Pressure exerted by a stream of balls

Demonstration

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

Read our standard health & safety guidance

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.

Procedure

Pour, by hand, a stream of balls, from a height of 30 cm or so, onto the inverted scale pan on the balance.

Teaching Notes

• 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