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tmztr
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The challenge is to build a fully GRAVITY-powered device that will shoot badminton birdies from the floor into an average basketball net, and given 2 minutes to shoot at least 10 birdies into the net. Physics experts are wanted in coming up with a design with this project.

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I assume by "gravity powered" you mean that the energy of the actual launch is caused by something falling, like a trebuchet (which, technically speaking, is powered by whatever lifts up the counterweight - probably human beings). Otherwise I'm afraid you're trying to violate Newton's third law... But yeah, trebuchets are sweet. Build one of those.

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I assume by "gravity powered" you mean that the energy of the actual launch is caused by something falling, like a trebuchet (which, technically speaking, is powered by whatever lifts up the counterweight - probably human beings). Otherwise I'm afraid you're trying to violate Newton's third law... But yeah, trebuchets are sweet. Build one of those.

 

Not necessarily. He could have a store of boulders high up, and after releasing the first one, some mechanism which resets the trebuchet (using the energy from the first boulder) and releases the next boulder. Then, after the initial setup, not work need be done by the user. (Obviously you need to do work to set it up.)

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That's what I mean. You need to do the work to set it up, and there's no way the work you do can be less than the work of throwing the boulders. And as soon as you run out of boulders, you need to do it again in order to fire again. The point is you're still lifting the counterweight each time, just indirectly: you lift the boulders, and the energy of the falling boulder is mechanically transferred to lift the counterweight. There's still no way to get gravity to "power" anything in the strictest sense, since you can't get anything out of it that you don't put in initially. But then again, we're obviously not talking the strictest sense, and I'm satisfied to say that gravity is a way of storing up potential energy which can then be released at whim, like in the device you describe.

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That's what I mean. You need to do the work to set it up, and there's no way the work you do can be less than the work of throwing the boulders. And as soon as you run out of boulders, you need to do it again in order to fire again. The point is you're still lifting the counterweight each time, just indirectly: you lift the boulders, and the energy of the falling boulder is mechanically transferred to lift the counterweight. There's still no way to get gravity to "power" anything in the strictest sense, since you can't get anything out of it that you don't put in initially. But then again, we're obviously not talking the strictest sense, and I'm satisfied to say that gravity is a way of storing up potential energy which can then be released at whim, like in the device you describe.

 

Since gravity is a conservative force, there is no way in principle to get energy out of it. You always have to put the energy in to begin with. So I don't think this is what the teachers were meaning. (This is even true for petrol - you need to create the plant/animal matter via photosynthesis, then put it under huge pressure for a few million years, before digging it up to refine and use.)

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I realize that. I was being facetious to begin with. I assumed teachers wouldn't assign students to violate fundamental laws of physics, which was my point. Although if I were a teacher, it would be funny to assign and see what they come up with.

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