Happy and Unhappy Balls

Currently we provide two different versions of this experiment. One version uses balls rolling down a long, inclined track (roughly 2 meters), and the other version uses balls that swing on a pendulum.

Track

trackwooden block
The experimenter places a ball on the track, roughly 1 1/2 meters above the cart. When the ball is let go, it will roll down the track and impact the wooden block. The "unhappy" ball collides inelastically with the block, and so it remains standing; the "happy" ball collides elastically, knocking the block over.

Pendulum
Pendulum

Here the rubber balls swing on a pendulum. Best results are usually obtained when the balls are initially positioned at about 45 degrees with respect to the vertical.