Final answer:
The principle of conservation of angular momentum is the rotary equivalent of Newton's first law of inertia.
Step-by-step explanation:
The principle of conservation of angular momentum is the rotary equivalent of Newton's first law of inertia. Just like linear momentum is conserved when the net external force is zero, angular momentum is conserved when the net external torque is zero. This means that the initial angular momentum of a system is equal to the final angular momentum when no external torque is applied to the system.