ok so my physics textbooks crap, it explains something in a tiny paragraph without any examples (I'm getting new ones). I have this question I'm stuck on. There is 2 balls that both have the mass of 1.5kg, one is not moving and one is moving at 4m/s - so ball A is about to collide with ball B. the questions are: the momentum of ball A just after collision, momentum of ball B just after collision and velocity of ball B after the collision? do I use force= change in momentum/time taken for the change equation. so for the first two im guessing the question is to find the change in momentum which is force x time taken for change. but what is the time taken for the change - is it 4m/s???? or do I need a different equation to find out the time it took for the change and then use that to find out the change in momentum?