A hockey ball of mass 200g travelling at 10 m s-1 is struck by a hockey stick so as to return it along its orignal path with a velocity at 5 m s-1. Calculate the change of momentum occured in the motion of the hockey ball by
Force and laws of motion
Change in linear momentum = Pfinal - Pinitial
= mvfinal - mvinitial
= m (vfinal - vinitial) = 0.2 kg(-5m/s - 10m/s) = - 3 kgm/s
Note that 10 m/s, the initial velocity is taken as positive, because we assume that direction as positive, but when the hockey ball returns the direction is changed and is opposite so the final velocity is -5 m/s.