Observation and investigation of a non-monotonic period function for an impact oscillator

A mathematical model for a bench-top impact oscillator (a steel pendulum and an aluminum barrier) that incorporates Hertzian contact is analyzed, and predictions derived from our model are compared with experimental data. We report our discovery of a new effect: the existence of a non-monotone period function for the annulus of periodic orbits in the phase plane surrounding the rest point that corresponds to the downward vertical position of the pendulum in the unforced, undamped case. From this non-monotonicity, we predict novel resonance response behavior for the forced, damped oscillator and verify our predictions by experiment.