Students switch to the c.m. reference frame by selecting the center-of-mass track in the reference frame menu. They then open a “world view” of the collision (bottom view in Fig. 1) where they observe that the pucks move in opposite directions at all times, first approaching the c.m. and then separating. Elastic collisions have separation speeds (radial velocity components) equal to approach speeds while inelastic collisions have slower separation speeds.