A pendulum is a weight suspended so that it can swing freely. When you push a pendulum sideways from its resting position, gravity tries to bring it back to its resting position. But the pendulum is too heavy to stop at the bottom of its swing, and so it swings back up the other way. Gravity then brings it back. This happens time after time, the size of the swing getting smaller and smaller until finally the pendulum comes to rest. But although the swings get smaller, the time to make a swing stays exactly the same, which is why it was used on many clocks in the past. It was a way of making clocks accurate.

You can try out a pendulum for yourself. Just go to a playground and get on a swing. Push hard - just once. You and the seat are not a pendulum. The swing chains are the same as a pendulum string. The video shows you more.

Galileo Galilei, a famous Italian scientist, was the first to investigate what a pendulum did. He did this in 1602 after watching a chandelier move to and fro in his local church. The idea was taken up by English scientist, Sir Isaac Newton, and he used it in working out the law of gravity.