1Q40.10-Rotating Platform and Weights


Video: Watch this demo

Instructor’s video

Description: This demonstration illustrates the principle of conservation of angular momentum (Ice skater effect).


  • Rotating platform
  • 2 Small dumbbell weights

Setup Procedure:

  1. Place the Rotating platform where no one will trip over it.

Demonstration Procedure:

  1. This can be done with either yourself or a student. In either case an assistant is necessary.
  2. Place the rotating platform on the floor and leave plenty of room on all sides for your arms (about 3 foot radius), and just in case you have to step off quickly.
  3. With one person holding the bottom of the platform with their foot, have the other step onto the platform.
  4. Have them align their body so their center of mass is right over the center of the platform, and make sure their feet are spread out.
  5. Give the person on the platform the two weights. Hold each weight out to the side so the hands are horizontal.
  6. The person on the floor should take the other’s arm, and give them a spin (not fast, but not too slow).
  7. The person on the platform should then bring both of their arms directly to their side, as fast as possible, and as uniformly as possible. As a variation, they may also bring their arms directly to their chest as quickly and uniformly as possible.
  8. If they don’t bring their arms in quick, they may loose their balance and need to step off.
  9. Note that they start to spin faster. This is due to angular momentum being conserved, since their weight distribution from center just changed.
  10. When finished, have the person on the platform bring the weights out again. Have the person on the floor bring them to rest.
  11. A variation of this demo is to have someone sit instead of stand. Whichever you prefer will be fine.