A light dowel rod hangs by a string from a fixed point. The bottom end of the dowel rod rests on a frictionless surface, in this case a small piece of very light foam, that supports the end of the dowel on the surface of the water.



The drawings above illustrate three possible resting positions for the dowel. When the dowel is placed in one of the three positions and released, where will it finally come to rest?

  • (a) with its upper end beneath the support point.
  • (b) with its center of mass beneath the support point.
  • (c) with its lower end beneath the support point.

After May 9, 2014, click Read More for the Answer.

 The answer is (a): the rod will come to a resting position with its upper end directly below the support point. This occurs because in the absence of friction acting on the lower end it will move so that the center of mass of the stick assumes its lowest possible position. This result is seen in the photograph below.


Any other position would result in a higher position for the center of mass, and therefore a higher resting potential energy.