When net external torque is zero, L = I*omega = constant. This principle explains:
- Ice skater: Arms in -> smaller I -> faster spin (same L).
- Platform problems: Person walks on a rotating platform, changing the system's MOI.
- Collision on rotating disc: A mass lands on a spinning disc, increasing I and decreasing omega.
- Bullet embedding in disc rim: Linear momentum is NOT conserved (axle force), but angular momentum about the axis IS.
Important: angular momentum is conserved for the system, not individual parts. Internal torques redistribute L among components without changing the total.
Energy is NOT conserved in these scenarios — internal friction or muscle forces can do work, changing KE even when L is constant.