A toy design that has existed since the middle ages. When the black rubber band is wound up, the the long horizontal arm swings around and the string wraps around the vertical rod. It keeps wrapping around till the mass at the end encounters the vertical rod, and then it stops and begins to unwind. Once it finishes unwinding, the rubber band is once again allowed to rotationally accelerate the horizontal arm, thus starting the cycle over.

When the black rubber band is wound up, the the long horizontal arm is set into an offset rotational motion. As it swings around the mass at the end of the string, the motion encounters the vertical rod and the string and mass being to wrap around it. It keeps wrapping around till the mass at the end encounters the vertical rod, and then it stops and begins to unwind. Once it finishes unwinding, the rubber band is once again allowed to rotationally accelerate the horizontal arm, thus starting the cycle over. This can be used to show transfer of energy between its potential and kinetic forms Updated Jan 17 2017