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Homework Help: Apparent weight on an elevator. Magnitude of its acceleration?

  1. Oct 2, 2008 #1
    1. The problem statement, all variables and given/known data
    You are standing on a bathroom scale inside an elevator. Your weight is 160 lb, but the reading of the scale is 123 lb.
    (a) What is the magnitude and direction of the acceleration of the elevator?
    ??? m/s2
    (b) Can you tell whether the elevator is speeding up or slowing down?
    do not know
    slowing down
    speeding up

    2. Relevant equations

    3. The attempt at a solution

    160lbs = 75.574kg

    123lbs: 55.792kg

    g= 9.8ms

    obviously, the elevator is accelerating downwards, reducing his apparent weight.

    his weight when on the ground is 740.625N.

    So, 740.625=55.792a
    a=13.27475. The question is asking for magnitude, but there is no x component so sqrt(a^2) would still be 13.27475

    Obviously, this isnt correct. Can someone point me in the right direction?
  2. jcsd
  3. Oct 2, 2008 #2


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    If it was slowing down while going up it could read the same. All you know is acceleration is less down on the scales.
  4. Oct 2, 2008 #3
    Ahh, you're right. That makes sense - I didnt think of that.

    I tried using -13.27, and subtracting 9.8. What am I missing with the calculation?

    Thank you for the tip!
  5. Oct 2, 2008 #4


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    You can divide by like equations.

    9.8/160 = x/123

    I know the units are silly but they cancel as long as your acceleration is to be in m/s2

    The difference then with 9.8 must be the acceleration in the same direction as g.
  6. Oct 3, 2008 #5
    Thank you so very much!!

    I was able to figure it out after a bit of tweaking.

    This place is the best =D. I've tried to go through answering peoples questions- but I find that I either cannot answer them well enough, or someone has already beat me to it. I'll try to give back as much as I can
  7. Oct 3, 2008 #6


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    You're welcome. Glad that it helped. All contributions welcome.

  8. Oct 3, 2008 #7
    Maybe I'm wrong, but I think it should be 546.762=75.574a so a=7.234.
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