All right, so, we just went over linear momentum in school, and one thing is confusing me: If I throw something at a wall, why does it bounce back? (I'm assuming both the object and the wall are infinitely hard and don't get deformed, and the wall won't move) It seems pretty straight-forward, but I can't get my mind around it, which makes me feel dumb. My dad tries to explain it with conservation of energy -- it has kinetic energy, and it can't just lose all that energy, so it has to keep some being kinetic energy, and that makes it go back. All right, so it has to. But I still don't understand how. I tried to ask him to explain what was happening in terms of forces instead, but he couldn't. The way I see it, is that the wall exerts normal force on the object thrown (ball). The normal force is a reaction force, right, so it ought only exert so much force on the ball that the ball stops and doesn't go through it. Why would it go ahead and exert extra force on the ball to make it bounce back? Where would it come from? I realize I'm probably just missing something simple, but could someone try to explain to me? It's really frustrating to not understand.