Concepts

Conservation of energy

Overview

A 1-kg mass pendulum is held up to the demonstrator's face and is released. Because of conservation of energy, the mass will not swing higher than the height at which it was released. In fact, because energy is lost during the swing, the mass 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
  
• [1] Long copper tube
  
• [2] Large 90-degree clamp
• [1] Aluminum rod
• [2] 90-degree clamp
• [1] Small wooden block
  

Classroom Assembly

1. Secure a large 90-degree clamp to a stable object, such as a table, cart, or lectern. Use the wooden block to protect the table finish, if necessary.
2. Make an upside-down L shape by attaching the aluminum rod to the copper tube with the other large 90-degree clamp.
3. Attach two 90-degree clamps to the aluminum rod and secure the rope between the two 90-degree clamps.
4. Use the clamp from step 1 to secure the copper tube to the stable object.
5. Set the rope length and tie the mass to the rope.
   

Important Notes

• The 1 kg mass is heavy and can cause serious bodily harm. Release, DO NOT PUSH, the mass away from your face. Note that it is difficult not to flinch when the mass swings towards your face. Practice this demo in advance and perform with extreme caution!
• Make sure the rope cannot slip out of the mass's hook.
  
• Take care not to hit benches and other objects with the pendulum.

Script

1. Lightly tap the mass on the table/cart/lectern to show it is a rigid object that could cause much pain.
   
2. Keeping the rope taut, pull the pendulum (1 kg mass) 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