Pressure exerted by a stream of balls
Practical Activity for 14-16
This demonstration models gas molecules hitting a wall.
Apparatus and Materials
- Toy shop marbles, steel balls or lead shot
- Spring balance, domestic or lever-arm balance
- Tray, a deep one, or lots of books
Health & Safety and Technical Notes
Either a domestic balance or a lever-arm balance can be used for this experiment. Turn over the scale pan so that the balls bounce off without collecting on the pan. Place a deep tray under the balance (or set up a barrage of books round the balance) to catch the balls.
Pour, by hand, a stream of balls, from a height of 30 cm or so, onto the inverted scale pan on the balance.
- As the balls hit the pan and bounce off they exert small impulsive forces. The comparatively great mass of the pan and balance smear out these forces into a steady push shown by the balance. Gas pressure results from impact forces like these spread over a surface.
- P = F/A, where P = pressure, F = force, and A = area over which force acts.
This experiment was safety-tested in March 2005