An astronaut with a mass of 100 kg throws a wrench with a mass of 2 kg at a velocity of 5 m/s. What is the resulting recoil velocity of the astronaut if both the wrench and astronaut were initially at rest together?

Answer 1

momentum after = momentum before = 0

0 = 2*5 - 100 v
v = 10/100 = 0.1

Answer 2

M1*V1 = -M2*V2.
100*V1 = -2*5,
V1 = -0.1 m/s.