Rotating Platform and Dumbbells

WHAT TO DO:

  1. STAND ON THE TURNTABLE WITH A DUMB-BELL IN EACH HAND.
  2. STRETCH YOUR ARMS STRAIGHT OUT AT YOUR SIDES.
  3. HAVE A FRIEND START YOU ROTATING SLOWLY.

WHAT HAPPENS WHEN YOU PULL THE DUMB-BELLS IN TO YOUR SIDES?

WHAT HAPPENS WHEN YOU STRIGHTEN YOUR ARMS AGAIN?

ANSWER:  When you pull your arms in, you speed up.

When you straighten your arms back out, you slow down again.

 

WHY DOES THIS HAPPEN?

 

Cautions, Warnings, or Safety Concerns:

  • The volunteer may get dizzy from spinning.
  • The volunteer should be careful in getting on/off the rotating platform.

Discussion:

Once the volunteer is spinning, he or she has some amount of angular momentum, L, which must be conserved. This is given by L = I*ω. When the volunteer brings his or her arms into the center, the moment of inertia I is decreased. Since L is constant, ω must increase, which is what we observe.

Original 1918 Museum Exhibit

Physics Lecture Demonstration Database