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Elevator/ Scale problem

  1. Feb 26, 2009 #1
    1. The problem statement, all variables and given/known data
    A 192 lb person stands on a scale in an elevator. What does the scale read
    a. When the elevator is at rest.
    b. when the elevator is accelerating up at 4ft/sec^2
    c. when elevator is accelerating down at 3ft/sec^2


    2. Relevant equations



    3. The attempt at a solution

    Im not really sure how to do this, can someone help me and show the formulas as well so I can learn it? Thanks
     
  2. jcsd
  3. Feb 26, 2009 #2

    Delphi51

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    When the elevator is at rest, the scale will read 192 pounds.
    When it is accelerating upward, it will read higher because you not only have the usual mg force pressing the person down but also the ma force necessary to accelerate him upward.

    This problem has some messy units - be careful with them. It will be quite difficult to figure out the mass. I'm just old enough to remember that when we had pounds the unit of mass was the "slug". Not joking! Maybe convert everything into metric units?
     
    Last edited: Feb 26, 2009
  4. Feb 26, 2009 #3

    PhanthomJay

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    The unit of mass in the US system of measure, is still the slug. 1 slug weighs about 32 pounds : W=mg, where m is in slugs, g is 32ft/sec^2, and W is in pounds.

    Nofinga911: What have you tried so far? The formulas will not mean much without your initial attempt.
     
  5. Feb 26, 2009 #4

    Thats just it, I dont know the formulas...It would help me if I saw it done by someone else with the formulas this way I have some time to studyit before my test tomorrow..it was one of two out of the 20 questions I was unable to get. I would be able to break the formula down if I saw it done.
     
  6. Feb 26, 2009 #5

    Delphi51

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    The formulas you need are F = mg and F = ma.
    Yes, let us see you put the numbers in and do some multiplying!
    Do you have the number for g in ft/s^2 ?
     
  7. Feb 26, 2009 #6
    The problem statement said that this was a 192 lb person, which means that we can assume that at rest on the surface of the earth, this person weight 192 lb.

    Delphi51, how do you figure that for the elevator at rest, the scale will read 10 lb? Why not 192 lb?

    By the way, Delphi51, I think you probably meant to say W=mg and F=ma, did you not?
     
  8. Feb 26, 2009 #7

    Delphi51

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    Terribly sorry - I meant to type 192! That must have been very confusing.

    What is the difference between F = mg and W = mg?
    Weight is the Force of gravity so either letter should be okay.
    The two forces must be added to get the answers.
     
  9. Feb 26, 2009 #8
    What is the difference between F = mg and W = mg?

    The difference, as I see it, is that when you write F = mg and F = ma, there is a natural equivalence implied that therefore mg = ma and therefore g = a.
     
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