Concepts

Conservation of energy

Overview

The 1 kg pendulum is held up to the demonstrator's face and released. Because of conservation of energy, the ball will not swing higher than the height at which it was released (in fact, because energy is lost during the swing, the ball falls slightly short of the demonstrator's face when it completes one full period). To paraphrase Leigh Palmer, physicists believe in conservation of energy and therefore have complete confidence that if they release the ball right next to their head that the ball will swing back and stop, not continue on and smash their head. The apparent danger of having one's head smashed gives the demonstration great crowd appeal.

Details

Equipment

• [1] 1 kg hooked mass
  
• [1] Rope
  
• [2] Lab stand
  
• [2] 90-degree clamp
  
• [1] Rod
• [2] Steel brick
• [1] Lab jack

Classroom Assembly

1. Assemble the stand for the pendulum mount using the lab stands, clamps, and rod.
2. Tie the mass to the rod.
3. Adjust the rope length to clear any obstacles.
   
4. Put the steel bricks on the lab stands to stabilize them.
   

Important Notes

• The 1 kg mass is heavy and can cause serious bodily harm. Release, DO NOT PUSH, the bowling ball away from your face. Note that it is difficult not to flinch when the ball swings towards your face. Practice this demo in advance and perform with extreme caution!
• Care must be taken not to hit benches and other objects with the pendulum.

Script

1. Keeping the rope taut, pull the pendulum (bowling ball) to one side until it touches your nose. Be sure to stand straight and still.
2. Release the pendulum, taking care not to move your face.
3. Remain very still while the pendulum swings through one full period.

Additional Resources