W6.1 A Question of Balance
Suppose that you support a meter stick horizontally by positioning one finger under one of the ends and the other finger about one-quarter of the way from the opposite end, as expertly demonstrated by Gwen in the photograph below.
Now suppose that Gwen moves her fingers slowly together until they touch. (The motion will be started in the configuration shown, with the meter stick extended off to the right side in the photograph.)
What will happen to the meter stick?
- It will fall off the right side in the picture above.
- It will fall off the left side in the picture above.
- Neither; it will end up balanced on Gwen’s fingers.
Solution
Reveal

Solution
The answer is (3): the meter stick will remain balanced as Gwen brings her fingers together, as seen on this MPEG video.
The finger closer to the center of the meter stick supports more of the weight, so that one has more frictional force with the meter stick and the other one slides. This continues until the sliding finger moves closer to the center, and the situation reverses. The two fingers alternately slide and stick until both fingers contact at the center.
Try placing a weight on one end of the meter stick and repeating this process. What will happen?
The same thing will happen, except that the fingers will come together under the center of mass of the weighted meter stick. In the simple case of the unweighted meter stick the center of mass is at the center of the stick. In fact, this is one way to find the center of mass of a complex object.