A ball of mass m strikes a vertical wall with a speed v at an angle of θ to the wall. The ball rebounds at the same speed and angle. What is the change in the magnitude of the momentum of the ball?

A. 2 mv sin θ
B. 2 mv cos θ
C. 2 mv 
D. zero

B

Momentum is a vector quantity so the angle and the direction are both relevant to the answer. Hence C and D can be eliminated. If θ = 900, then the ball is just rolling down the wall and there is no change in momentum. Hence B is correct (cos900 = 0).