The rattleback is a top that defies convention. It spins freely in one direction, but if spun in the other direction, its rotational motion becomes unstable; it starts to wobble and then will begin to spin in the opposite direction. In this regard, the rattleback seems to defy the physical law of conservation of angular momentum.
The rattleback is shaped as an elongated asymmetric semi-ellipsoid with a flat top. The unintuitive reversal of the rotational motion occurs because of its asymmetry. The asymmetry of the rattleback and frictional force from the table surface cause the rotational motion of the rattleback to become unstable. The friction transfers energy from the spinning (rotational energy) to the wobble (vibrational energy). The vibrational energy is then transferred back to rotational energy but in the preferred direction. Rotation in the preferred direction can also be initiated by pushing down on one end of the rattleback. This vibration is then transferred to a rotational energy.
The rattleback provides an opportunity to demonstrate many physics concepts in the classroom.