Why is angular momentum conserved?
Angular momentum describes motion about a chosen point or axis. Total angular momentum is conserved when the net external torque about that point is zero. This explains why rotation speed can change when an object's mass distribution changes.
The role of moment of inertia
For suitable rigid-body rotation around a fixed axis, angular momentum equals moment of inertia multiplied by angular velocity. Moving mass closer to the axis reduces the moment of inertia, so rotation speeds up if external torque is absent. A skater pulling in their arms illustrates this. The increase in rotational kinetic energy is not free energy: the skater performs work while drawing the arms inward.
Examples from space and daily life
Satellite attitude control, stellar collapse, and rotating wheels are studied through angular momentum. Conservation should not be interpreted as an unchanging rotation axis in every situation; vector direction and reference point matter. Ignoring friction, motor torque, or gravitational torque can invalidate a calculation. Bicycle stability also cannot be reduced to gyroscopic effects alone, since steering geometry and rider corrections contribute.