# Law of conservation of angular momentum

Newton’s second law for rotational motion,

So, if the net external torque on a particle (or system) is zero, then

i.e., = constant

Angular momentum of a system (may be particle or body) remains constant if resultant torque acting on it is zero.

As L = , so if then = constant ⇒ I1/ω.

Since angular momentum remains constant, so when I decreases, angular velocity ω increases and vice versa.