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Homework Help: Diameter of a wire

  1. Aug 16, 2009 #1
    1. The problem statement, all variables and given/known data
    Find the minimum diameter of an l = 17.2 m long steel wire that will stretch no more than 7.38 mm when a mass of 323 kg is hung on the lower end. (Hint: The Young's modulus of steel is 200.0 GPa.)

    2. Relevant equations

    Youngs = (Force / Area) / (change L / L)
    = (mg / pi*r^2) / (change L / L)

    3. The attempt at a solution
    Simple enough... just plug and chug right?
    just to clarify, 200.0GPa = 2x10^11 N/m^2 correct?
    and 7.38mm = 0.00738m
    If so,

    Then after solving for r... i got .006853m, which is wrong. And i did multiply by 2 for the diameter. 0.0685 is also wrong, so is .685m

    What am i doing wrong??????? or am i just way off??

  2. jcsd
  3. Aug 16, 2009 #2
    What value of g did you use?
  4. Aug 16, 2009 #3
    I used the same formula as you did and I get 0.0069 m. Do you know the correct answer?
  5. Aug 16, 2009 #4
    Sorry for not replying earlier... fell asleep.
    I used 9.81 as g, and idk what the right answer is... =(
  6. Aug 16, 2009 #5


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    Staff Emeritus
    Science Advisor
    Homework Helper

    You have the right answer. Maybe you have the wrong number of significant figures, or are rounding incorrectly.

    g=9.81 gives 6.86 mm, and g=9.80 gives 6.85 mm for the diameter.
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