RMS/Mean Free Path
Properties of Matter

Mean free path of marble in a tray

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

Marbles in a tray illustrate the meaning of 'mean-free-path'.

Apparatus and Materials

  • Two-dimensional kinetic model kit.

Health & Safety and Technical Notes

Students should be fully prepared for the experiment so that they are careful not to send marbles out of the tray.

Read our standard health & safety guidance


  1. Keep the tray flat on the table and agitate it so the marbles move around and collide with one another.
  2. Concentrate your attention on the marble whose colour is different from that of the others. Watch to see the sort of path it follows.

Teaching Notes

  • Carelessness can, of course, lead to chaos with marbles all over the lab. However, if students are given a short time to make observations as well as the task of writing a description of what they see the marble doing, this experiment can be fruitful.
  • You will want students to see the random changes of direction made by the marble after each collision. Draw to their attention the difference between the total distance travelled by a marble and its displacement from a starting point.

This experiment was safety-tested in August 2006

RMS/Mean Free Path
can be analysed using the quantity RMS Speed
is used in analyses relating to Brownian Motion
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