if perpetual motion is impossible, why/how do planets orbit and spin continually?

Question

if perpetual motion is impossible, why/how do planets orbit and spin continually?

In: Physics

Answer 1

The others have answered in detail about the planets.

Perpetual motion is NOT impossible, in fact physics says that in theory an object will move (or spin) forever (in space) if there’s no force acting on it. And, as everyone else has pointed out, there’s always a force, however small – space does have a very low density of atoms, dust, particles, so moving through space does encounter some friction.

But anyway, the “perpetual motion” rule you quoted refers to perpetual motion MACHINES, which would be devices that spin or push or create energy forever, without a power source. You always have a power source, because there’s always friction and resistance, so the energy to push against friction and resistance has to come from somewhere.

The underlying principle is actually [conservation of energy](https://en.wikipedia.org/wiki/Conservation_of_energy). That’s why we can’t have perpetual motion MACHINES.