1D50.20 Whirligig


Circular motion, centripetal forces


A weight is attached to the end of the string. The weight moves up and down depending on the period of rotation of the ball on the other end of the string. When the ball spins with a certain period, the weight reaches equilibrium. Note: the ball and weight are visible in a large lecture hall, but the alligator clip is not.



  • [1] Tennis ball on a string threaded through a glass tube
  • [1] Alligator clip
  • [1] 200 g mass

Classroom Assembly

  1. Attach the 200 g mass to the end of the string and place it on the table.
  2. Adjust the string so that the distance between the weight and the glass tube is approximately 6".
  3. Attach the alligator clip above the weight at the base of the glass tube. The alligator clip will act as a reference to show how far the weight has moved.

Important Notes

  • Practice this demo in advance. Smooth motion is required.


  1. Holding the glass tube vertically, begin to spin the ball.
  2. Lift the weight from the table while you are spinning the ball. The weight will move up or down depending on the period of rotation of the ball.
  3. Adjust the period of the ball until the alligator clip has returned to the base of the glass tube.


Additional Resources


  • PIRA 1D50.20


  • Don't attempt this at home!

Last revised

  • 2018


  • Original construction: a string was sewn to a tennis ball. The tube of glass is approximately 4" long and is coated in masking tape in case of breakage. The ends of the tube of glass were fire polished for smoothness. A loop was tied in the end of the string to hold the weight.

Related AV

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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.