Quote: a 30-kg girl and a 25-kg boy face each other on Friction-free roller blades. the girl pushes the boy, who moves away at a speed of 1m/s. what is the speed of the girl?
Momentum is conserved.
Before the push the total momentum of the boy and the girl is zero since they aren't moving.
After the push the boy has momentum of (25kg)(1m/s) = 25 kg*m/s
The girl must have momentum equal and opposite to this since their sum must remain zero.
(25 kg*m/s) = -(30kg)(v
girl m/s)
v
girl = -(25/30)m/s = -5/6 m/s where negative sign indicates the girl moves in the opposite direction from the boy.
+6251 
