Question

On an ice skating rink, a girl of mass 50 kg stands stationary, face to face with a boy of mass 80 kg. The children push off of one another, and the boy moves away with a velocity of +3 m/s.

What is the final velocity of the girl?

–1.9 m/s
+1.9 m/s
–4.8 m/s
+4.8 m/s

Answer 1

Use conservation of momentum. Initially at rest so total momentum = 0:

80(3) + 50 v_girl = 0 → v_girl = - (80·3)/50 = -4.8 m/s.

Answer: –4.8 m/s.