So, I am trying to grasp the concept of conservation of momentum. I need someone to just look over my reasonings (and correct me for any errors) for the following situation. Let's say there are two objects, A and B. A is moving and then it has a head-on collision with object B, which is stationary. There are no external forces acting upon the objects. So, I'm trying to figure out which of the following situations are possible. -A and B move in the same direction - I say this is possible if object A has a mass that is much greater than B. This way, when the objects collide, object A's velocity barely gets changed after colliding with B. -A and B move in opposite directions - This is possible - I saw a lot of demonstrations showing this result. -A moves and B is stationary - I think this is possible. It's like the reverse of the first situation. The mass of A is a lot smaller than B so that it just bounces off B. -A is stationary and B moves - I think this is possible. Elastic collision? It's like that toy where you pull back one ball, and the one on the opposite end flies off at the same veleocity. -A and B are both stationary - This one I'm not sure about. Isn't it possible that the KE turns into other forms of energy. But then, the conservation of energy formula wouldn't work. m1v1 + m2(0) = m1(0) + m2(0) m1v1 = 0. That means the ball either has no mass or no velocity, which wouldn't work for the problem.