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Homework Help: Different young modulus of rod - Need a proper explanation?

  1. Nov 27, 2014 #1
    1. The problem statement, all variables and given/known data
    A rod is made up as shown [pic attached]
    Both have same cross-sectional area. Ep and En are Young modulus.
    The rod breaks when total extension = 3.0mm
    Greatest tensile stress that can be applied before rod breaks???

    2. Relevant equations
    Young modulus = stress / strain

    3. The attempt at a solution
    Ans: 5.7 x 10^6 Pa.
    Won't it break when the one of the 2 materials breaks? Can you plz tell me the concepts to be used. How to solve?
  2. jcsd
  3. Nov 28, 2014 #2


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    The Young's modulus tells you how much the material stretches for a given applied tensile stress.

    If you apply the same stress to the rod shown in the OP, which material is going to stretch more?

    The problem is asking you to find how much stress can be applied to the rod so that the combined stretch of the nylon and the glass-reinforced plastic equals 3 mm. Remember, the strain is equal to the change in length of the material divided by the original length (unstretched).
  4. Nov 28, 2014 #3


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    Sure, but you don't care which breaks, or even the fact that it breaks. What matters is that the max allowed extension of the rod as a whole is 3mm.
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