# Perpetual Motion From Satellite Revolution

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 P: 3 This is going to sound very far fetched, and I know very little about the laws of physics, admittedly (I don't even know if I'm posting this in the right area), but curiosity compels me. I was wondering if it would be possible to harvest energy from the revolution of a nearby satellite as I've illustrated here: http://i.imgur.com/Z3u5i33.png I use the moon as an example here, but that could be replaced with any nearby satellite. Would this work, in theory?
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 6,756 I'm not sure what your sketch is trying to show. However, it is the policy at PF not to discuss PM and the like. In any event, if you try to extract energy from an orbiting body, you are going cause that body to lose altitude over time. After all, it takes a fixed amount of energy per unit mass to put something into a particular orbit, so while your scheme may be long-lived, it cannot be 'perpetual'.
 P: 3 Yeah, that's why I put it in quotes. While my understanding of physics is very limited I felt of course it couldn't last forever. I don't literally mean PM in this case (and that goes twicefold since it's disallowed), but simply a long term means of energy extraction. You seem to have gotten the right idea from my rudimentary illustration, and I thank you for your input.
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Perpetual Motion From Satellite Revolution

 Quote by LotoSage I use the moon as an example here, but that could be replaced with any nearby satellite. Would this work, in theory?
We are already extracting that energy in practice:
http://en.wikipedia.org/wiki/Tidal_power‎
 Mentor P: 12,017 We'd have to expend more energy putting the satellites into orbit than we'd get back, so there's no point.
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 Quote by LotoSage I use the moon as an example here, but that could be replaced with any nearby satellite. Would this work, in theory?
It's all a matter of degree. The Moon is the largest object that will ever (could ever) be in orbit around the Earth. Any event that could introduce a comparable size of satellite would spell the end of the world for humans. Tidal surges or whaaaaat?

Tidal effects from the Moon (And the Sun, aamof) are many orders of magnitude greater than from anything else out there so those are the ones to go for. And we already do. From our point of view, the energy is 'inexhaustible' but it's not PM (verboten in PF).

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