Work Step by Step
There are no external forces acting on the pair, so their momentum is conserved. The initial momentum is zero. Let’s say the boy moves to the right. His final momentum is mv = (40 kg)(1 m/s). The girl’s final momentum must be (20 kg)(-2 m/s) so that their net final momentum is still zero. In other words, the girl moves in the opposite direction at a speed of 2.0 m/s.