1. The problem statement, all variables and given/known data A question came to my miind: Wouldn't everything be conserved? I know that the law of momentum conservation only applies when the system doesn't have any external forces or the net of them is equal to 0. Lets just say that someone hits the ball with a bat, He applies a force to the ball and the same happens to the bat ( Newton third law). So the momentum is conserved. An object is moving on a surface that has friction, The surface tries to stop the object and the same happens to the surface. So the momentum is conserved. An object is flying at a speed of v. The air tries to resist its motion with a force and the object also pushes air molecules with a force f. So Is the momentum conserved? ( I guess) When do we actually say that the momentum changed? what am I missing here? Should I exclude the air and friction from my system and just consider the object is my system then it is appropriate to say that the momentum changed? but if I take as a whole then I should say that the momentum is conserved? 2. Relevant equations P = mv F * t = m dv F = -f It would be great if you can provide some examples so I can figure it out. Also point out stuff don't directly go the the final point.