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Homework Help: Young's modulus, stretching question.

  1. Sep 2, 2013 #1
    1. The problem statement, all variables and given/known data
    Three separate members of steel, copper and brass are of identical dimensions and are
    equally loaded axially.Young’s moduli for the materials are:
    steel, 210,000 N/mm^2
    copper, 100,000 N/mm^2
    brass, 95,000 N/mm^2
    If the steel member stretches 0.13mm,
    calculate the amount of elongation in the copper and brass members.

    2. Relevant equations
    The equation i believe to be relevant;
    Young's modulus=Stress/Strain

    3. The attempt at a solution
    I am confused how to approach this question at first i believed i had to find the stress by using the elongation of 0.13mm, and because all the members would be under the same stress i could then input that information into a youngs modulus equation for the other two members. However the young's modulus equation requires strain not just the elongation and i don't know the original length.

    The only way I can think of solving this question would to assume that the strain = 0.13 and solve the question like so

    Steel young's modulus= stress/strain

    Starting with brass as an example
    Young's modulus = stress/strain
    strain= stress/young modulus
    Strain= 27.3/95 =0.29 mm

    and Copper =0.27mm

    Is this assumption correct, if not any hints on how i could go about solving this problem?
    Thank you very much.
  2. jcsd
  3. Sep 2, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You don't need to know the original length, just that it is the same for each. Accordingly, the elongation will be proportional to the strain.
  4. Sep 2, 2013 #3
    Oh haha, thank you very much. My misunderstanding =D
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