**PURPOSE: **To show that the instantaneous velocity of an object executing uniform circular motion is tangent to the circle, and to illustrate centripetal force.

**DESCRIPTION: **Swing the ball around your head in a horizontal plane, demonstrating uniform circular motion. The required inward force on the string represents centripetal force. If you release the ball, how will it tend to move after it is released?

**SUGGESTIONS: **Suggest three possibilities: (1) the ball will tend to move away from the center of the circle, (2) the ball will tend to continue to move around in a circle, or (3) the ball will move away tangent to the circle at the point where it was released. Let your students vote before doing the experiment.

**REFERENCES: ** (PIRA 1D55.15)

**EQUIPMENT: **String ball on string, as photographed.

**SETUP TIME: **None.