When two identical balls collide with equal inital speed on a horizontal groove, one can observe three trajectory types depending on the groove width. To explain this observation, we derive velocity diagrams of the balls motions from Newton's laws of translation and rotation and kinematics of rigid bodies in a three-dimensional vectorial representation and compare them with experimental results. The velocity diagrams and an introduced determinant allow to discriminate between the trajectory types and to understand the interplay between translation and rotation after the collision of the balls.The observation of these trajectory types is 1 arXiv:1909.11471v1 [physics.class-ph]