A boy throws a 4-kg pumpkin at 8m/s to a 40kg girl on roller skates, who catches it. At what speed does the girl then move backwards?

Answer 1

Assume the girl was facing the boy, and was stationary before catching the pumpkin.

Her initial momentum is zero.
Momentum of the punmkin
= 4 kg * 8 m/s
= 32 kg-m/s

Combined mass of girl and pumpkin
= 4+40
= 44 kg

Final velocity
= 32 kg-m-s^-1 / 44 kg
= 8/11 m/s