How Does a Rattleback Work?

The Rattleback takes the shape of an elongated asymmetrical semi-ellipsoid – a squashed sphere cut in half horizontally – with a flat top. When placed flat-side up on a smooth level surface and spun clockwise, it begins wobbling and spinning in the other direction. When a finger pushes on one side of the Rattleback and is then released, the momentum it creates causes the Rattleback to wobble and spin counterclockwise.

This occurs due to the asymmetric nature of the Rattleback. Its asymmetricity and the frictional force from the flat surface causes the rotational motion of the rattleback to become unstable. The centre of mass is offset from the contact with the surface and causes the friction force to be biased in the direction of the center of mass. This friction transfers the energy from the clockwise spinning (rotational energy) to the wobbling (vibrational energy), then back to spinning – this time in a counterclockwise motion due to its offset centre of mass. A shorter version of this process occurs when the Rattleback is pushed down on one side. This creates vibrational energy which is then transferred to rotational energy in a spinning counterclockwise direction.