There are 3 types of collision that you need to know about at A-level... Perfectly Elastic collisions (A special case) All momentum is conserved (not surprisingly - it always is!) Kinetic energy is conserved (that's what makes this special). Relative speed of approach = relative speed of separation. (So if one is catching the other at 10m/s before the collision, it will be moving apart from it at 10m/s after the collision). /**/ Perfectly Elastic collisions are surprisingly common. All collisions between atoms are Perfectly Elastic according to the Kinetic Theory of Gases. Perfectly Inelastic collisions (Another special one) All momentum is conserved (as always). Kinetic energy is not conserved. The relative speed of separation is zero. (In English, that means the objects stick together after the collision so just consider them as one object whose mass is the same as that of the two original masses combined). /**/ Inelastic collisions (The usual old case) All momentum is conserved (again). Kinetic energy is not conserved (again). You can't say anything about the speed at which they leave each other without doing a calculation.