Internal Degrees of Freedom During Collisions

This demonstration is designed to help students understand what goes on at the microscopic level during collisions. The system on the left consists of an array of lead fishing sinkers suspended from springs inside a rigid frame. This can be thought of as representing a crystalline solid (the jiggling masses on springs representing lattice vibrations). The frame is mounted on a roller cart with an elastic steel bumper at one end. The cart is rolled across the lecture bench and collides with a fixed barrier, whereupon most of the initial kinetic energy is transferred irreversibly to lattice vibrations of the crystal model.

A more elastic collision results if the vibrating crystal model is replaced with rigidly mounted objects of comparable total mass. The photo on the right shows the same roller cart with a set of masses which have been wedged into a suitable holder to prevent them from sliding with respect to the cart during collision. The rebound kinetic energy of this system is nearly equal to the initial kinetic energy, in contrast to the vibrating crystal model, which absorbs most of the energy during the collision.