A celt or rattleback is one of the most fascinating physics toys!
It is a long, thin, half-elliptical shape formed in plastic. It has a slightly offset center of gravity, which causes it to spin in a directional rotation of choice, usually counter-clockwise. Suppose you attempt to turn the celt or rattleback in the clockwise direction. In that case, strong preference will cause it to “rattle” back and forth after few rotations, stopping and then reversing the course back to the preferred counter-clockwise direction. Also, if you push down on one of its ends from rest, it will begin spinning.
Rattlebacks make a great desk toy and conversation starter.
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