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Homework Help: Conservation of momentum astronaut problem

  1. Mar 29, 2008 #1
    1. The problem statement, all variables and given/known data
    An astronaut in his space suit and with a propulsion unit (empty of its gas propellant) strapped to his back has a mass of 143 kg. During a space-walk, the unit, which has been filled with propellant gas, ejects some gas with a velocity of +30.9 m/s. As a result, the astronaut recoils with a velocity of -0.265 m/s. After the gas is ejected, the mass of the astronaut (now wearing a partially empty propulsion unit) is 167 kg. What percentage of the gas propellant in the completely filled propulsion unit was depleted?

    2. Relevant equations


    3. The attempt at a solution
    -.265(astronaut + gas) = 30.9 (gas)
    -.265(143 + x) = 30.9x
    -37.846 -.265x = 30.9x
    31.165x = -37.846
    x = 1.214 kg

    i dont really know where to go from here... can someone please help?
  2. jcsd
  3. Mar 29, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You need to distinguish between the portion of gas left in the propulsion unit and the portion of gas expelled. You should be able to immediately figure out the portion in the container from the data given. Use the conservation of momentum equation to find the portion expelled.
    Correct this equation. The "gas" on the left is the portion left in the unit, which should not be an unknown. And get rid of that minus sign on the left.
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