SAT Physics Subject Test

Chapter 6 Curved and Rotational Motion

CONSERVATION OF ANGULAR MOMENTUM

Newton’s second law says that

so if Fnet = 0, then p is constant. This is conservation of linear momentum.

The rotational analog of this is



So if τnet = 0, then L is constant. This is conservation of angular momentum. Basically, this law says that if the torques on a body balance so that the net torque is zero, then the body’s angular momentum can’t change.

The most common example of this phenomenon is when a figure skater spins. As the skater pulls her arms inward, she moves more of her mass closer to the rotation axis and decreases her rotational inertia, I. Since the external torque on her is negligible, her angular momentum must be conserved. Since L = , a decrease in ω causes an increase in, and she spins faster.