why

Dear student,

the time rate of change of angular momentum of a system of particles is equal to net external torque on the system. This is what is known as Newton's second law in angular form for a system of particles. It, then, follows that the angular momentum of the system will be conserved, if net external torque on the system is zero. Though, there is no external torque on the system, the particles inside the system may still be subjected to forces (torques) and,  therefore, may undergo multiple change in velocity (angular velocity).

Hope this helps.

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