Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

The swinging pendulum

Demonstration

The simple pendulum used as a stimulus to discuss energy transfers.

Apparatus and materials

Metal or wood blocks to hold the pendulum cord, 2

Simple pendulum bob, 1 - 2 cm lead, iron or brass

Cord for pendulum, at least 1 m or longer

Retort stand, boss, and clamp

G-clamp

Health & Safety and Technical notes


Any attempt to fix the support to the ceiling requires two persons - one to hold the ladder or steps.
 
The use of a brick as a pendulum bob would be unwise. The brick may rotate and present a rough edge or corner to the demonstrator.

swinging pendulum

This demonstration is most successful when the energy leakage is kept to a minimum. This can be done by using a massive support and by ensuring that the cord is firmly clamped. Clamp the pendulum cord between a pair of blocks with the G-clamp, keeping the lower edges of the two blocks flush. 

If it is possible to clamp the cord from the ceiling, the support will be even better.

a Clamp the pendulum to a rigidly held retort stand.
 
b Let the pendulum swing back and forward and encourage students to discuss the transfers taking place.

 

 

Teaching notes


Apparatus1 Do not do any timing of the pendulum, nor discuss the periodicity. The energy transfers are the focus of this demonstration.
 
2 Students should look for the energy transfers as the pendulum bob moves. It goes from a gravitational store at the top of its swing to kinetic energy which is greatest at the bottom of its swing. Then back to increasing gravitational energy as it rises to the top of its swing on the other side.
 
3 Apart from at the top and bottom of the pendulum's swing, the bob has a mixture of gravitational energy and kinetic energy. As the bob swings one gradually increases as the other decreases.
 
If the pendulum is fixed as described, it should rise to the same height on either side for a good number of swings. As long as this continues, all the gravitational energy is transferred to kinetic energy and back again. The total amount of kinetic energy plus gravitational energy remains constant.
 
This is one of the few demonstrations which, for a few oscillations before energy is transferred to the support, illustrates the Principle of Conservation of Energy.
 
4 An entertaining extension to this experiment is to hold the pendulum bob at 'nose' height and to leave go of the bob, taking care not to push it. The bob will come back to the same point just in front of your nose, as long as you don't flinch. If the bob is more massive the effect is more dramatic, and illustrates that energy cannot be gained from outside the system.
 
This experiment was safety-checked in November 2005