Happy and Sad Balls

A ramp suspended in the air with a wood block and 2 balls.

PIRA: 1R40.30

Materials: Rod Stand, Rod, Ramp, 1 Bouncy Ball, 1 Dense Ball, 1 Wood Block

This demonstration of momentum transfer uses the Arbour "Happy" and "Sad" balls. These balls are vulcanized rubber. At room temperature, the "Happy" ball has a large coefficient of restitution (it bounces good), while the "Sad" ball has a coefficient of restitution of almost zero (it doesn't bounce). Both balls are of the same size and mass. Adjust the Recoil ramp so that when rolled from the top, the "Sad" ball barely fails to knock the Recoil Board over. Be sure to set this up before class. Otherwise it may look like you are deliberately rigging the demo (which you are).

Ask your students which ball is more likely to knock the board over. Many are surprised to see the "Happy" ball topple the board when the "Sad" ball failed. This is a very clear demo of the fact that recoiling objects transfer more momentum than those that don't.