# Galileo's pin and pendulum

##### Class practical

Friction spoils the downhill and uphill experiment of a rolling ball on a curved track. This is Galileo's almost frictionless version of it.

#### Apparatus and materials

Nail, 15 cm or dowel rod

Bosses, 2

Retort stand

G-clamps, 2

Metal strips used as jaws, 5 cm, 2

Pendulum (larger bobs work better)

#### Health & Safety and Technical notes

Instruct the class to set up their apparatus so that the pendulum will not swing too close to a neighbouring group.

For this experiment to be a success it is essential to have a massive and very rigid support for the pendulum and for the 'pin'. Otherwise energy is lost at the support and the experiment fails miserably.

The best arrangement for clamping the pendulum thread is between the two metal plates acting as jaws.

#### Procedure

a Set up the pendulum as illustrated. Hold the pendulum-bob to one side and release it. Allow the pendulum to swing for a few cycles and then interpose the nail or dowel rod to interrupt the swing. You should find that the bob rises to the same level from which it started. (It may be easier to release the pendulum from a fixed point and measure the rise on the far side in two separate experiments, with and without the nail or dowel in place.)

#### Teaching notes

1 This is a famous experiment said to have been performed by Galileo. It was a stroke of genius. Friction made experiments with a rolling ball on a track unconvincing. (See the experiment Galileo's rolling ball.)

Using a pendulum reduces the frictional forces to those of the support and the surrounding air. For a massive pendulum, moving slowly, the air resistance dissipates little energy.

2 When the nail or dowel is interposed, the pendulum which has been swinging down in the shallow arc has to climb up a steep arc. Students can look to see whether the heights are the same for both arcs, or measure the heights reached, demonstrating conservation of energy.

This experiment was safety-checked in November 2005