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Homework Help: Spring constant of a horizontal wire

  1. Mar 27, 2009 #1
    1. The problem statement, all variables and given/known data

    When a mass "m" is hung from the centre of a horizontal wire, length "L". How far will the centre of the wire stretch downwards, in terms of m, L, E (youngs modulus) and A (cross sectional area of wire)

    2. Relevant equations

    mg = Tcosθ
    When m is the mass hung from the wire
    And T is tension in the wire


    Where E is youngs modulus
    F will be the tension in the wire?
    ΔL is the amount the wire stretches
    Lo is the original length of wire

    3. The attempt at a solution

    mg = Tcosθ


    but is it posible to change this equation so it is only in terms M,Lo,E,A

    Or find the effective spring constant of the wire
    Last edited: Mar 27, 2009
  2. jcsd
  3. Mar 27, 2009 #2
    This can be a wickedly hard problem. How tight is the horizontal wire before the mass is hung on it?
  4. Mar 27, 2009 #3
    No value given. Possible add another variable (Original tension)
  5. Mar 28, 2009 #4


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    Syd, welcome to PF!

    I have a few comments and questions.

    1. It sounds like we assume zero tension initially, since none was given.

    2. How are you defining θ? I'm guessing it's the angle of the wire w.r.t. horizontal, but I'm having trouble with this equation:
    Did you draw a free-body diagram first? How many forces does it have?

    3. After eliminating T, we are left with 2 unknowns, θ and ΔL. Can you find another equation (using geometry) to relate them?
  6. Mar 28, 2009 #5


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    Also note that the linear stress-strain relation is only valid within the elastic regime, which means that it is typically applied for very small values of ΔL/L (ΔL/L<<1).
  7. Mar 28, 2009 #6
    It is a spring only withing the elastic range, so by definition, asking for a spring constant restricts the discussion to the linear elastic range.
  8. Mar 28, 2009 #7
    Draw the picture, first with the wire horizontal and then with the wire stretched to the extended position with the weight in place. Then think about how much the wire has been stretched, and also draw an FBD in the drooped position.
  9. Mar 30, 2009 #8


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    There is no reason to speak in terms of a spring constant if the stress-strain relation is used (and required by the problem statement). Linearity and smallness are typical assumption that the OP needs to be aware of; the spring constant is not.
  10. Mar 30, 2009 #9
    Actually, the linear range is a part of the stress-strain relation.
  11. Mar 31, 2009 #10


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    If you mean "the stress-strain relation" that Syd should probably use in the solution, then I agree. If you mean "the" general "stress-strain relation", then I disagree.
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