Conservation of Angular Momentum

Conservation of angular momentum: if net external TORQUE on a system is zero, total L is conserved.

L = Iω. If I changes, ω compensates so that Iω stays constant.

Examples: figure skater pulls in arms → I drops → ω rises. Diver tucks → spins faster.

Holds about any FIXED axis or about the COM.

In the absence of external torque, an isolated rotating body keeps spinning forever.

Earth's day length is slowly INCREASING (~2.3 ms per century) because tidal friction transfers angular momentum to the Moon's orbit.

Astronomical examples: pulsars (collapsed stars) spin extremely fast because of conservation as r shrinks during collapse.

Conservation of L is INDEPENDENT of conservation of energy or linear momentum.