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Homework Help: Astronaut Momentum

  1. Apr 29, 2009 #1
    1. The problem statement, all variables and given/known data

    An 89 kg astronaut is working on the engines of a spaceship that is drifting through space with a constant velocity. the astronaut turns away to look at the earth and several seconds later is 43.8m behind the ship, at rest to relative to the spaceship. The only way to return to the ship without a thruster is to throw a wrench directly away from the ship. The wrenches mass is .690 kg, and the astronaut throws the wrench with a speed of 21.9 m/s.

    How long does it take the astronaut to return to the ship ?

    2. Relevant equations

    mv1 = mv2

    3. The attempt at a solution

    i tryed (.690)(21.9) = (89)(x) and got the velocity of the astronaut .17 m/s so i multiplied that by the distance 43.8m and got 7.43 s is this correct ??
  2. jcsd
  3. Apr 29, 2009 #2
    First part seems good but I think you've got the last part muddled up.

    If the speed of the astronaut's recoil is 0.17m/s then you need to divide the distance by this number to find the time.
    speed = dist/time

    time = dist / speed

    If you reflect on your answer you should see that it isn't reasonable to cover 43m in 7 seconds moving at less than a metre per second.
  4. Apr 29, 2009 #3
    of course ! thanks a lot
  5. Apr 29, 2009 #4
    time = distance / velocity...

    edit: oh nevermind, a lil' too late
    Last edited: Apr 29, 2009
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