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Homework Help: Astronaut throwing a Wrench in space problem again

  1. Dec 4, 2015 #1
    1. The problem statement, all variables and given/known data
    An astronaut with a mass of 90 kg (including spacesuit and equipment) is drifting away from his spaceship at a speed of 0.20 m/s with respect to the spaceship. The astronaut is equipped only with a 0.50-kg wrench to help him get back to the ship.

    2. Relevant equations
    conservation of momentum m1*v1=m2*v1

    3. The attempt at a solution
    unfortunately i really have no good an attempt. i know that the Δv has to = 0.1
    and that the wrench has to exert a force on the astronaut in the opposite direction he is traveling but what i do not get is how the conservation of momentum equation is used since all the variables are known? (90kg+.5kg)(.2m/s)=(90ks)(.1m/s)

    i feel like i should be using the equation F=m*Δv/t since i need to get a Δv in it somewhere but beyond that i am stumped
  2. jcsd
  3. Dec 4, 2015 #2


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    Staff: Mentor

    Hi jmattsen, Welcome to Physics Forums.

    Your problem statement is vague or incomplete. You haven't specified precisely what is to be determined, and in what reference frame it is to be given. Presumably you need to find the minimum speed that the astronaut needs to throw the wrench in order to return to the spacecraft, but is that speed to be with respect to himself or the spacecraft? And is the wrench's mass included in the astronaut's "spacesuit and equipment", or is it to be counted separately from that 90 kg?
  4. Dec 4, 2015 #3
    whoops, i forgot the other half of the question.

    With what speed must he throw the wrench for his body to acquire a speed of 0.10 m/s?
  5. Dec 4, 2015 #4


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    Science Advisor

    That's the correct approach but where is the term for the spanner? and check your signs.. Your equation says the astronaut is still moving away from the ship, just slower than before, he's still in trouble! The question doesn't say it explicitly but I'd assume he wants to throw it fast enough to start moving back towards the ship.
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