Imagine that you have a a dougnut shaped rod, that is has a threaded pattern in the surface. You can think of it as a threaded rod that you bent to a circle. But before bending, you placed a object in its threads. It is a threaded ring, with two bar magnets attached to its surface, one inside the circle and one outside. The ring can rotate freely on the doughnut.

Now, you give a nudge to one of the bar magnets. As it moves it is repelled by the other magnet. Does the ring rotate forever?