The BIG pendulum

What the Activity is for

This activity involves a discussion of the energy changes associated with a pendulum moving to and fro and probes the idea of energy dissipation.

What to Prepare

• a simple pendulum hung from the lab ceiling with a big pendulum bob on the end of a strong, thin length of cord
• this linked interactive object and the means to display it

The impact of this interactive demonstration depends on the quality of the pendulum. A big bob on the end of a long cord will sweep impressively back and forth, capturing the attention of the pupils. (A colleague, who likes to have maximum impact with his demonstrations, uses a ten-pin bowling ball as his pendulum bob!)

What Happens During this Activity

Set the pendulum up at the front of the room and gather the class around. Give the pendulum plenty of build-up (the best pendulum you'll see in your lifetime) and set it going to and fro. Describe the energy changes:

Energy is continuously shifted from the gravitational store of the bob (in the Earth's field) to the kinetic store of the bob, and back to the gravitational store … and so on.

Focus attention on the fact that as time goes by the bob does not reach the same height on its return path. It seems that the gravitational and kinetic stores are leaking.

Emphasise the point by asking a pupil to take a steady nerve test. This involves the pupil holding the bob to his or her chin and releasing it. As the bob sweeps back towards the chin, will the pupil have the nerve to hold still, convinced that it is impossible for the bob to return to a height greater than its starting point? Does the pupil really believe in physics?

Of course, the bob always falls short and this leads to discussion of energy dissipation. As the bob sweeps back and forth, warming occurs. The energy level in the gravitational and kinetic stores is gradually reduced as energy is shifted to the thermal store of the surrounding air.