Untitled Document

Physics Demo Number: 111

Approximate

Run Time: 10 min

Yet Another Uniform Circular Motion Option

Demo Description

A tennis ball is tethered to a light cork via a string which passes through a smooth tube which may be held (vertically aligned) in the demonstrator's hand.

Initially the cork is wedged against the top of the tube so that the string tension can equal the weight of the tennis ball with the help of contact force between the tube top and cork.

The tube may be twirled in small cyclic motions about a vertical axis parallel to the tube, while the tennis ball hangs vertically below the tube.

If done properly this tube motion will cause the cork to undergo uniform circular motion at the end of some nonzero length of the string .

Scientific Principles

Uniform circular motion of a mass requires a center pointing force at right angles to the motion.
A light object can balance the weight of a heavier body if the light object can move dynamically.

Equipment

Tennis ball ,cork. tube , string.

Equipment Location

Kit (111) on [E-4-6].

Instructions

A tennis ball tethered to a cork cylinder via a yellow string is pictured.

The tennis ball may be held in a hand along with the black handle on the lower half of the hollow copper tube which has the tethering string passing through it.

One may then whirl the cork cylinder in orbits around the flared tip of the tube, with the tension in the string providing any required central forces to maintain the motion of the cork. The apparatus is housed in Kit (111) on [E-4-6].

For classes that can handle equations, one may let the tennis ball hang down while keeping the cork rotating and comment on how the dynamic system of cork plus string manages to balance the weight of the heavy tennis ball vertically.

For safe operation , only whirl the light cork around in orbits.

The cork is far less likely to do damage to the body than the heavier tennis ball.