We already know two powerful conservation laws, namely, the conservation of energy and the conservation of linear momentum. Let us study another conservation law, namely the law of conservation of angular momentum.
Torque is written in terms of angular momentum as
If no net external torque acts on the system then,
i.e., if the net external torque acting on a system is zero, the angular momentum 
of the system remains constant, no matter what changes take place within the system.
Next, we consider a rigid body rotating about an axis AB. Let the angular velocity of the body be w. Consider the ith particle going in a circle of radius ri, with its plane perpendicular to AB. The linear velocity of this particle at this instant is vi = ri w. The angular momentum of this particle about AB:
The angular momentum of the whole body about AB is the sum of their components i.e.,
