The angular momentum of a rotating rigid body is, L = I ω
The torque acting on a rigid body is, τ = dL/dt
When no external torque acts on the system, r = dL/dt = O
L = I ω = constant
Total angular momentum of the body = constant
So, when no external torque acts on the body, the net angular momentum of a rotating rigid body remains constant. This is known as law of conservation of angular momentum.