OpenStax College Physics for AP® Courses, Chapter 8, Problem 45 (Test Prep for AP® Courses)

This is College Physics Answers with Shaun Dychko. Momentum is conserved whenever there is no net external force acting on a system. But in this question there is a force acting on the system which is external to the system and that is the friction due to this rough table top. It's exerting a force like this and so this mass, mass A, is experiencing a net force on it. It's slowing down before it collides with this thing here. You can consider its momentum at this point here, it'll be something -- it'll be mass A times whatever velocity it has at that point, we'll call it x equals zero. Then just before it collides with his block here, its momentum has changed even before any collisions have occurred. So since that's the case, that means that momentum is not conserved because its momentum is changing. At these two positions, x equals zero and x equals whatever x two this is before the collision, this momentum has changed before the collision. So that's a clue that momentum is not conserved. The other clue is that there is a net external force acting on the system. Then after the collision it's the same issue where these two objects are being acted upon by the friction force. But during the collision, since they're not sliding during the collision and this is just the brief instant of contact between the two objects, during that moment there is no friction acting on it because it's not sliding; it's just kind of deforming and colliding. So during that instance, momentum is conserved. So we're going to give an answer A here.