Angular Oscillation in a Torsion String

Angular oscillation in a torsion string is shown by a rigid body suspended from a string that twists as it moves. The twisting of the string provides the restoring force. The string is ideally massless, and in practice its mass is much smaller than the rigid body's mass, so it is neglected.

A torsional pendulum is a rigid body, like a top, suspended from a string that is assumed to be massless — an assumption that is valid if the rigid body's mass is much larger than the string's mass.

When the top is twisted about the string's axis and released, it oscillates between two angles. The restoring torque is due to shearing of th...