Collisional vibrational excitation at the introductory physics laboratory

Abstract We present an approach to teaching inelastic collisions in the introductory physics laboratory based on the use of a target with internal structure. This allows freshman Science, Technology, Engineering and Mathematics students to explore analogies between collisions of gliding blocks on an air track and atomic and molecular collisions. A target simulating a diatomic molecule is assembled by connecting two gliding blocks by a light spring. The positions of the components of the collision system are determined as a function of time by video analysis. In this way, the student can go beyond the usual association between inelastic collision and dissipative force. The student performs on the air track the equivalent of a collisional vibrational excitation, in which part of the projectile’s initial kinetic energy is stored as internal energy of the target in the final configuration. We also discuss the consequences of introducing a new time scale related to the existence of internal structure in the target and, in this context, the so-called impulse approximation in many-body collisions. Finally, we explore the nature of the projectile–target interaction, mediated by rubber bands, and highlight the anharmonic character of the interaction.