This has always bugged me, and I don't think that I've ever gotten a satisfactory explanation (even though it's a really simple issue): Why do things bounce? The way I see it, you have a ball falling towards the ground. The ground has no momentum, the ball has momentum pointing towards the ground. When the ball hits, the momentum is transferred from the ball to the ground. The ground hardly moves, because the earth is massive, so the change in velocity is negligible. Yet, the net velocity in the system is now zero: how does the ball end up with a momentum opposite (minus a bit) of its original momentum? On a similar note, same scenario: a ball is bouncing, the ball has a mass m and a directed velocity vector v leading to a momentum of p. The ball hits the ground. How are the forces resolved? In other words, obviously some forces cause the ball to reverse direction, and I know that they are equal and opposite, but how does the ball get a double dosage of this force (enough to cause it to stop and then reverse direction) and how are the forces calculated (I know that force is momentum over time, but how do we find the time?) Thanks.