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Stress and Strain/ Hooke's Law Question

  1. Nov 20, 2009 #1
    1. The problem statement, all variables and given/known data

    A 50 kg traffic light is suspended above an intersection by a continuous steel cable. The cable has a diameter of 1.0 cm, and the light is depressed 12 degrees below the horizontal. What is the percentage increase in the length of the cable due to the mass of the traffic light?

    2. Relevant equations

    Stress=Force/Area
    Strain=Change in length/original length
    Young's elastic modulus(E)=Stress/Strain
    Area = (pi) x (r^2)

    E for steel is 200 x 10^9

    3. The attempt at a solution

    Well I found the stress and strain, but now I am lost.
     
  2. jcsd
  3. Nov 20, 2009 #2

    Q_Goest

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    Hi swifel Welcome to the board. Can you calculate the tension in the cables first? Can you then find the stress in the cable? Once you do that, do you know how stress, strain and modulus relate? Can you then determine the length change?
     
  4. Nov 20, 2009 #3

    mgb_phys

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    are you given the value for steel or did you look it up?
    I think you are overreading the question and it is simple a triangle geometry question
     
  5. Nov 20, 2009 #4
    I was given the E value for steel.
     
  6. Nov 20, 2009 #5

    ideasrule

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    The question wants the percentage increase in the cable's length and you've found strain, which IS the percentage increase in length.
     
  7. Nov 20, 2009 #6

    Q_Goest

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    hi ideas.
    where above do you see strain being calculated?
     
  8. Nov 20, 2009 #7

    mgb_phys

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    In that case you can easily find the stress (weight of light and area)
    Then with E you can find strain, strain is the relative increase in length
     
  9. Nov 20, 2009 #8

    ideasrule

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    The OP said he calculated the stress & strain. He probably had trouble calculating the absolute change in length of the cable, not realizing it wasn't necessary to calculate percentage change.
     
  10. Nov 20, 2009 #9

    Q_Goest

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    lol ... yep...
     
  11. Nov 21, 2009 #10
    Wow, I already had the answer? Now I feel dumb, oh well. Thanks for the help guys, I really appreciate it.
     
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