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Homework Help: Newton's Laws

  1. Feb 25, 2010 #1
    1. The problem statement, all variables and given/known data

    Bill gets into an elevator on the 40th floor of a building and it begins moving at t = 0s. The figure shows his apparent weight over the next 12s.

    Is the Elevator moving up or down originally?
    What is his weight?
    How far does he travel over the first 12 seconds

    graph.jpg

    2. Relevant equations
    F = ma


    3. The attempt at a solution

    Part A is taken care of.

    But for Part B (and therefore part C), I assumed that his weight at t = 0 was his mass, so divided that by gravity, but that answer was spit out (automated grading system). I'm assuming that this is because the elevator starts moving at 0, but if that's the case I'm not sure what to do about his original mass.

    I know once I have that I can use the difference at each interval to find the acceleration and therefore distance, but it's the original mass that's confusing me.
     
  2. jcsd
  3. Feb 25, 2010 #2

    Delphi51

    User Avatar
    Homework Helper

    Three conditions are possible: upward acceleration (highest weight), constant speed (normal weight), and downward acceleration (lower weight). You should be able to pick out the middle one on the graph and that will be your normal weight.
     
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