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Homework Help: Conservation of momentum and astronaut

  1. Nov 5, 2008 #1
    1. The problem statement, all variables and given/known data
    An astronaut is a distance L from her spaceship, and it is at rest with respect to the ship, when she discovers that her tether is broken. She tosses a wrench with a spees Vw in the direction opposite that of the ship in ordr to propel herself back to the ship. The astronaut has a mass MA, and the wrench has a mass MW.
    1) What is the initial momentum (before toss) of the astronaut + wrench system? What is the final momentum?
    2) Use conservation of momentum to solve for the final speed. VA of the astronaut relative to the ship, in terms of MA, MW
    , and Vw.
    3) How long does it take her to reach the ship? Express your answer in terms of L, MA, mw, and VW?
    4) How far has the wrench traveled from its original position when the astronaut reaches the ship? Express your answer in terms of L, MA, and mW?
    2. Relevant equations
    conservation of momentum: Pi=PF if Fnet=0
    3. The attempt at a solution
    Please tell me if I've made any mistakes:
    1) the initial momentum if zero since everything is at rest before she throws the wrench, and since it is an isolated system I have the following:
    PF=Ma(Va)+ mw(vw)= Pi=0
    2) final velocity is:
    3)how long it takes her to reach her ship:
    (there is no acceleration due to gravity because they are in space)
    0=L- (MwVw/ma) t
    L= (MwVw/ma) t
    I divided, inverted and multiplied, and came up with the following:
    t= Lma/mwvw
    4) I don't have any idea how to approach this part, and will appreciate any hints. Thank you. :smile:
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 5, 2008 #2
    1,2,3 correct: 4: you know the speed of the wrench, and the time the astronaut travels.

    Or, you know the ratio of their (constant) speeds, so the ratio of the distances they travel must be the same.
  4. Nov 5, 2008 #3


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    Gold Member

    You already solved for time T it took for the astronaut to reach the ship which will be the same time for the wrench. So simply use your momentum equation and solve for V of the wrench. and dx/dt = v of course.
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