1Q40.30-Rotating platform and bicycle wheel


Video: Watch this demo

Instructor’s video

Description: This demonstration shows the conservation of angular momentum.
The only angular momentum of the system is that of the bicycle wheel alone. When the student turns the bicycle wheel there must be no net torque acting on the system.
The angular momentum after the twisting must be equal in magnitude and direction to that of the initial state.
For this to happen, the student will start to spin.


  • Rotating platform
  • Bicycle Wheel with matching string pull cord.

Setup Procedure:

  1. Place the Rotating platform where no one will trip over it.
  2. Wind up the bicycle wheel with the string.

Demonstration Procedure:

  1. An assistant is necessary.
  2. Place the rotating platform on the floor and leave plenty of room on all sides for their arms (about 3 foot radius), and just in case they have to step off quickly.
  3. Hold the bottom of the platform with your foot and have your assistant 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 bicycle wheel to the person on the platform. Have them hold it so the two handles are horizontal (bicycle wheel vertical).
  6. Pull the string cord with the handle to get the bicycle wheel spinning fast.
  7. Have them slowly turn the wheel to the side and note that the person starts to turn in the direction that they turn it.
  8. This is because the bicycle wheel is exerting a force on the person, which acts as a torque on the platform.
  9. When finished, take the bicycle wheel from the person and have them step down.