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What is the magnitude of the elevators acceleration

  1. Feb 18, 2009 #1
    1. The problem statement, all variables and given/known data

    When you weigh yourself on good old terra firma (solid ground), your weight is 160lb . In an elevator your apparent weight is 140lb. What is the magnitude of the elevators acceleration.

    2. Relevant equations

    For this problem would we use the equation Fn= mg+ma

    3. The attempt at a solution
     
  2. jcsd
  3. Feb 18, 2009 #2

    LowlyPion

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    Welcome to PF.

    That equation is ok.

    Be careful what you use for g since your givens are in lbs.
     
  4. Feb 18, 2009 #3
    i used the equation listed above but i get keep getting a wrong answer.

    i used 160lb for Fn and 123 for mg i not really sure what to use for ma. I was also wondering if I should change the units from pounds
     
  5. Feb 18, 2009 #4
    ma = -mg + Fn
     
  6. Feb 18, 2009 #5
    they gave you mg and Fn, Fn is your apparent weight. mg is normal weight
     
  7. Feb 24, 2009 #6
    so they give me Fn= 63.5kg and mg= 72.6kg i plugged it in to find ma and i came up with .123 and it is still wrong. Can you help me out and see where i am making my mistake
     
  8. Feb 24, 2009 #7

    Redbelly98

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    Note that Fn is a force, while "63.5 kg" is a mass. So they cannot be equal.

    Did you use selter01's equation,

    ma = -mg + Fn ​

    ?

    m=72.6 kg, and you're trying to find "a".
     
  9. Feb 24, 2009 #8
    I used that equation how would i express 63.5kg as a force so i could use it for Fn I am stuck and do not know where to go from here
     
  10. Feb 24, 2009 #9

    Redbelly98

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  11. Jun 17, 2009 #10
    as we know that the equation relating the Apparent and the Real weight is given as
    Fnet=W(real)-W(apparent)...............1

    but
    W(real)=Mg........................................ ...........2
    and F(net)=Ma......................................... ......3
     
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