A measure of the momentum of a body in rotational motion. The angular momentum of rigid bodies is conserved; thus, a spinning sphere will continue to spin unless acted on by an outside force. Changes in angular momentum are equivalent to torque. In classical mechanics, angular momentum is equal to the product of the angular velocity of the body and its moment of inertia around the axis of rotation. It is a vector quantity; the vector points up along the axis of counterclockwise rotation. In quantum mechanics, the angular momentum of a physical system is quantized and can only take on discrete values. See also Planck's constantspin.