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Homework Help: Center of mass question (man on a boat)

  1. Oct 3, 2011 #1
    1. The problem statement, all variables and given/known data

    A man with mass m1 = 60 kg stands at the left end of a uniform boat with mass m2 = 169 kg and a length L = 2.7 m. Positive x is pointing to the right. Assume there is no friction or drag between the boat and water. After the man walks to the right edge of the boat, what is the new location the center of the boat?
    (The boat lies on the x-axis, so they're looking for the absolute position of the center.

    2. Relevant equations

    The absolute location of the CM will not change.

    3. The attempt at a solution
    First the CM equation (169*1.35)/(169+60) gives me x = 1 (relative to the boat) as the position of the center of mass, which I use as a reference point for the x-axis.
    When the man moves to the right, I plug in (60*2.7+169*1.35)/(169+60) and I get x = 1.7 relative to the boat. Since the center has moved to the left, I do 1.7 - 1 = 0.7 to find the distance the center has gone. Finally I subtract that change in distance from the original position with 1.35 - 0.7 = 0.65. But SmartPhysics says this is wrong. Does someone have any further insight here? Thank you!
  2. jcsd
  3. Oct 3, 2011 #2


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    Staff: Mentor

    The question doesn't provide an origin against which positions are to be measured, so how is 'absolute position' to be interpreted? Is this the precise and complete wording of the problem?
  4. Oct 3, 2011 #3
    because this is a question with multiple parts, and the answers to the first parts are based on the reference points of the boat's initial position, with the left of the boat starting at x=0. I guess there's a chance that SmartPhysics has an error, but I'm completely stumped.
    Last edited: Oct 3, 2011
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