Yardstick, meter stick, cane, broomstick, PVC pipe, or any stick of a similar general size
Clay or other weight
To Do and Notice
Support the stick with both hands, resting the ends on just your index fingers. Slowly slide your fingers together until they meet. Your fingers will meet under the stick’s center of gravity.
Attach the weight or a piece of clay to some point on the stick. Again support the stick on two fingers, and then slide your fingers together to locate the new center of gravity. Move the weight or piece of clay to some new place on the stick. Repeat the experiment. Your fingers will always meet right under the center of gravity.
What's Going On?
The stick’s center of gravity is the place where you can balance the stick on just one finger. When you first support the stick with two fingers, one finger (the one that is closer to the center of gravity) will generally be holding a little more of the weight than the other. When you try to move your fingers closer together, the one that is carrying less weight will slide more easily. This finger will continue to slide more easily until it gets closer to the center of gravity than the other finger, at which point the situation will reverse and the other finger will begin to slide faster. Your left and right fingers simply alternate moving until they meet at the center of gravity, where both fingers support equal weight.
Often, an object's center of gravity is also the center of its length. The next time you need to carry something uniformly long and thin—like a wooden pole or a length of pipe or metal (or, if you're a tightrope walker, that really long balancing stick)—finding the center of gravity by sliding your hands together near the middle of the object can help you find its center. If you need to carry shorter lengths in your car, you can find the approximate center of the material this way, have it cut, and be confident that you've got two roughly equal-sized pieces.