Lissajous Figures with a Pendulum (5)

Objective: To draw complicated pictures using two perpendicular pendulums

Outcomes: Demonstrate how various forces can affect the movement of objects

Materials: Black sheet of construction paper; styrofoam cup; salt; string; nail

Doing it: Tie a long piece of string between two chairs, so that it hangs a few feet above the floor. Then tie another piece of string into a loop around the first piece. Your new pendulum should look like a Y. Now attach your pendulum bob, which will be the cup and salt. Attach the cup using anything, such as a paper clip poked through each side or a thin piece of twine. Try to make the cup balanced. Now, fill the cup with salt. Poke a small hole in the bottom of the cup with the nail. Salt should begin to flow freely. Pull the cup a bit in any direction and let it go. You will watch as a Lissajous pattern is formed. The number of different patterns that can be made by such a setup is huge.

Explanation: The Y shaped string is really two pendulums at right angles. One will have a longer period than the other. This means the cup will never quite arrive at the same point twice, and is further complicated by the fact that as salt flows, the mass of the pendulum bob is changing. Eventually you will have nothing but a smear of salt. Pendulums have been used for many things in machines, most famously their use as a time keeper for pendulum clocks.

Tips:
-Use smaller grained salt for this experiment (or sugar), do not use rock salt.
-To make the salt movement uniform, try inserting a funnel into the hole and allowing the salt to fall through it
-The higher the cup in above the ground, the more the salt crystals will scatter when they fall. Lower it until the salt grains scatter less than a few cm (less than half a foot is a good height)
-Try using a section of carpet (or something similarly padded) to catch the salt pattern, this will reduce any movement in the salt grains as they land and will create a more distinct pattern.
-Use a dark background for the salt to land on.