# ScienceNostalgia101

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## Posts posted by ScienceNostalgia101

1. ### Where does one body of water end and the next begin?

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?

On 8/28/2019 at 2:27 PM, swansont said:

Yes. The issue is how much, and is it worth the fuss. QuantumT mention a bike dynamo, which lit a small light and added a noticeable load to one’s pedaling. I remember a demonstration in high school where a tandem bike was hooked up to a generator. The pair struggled lighting up an incandescent bulb, and a hair dryer basically brought them to a stop.

Humans just don’t generate all that much power. There’s a reason we went with animals and machines.

A 100 kg person going up 3 flights of stairs - 10 meters - does just under 10000 joules of work. If that happens in 9.8 sec, that’s a kilowatt. How long can a person do that kind of exercise? In an hour, they will have generated around 10-12 cents worth of electricity (it will vary with local market conditions)

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?

4. ### Implications of movie physics

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?

5. ### Exact scientific definition of weight

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?

6. ### Potential energy of parallel plates?

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.

7. ### Universal Law Of Gravitation

This is school related, but I ask it as a teacher, not a student.

The above is considered the standard form of the Universal Law Of Gravitation. However, if someone gave the following...

...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?

8. ### Did irrigation contribute to California wildfires? (Disclaimer: Speculative)

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?

9. ### Volcanic power?

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?

10. ### Harnessing the power of hurricanes

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.)

11. ### Giant solar collectors in space?

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.)

12. ### Giant solar collectors in space?

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?

13. ### Giant solar collectors in space?

6 hours ago, mistermack said:

Since there's no prospect of a large population moving to the Sahara any time soon, the only way you could make solar energy pay on a gigantic scale would be to lay an enormous cable across the Mediterranean and up through France to central Europe.

The cost would be phenomenal, and it would only make sense, if there were stable reliable governments in North Africa. None of that is likely to happen.

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.

14. ### Giant solar collectors in space?

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

15. ### Giant solar collectors in space?

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?

16. ### Giant solar collectors in space?

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.

17. ### Induced emf in airplane wings

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?

18. ### How fast does groundwater contamination spread?

I ask because I keep hearing of groundwater contamination, yet never hear of how quick the process is, vs. how gradual the process is.

19. ### Volcanic power?

Please pardon the crude illustration, this is merely meant to make it clearer where I'm going with this.

I'm picturing a power source; I'm not sure whether it's already covered under the "geothermal" umbrella or not; that uses the heat of volcanoes to turn turbines through steam.

The idea is that water would be pumped into an active volcano, and a tube would be placed at or near the top to contain the resulting volcanic gases (or at least most of them) and use the force applied by their escaping; or at least from the steam escaping; to turn turbines.

This leaves me with a few questions.

1. Would this be a "renewable" resource, wherein heat from the mantle will continuously re-melt the magma frozen by the water, or would too thick a layer of frozen rock eventually block the heat permanently?

2. Either way, would the amount of power required to pump water to the summit of a volcano be greater than or less than the amount of power generated by the escaping gases?

3. If seawater and/or sewer water were used, would the effects thereof on the chemical composition of said magma be carried by the mantle toward other volcanoes?

20. ### Rainwater collection

On 10/23/2018 at 10:56 AM, DrP said:

Being honest - the surface area of the combined rooftops across town is quite a large area already...  all that water gets collected by the guttering on the run off of the roofs. Putting a big bucket over the city won't collect that much more...   It maybe needs to be directed somewhere other than the drainage system/sewer once it runs off the roof?...

....  maybe by collecting it somewhere and pumping it off to a reservoir or something.

Indeed, that's why I've since replaced the "over a city" part with "at sea." The "under the ITCZ" qualifier is simply because you get more rain there than at the polar front. However, the key point is the "at sea" one. Here's a quick visual of what I'm talking about, which I probably should've used sooner...

Since seawater is 1.02 to 1.03 grams per mL, while water is just 1; and since volume is proportional to r^3; my idea was that the column height for freshwater could be 1.0066 to 1.01 times that of the surrounding seawater, if hull mass is considered negligible. (Depending on necessary thickness, resilience, and lightweight nature of the material.) For a, let's say, 100m seawater column height, this could correspond to a 1 metre water level difference, allowing it to be pumped to a 1 metre tall reservoir with a net zero vertical displacement. (Misinterpreted the definition of siphoned, but the idea is to avoid using more pump power than necessary.)

Of course, on further consideration, it's doubtful one could use a large enough (by surface area) region of Earth for 1m to work for the "offset ocean level rise" purpose anyway. Earth's surface is 155 million square km; if it all rose by 2m, (ie. 0.002km) that'd still be 310 thousand cubic km. You'd need a 68km by 68km by 68km reservoir to store that. Highest mountains are less than 9km, so it'd be more like 9km by 204km by 204km. Any geology majors here know of any large valleys surrounded on all sides by material impermeable to water? Or at least of large enough regions of the Earth's surface from which water can't seep into the surface from above to save on floor construction?

However, for the "alternative to desalinization" purpose, that still leaves behind the question of whether or not a pump could use electrical power more efficiently than desalinization.

. . .

For the record, I'm well aware that most uses of desalinized water result in it evaporating and eventually forming rain again.

21. ### Implications of movie physics

Gah, I forgot all about the thermal effects of all that air resistance.

Anyone know more about air resistance's effects on bullet acceleration?

22. ### Turning planets or moons into convex mirrors?

On 10/20/2018 at 8:13 PM, pavelcherepan said:

Even if you did that, the planet would still keep its spherical shape overall.

True, but I was wondering more about convex-mirror-esque reflection than about shape.

23. ### Rainwater collection

The idea is more so that the water would be siphoned off to said reservoirs before it gets beyond that point.

I'm not sure why it would need to be an outright boat, though. If it were a simple concave-up structure, wouldn't buoyancy itself act as a restoring force toward equilibrium if it even so much as began to tilt to one side?

Also, don't desalinization plants continuously use electricity while desalinating  water? Wouldn't constructing a rain collector be an investment in the short run that pays off in the long run through savings on electricity? Or would water siphoning consume even more electricity?

24. ### Rainwater collection

I previously discussed, as part of a broader thread about flood mitigation/adaptation, the notion of concave-up ellipsoids over cities to collect rainwater and prevent flooding. It was noted that they would be weak at the bottom, because of the immense pressure of all the surrounding water. On further consideration, cities are probably not the best place to put them in light of that.

However, thinking about this further, I've since come up with another idea; a concave-up rain-collector (whether as ellipsoid or half-cylinder, depending on whichever is more practical) at sea, directly underneath the intertropical convergence zone.

This way, rainwater, distillated by nature, could be siphoned off to artificial reservoirs on land. This would kill two birds with one stone; the need for desalinized water, and the rising ocean levels. (Imagine how much by which they could be decreased if a disproportionate chunk of the 100 inches of rainwater that would otherwise annually fall into the ocean were moved to reservoirs on land!)

As well, another advantage to making it at sea is that both buoyancy and pressure at a point in space relate to the mass above that point in space. Therefore, even though fresh water is less dense than salt water, this would not matter, as any rate of freshwater-siphoning adequate to keep the rain-collector afloat would be adequate to keep the pressure on the inside of its walls comparable to the pressure on the outside of its walls, hence the forces pointing outward and forces pointing inward being nearly in balance.

So why hasn't such a project been pursued?

25. ### Angular equivalent of "vector components?"

Is there any mathematical formula (software notwithstanding) into which you could plug the latitude/longitude differences, from said position of direct sunlight, to get the solar angle, not unlike how you can use the pythagorean theorem to find a vector's magnitude from the magnitudes of its components?

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