When the net external torque on a system is zero, the total angular momentum is conserved: L = I*omega = constant.
Classic Applications:
- Ice skater: Arms pulled in -> I decreases -> omega increases (spins faster)
- Diver tucking: Same principle — reduced I means faster rotation for somersaults
- Disc + person system: Person walks on rotating disc, system I changes, omega adjusts
- Neutron star formation: Collapsing star — radius shrinks drastically, spin rate increases enormously
Important: L is conserved for the system, not individual parts. If internal torques redistribute angular momentum between parts, the total remains constant.