This demo was made for PHYS 216 to show the centre of percussion. There are two ways in which the demo can be used. First of all, the rod can be used as a simple pendulum suspended from different holes along the rod. The holes are drilled at 0, L/8, L/4, 2L/3, 13L/18, 5L/6, as well as a hole at L/(2sqrt(3)). Places which should have the same period are painted the same colour. The hole painted with a blue stripe is the point with the shortest period. The various periods can be calculated, timed, and compared. It usually works best to wait for ~15 oscillations to make a measurement. Secondly, the pivot can be placed on the tracks at the top of the demo, allowing the pivot to move freely. A rubber mallet is then used to strike the bar in various places. When the bar is struck above the centre of percussion, the pivot swings the same direction initially. The opposite is true when the bar is struck below the centre of percussion. When struck at the centre of percussion (=2L/3), the pivot initally does not move, since the initial forces cancel.