Whirligig

Equipment: A tennis ball on a string that is threaded through a glass tube, an alligator clip and a 200 gram weight.

What it does: A weight is attached to the end of the string. The weight moves up and down depending on the period of rotation of the ball. When the ball spins with a certain period, the weight reaches equilibrium.

Concepts Demonstrated: Circular motion; centripetal forces.

Classroom assembly:

1. Attach the 200 gram weight to the end of the string and place it on the table.
2. Adjust the string so that the distance between the weight and the glass tube is approximately 6".
3. Attach the alligator clip above the weight at the base of the glass tube. The alligator clip will act as a reference to show how far the weight has moved.
4. Holding the glass tube vertical, begin to spin the ball.
5. Lift the weight from the table while you are spinning the ball. The weight will move up or down depending on the period of rotation of the ball.
6. Adjust the period of the ball until the alligator clip has returned to the base of the glass tube.

Cautions: Practice this demo in advance. Smooth motion is requried.

Setup Time: Short.

Difficulty: Straightforward.

Visibility: Medium. The ball and weight are visible in a large lecture hall, but the alligator clip is not.

Related demonstrations:

References: PIRA 1D50.20

Original Construction: A string was sewn to a tennis ball. The tube of glass is approximately 4" long and is coated in masking tape in case of breakage. The ends of the tube of glass were fire polished for smoothness. A loop was tied in the end of the string to hold the weight.