1Q40.10 Rotating Stool and Weights


Conservation of angular momentum


A person sits or stands on the rotating stool (or platform) and holds the weights with his/her arms extended. After the stool is spun, the person brings his/her arms closer to the body. By moving masses closer to axis of rotation, the moment of inertia of the system is decreased, so the angular speed of the system must increase in order to conserve angular momentum.



  • [1] Low-friction stool, platform, or chair
  • [2] Weight

Classroom Assembly

  1. Place the low-friction platform, chair, or stool on a clear spot.
  2. Level the chair, if applicable.

Important Notes

  • Make sure the person demoing the wheel won't hit anything while rotating and has room to stumble off the platform if it is used.


  1. Get a volunteer onto the low-friction apparatus.
  2. Hand the volunteer two weights and ask him/her to hold one in each hand with arms extended.
  3. Spin up the rotating apparatus.
  4. Ask the volunteer to brings his/her arms closer to the body. Notice the increase in angular velocity of the system.


Additional Resources


  • PIRA 1Q40.10


  • Don't attempt this at home!

Last revised

  • 2017


Related AV

Related demos

  • Angular Momentum Wheel

If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.