Question

An astronaut with a mass of `75 kg` is floating in space. If the astronaut throws an object with a mass of `9 kg` at a speed of `1/8 m/s`, how much will his speed change by?

Answer 1

Assuming that the astronaut was initially stationary or moving parallel to the object's motion:
astronaut's change in speed = change in momentum / astronaut's mass = (1.125 kg m/s) / (70 kg) = 0.016 m/s

Explanation:

Change in momentum = mass x change in velocity

object's change in momentum = 9kg x 0.125 m/s = 1.125 kg m/s

This momentum is conserved, so the astronaut's velocity is changed in the opposite direction to the object's motion.

Assuming that the astronaut was initially stationary or moving parallel to the object's motion:

astronaut's change in speed = change in momentum / astronaut's mass = (1.125 kg m/s) / (70 kg) = 0.016 m/s

There is a problem in the wording of this question, in that we are not told the astronaut's initial velocity. If the astronaut was not initially stationary or moving parallel to the object's motion, we don't have enough information to calculate the change in speed.