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Weight Change in an Elevator

  1. Oct 14, 2008 #1
    1. The problem statement, all variables and given/known data
    A woman stands on a scale in a moving elevator. Her mass is 58.5 kg, and the combined mass of the elevator and scale is an additional 815 kg. Starting from rest, the elevator accelerates upward. During the acceleration, the hoisting cable applies a force of 9510 N. What does the scale read during the acceleration?


    2. Relevant equations
    Fnet = ma


    3. The attempt at a solution
    I found the acceleration of the entire elevator by doing 9510 - 815(9.8) = 1523. I divided this by 815 to get 1.8687. It is in the negative direction, so it is -1.8687 If this is the acceleration of the elevator, it must also be the acceleration of the person. As a result, I did Fnet = ma, Fnet = (58.5)(-1.8697) and got Fnet = 682.31895. I put 682.31895 as the what the scale reads but it says I got the answer wrong. What did I do incorrectly?
     
  2. jcsd
  3. Oct 14, 2008 #2

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    How many scales have you stepped on that give a reading in Newtons? When you are in an elevator that starts going up, do you feel momentarily heavier or lighter?
     
  4. Oct 14, 2008 #3
    The space to fill in the answer requests that it be given in Newtons.
    I believe you feel momentarily heavier.
     
  5. Oct 14, 2008 #4

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    You forgot about the mass of the woman.
     
  6. Oct 14, 2008 #5
    Is that because it says that the elevator and the scale provide an additional mass?
     
  7. Oct 14, 2008 #6

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    Yes. That's why I highlighted the word additional.
     
  8. Oct 14, 2008 #7
    Code (Text):
    9510 - 815(9.8) = 1523. I divided this by 815 to get 1.8687.
     
    So basically just add 58.5 to each of those calculations where I used 815?
     
  9. Oct 14, 2008 #8

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    What does that get you?
     
  10. Oct 14, 2008 #9
    It accounts for the weight of the woman in all of my calculations.
     
  11. Oct 15, 2008 #10
    So is the answer 636.8895?
     
  12. Oct 15, 2008 #11

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    I get a very slightly different answer from your 636.8895 Newtons.

    From your first post, it appears you are using some electronic form that tells you whether your answer is correct. Did it accept 636.8895 Newtons? If it did not accept that, show your work.
     
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