1D50.40 Waiter's Tray


Forces in circular motion


A glass of "wine" or water is placed on a waiter's tray, and when the tray is swung in a full circle, the glass does not move and the water does not fall out.



  • [1] Waiter's tray
  • [1] Glass filled with water or "wine"

Classroom Assembly

  1. Fill the glass with water or "wine" and place on the waiter's tray.

Important Notes

  • Practice this demo in advance. Confidence and a smooth motion are required. Do not hit your leg with the tray.


  1. Swing the waiter's tray along a vertical arc, starting with a low amplitude and working up to a large amplitude.
  2. Continue to increase the amplitude of the swing until the waiter's tray swings in a full circle.
  3. Swing the waiter's tray in other directions for variety.

This is a good demo for "hamming it up." Tony Arrott did this very well. Starting with an empty plastic cup he swung the tray slowly so that the cup flew off. He then picked up the cup, filled it with water and repeated the demo properly. This act created great tension in the audience and made for good theatre.


Additional Resources


  • PIRA 1D50.40


  • Don't attempt this at home!

Last revised

  • 2018


  • Original construction: purchased. The "wine" is water with red food colouring.
  • This tray comes from Athens where it was used by a waiter in a restaurant to carry meals from the kitchen to the table. This demonstration is a nice example of the use of physical principles in daily life.

Related AV

Related demos


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.