The rattleback (also known as the Celtic stone, or wobble stone) is a fascinating object. You spin it in one direction, it slows down, wobbles, and then starts spinning in the opposite direction!
Check out a video of a rattleback:
The counter-intuitive behaviour of the rattleback is difficult to explain in a physics sense. I gave it a stab once, and what I came up with is that the rocking motion of the rattleback creates a dynamic imbalance that causes it to spin the other way. It's a chain of dynamic events, and understanding the rattleback means understanding the chain of events that cause it to first slow down, and then spin in the other direction. Spinning it in one direction results in it just spinning normally. But spinning it in the other direction causes a secondary rocking motion to be set up which causes the rattleback to slow down, stop, and then spin the other way. This is all due to the non-symmetric shape of the rattleback, which creates uneven mass distribution.
A detailed explanation of the rattleback is the stuff of advanced physics courses. Indeed, its behaviour defies intuitive explanation.
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