Ice Skater (L-consv)

To demonstrate the conservation of angular momentum.

Turntable Dumbbells

Equipment Location:

Have a student stand on the turntable with arms extended. Give the student a medium push (no too fast!) and ask the student to slowly bring her arms in. As she does, she will speed up. Extending the arms, the student will slow back down. Next have the student hold 5-lb dumbbells, one in each hand. Again give the student a small push with her arms extended. When she brings her arms in this time, her speed will increase significantly due to the larger change in rotational inertia.

Using a low I person helps as the change in speed will be greater. Also, emphasize that kinetic energy is NOT conserved when the arm position is changed. Energy is supplied or sunk by the arm muscles.

Relevant derivations/explanations:
L=I w and conservation of angular momentum, L = L', so I w = I' w'. Ignoring the mass of the arms (not really valid, but....) the inertia for the dumbbells is I = MR2. So, If R'w.