Drawing Board

by Aiona

Last weeHarmonogramk I snuck up to drawing board a few minutes after closing with one of my closest friends, Daniel, who is also one of the biggest physics geeks I know. In the void and lonely mezzanine we boldly experimented with the one thing we were explicitly told never to do… we moved the weight.  It was the dawn of a new fascination with this simple exhibit, and its complex mathematical properties. The following is a brief geek out session about the physics of drawing board: When we first learned how to facilitate this exhibit, Sarah told us about how the board has three degrees of freedom, movement along the x-axis, movement along the y-axis, and torque. When the board is moving along both the x and y axes it creates a circle, while torque will create figure eights and more complex patterns.  As friction slows the swinging of this giant rectangular pendulum, the shapes become smaller and smaller until they shrink to a point and all that is left is a crazy psychedelic image.  This type of pattern is called a harmonogram, and here’s a really nifty applet where you can play with them: http://www.comsewogueyouthclub.net/encyclo/640×4801.html. The drawing board at the Exploratorium is basically a giant real life harmonogram maker, except that it’s been specifically tuned to have certain properties. According to the museum website, the weight on the board and the length of the chains are specifically designed so that the period of the torque is twice the period of the pendulum motion. You can destroy this by sitting (very very still) on the drawing board while it’s moving. The result is a more complex harmonogram that can do more loops than just a figure eight. Moving the weight off center will cause the pattern to morph more rapidly than normal. I have no idea why that is, but please please post a response if you do. I hypothesize that the torque is not entirely independent of the pendulum motion and that motion of one sort either decays faster than the others, or can be translated into it.  I also noticed that if I start the drawing board moving along only one axis it will start out as a line, morph into a circle, and end up moving in a line along the other axis. Why does it do that? Please, please, physics geeks of the museum, answer all my questions about drawing board!