Orbital Perpetual Motion Generator

  • #1
I'm probably going to make a fool of myself for asking this, but that's what the anonymity of the internet is for right? (That and other things that you probably won't find at a physics forum)

If I understand correctly, and I may well not, the reason you cannot build a device that will generate movement indefinitely is that any kind of process which generates energy requires a change of state of mass, from a higher energy state to a lower energy state, or else a conversion of mass to energy. Both of these are unsustainable because eventually all the mass will be in a lower energy state or converted into energy.

But isn't gravity kind of the exception to this rule? In theory a body in a perfect orbit can continue in that orbit indefinitely, and generate kinetic energy in the process.

So armed with my high-school education I gave it a little thought, and I'm wondering if it wouldn't be possible to turn a large body orbiting another large body into a giant generator by magnetising it and building an enormous cylindrical tunnel around it, so that it generates a current as it orbits. Of course the magnetic resistence would change the radius of the orbit, but in theory that could be accounted for... or is this the weakness of the idea, that the magnetic drag would always throw off the balance of momentum and centripetal force and crash any body attempting such an orbit?

I could (and may) attempt the maths myself, but I thought I would throw it out there for greater minds who could also tell me what other problems there are with this.

Let me know, thanks.
 

Answers and Replies

  • #2
mathman
Science Advisor
7,870
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But isn't gravity kind of the exception to this rule? In theory a body in a perfect orbit can continue in that orbit indefinitely, and generate kinetic energy in the process.
Not true. It may continue in the orbit, but does not generate kinetic energy.
 
  • #3
2,685
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But isn't gravity kind of the exception to this rule? In theory a body in a perfect orbit can continue in that orbit indefinitely, and generate kinetic energy in the process.
Nope, gravity follows the rule perfectly. There is no theory that says two bodies will continue to orbit indefinitely. All orbits decay, it's just that depending on the size of the bodies the time it takes can vary from minutes to billions of years.

The orbiting bodies aren't "generating" kinetic energy. They have kinetic energy - a finite amount.

Technical problems aside, as soon as you start drawing on the kinetic energy of the orbiting bodies you start to destroy their orbital paths.

The energy that was allowing them to orbit then becomes energy for you to use - there's only so much of that around and it's one or the other.
 
  • #4
K^2
Science Advisor
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On the other hand, there is more energy in Moon's orbit than we could conceivably use in any foreseeable future. So if you have an idea on how to draw on that energy, we can still use it.

Yes, that's probably what people said about fossil fuels 100 years ago, and maybe 10,000 years from now the Moon will be brought to such a low orbit from all the energy consumption that it will begin to siphon off Earth's atmosphere. But hey, maybe by then we'd be living in space colonies and start construction of Dyson Sphere. So I say, go for it.

Edit: On second thought, there is that pesky angular momentum conservation, and the only other thing to take up the difference is Earth. Earth-Moon energy is minimized when Earth is tidally locked to the Moon, so you'd be actually increasing energy of the Moon, and stealing the energy from Earth's own rotation, increasing the duration of Earth's Day. There are many TW of energy already being dissipated by tidal waves, however, so we might as well start with capturing tidal energy and using that. And hey, people already do that. So I guess you're just a little late with your great idea. But keep them coming. I like their scale.
 
  • #5
1
0
The moon is slowing drifting away from the earth, which will happen even faster if we ever decide to mine it, so wouldn't something that will bring it back (at least up to a point) be a good thing?
 

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