PURPOSE: To show that a sphere rolling on a rotating disc will move in circles.

DESCRIPTION: The ball can be placed at the center of the rotating disc and tapped to initiate the motion or it can be loosely trapped with your fingers at about half the radius of the disc, and then released. The frequency of the circular motio n does not depend on either the radius or the mass of the ball, or on the radius of the orbit, but only on the frequency of the rotating disc.

SUGGESTIONS: Leveling is significant. If the circular orbit rapidly migrates to the edge of the rotating disc, shim the base of the rotator using available materials, such as paper and cardboard sheets. Check level by placing ball on stationary disc and noting the direction it rolls.

REFERENCES: (PIRA unknown.) See Demonstration Reference File for several papers discussing aspects of this problem, including derivation of the equations.

EQUIPMENT: Base from rotating stool, billiard ball, and large plastic disc with center peg to locate disc on base.

SETUP TIME: 5 min.