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Homework Help: What is the mass of the woman?

  1. Oct 31, 2005 #1
    A woman stands in an elevator that has a constant upward acceleration while the elevator travels upward a distance of 18.0 m. During the 18.0-m displacement, the normal force exerted by the elevator floor does 8.25 kJ of work on her and gravity does 7.35 kJ of work on her.

    a:What is the mass of the woman?

    b:What is the normal force that the elevator floor exerts on her?

    c:What is the acceleration of the elevator?

    Once again, I am clueless as how to approach this problem.
  2. jcsd
  3. Oct 31, 2005 #2

    Chi Meson

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    Remember that gravity's work is negative in this situation.
    Start writing the formulas for what you know:

    a: The force of gravity is found by multiplying "what" times 9.8? Force times distance equals "whatelse"? Go calculate the mass!

    b: same thing, different force.

    c: two forces. What's the net force?
  4. Oct 31, 2005 #3
    part a: force x distance= work done, right? so work/distance=force?, so 7.35/18=.4083

    still don't know how to calculate mass.
  5. Oct 31, 2005 #4


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    Use Newtons 2nd law to determine the mass.
  6. Oct 31, 2005 #5
    sum of the forces= mass * acceleration
    but I dont know the force or the acceleration
  7. Nov 1, 2005 #6


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    a) Write down an expression for the gravitational force on the woman. That force acts over a distance, so write down an expression for the work done by that force.
  8. Nov 1, 2005 #7

    Chi Meson

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    Similarly to vertigo's hint, think what was the change in gravitational potential energy on the woman, because this is equal to the (negative of the) work done by gravity.
  9. Nov 1, 2005 #8
    Well, don't think I'm being harsh with you ledhead, but you should read your courses and your book once again.
    I mean, this kind of exercice is a direct application of what you should have learned.
    At least, it is here, in France.

    Here is a good advice to help you solve almost all mechanics related problems:
    always do a diagram of the situation, it helps a lot in most situations.

    Another good advice in physics is to use dimensional equations: this way you are always sure the equation you use gives you a coherent result in term of units.

    Just some advices to help you ledhead, but do not hesitate to ask more questions if you feel the need!


    Ps: Excuse me for my not-so-good english :p
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