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Homework Help: Two people in spaceshift shift seats

  1. Mar 24, 2009 #1
    Two people in a spaceship shift seats....

    1. The problem statement, all variables and given/known data
    Simma (mass 60kg) and Stan (mass 90kg) are testing an ultralight spacepod. They swap seats, with the seats being 4.0m apart, located at equal distances from the center of mass of the space-pod. The space-pod's mass is 50kg. Why does the space-pod not move after they take their new seats? How far does it move and which way? All observations are in the frame in which the space-pod was initially stationary.

    2. Relevant equations
    p[tex]_{1}[/tex] + p[tex]_{2}[/tex] + p[tex]_{3}[/tex] = [tex]\Sigma[/tex]P
    center of mass = m[tex]_{2}[/tex] / m[tex]_{1}[/tex] + m[tex]_{2}[/tex] x d

    3. The attempt at a solution
    There are two questions here: How far does the spacepod move when Simma and Stan change seats, and why does it stop after the take their new seats.

    Unless I'm over looking something, the answer to the second question is simply that the net momemtum = 0, so the spacepod is at rest.

    In order to find out how far it moves, I figured I could try to figure out the change in position of the center of mass, and that is how far the spacepod moves? Not really sure how to approach it.
    Last edited: Mar 24, 2009
  2. jcsd
  3. Mar 24, 2009 #2
    Re: Two people in spaceshift shift seats....

    Anyone? Just need a little steering in the right direction, the rest I should be able to do by myself.
  4. Mar 24, 2009 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Re: Two people in a spaceship shift seats....

    Yes, that is exactly how to approach it :smile:

    Since the center-of-mass of pod+Simma+Stan does not move,

    Δcom of pod = -Δcom of (Simma & Stan)
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