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.
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