Does a wheel keep turning on a planet without wind?

Answer

Hi Paul

Indeed, a wheel on a planet without an atmosphere that is not in contact with the ground will experience no rolling resistance or air resistance. However, it will still lose energy due to a (small) resistance from the axis around which it rotates and thus slow down slowly and come to a stop.

If you could also eliminate this energy loss, the wheel would indeed keep spinning. However, this does not mean that you have infinite energy. The energy of a wheel is given by the following formula:

E=I*w²/2

Here I represents the mass moment of inertia and w the rotational speed. A large and heavy wheel has a greater moment of inertia than a small and light wheel. You can see that that formula is very similar to that of the kinetic energy of an object moving straight ahead.

E=m*v²/2

Here m is the mass and v is the speed.

The energy is given by the formula and is thus not infinite. If you were to use the wheel to drive a generator to generate electricity, the wheel would slow down as it releases energy to the generator, it could supply power until it has lost speed. For example, think of your bicycle dynamo, if you spin your bicycle wheel and then let it go, it will come to a stop after a while. If you spin the same wheel, but with the dynamo against it, it will come to a stop earlier. This is because energy is not only lost through friction with the air and axle, but also because the dynamo takes energy from the wheel and converts it into radiant energy (light).

Here you can see that the energy is not infinite, because an infinite energy source would not be exhausted. This would solve our energy problem, but unfortunately is not physically possible.

Wheels can also be used to temporarily store energy, see https://nl.wikipedia.org/wiki/Vliegwiel

Kind regards

Henri