The simple pendulum

A simple pendulum is a small metal ball, called the bob, hanging from a rigid (firm, not wobbly) support by a long thread.

A pendulum at rest is in its mean position. Move the bob a little to one side and let go, and it swings to and fro: this is oscillatory motion. It is also periodic motion, because it repeats its path after a fixed interval of time.

Call the mean position O and the two ends of the swing A and B. The pendulum has made one oscillation when the bob goes from O to A, turns, goes all the way to B, turns, and comes back to O. Going from A to B and back to A is also one oscillation. Either way, one oscillation is a complete there-and-back swing, not just one side.

The time the pendulum takes to complete one oscillation is its time period.

Activity 8.2 measures it. Tie a heavy metal ball or stone to a string about 150 cm long and fix the other end to a rigid support, so that about 100 cm hangs between the support and the bob. Let the bob come to rest.

Pull the bob gently to one side and release it — just let go, don't push — keeping the string tight. Measure the time for 10 oscillations, and repeat three or four times. Why 10? One swing is too quick to time accurately; timing ten and dividing is much more accurate.

Divide the time for 10 oscillations by 10 to get the time period. You find it comes out almost the same every time.

You have just repeated a famous experiment first done by Galileo, except that you used a watch and he used his own pulse.