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Problem about Force, Work, and Energy

  1. Jan 9, 2013 #1
    1. The problem statement, all variables and given/known data

    MJ is falling from the highest point in his jump.
    h1(the highest point) = 2m, and the scale reads 0N,
    h2 = 1m, and the scale reads 2500N,
    and h3=?m, and the scale reads 800N.
    MJ is at rest at h3

    2. Relevant equations

    E1+W of point2-3= E3



    3. The attempt at a solution

    mgh1+FN(h2-h3)+Fg(h2-h3)= mgh3, because there is no KE1 or KE3,
    80(10)(2)+2500-2500h3-800+800h3= 800h3,
    3300= 2500h3, h3 = 1.32m (obviously incorrect as h3 is higher than h2)
     
    Last edited: Jan 9, 2013
  2. jcsd
  3. Jan 9, 2013 #2

    haruspex

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    What 'scale' is this - what is it measuring? Should the 800m be 800N?
     
  4. Jan 9, 2013 #3
    Sorry, the question is talking about a scale as in a bathroom scale, which measures FN in Newtons
     
  5. Jan 9, 2013 #4

    haruspex

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    Then my next question is how exactly the heights are defined. At h2, he must be in contact with the scales, so the height is not being measured from scales to feet. If it's from scales to anything higher than the ankle it will depend on posture - maybe he's crouching at h2, and the heights are measured to the head. But a reasonable guess is that he's upright in each case and the heights are measured to c of g.
    I don't see how you can write the equations you have in your attempt. They imply e.g. that the scales supplied an upward force of 2500N for the entire distance as he descended from h3 to h2.
     
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