A ball of mass 50 g moving at a speed of 2.0 m/s strikes a plane surface at an angle of incidence 45o. The ball is reflected by the plane at equal angle of reflection with the same speed. Calculate (a) the magnitude of the change in momentum of the ball (b) the change in the magnitude of the momentum of the ball.
Mass of the ball, m = 50 g = 0.05 kg
Initial velocity of the ball, u = -2 cos45° î + 2 sin45° ĵ
Final velocity of ball after reflection, v = 2 cos45° î + 2 sin45° ĵ
(a.) Change in momentum = m(v-u)
= 0.05×[(2cos45° î + 2sin45° ĵ ) – (-2cos45° î + 2sin45° ĵ)]
= 0.2cos45° î = 0.14 î
Magnitude of change in momentum = 0.2/√2 =0.14 kg m/s
(b.) Magnitude of initial momentum = 
= 0.05×2
= 0.10 kg m/s
Magnitude of final momentum = 
= 0.05 × 2
= 0.10 kg m/s
Change in magnitude of momentum = 
= 0 kg m/s