Something I am doing wrong?
"A 65kg boy and his 40kg sister both wearing roller blades, face each other at rest. The girl pushes the boy hard, sending him backwards with a velocity of 2.90 m/s toward the west. Ignore friction.
What speed does the girl go?"
I cam up with m1(v1i - v1f) = m2(v2f - v2i)
Solved for v2f and got 3.9 m/s
But the correct answer is 4.71 m/s
?? Any help?