Explanation:
Total mass of child and car = 50 + 120 = 170 kg .
initial velocity u = 4.5 m/s
initial momentum = mass x velocity = 170 x 4.5 = 765 kg m/s
final velocity = - 4.5 m/s [ - ve is due to reversal of direction ]
final momentum = - 765 kg m/s
change in momentum = final momentum - initial momentum
= - 765 - ( 765 )
= - 1530 kg m/s .