Angular momentum is a vector quantity and its direction is determined by applying the right-hand rule.
In the SI system of units, angular momentum is measured in kg.m2.s−1. In contrast, linear momentum is measured in units of kg.m.s−1 so the two are not compatible and cannot be added.
The angular momentum of a symmetrical body, such as a spinning flywheel, can be calculated as the product of the body's moment of inertia and its angular velocity.
To calculate the angular momentum of a rigid body or system, a point called the origin is chosen. For convenience this might be on the axis of rotation of the rigid body or the center of mass of the system. The distance from the origin to each element of the rigid body or system is determined. This is then multiplied by the transverse component of the linear momentum of each part. The angular momentum of the rigid body or system is the vector sum of all the elements. (wikipedia)
Excellent article on angular momentum
See Also
Motion
orbital angular momentum
rate of rotation
Revolution
rotate
Rotation
rotational velocity
Spin
spin direction
whirl