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Newton's law, normal force, acceleration

  1. Sep 29, 2011 #1
    A 75 kg person is standing on a bathroom scale in an elevator. The scale is calibrated to read in Newtons. The elevator is accelerating upward at 135 m/s2. What is the reading on the scale? What apparent value of g does the man give?



    The Free Body Diagram
    Normal on the man, upward
    Weight mg of the man, downward



    My Attempt/Logic
    The only two forces acting on the man are normal force and weight.

    FN - mg = ma
    fN = mg + ma, where N is the reading on the scale.

    FN = (75)(9.8) + (75)(135)
    FN = 10860 N

    The solution reads 8.5x102 N? Am I doing something wrong? This problem seems very basic, but I am obviously missing something.
     
  2. jcsd
  3. Sep 29, 2011 #2

    Doc Al

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    Staff: Mentor

    Are you sure the acceleration wasn't 1.35 m/s^2. 135 m/s^2 is not very realistic.
     
  4. Sep 29, 2011 #3
  5. Sep 29, 2011 #4
    It is much closer with 1.35 m/s2, However it is still a little off??? I am not sure if there is even anything wrong with the way I am doing it :( it seems correct?
     
  6. Sep 29, 2011 #5

    Doc Al

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    Staff: Mentor

    If you take g = 10 m/s^2 it's pretty close. (But it definitely says 135 m/s^2.)
    Your work is perfectly fine, and given the data, so is your answer.
     
  7. Sep 29, 2011 #6
    Thank you so much! It's very surprising that the solution could be wrong because if you read the preface of the packet it says that this learning guide has been edited many times over the course of many decades!!!
     
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