100. Two Balls Hanging

This pretty demonstration illustrates two principles: one, the period of a conical pendulum is the same as a linear pendulum; and two, all the momentum is transferred in an equal mass elastic collision.

Two identical ivory balls hang side by side from the ceiling of the lecture hall. You can illustrate the equal mass collision by pulling one out and letting it collide with the other, as in the Collision Balls demonstration. Now pull both balls back, away from the class, hold one lightly with your fingers, and collide the other into it from the side. The colliding ball will stop "dead" and swing toward the class in linear pendulum motion, and the struck ball will swing around in conical pendulum motion. Since the periods of the motions are the same, the balls will again collide at the end of the swing and exchange motions. The situation will continue for some time with the balls exchanging conical and linear pendulum motion at successive collisions.

Available only in the Knudsen Lecture Halls and PAB 1425