Demos: 1Q-06 Ball on Stick

Attempting to balance a stick on your finger is easier when the stick is weighted at its upper end. The inertia of the heavier ball allows the lower end of the stick to be moved rapidly back and forth to keep the support point under the center of gravity. Turning the stick over shows that it is almost impossible to maintain the balance.

Directions: Place the unweighted end of the stick on your finger and show that it is relatively easy to maintain the balance. Then invert the stick and show how difficult it is to balance it.

Suggestions for Presentation: If you have already presented the ideas of center of gravity and stability, the student might be led into thinking that the center of gravity being lower would lead to a greater stability. Demonstrate that in this case just the opposite occurs. Then ask what the differences might be.

(It is true that if the stick has a perfectly flat end and were to be set on a table and balanced, it would be more stable if the mass were lower. However, in this case we are dealing with bringing the system BACK into equilibrium after it has been displaced.)

Applications: Think about a circus performer attempting to balance a person sitting in a chair attached to a long pole. The person on the bottom moves quickly back and forth under the chair to keep it balanced.

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