A rattleback is a spinning top with a preferred direction, either clockwise or counterclockwise. Its asymmetrical shape and weights cause it to become unstable when rotated in the non-preferred direction, leading to a mid-spin reversal. Rattlebacks have fascinated people for thousands of years and were even used for divination.
A rattleback, also known as a wobblestone or celt, is a type of spinning top that has a preferred direction in which to spin. That direction varies based on the particular rattleback and can be either clockwise or counterclockwise. If rotated in the non-preferred direction, the top will stop and begin rotating in the preferred direction. Rattles are ancient toys that have fascinated people for thousands of years primarily because of their apparent defiance of the laws of physics.
A rattleback is semi-ellipsoidal in shape: flat at the top and curved at the bottom, and significantly longer than it is wide. Ancient rattles were typically made of stone, but today they can be made of almost any hard material, including plastic, wood, and glass. One can also be made with a wrapped stick of bubble gum. The rattleback works due to some sort of asymmetry, both in the base itself and in the form of asymmetrical weights attached to the top of the base. This asymmetry causes the rattleback to become unstable when rotated in one direction.
No matter which direction a rattleback is turned – whether it’s the “right” or the “wrong” direction – it will spin a bit unsteadily. When rotated in the non-preferred direction, it will wobble, slow down, and start spinning in the preferred direction; but even when it spins in the preferred direction, it will spin fine for a while, wobble and stop spinning. A wooden or stone rattleback typically slows down from friction before becoming unstable in the preferred direction of rotation. A glass rattle, however, can change direction of rotation a few times before slowing down.
Rattlebacks seem mysterious because people expect something that spins in one direction to keep spinning in the same direction until a force intervenes to stop it. For this reason, rattles have sometimes been considered supernatural and used as a means of divination. There have been many physics papers published on the subject of the rattleback and most people today recognize that it is simply a toy and that its seemingly abnormal movement has a rational explanation. However, rattlebacks continue to amuse many with their amazing mid-spin reversal.
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