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Homework Help: Elevator problem

  1. Jul 23, 2013 #1
    1. The problem statement, all variables and given/known data
    A woman stands on a scale in a moving elevator. Her mass is 50kg and the combined mass of the elevator accelerates upward. During the acceleration , the hoisting cable applies a force of 9350N. What does the scale read during the acceleration ?

    2. Relevant equations
    F net= T-MG= Ma
    Apparent weight = mg(a/g+1)

    3. The attempt at a solution
    First we find a and the plug in the a in the apparent weight and we find the scale reading
    first attempt: 9530-(50+815) = (815+50)a
    and a= 1.22m/s^2
    and the apparent weight = 9532N
  2. jcsd
  3. Jul 23, 2013 #2

    rude man

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    You are mixing up Newtons with kg in this equation.
    Your problem doesn't state it but I gather the mass of the elevator is 815 kg.
  4. Jul 23, 2013 #3


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    Where does the number 815 come from?
    Please work with units, it is easier to spot errors that way. For example, you cannot subtract 50kg from 9350N, that does not make sense.

    It would be impossible to stand on a scale in that case (that is the weight of a car), the result cannot be true.
  5. Jul 23, 2013 #4
    815 comes from the combined mass of the elevator and the scale .
    a=1.22 m/s2
    Apparent weight=N=Mg(a/g+1)
    =(865*9.8)(1.22/9.8 + 1)
    Is that corrected ?
    Last edited: Jul 23, 2013
  6. Jul 23, 2013 #5

    rude man

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    You mean 815 is the mass of the elevator, I think.
    That is finally correct.
    No. She's not going to weigh much more than at rest. And the scale reads in kg, not N.
    So the answer is not much more than 50 kg.
  7. Jul 24, 2013 #6


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    865? Isn't that going to give you the apparent weight of the elevator+passenger?
  8. Jul 24, 2013 #7
    so you mean the mass is 50 not 865( 815+50) ???
  9. Jul 24, 2013 #8

    rude man

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    Yes. You're not measuring the weight of the elevator, right?
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