Mechanical apparatus for the fold catastrophe demonstration

Abstract

We present and discuss a non-expensive mechanical experiment which is very suitable for a classroom demonstration of the fold catastrophe behaviour. The apparatus consists of a bar attached to a pulley, to which a hanging body is connected by means of a thread wrapped around its rim. The system may perform stable oscillations or, else, the body may continuously fall down, while the pulley executes a sequence of 360° rotations. This behaviour is analysed in connection with the shape of the potential, which is a good example of a fold catastrophe potential. For the system to perform stable oscillations, that shape reminds a clothes washboard, with local maxima and minima, a potential well known in superconducting quantum computing. We provide some hints for the instructor to explore the device in lab classes.