To demonstrate conservation of energy in a very dramatic way.
Giant pendulum suspended from ceiling of lecture hall, ladder with plywood backdrop.
5 min. Note: The pendulum must be hung after the stage is rotated.
The instructor backs up against the ladder/plywood backdrop, holds the pendulum bob up to his or her chin, and releases it. Because of conservation of energy the bob will swing across the stage and return to its original position adjacent to the instructor's chin, but without hitting his or her chin. Despite the wariness of the students, the pendulum bob cannot rise to a height greater than its original height, and the instructor is safe.Try giving the pendulum a subtle push as you release it, so that the first time the experiment is done the bob comes back and strikes the plywood backdrop.
PIRA 1M40.10) See also Demonstration C8-02 for classroom version.