Angular Momentum

Angular momentum is a physical quantity that describes motion around a chosen reference point or axis. It is the rotational counterpart of linear momentum. Its value depends on how mass is moving and where that motion occurs relative to the reference point. Angular momentum has both magnitude and direction, so contributions from different moving parts must be combined with their directions taken into account .¹

For a rigid body rotating about a fixed axis, the component of angular momentum along that axis equals its moment of inertia multiplied by its angular velocity. Moment of inertia describes how mass is distributed relative to the axis, while angular velocity describes the rate of rotation. Angular momentum therefore captures more than spinning speed alone .¹

A net external torque, the turning effect of external forces, changes a system’s total angular momentum. If that net torque is zero, the total is conserved. Individual parts can exchange angular momentum while their combined total remains unchanged. Conservation applies to the whole system under the stated condition, not necessarily to every part separately .²

A spinning skater illustrates the distinction between angular momentum and rotation speed. When external torque is negligible, drawing the arms inward reduces the moment of inertia and increases rotation speed while angular momentum stays approximately constant. Changing body configuration can therefore change how quickly a person rotates without requiring an increase in total angular momentum .²