change in momentum?

Question

A 1kg ball hits a surface perpendicularly with a speed of 3m/s and bounces back with a speed of 2m/s. The ball undergoes a change in momentum of
1) 1kg*m/s
2) 5kg*m/s
3) 3kg*m/s
4) 6kg*m/s
Explain~

Answer 1

p - p = mv - mv' = m(v - v') = 1(3 - (-2)) = 5 kg-m/sec (2)

This results because velocities are vectors; so we pick one direction as positive and the opposite direction has to be the negative direction. I chose v > 0 as down; so the rebound v' < 0 in the up direction.