Angular momentum needs a no-external-torque axis
- 01
The tempting model
Conserving angular momentum about an axis with a nonzero external torque.
- 02
Why it feels right
Angular momentum sounds like the rotational version of momentum, so it gets treated as always conserved.
- 03
Where it breaks · how to catch it
You conserve L without checking torque about the exact axis you chose.
- 04
The correct model
ΔL about an axis equals external angular impulse about that axis. Pick an axis where external torque is zero or negligible.
- 05
Repair · see it, then test it
worked example A falling rod pivots about a fixed end. Is angular momentum about the pivot conserved?
Tempting move
Yes, because it rotates.
Correct move
No if gravity exerts torque about the pivot; mechanical energy may be a better tool.
Test the repair ROT-05 · transferMake a fresh prediction before referring to the worked example.
Then practise it on the ladder
R36Angular momentum axis checkadds: Angular momentum conservation depends on external torque about the chosen axis.