The swinging pendulum
Practical Activity for 14-16
The simple pendulum used as a stimulus to discuss changes in the ways that energy is stored.
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
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.
This demonstration is most successful when the energy dissipation 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.
- Clamp the pendulum to a rigidly held retort stand.
- Let the pendulum swing back and forward and encourage students to discuss the transfers taking place.
- Do not do any timing of the pendulum, nor discuss the periodicity. Changes to the ways that energy is stored are the focus of this demonstration.
- Students should look for the ways energy is stored. Energy is stored gravitationally at the top of its swing, and kinetically (and gravitationally) at the bottom of its swing. The change in energy stored gravitationally is equal to the energy stored kinetically if there is no energy dissipated.
- Apart from at the top and bottom of the pendulum's swing, the bob has a mixture of energy stored gravitationally and energy stored kinetically. 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 energy stored gravitationally is transferred to energy stored kinetically and back again. The total amount of energy stored kinetically plus energy stored gravitationally 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.
- An entertaining extension to this experiment is to hold the pendulum bob at
noseheight 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-tested in November 2005