Concepts

Conservation of energy

Overview

The bowling ball 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. The demonstration usually draws an audible response from the audience.

This version of the demo can only be done in B9200, which has a hook from which we can suspend the bowling ball.

Details

Equipment

• [1] Bowling ball with eye bolt and carabiner
  
• [1] Hook (pre-mounted in B9200)
  
• [1] Steel cable
  
• [1] 5/16" nut driver
• [1] Telescoping pole
• [1] Set of binoculars

Classroom Assembly

This is a two-person job. One person does the setup while the other person makes sure no one gets in the way.

1. Use the pole to mount the cable on the hook. The rubber end should go on the hook.
   
2. Make sure the cable is well-secured. Use the binoculars and tug on the cable a bit.
   
3. Adjust the cable length using the nut driver. Tighten all the nuts.
4. Clip the bowling ball in position.

Important Notes

• The bowling ball 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.
• When mounting and dismounting the cable, make sure no one is in the way. If the cable falls off the pole, it could hurt someone. A person bumping the cable-wrangler could result in the cable and the pole dropping.
• The hook is mounted fairly close to the screen. Make sure the ball's swing won't brush the cable against the screen.
  

Script

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

Additional Resources