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on Sep 15, 2016

Perpetual motion is possible, perpetual work is not.
In space wheel can spin practically forever and the only reason it will slow down is the small amount of gas atoms in the vacuum of space. If you try to get any work out of the wheel, it will eventually slow down to a stop. There's no free lunch!

on Sep 17, 2016

In addition to the small amount of gas atoms, the fact that the wheel has mass will cause it to slow down due to tidal friction from the presence of gravity, however slight, presented by other mass in the universe. Without other mass, there is no way the wheel could be said to rotate.

on Sep 17, 2016

In space no one can hear your bearings scream.

Friction is always going to be the impassable gate to perpetual motion. As diacad says friction from the atmosphere that does exist will eventually stop the wheel from spinning.

Then there's the question of how you've constrained the wheel to spin in a wheel like manner (if you weren't going to constrain it then a rock or golf ball or a sheep would work just as well as a wheel).

Moving parts in space create all sorts of problems because space isn't actually cold, it's just mostly empty. Which is the opposite of terrestrial cold where positive pressure increases the density of the atmosphere.

The practical effect of creating heat in an ultra high vacuum is that there's not much to share the heat with, which is how heat dissipates in a positive pressure environment. Since it has nowhere to go it just sort of hangs about and exacerbates the friction problem. The same is true of electronics in space, keeping them cool is extremely difficult.

In short; tossing unconstrained spinning sheep or something into space is pointless as it'll just stop spinning anyway and it'll be really hard to get to. If you constrain it the heat buildup from its rotation will result in pretty quick failure.

Either way, space doesn't cancel physics, it just makes physics behave differently. Physics will not allow perpetual motion anywhere and you can't have any where if there's noting there.

on Sep 17, 2016

A form of perpetual motion does exist in nature all around us. Any temperature above absolute zero involves molecular motion, and its effects can be observed under magnification when molecules impact microscopic particles. It is called Brownian Motion. Of course, this perpetual motion can not be tapped for useful work.

on Sep 17, 2016

Brownian Motion is not perpetual motion. In point of fact, Brownian Motion is the exact opposite of perpetual motion.

Brownian Motion requires particles expressing the phenomenon to be supplied with constant external power that is provided by atoms already moving around in the same area. Those atoms in motion have to be accelerated by something. Atomic decay, fusion and associated energies (mostly from Sol, aka The Sun, to Earthlings), fission or a wizard are all common means of accelerating those atoms. The point is those atoms require external energy to move.

It's a inefficient way to travel but that's always the case. The atoms knocking the Brownian particle around have to have enough energy to pass some off to the particle and still remain in motion themselves and/or bond to something else in the area.

So it's perpetual motion until there's no more energy available. Like coal fired stove.