Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Mean free path of marble in a tray

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.

This kit may be obtained from science apparatus suppliers. It consists of a set of shallow trays (preferably fitted with a thin cork sheet on the base to reduce the noise-level) and a dozen or so marbles, all bar one of the same colour.



a Keep the tray flat on the table and agitate it so the marbles move around and collide with one another. 

b Concentrate your attention on the marble whose colour is different from that of the others. Watch to see the sort of path it follows.

Tray and marbles

Teaching notes

1 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. 

2 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-checked in August 2006


Related guidance

Estimate of molecular size: a more formal method

Mean free path

Further discussion of mean free path

Proof of R = sN


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