The (magnitude of the) angular momentum for a particle moving in a circle about the centre is mvr, where v is the speed of the particle and r is the radius of the circle.
The (magnitude of the) angular momentum for a rigid body rotating about an axis is Iw, where I is the moment of inertia of the body about the axis and w is the angular speed of the body.
Analogous to Newton 2 for a particle, the Principle of Angular Momentum relates the resultant external torque (L) on a body about an axis to the angular acceleration (dw/dt) of the body about the axis by L = Idw/dt where I is the moment of inertia of the body about the axis.
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