Mean free path of marble in a tray
Practical Activity for 14-16
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.
- Keep the tray flat on the table and agitate it so the marbles move around and collide with one another.
- Concentrate your attention on the marble whose colour is different from that of the others. Watch to see the sort of path it follows.
- 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