# "Perpetual Motion" From Satellite Revolution

by LotoSage
Tags: perpetual motion, revolution, satellite
 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?
 HW Helper Thanks P: 5,151 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.
P: 3,406

## "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
 PF Gold P: 10,763 We'd have to expend more energy putting the satellites into orbit than we'd get back, so there's no point.