The sand pendulums consist of two parts: a position-limiting pendulum and a free pendulum. When the swinging plane of the position-limiting pendulum and the singing plane of the free pendulum overlap, the sand leaking from the sandglass will draw out a straight line. When the two swinging planes do not overlap, the sand of the sandglass will draw out various different patterns. Swing of the sandglass follows a strict law. The period for back-and-forth swinging of the sand pendulums is manly determined by the period of the position-limiting pendulum.
The pattern drawn by the sand pendulums is related with the length of their swinging lines and the position of the common point of the two swinging lines. Sand pendulums with four groups of different parameters are designed here, and you can see four distinct sand pendulum patterns at the same time. You can also adjust the position of the common point to observe change of the pattern.