Help me plz.
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This question is a good example of the difference between 'speed' and 'velocity'.  This one is all 'velocity', and you've absolutely gotta keep the directions straight.

The object started out moving forward at 10 m/s.  It ended up moving backward at 2 m/s.  What was the change in its velocity ?

The change was 12 m/s backward !  

Momentum = (mass) x (velocity).

Change in momentum = (mass) x (change in velocity)

This change in momentum = (0.1 kg) x (12 m/s backward)

This change in momentum = 1.2 m/s backward  or  1.2 Ns  (B)

a and b sometimes have a positive value