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What is her speed relative to the ice surface?

  1. Apr 22, 2009 #1
    A 44 kg girl is standing on a plank that has a mass of 170 kg. The plank, originally at rest, is free to slide on a frozen lake, which is a flat, frictionless supporting surface. The girl begins to walk along the plank at a constant speed of 1.37 m/s relative to the plank.
    What is her speed relative to the ice surface?

    My attempt:
    The horizontal force the girl exerts on the plank is f=1.37(44)=60.28N
    so the speed of the plank relative to the ice should be 60.28=170a a=.354ms
    and her speed relative to the ice should be 1.37-.354=1.016
    but this is incorrect, what am i doing wrong?
     
  2. jcsd
  3. Apr 22, 2009 #2
    Re: friction

    Here's at least one error in your thinking: How can a force be a speed times a mass?
     
  4. Apr 22, 2009 #3
    Re: friction

    where do i begin then?
     
  5. Apr 23, 2009 #4
    Re: friction

    would momentum be involved?
     
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