The rotational analog of linear momentum. Angular momentum is a vector. The time derivative of the angular momentum is equal to the vector sum of the torques acting on the system. If the torques on the system sum to zero, then the angular momentum of the system is constant.
The Planar Approximation
For an introductory class, it is sufficient to understand
Angular Momentum of a Point Particle about a Fixed Axis
Suppose we have a point particle of mass m with a displacement vector
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measured relative to a fixed axis