Two disks with sizes proportional to their masses are assumed to interact only when they collide. An elastic collision, conserving kinetic energy and momentum, results when the disks make contact with one another. The lengths of the arrows are proportional to their velocities. The center of mass of the two-body system is marked with a small yellow disk. The paths after the collision (if any) are shown and you can see that the center of mass moves in a straight line throughout. This is implied by Newton's first law, since the composite system is not subject to any external forces.