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Homework Help: Obtaining compressive and tensile stresses in beams

  1. Nov 18, 2011 #1
    1. The problem statement, all variables and given/known data

    In calculating stresses, how do we determine when we have a negative in front of the question and when we do not have it. I am really confused, meaning I have not understood the concept, but the examples I am looking at some have got σ= My/I whilst others have σ= -My/I.

    2. Relevant equations

    σ= My/I

    σ= -My/I

    Hopefully they get uploaded properly. The first two use the first equation, whilst the the other two use the second equation





    Thanks for the help.
  2. jcsd
  3. Nov 18, 2011 #2
    Just understand that:
    • Compression is always Negative
    • Tension is always Positive

    I always just keep my numbers positive in calculation until the end and if I am solving for a compressive stress I add the negative.

    Hope this helps!
  4. Nov 18, 2011 #3
    Ah right okay. So basically if they ask for max or anything always use σ=My/I , and then if it is compression, add the negative sign to the final answer. Cheers mate
  5. Nov 19, 2011 #4
    Yup you got it!
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