**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 motion 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 critical. Use a bubble level first, then check by dynamic leveling. The ball will migrate 90 degrees counterclockwise from the low side when the platform rotates counterclockwise.

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

**EQUIPMENT: **Record turntable with aluminum disc, pingpong ball, two-axis leveler with bubble level.

**SETUP TIME: **10 min.