Angular momentum is a property characterizing the amount of spin of an object or system of objects in motion about an axis. The Earth has orbital angular momentum by reason of its annual revolution about the Sun and spin angular momentum because of its daily rotation about its axis.

The magnitude of the angular momentum of an orbiting object is defined as the product of mass (m), linear velocity (v), and radius(r), or simply

The radius is the size of the spinning object from the center of gravity. For a given object or system isolated from external forces, the total angular momentum is a constant, a fact that is known as the law of conservation of angular momentum. A rigid spinning object, for example, continues to spin at a constant rate and with a fixed orientation unless influenced by the application of an external force. A figure skater spins faster, or has a greater angular velocity, when the arms are drawn inward, because this action reduces the radius of the body while the skater's angular momentum remains constant.