a ball of mass 200g falls to the gound and just before striking the ground its speed is 5m/s. It rebounds with a velocity of 4m/s. What is the magnitude and direction of the change in momentum. I have the momentum before the rebound as -1kg/m/s and after as 0.8kg/m/s but I don't know how to work out the change or the direction? Can anyone help me do it?