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Homework Help: Conservation of Momentum and Relative Velocities

  1. Feb 9, 2015 #1
    1. The problem statement, all variables and given/known data
    An 82-kg lumberjack stands at one end of a 380-kg floating log, as shown in the figure (Figure 1) . Both the log and the lumberjack are at rest initially.

    If the lumberjack now trots toward the other end of the log with a speed of 2.7 m/s relative to the log, what is the lumberjack's speed relative to the shore? Ignore friction between the log and the water.

    Express your answer using two significant figures.

    2. Relevant equations
    Conservation of Momentum

    3. The attempt at a solution
    m1v1' + m2v2' = 0
    m1v1' = -m2v2'
    (82)(2.7) = (-380)(v2')
    v2' = -0.5826

    velocity in respect to shore = 2.7 + (-0.5826) = 2.1 m/s
    For some reason this answer is incorrect. I also tried 2.7 + 0.5826 = 3.3 m/s, but that is wrong also. Is there something that I am doing incorrectly before this last step?
  2. jcsd
  3. Feb 9, 2015 #2


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    2017 Award

    the 2.7 m/s is w.r.t the log, right ? But the 0.58 m/s can not be wrt the log. !
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