AP Physics C: Mechanics / Unit 6: Energy and Momentum of Rotating Systems
6.4 Conservation of Angular Momentum
Notes
With no net external torque, $L$ is constant: $I_i\omega_i=I_f\omega_f$.
- Pulling mass inward lowers $I$, so $\omega$ increases.
Practice
A skater pulls in her arms. Her spin rate:
$I$ drops, $\omega$ rises.
$I$ goes from 6 to 2 kg·m² while spinning at 1 rad/s. The new $\omega$:
$6(1)=2\omega$.
When the skater pulls in her arms, her rotational KE:
$\displaystyle \frac{L^2}{2I}$ rises as $I$ drops.
A person walks to the center of a spinning merry-go-round (no friction at axle). Its $\omega$:
$I$ of the system drops.
Ready for more? Take the Unit 6 test.