|MadSci Network: Physics|
Well, when people talk about perpetual motion being impossible I think they mean one of two (equivalent) things:
Now the first of these is, in a way, more fundamental. It's connected with basic ideas about the conservation of energy (or mass/energy in relativity) - you can't continue to take energy away from something without eventually running out.
The second is less fundamantal and more practical, and is what you have in mind for the earth moving about the sun. If we take the first idea above as true then you can see that it only stops an `isolated machine' from going on forever if you take energy away from it.
In practice any machine we make on earth will have some friction. This takes energy away in the form of heat. I bet one of the best things we make for low friction are the gyroscopes in the old inter-continental ballistic missiles. They have air bearings and, once spinning, are designed to continue until, if not the end of the world, at least for the lifetime of the gyroscope.
The earth going around the sun sounds like a pretty good candidate for something with no friction, so maybe you are right in that it is a perpetual motion machine. However, that would only be true if it wasn't losing energy in some way, and unfortunately it is...
You see, space is not a vacuum. It is almost empty, but there are still small particles whizzing around, as well as larger things like meteorites. Many particles come from the sun. As the earth moves round the sun it will collide with these particles (`sweeping them up', if you like) which will slow us down very slightly.
However, the effect is tiny. There may be other ways that the earth is slowed too (look up `tidal friction' in an astronomy textbook) but they affect mainly the rate of spin of the earth, rather than the rotation of the earth around the sun. Long before we slow down enough to get near the sun, the sun itself will have expanded to a `red giant' and cooked us...
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