ScienceNostalgia101

https://www.youtube.com/watch?v=ci8ZXxYDajQ So in this Southland scene, a stolen taxi is aimed directly at a police car and driven toward them for a head-on collision. The cops swerve to avoid this. I'm just wondering now; is there any way to deploy spikes in the front of police cars, with springs behind the spikes, such that the engine and/or tires of the vehicle in front of them can be punctured, while the springs absorb the impact so the car's speed doesn't change as abruptly as that of the vehicle in front of them? That way, if this sort of thing happens in real life, they can deploy the spikes instead of swerving and stop the criminal sooner.

So we as a society have categorized weight into "underweight," "healthy," and "overweight." We assume there is an interval of weight that is healthier, in and of itself, than weight above or below it. However, what I'm left wondering is; how do we distinguish the effects of weight itself from the high cholesterol that often happens to; but does not always; correlate with it? I was once underweight. I tried to put on the weight by eating a lot of fatty foods; ones that happened to be especially high, however, in saturated fats. That left me with high cholesterol while I was still underweight. It stands to reason that one can be overweight with low cholesterol. Americans also eat a lot of foods high in saturated fat, so it stands to reason the overweight Americans will be more likely to have high cholesterol. So how, then, do they know that the "weight" is what's causing their health problems, and not, you know, the cholesterol that tends to go with it? Have there been any studies on people who became overweight by, let's say, overeating avocado? Overeating peanuts? Overeating fish? Or any other such foods that are low in saturated fat and high in unsaturated fat?

Time and time again, some part of the world's forests are on fire. I've discussed in this thread the notion of a network of water pipes above the forest, and I've been told it's unfeasible. I'm not sure if that has changed, but in the meantime, I intend to focus on the other options. A. Would increased logging of temperate forests significantly reduce the amount of combustible material in the long run? I've heard it temporarily increases it before decreasing it; why is that? (I would assume they'd find use for the wood, if not by incentivizing paper use as an alternative to other materials, then by incentivizing ) B. What if they created a large criss-crossed pattern of permanent firebreaks such that the larger "forest" is then divided into a set of smaller forests, such that only one of these smaller forests can catch fire at a time, such that the forests can still use fire to replenish themselves without presenting as much of a danger to human lives?

What about enough heat to boil water for cooking food? That's useful even when it's warm. Also, if every household had a concave mirror for a rooftop, more sunlight would be absorbed by the boilers and less would be absorbed by anything else. Wouldn't that reduce the urban heat island effect? Could it also reduce the city's albedo to below what that of the pre-existing natural environment would have been? As for hail being not a big deal to solar panel rooftops, how come it's such a popular anti-solar talking point? Engineering angle aside, would replacing solar panels with concave mirrors do more good to the pro-solar movement or more harm from a PR standpoint? EDIT: And in light of Australia's wildfires, should I bump the "harnessing the energy of forest fires" thread or continue that discussion here?

A number of my past posts relate to hypothetical alternative sources of electrical power. I'm wondering if it'd be more efficient (pun intended) to coalesce them all into one megathread, such that people could compare and contrast these hypothetical proposals. I'd like to reiterate all these proposals, and a few new ones: A. One common criticism of household solar panel rooftops is that they're vulnerable to hail damage. I'm not sure if there's some way to mitigate this vulnerability that the fossil fuel industry doesn't want us to know or whatever, but in the meantime, since thermal solar isn't as vulnerable to hail damage, would a better approach be to have some rooftop concave mirror and/or rooftop convex/fresnel lens, to boil water? (This might also be more efficient twofold in having a direct source of heat, instead of having to convert heat into electricity and then electricity into heat.) If fire safety's the issue, would it be worthwhile to have the water supply BE the convex lens; namely, a watertight (apart from some outlet/intake tubes at the top) roof filled with water, such that the point of convergence is some dark surface within this roof that happens to be surrounded by water? B: So lightning is a form of direct current, is it not? If so, does that mean that some device designed to attract lightning, and run it through some electrolysis device, would be able to convert its electrical energy into chemical energy?

In introductory chemistry at university I heard of an equation called the Clausius-Clapeyron equation relating vapour pressure to temperature of a substance. I vaguely recall hearing it applies to other liquids than just water. What I do not recall hearing is whether or not it applies solely to substances in their liquid phase. Does it apply to solids? Does it apply to solids dissolved in liquids? Does it apply to liquids dissolved in other liquids?

Let's say you're graphing some function... for simplicity's sake, some quantity over time. There's a time interval that needs to be skipped because nothing happens during that time interval. What would be the proper way to skip that interval on the x-axis? I've seen zig-zags, dotted lines, etc... and I would like to be filled in on what is the correct way. Thank you in advance.

What about vertical structures designed to capture the moving air and use it to turn turbines? Would that be more practical?

https://www.nhc.noaa.gov/refresh/graphics_at5+shtml/115127.shtml?cone#contents So at this moment Hurricane Dorian is forecast to make a direct hit on Lake Okeechobee. If the floodwaters extend from the lake to the ocean, they'll essentially be the same body of water. But then I remembered, lakes are connected to rivers, which are connected to the ocean. This got me thinking; where does one body of water end and the next begin? Where does a river end and the ocean begin? Is there some threshold of salinity? If so, does that mean the boundary between a river and the ocean shifts as that threshold shifts? How do we distinguish rivers from lakes/ponds? Is there some metric of the velocity of the water? Does increasing/decreasing flowrate therefore shift this boundary?

Which suggests it would take a long time to turn a profit, but doesn't suggest that it won't eventually do so, especially if someone would otherwise exercise a comparable amount of time anyway. Why does no one want to invest in this?

We have coal, oil, gas, nuclear, and thermal!solar power plants that work by burning these boiling water, and turning turbines. We have wind and hydroelectric power plants that use their respective fluids to turn turbines directly. And yet, when people want to work out, they get on devices that USE electricity to run a motor. Something isn't right here. Is there any way to design a treadmill and/or stationary bicycle that can generate electricity from the user's motion?

Forgot about this thread until now. Here's another one. At 6 minutes and about 20 seconds in, the fish claim they're going to "roll" the bags by swimming in water that's inside the bags. I guess the idea is that they're going to swim in a line that does not cross the center of mass, but that still leaves behind a question. If in swimming forward they push water backwards, is there any way for the torque they generate by this action force to not be cancelled out by the water-pushing reaction-force?

Despite my background in physics, I've never fully wrapped my head around what weight's supposed to be, other than "not the same thing as mass." It used to hardly even matter, but now I need to know for my students. For instance, if an astronaut is in orbit around the Earth, and the centripetal force is provided by gravity at precisely the right magnitude for the astronaut to feel "weightless" in this non-inertial reference frame, said astronaut would experience "weightlessness," but force of gravity would not be zero. What would be zero is the normal force, as there is no need, until the astronaut encounters an object in spite of weightlessness, for a normal force. So is weight supposed to be the normal force, the gravitational force, or something else?

I ask this as a teacher, not a student... if you have three sets of parallel plates... one of +2a and +a electric potentials, one of of +a and -a electric potentials, and another of +a and 0 which has the higher potential energy? I assume it's +a and -a as it relates to potential difference, I just want to make sure.

This is school related, but I ask it as a teacher, not a student. https://wikimedia.org/api/rest_v1/media/math/render/svg/8c6ee5510ba3c7d6664775c0e76b53e72468303a The above is considered the standard form of the Universal Law Of Gravitation. However, if someone gave the following... https://wikimedia.org/api/rest_v1/media/math/render/svg/ebf0689fbd05781a129e2df24ef5bd8b7edf2f93 ...except without the function notation or r-hat notation, would this count as merely derived from the Universal Law of Gravitation, or as a form of it in and of itself?

One of the advantages to farming in a dry state like California is that with less rain, you have so much more sunlight. One of the disadvantages is that you have to use a hell of a lot of non-salty water just to make it work. What I'm wondering is, what's stopping it from seeping into the groundwater, and gradually shifting toward non-farmland vegetation that could use it too? I look at the fact that California is on fire again; it seems to be an annual thing; and I find it odd that such a supposedly dry state could have naturally built up enough combustible vegetation in the first place to sustain several years' worth of forest fires. I cannot think of any other state as fire-prone as California. Could irrigation have played a role in this, or am I just barking up the wrong tree?

Wouldn't drilling into the side cause the pressure from all the lava at and above that point to force it out of the hole? I get that it's more viscous than water, but still, that's an enormous pressure gradient at the hole. I'm not sure what you'd need to predict... that the volcano won't erupt before installation is complete? If so, why not install the setup remotely?

Subject came to mind again because of post-tropical storm Oscar. How expensive would any materials that can withstand hurricane force winds for the purposes of structures like these be? (For what it's worth, I accept that the "rows of wind turbines to prevent hurricane development" option is out of the question.)

Well, being that particles dissolved in water are known to cause turbidity, I would guess either they collected particulates from the air or bacteria/viruses added impurities. Not sure if saltwater could prevent that... probably not. And being that "convex lens increases converging power focusing light on smaller area" was the point of my idea of using a concave mirror in the first place, I think I might as well abandon that idea. Still though, why can't they make more solar collectors in Australia? There's vast amounts of unused desert out there, and even if they can't use all of the electricity themselves, could they use it to generate hydrogen gas they can then sell to other countries? Surely a boat carrying the gas should be reasonably easy to keep afloat. (Assuming at-sea hydrogen pipelines aren't an option.)

Would the seawater be just as likely to turn green if it were filtered or no? Is it a function of the microbes, the salinity, or just being left out in the open long enough? Interesting concept on Bingham Canyon; would drawing attention to that idea make it more likely to get implemented or less so?

You can't pay them to accept the presence of armed guards protecting the cable? Ok, so maybe North Africa is not the best place. What about other massive deserts, like the aforementioned central Australia? If you constructed a concave mirror there, whether by conventional means or electric repulsion shaping a sheet of metal, and used the electricity to pump seawater out of the ocean and into the concave mirror, would the water then double as a convex lens, increasing its converging power, increasing the amount of water pumped into the concave mirror? Could the excess water then be desalinized and pumped toward neighbouring countries? I ask because for the prior rainwater collection thread I calculated that the estimated sea level rise is by 310 thousand cubic kilometres... however, a half-sphere with a radius of 53 kilometres wide could accommodate all of that without overflowing.

So why aren't they being done more, then?

Interesting. All of China? Does that mean I should be able to see it in 2020 if I visit Shanghai? Speaking of the idea of a large concave mirror, here's another thing I was thinking about. The surface area of the Sahara desert is 9.2 x 10^9 square metres. Sunlight's radiant intensity is 1050 watts per square metre. Doesn't that suggest that a solar collector the size of the Sahara could have a power of 9.7 x 10^12 watts? Wouldn't that be several orders of magnitude more power than all the USA's nuclear power plants combined, being that 805 billion kilowatts are 2898000000000000 joules per year, or 91832078 per second? What's the cheapest highly-reflective material with which you could line the Sahara, assuming you could eventually make this pay for itself?

This video showed the construction of a modestly large concave mirror using a fairly meticulous process of carefully placed reflective foil. However, I would presume that, the larger the scale on which it's conducted, the less any wrinkles in the structure matter, at least for the purposes of collecting sunlight. I'm wondering now; is there any cost-effective way to have outer space robotics (Canadarm, etc.) assemble a giant amount of reflective material in outer space? Would electrostatic repulsion and/or magnetic attraction serve to force a giant sheet of reflective material into a concave shape? Could this concave mirror then be used to melt the sands of the Sahara into glass for the purposes of the aforementioned water reservoir idea? Alternatively to mirrors, what about lenses? If one could, using some space elevator or other means, carry a mold impermeable to water into space, and pump a giant amount of seawater into it before it freezes, could this then be used as an outer-space converging lens made of ice, to concentrate sunlight into some massive solar collector? "Geostationary" orbits aside, is there any way to position either's orbit such that it's always at the same angular position relative to the sun? This is not to condone either such project, as I'm guessing there are potential side-effects of which I'm not yet aware, I'm just asking out of curiosity.

So according to this video, flying perpendicular to a magnetic field, including a constant magnetic field at constant velocity, induces emf. Assuming it's right... 1. Would the resulting electric field (in volts per metre) be concentrated in the wingtips, concentrated at the center, or constant from the left tip to the right tip? 2. Would the airplane essentially be functioning as a capacitor as a result? If an airplane were flying through a stronger magnetic field (let's say several teslas) would this in turn cause it to form lightning with surrounding clouds? If it were between two clouds, would it attract electrons in one, repel them in another, and therefore function as part of a complete circuit? 3. In the event an airplane were flying through stronger and stronger magnetic fields, what would become more harmful more quickly? The induced emf or the magnetic field itself? Would the charges being separated have any reason to flow through the bodies of the people on board, whether during or after said charge separation? Would electrons being concentrated on one side of the craft eventually disintegrate the side with fewer electrons in it?