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Stretching of a Steel wire

  1. Mar 27, 2015 #1
    1. The problem statement, all variables and given/known data
    A steel wire 2.9mm in diameter stretches by 0.038{\rm \\%} when a mass is suspended from it. The elastic modulus for steel is 2.0×1011N/m2

    *that {\rm \\%} value is in my homework, I assumed it was a mistake and was meant to be mm
    2. Relevant equations
    young's module for elasticity F/A=E(Δl/l)

    3. The attempt at a solution
    So to answer this I think I need to find F because that equals mg, so I'll just divide F by 9.8.

    However I'm not given l so inserting the numbers into the equation I get
    F/ [(.5*.0029)^2*π]=2.0*10^11{[(l+.00038)-l]/l}
    There are two unknowns and I don't know about other equations to answer this problem, usually original length is given so I'm very confused.
  2. jcsd
  3. Mar 27, 2015 #2


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    It's not clear what you are supposed to find here, F or the length of the wire.
  4. Mar 27, 2015 #3
    The exact question is "How large is the mass?"
  5. Mar 27, 2015 #4


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    What if instead of the wire stretching by 0.038 mm, it stretched by 0.038% of its original length?
  6. Mar 27, 2015 #5
    That makes so much more sense and I was able to solve it, thank you so much I was working on this problem forever :)
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