alternator with Thermoelectric Generators or TEGs Porsche 919 Hybrid LMP1 Le Mans prototype... Footnote The suggested duplicate is related, but still distinctly different. The order of magnitude of energy available to recover from an internal combustion engine is significantly greater than within the GPU of a video card. As such, the economies of scale are different and different solutions are therefore possible.", "question_score": 12, "question_tags": ["mechanical-engineering", "electrical-engineering", "thermodynamics"], "choices": {"A": "This is to make sure they know what the foundation is made of. For all they knew there may have been an old tunnel underneath that would have collapsed when the new building is put on top. London is built on top of an old marsh, this type of soil is very prone to sinking and uneven settling, digging down and reinforcing the foundation alleviates that. It also ensures the foundation is uniform under the building to avoid a new tower of Pisa. Given the age of the city it may have been to scour the land for potential archaeological...", "B": "As with all good things, it depends. If you can assume that your supports are totally stiff and that the loading on the shelf will be approximately uniform, then you basically have the following structure: A rectangular cross-section (such as a plank) will behave equally under positive or negative bending moment, so your objective should be to balance both. To do so, you want your main span to be $2\\sqrt2 \\approx 2.83$ times the cantilevers. This is found by calculating the cantilever required to offset half of the bending moment due to a uniform load along a simply supported beam:...", "C": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...", "D": "As with any new technology, the cost is the big driver here. In addition, these devices produce electricity which is a form of energy that a typical internal combustion automobile can only utilize for ancillary equipment. This would effectively improve fuel efficiency but the gain would be relatively minor. Engineers are generally reluctant to use expensive new technologies that are relatively untested when existing methods are sufficient for achieving the goal. In this case, most automobile manufacturers strive to produce a cost-competitive product. The people who are most concerned with fuel efficiency will tend to consider a hybrid (or all-electric)..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/389/thermoelectric-technology-to-harvest-energy-from-internal-combustions-engines"}
{"id": "engineering_1718", "domain": "engineering", "question_title": "What does "$\\pm$ 0.5% F.S." mean?", "question_body": "I've seen it in several data sheets - it is a measure of error of some kind, of course. The problem is I dont know the exact meaning. I've seen it in the context of repeatability, accuracy and linearity. An example is the following data sheet: smc data sheet (On page 3)", "question_score": 12, "question_tags": ["nomenclature"], "choices": {"A": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "B": "I will expand on DKNguyen answer, because to my knowledge also the two reasons are: reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation) change the joint tightening characteristics (see joint diagram). reduce contact stresses on surfaces. The basic idea is that since contact stress is defined as: $$\\sigma = \\frac{F}{A}$$ Obviously the larger the area the less the stress. Also, like Nuclear Hoagie pointed out the area changes with a square law of the diameter. UPDATE 2 - Calculation for M6 bolt (thanks to BenC) *The question gathered enough interest for me to carry out a...", "C": "Electricity production optimization is a very complex subject. It is also affected by many parameters , which I will try to outline below. TL;DR: Demand is constantly monitored and supply is constantly adjusted TL;DR 2: Lshaver's excellent post is a suggested reading after reading this, because it expands and explains what happens at timescales ranging from the micro-second to minutes timescales. Changes in energy demand during the day First of all, the energy demand during the day, week, month of year can change quite a lot. Below is a typical day in New England. Fluctuation in power energy consumption a...", "D": "FS = FULL SCALE = maximum reading. It means that the accuracy is such that the reading is probably within + or - 0.5% of the FULL SCALE reading. This is a very important and easily overlooked qualification of the result. If I have a reading of 1 Volt and the accuracy is +/- 0.5% it means that the actual result should lie in the range 1 - 0.5% x 1 to 1 + 0.5% of 1 = 0.995V to 1.005 V However - if I measure the result on the 10V range then 0.5% of 10V = 0.5% of..."}, "answer": "D", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/1718/what-does-pm-0-5-f-s-mean"}
{"id": "engineering_1767", "domain": "engineering", "question_title": "At what point does an I-beam becomes a H-beam?", "question_body": "According to BS5950, a beam section can be classified as plastic, semi-compact, compact or slender. For the same section area, a H-beam can take axial compression (without buckling) better than an I-beam, and as such, uses a different strut curve in the code: Now, I understand that a H-beam has a wider flange compared to an I-beam, but at what point, precisely, does this transition from I- to H- occurs? For example, is a 400x300 (depth x width) beam considered a H- or an I-beam? Update: Extracted from BS5950 guide, the following table shows H-beams (also known as universal columns, some of which with depth greater than width. This is the reason why I don't believe the differentiation is so straight forward.", "question_score": 12, "question_tags": ["civil-engineering", "steel", "beam"], "choices": {"A": "I'm going to set aside the potential legal liability aspects of your question. In part because you didn't ask about them, but also because liability will vary based upon jurisdiction. Obviously, consult an attorney familiar with the relevant law. Ethically, I think you've done everything that you're obligated to do. You have contacted the customer and notified them of an unsafe situation or configuration of the product. And you have (strongly) advised them that they need to discontinue using the older product in the unsafe configuration. If the customer remains insistent about using the product in an unsafe manner, you...", "B": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...", "C": "BS5950-1:2000 Clause 1.3.23 defines an H-section as having \" an overall depth not greater than 1.2 times its overall width \", and Clause 1.3.25 defines an I section as having \" an overall depth greater than 1.2 times its overall width \". Note that at exactly a ratio of 1.2, it would be an H section not an I section.", "D": "So there's an incorrect assumption underlying your question. In an ideal world the lifting capacity required of the jacks would be the self-weight of the bridge divided by the number of jacks (+ allowances for wind/snow, etc.). And the assumption there is that the lifting capacity is equivalent only to the weight of the bridge. The problem is that if anything goes wrong, you're likely to see a catastrophic failure of some sort which could lead to irreparable damage. Real world lifts don't operate in that \"ideal\" manner, and instead rely upon a safety factor in order to make sure..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/1767/at-what-point-does-an-i-beam-becomes-a-h-beam"}
{"id": "engineering_2015", "domain": "engineering", "question_title": "Why are most standard bolt threads single start?", "question_body": "When looking at thread descriptions, one of the basic properties is always the number of thread starts. As far as I could tell, all of the major standard bolt threads are single-start. This includes: Unified Standard (UNC, etc.) National Pipe Thread (NPT, NPS) British Standard I only found one standard thread that can also come in a multiple-starts: ACME . What are the reasons why single-start threads are so common and multiple-start threads are rare? I am specifically interested in bolts and other fasteners.", "question_score": 12, "question_tags": ["threads", "fasteners"], "choices": {"A": "If the receiver does not detect the sub-carrier for the \"colour burst\" signal which is transmitted during the horizontal blanking period the receiver switches on the \"colour-killer\" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise. Figure 1. The colorburst signal is transmitted on the \"back porch\" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately...", "B": "Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with...", "C": "As Dave Tweed points out, the ratio of torque to tension is lower the lower the lead angle is. Since the important measure of bolt tightness is generally the tension in the bolt, we want to achieve that minimum pretension with the least effort possible. Assuming we have to maintain a certain shear area of the thread (so the the threads are stronger than the bolt when fully engaged) having two starts means we double the lead angle and greatly increase the amount of force required in the wrench to tighten the fastener appropriately. On its own though, this isn't...", "D": "The problem is that the formula $P=I\\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\\ V\\ \\cos(\\phi), $$ where $\\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual..."}, "answer": "C", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2015/why-are-most-standard-bolt-threads-single-start"}
{"id": "engineering_2071", "domain": "engineering", "question_title": "Is interference between aircraft an issue for fly-by-wireless technology?", "question_body": "I was reading up on fly-by-wire development, and I saw a short section about fly-by-wireless technology. It seems like a great idea, with the potential to lower costs, weight and complexity. I can see a possible scenario where it could be an issue, though: Two aircraft are very close together (e.g. on a runway or flying in formation). One pilot transmits commands through the aircraft's fly-by-wire system to other parts of the aircraft. The other aircraft accidentally receives the signal because it is so close. Things get very bad very quickly. The thing is, I haven't been able to find any technical specifications regarding fly-by-wireless systems, and I have no idea if the transmission would be powerful enough to reach the other craft, nor if it would then be interpreted as actual data sent from that aircraft's pilot. Is this cross-interference between fly-by-wireless systems possible? If so, how can it be mitigated?", "question_score": 12, "question_tags": ["electrical-engineering", "aerospace-engineering", "emc", "telecommunication", "product-testing"], "choices": {"A": "Cross-interference between aircraft is a high unlikely event because all commercial aircraft designs have to pass DO-160 environmental testing requirements. Among the DO-160 testing specification is EMI/EMC Testing. These tests include Radiated Emission, Interference and Immunity Testing. Part of these tests are to answer \" Two aircraft are very close together \" and \" The other aircraft accidentally receives the signal because it is so close. \" Below is a picture of an anechoic chamber used to test aircraft. Fly-by-wire allows the aircraft control system to access and command monitoring and control systems. The monitoring and control systems have a...", "B": "As it happens, I just recently went through that calculation myself for a different site. Given the following facts from a quick web search, it isn't difficult to work out the numbers. The maximum efficiency of a (large) windmill is about 40%. The density of air is 1.225 kg/m 3 You need about 50 mW (10 mA at 5V) to light up an LED First, we'll need about 50 mW / 0.40 = 125 mW of air power flowing through the windmill to create the electricity we need (ignoring other factors such as the actual efficiency of a small windmill...", "C": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...", "D": "I think you are talking about roundabouts, not traffic circles. It is baffling to those of us in the UK that Americans think roundabouts are a new idea. In the UK we have so many variants, from mini-roundabouts all the way up to full motorway junctions (a giant roundabout above or below the motorway). So do roundabouts take up more space? Not necessarily, this is a mini roundabout: It's nothing more than a slightly domed area of paint on the road, no lights are necessary, you can actually drive straight over the top of it rather than around it, its..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2071/is-interference-between-aircraft-an-issue-for-fly-by-wireless-technology"}
{"id": "engineering_2114", "domain": "engineering", "question_title": "How to calculate lever force when lever has uniformed distributed load?", "question_body": "We have a simple class 1 lever: $$\\begin {array}{c} \\text {5,000 kg} \\\\ \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\, \\downarrow \\\\ ==================== \\\\ \\hphantom {==} \\triangle \\hphantom {==============} \\\\ \\vdash \\text{1 m} \\dashv \\vdash \\hphantom {======} \\text{4 m} \\hphantom {======} \\dashv \\\\ \\end {array} $$ The lever ($===$) is 5 m long. The fulcrum ($\\triangle$) is 1 m from one end of the lever. The lever has a object sitting uniformly upon it weighing 5,000 kg. How do I compute the upward force that needs to be exerted at the end of the 1 m side of the lever to keep the lever stationary? $F = (W \\times X)/L$ is simple when the weight is applied at the very end of the lever. But what happens if the weight is distributed along the lever? Our final goal is to tether the free end (on the 1m side) to keep the lever level and we need to know how strong the tether should be.", "question_score": 12, "question_tags": ["mechanical-engineering", "statics"], "choices": {"A": "Answer: Yes , this is exactly how it is done in the real world. What you have described is what I do in my job to check systems in CAD. Since you indicated you would like me to step through my design process, I have detailed it below. Note that most of this does not involve CAD. CAD is invaluable, but only if you are prepared to get out the pencil and paper first. Also note that this is just my design process, it is by no means the only way to go about things. Preparation When I begin any...", "B": "Since the mass is 5k kg and the lever is 5m, this makes it quite easy to simplify because it is exactly 1k kg per m. The leftmost 2k kg (2m) of the mass has its center of mass exactly above the fulcrum so can be ignored as it provides no contribution to the moment. This leaves 3k kg (3m) spread from 1m to 4m on the right side. The center of mass will therefore be at 2.5m. Now it's super-simple, assuming you want the moment when the lever is level (i.e. when gravity is pulling straight down, perpendicular to...", "C": "OP injection molding tag is correct. OBall uses injection molding and plastic welding. The OBall is the invention of David E. Silverglate. Toy Ball Apparatus with Reduced Part Count Reduced image from Kids II . It consists of four identical, flat, injection molded, pentagon and hexagon shapes with circular (or elipitical) holes, which are shaped and plastically welded into spheres. Pentagon and hexagon edges are the same size and individual connected circles are only connected along one edge. The four shapes are clearly shown in colors above and from the patent. Solid lines on each part are hard connections, while...", "D": "In the video you've linked to, the spheres are seen on both the leading and trailing edges of the propeller: I expect they are intentional - there are a number of ways to attach a propeller without having to disturb that surface, or using flush caps. Cavitation is caused by a drop in pressure. This would be seen on the trailing edges of the propeller, and is worse on the outer edge of the propeller which is moving faster through water than the inner area. I doubt these spheres have any effect on cavitation at all. It is either a..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2114/how-to-calculate-lever-force-when-lever-has-uniformed-distributed-load"}
{"id": "engineering_2690", "domain": "engineering", "question_title": "Will more electricity be generated by using a lens to focus sunlight onto solar cells?", "question_body": "I have been wondering about this question for quite some time. Assuming an ideal case, the energy from photons hitting solar cells is converted into electric energy as described by the equation: $RI^2t=W\\equiv E=\\hbar\\nu$ where $\\nu$ is the frequency of photons. Using a lens won't increase the frequency of photons, thus no extra electricity is generated. Am I correct in thinking that no extra electricity will be generated by solar cells when a lens is used to focus light onto them?", "question_score": 12, "question_tags": ["electrical-engineering", "optics", "photovoltaics", "solar-energy"], "choices": {"A": "Your equation is partly correct. You've calculated the energy per photon ($\\hbar \\nu$), but you've neglected the number of photons. That's why the units don't match (power is energy per unit time, while you've only got energy for each photon). The ideal power (energy per unit time) depends on the area of the solar panel, $A_p$, the number of photons striking it per unit time ($\\Phi$) and the energy of each photon, $E$, such that $W_{Ideal} =A_p \\cdot \\Phi \\cdot E$. A lens or mirror can focus light (a flux of photons) onto a small area. Under really ideal conditions,...", "B": "As others stated before, induction loops are the primary - most reliable method: the coils (usually just several loops of wire) embedded in the road; fed given frequency from a generator, in presence of metal the frequency of the LC circuit changes and the sensor circuitry detects the change of frequency, producing a presence signal. In some cases these may fail to detect bicycles, but they are by far most common as they aren't affected by weather (or more precisely, the detection circuit tunes in to slow changes of frequency caused by weather) and are immune to accidental false positives....", "C": "If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...", "D": "On this processor, the register that holds the conversion result is 16 bits wide. A right-justified result means that bits [( N -1):0] (where N is the number of bits of precision) of the register contain the ADC value and the most-significant bits of the register are set to zero. A left-justified result means that bits [15:(16- N )] of the register hold the result, and bits [(15- N ):0] are set to zero. For example, if your actual conversion result is 0x123, it would be read as 0x0123 if the register was right-justified, and as 0x1230 if it was..."}, "answer": "A", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2690/will-more-electricity-be-generated-by-using-a-lens-to-focus-sunlight-onto-solar"}
{"id": "engineering_2725", "domain": "engineering", "question_title": "How long does it take for dust to settle out of the air?", "question_body": "In order to make this a manageable question, let's add a few simplifications. The dust particles can be well described as uniform spheres of radius $R$ and density $\\rho$. The space is enclosed and there is no bulk flow, i.e the air is still in a macroscopic sense. The air is at the standard temperature and pressure (STP) ; $T=20\\ ^\\circ\\mathrm{C}$ and $P=1\\ \\mathrm{atm}$. Under these conditions, what is the settling time for dust particles? At what size/density does Brownian motion of the air become important?", "question_score": 12, "question_tags": ["fluid-mechanics", "air-quality"], "choices": {"A": "Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste...", "B": "Solid particle settling time in air depends mainly on the size of the particle. Different forces become significant depending on what size range you're talking about, so it's hard to give an answer that's both concise and accurate. I'll do my best to synthesize the important points rather than parrot a reference; that said, where practical applications in the field of air quality are concerned, the text I recommend is Air Pollution Control by Cooper & Alley . In particular, I'm going to pull many of the details for this answer from Section 3.3: Particulate Behavior in Fluids. Gravitational Settling...", "C": "Electricity production optimization is a very complex subject. It is also affected by many parameters , which I will try to outline below. TL;DR: Demand is constantly monitored and supply is constantly adjusted TL;DR 2: Lshaver's excellent post is a suggested reading after reading this, because it expands and explains what happens at timescales ranging from the micro-second to minutes timescales. Changes in energy demand during the day First of all, the energy demand during the day, week, month of year can change quite a lot. Below is a typical day in New England. Fluctuation in power energy consumption a...", "D": "The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for..."}, "answer": "B", "distractor_source": "same_domain_answer_pool", "source": "stackexchange", "license": "CC-BY-SA 4.0", "url": "https://engineering.stackexchange.com/questions/2725/how-long-does-it-take-for-dust-to-settle-out-of-the-air"}