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Homework Help: Momentum Relative to Surface / Plug-and-Chug

  1. Mar 10, 2009 #1
    1. The problem statement, all variables and given/known data

    Stevie stands on a rolling platform designed for moving heavy objects. The platform has mass of 76 kg and is on a flat floor, supported by rolling wheels that can be considered to be frictionless. Stevie's mass is 43 kg. The platform and Stevie are stationary when she begins walking at a constant velocity of +1.2 m/s relative to the platform. (a) What is the platform's velocity relative to the floor? (b) What is Stevie's velocity relative to the floor?

    2. Relevant equations

    (p)i = (p)f Conservation of Momentum

    3. The attempt at a solution

    Used Conservation of Momentum, but I am having trouble establishing a reference point.

    43x1.2 + 76xV2f = 0 - That's for the the platform and Stevie.

    .521x119 = 43xV1f + 76xV2f - That's for the entire system....I think.

    As stated above, I think my problem is finding the reference point. I would normally spend more time with this one by myself, but I have a quiz on this fairly soon.

    The answers:
    (a) −0.43 m/s
    (b) 0.77 m/s
  2. jcsd
  3. Mar 10, 2009 #2
    Your first equation would give you the correct answer if you considered that the platform has additional mass as a result of someone standing on it.
  4. Mar 10, 2009 #3
    Of course it would.

    You're the man BishopUser. Thanks dude.
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