The upward-driven disk, a steadily forced chaotic pendulum

Editor's Note: A simple plastic disk, wedged between two wheels and driven upward by a small electric motor, demonstrates remarkably complex behavior. By varying the initial conditions, one can observe steady rotation, periodic motion, or chaotic motion. This paper introduces a simple model for the behavior that also points out how it can mimic the solutions of the Lorenz equations, which are frequently used to describe fluid convection. There are still plenty of open questions, especially regarding the best model for frictional damping, and the easy and inexpensive construction could make this the starting point for many student projects.