Rattleback, 'celt' or 'wobblestone'

The pictures show a natural rattleback (left) and celts (right).

Natural stone.

Celts (stone adzes).

A rattleback is an elongated stone that can be found occasionally on beaches and in streams all around the world. If set to spin in one direction, the stone will start to rattle, come to a stop, and reverse its direction of spin. The motion is complex and difficult to analyze.

A model can be made with a plastic spoon bowl and two coins. The coins are placed off axis on opposite sides near the points.

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