Consider two concentric circles of radius r 1 and r 2 as might be drawn on the face of a wall clock. Suppose a uniform rigid heavy rod of length l = | r 2 − − r 1 | is somehow constrained between these two circles so that one end of the rod remains on the inner circle and the other remains on the outer circle. Motion of the rod along these circles, acting as guides, is frictionless. The rod is held in the three o'clock position so that it is horizontal, then released.