Ball on a String

Equipment: A tennis ball on a string.

What it does: Swing the ball on the string to show circular motion. Release the ball to show the instantaneous velocity of the ball.

Concepts Demonstrated: Circular motion; the instantaneous velocity of an object moving in a circle.

Classroom assembly:

1. Hold onto the end of the string.
2. Swing the ball in a vertical path.

1. Hold onto the end of the string.
2. Swing the ball in a horizontal path.
3. Release the string. The ball will continue along a roughly horizontal path (the ball will fall, but it is not as obvious as when the ball is released in a vertical swing).

• When demonstrating circular motion, swing the ball in a vertical path to increase visibility.
• When demonstrating the instantaneous velocity of an object moving in a circle, swing the ball in a horizontal path to avoid complications due to gravity (the ball will travel along a parabolic path when released during a vertical swing whereas it will travel along a roughly horizontal path when released during a horizontal swing).

Cautions: It is easy to hit a student when releasing the ball. Practice this demonstration in advance.

Setup Time: Short.

Difficulty: Straightforward.

Visibility: High. This demonstration is appropriate for large lecture halls.

Related demonstrations:

References: PIRA 1D50.10

Original Construction: A string was threaded through a tennis ball.