1. The problem statement, all variables and given/known data I solved this problem simply by substituting the initial and final velocities in vector form and applying the principle of conservation of momentum, but there's something I don't understand about this problem. "Two small smooth spheres A and B have equal radii. The mass of A is 2m kg and the mass of B is m kg. The spheres are moving on a smooth horizontal plane and they collide. Immediately before the collision the velocity of A is (2i – 2j) ms-1 and the velocity of B is (–3i – j) ms-1. Immediately after the collision the velocity of A is (i – 3j) ms-1. Find the speed of B immediately after the collision." I thought that the components of velocity perpendicular to the line of centres of the spheres would be unchanged in the collision, at least, this is what it says in my M4 book. The vertical component of A's velocity before is -2j, but after colliding with B, it's vertical component is -3j. Why is this? Thanks. 2. Relevant equations v = eu, conservation of momentum 3. The attempt at a solution If you simply apply the conversation of momentum principle with the velocities in vector form, you get the correct answer of √2 m/s. But why do the velocities perpendicular to the line of centres change?