A system consisting of two pendula attached to an air cart is mathematically analyzed, and the coefficient of restitution is shown to pass through a deep minimum. The solution to the small angle equation of motion is transcendental and provides an exercise in graphical methods for the beginning mechanics student.
Gruebel, R.; Dennis, James C.; and Choate, L., "A Variable Coeficient of Restitution Experiment on a Linear Air Track" (1971). Faculty Publications. 17.