I am confused about problem number seven part b in this pdf. http://aplusphysics.com/ap1/Problems/AP1 Momentum.pdf I read the given answer, and I am still confused. I don't know how you are supposed to get 0 m/s for the velocity of the block. Why do you use negative impulse for the block when you use positive impulse for the sphere? When do you use negative impulse? Why don't you use the mass of the entire system, which would be the block and the ball? This has to do with it being elastic/inelastic, but I don't know how you can tell from an impulse graph. Help??