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Equal area axis

  1. Jun 14, 2017 #1
    1. The problem statement, all variables and given/known data
    For this shape , it's clear that the centroid and the horizontal line of equal axis lies on the same horizontal line , am i right ????

    2. Relevant equations

    3. The attempt at a solution
    I'm not sure . correct me if i am wrong .

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  3. Jun 14, 2017 #2


    Staff: Mentor

    Do you mean "line of equal area"?
    Anyway, yes, symmetry should convince you that you're right if there's no other information given.
  4. Jun 14, 2017 #3
    Are you familiar with plastic analysis ? Zp here is the plastic modulus .

    I use another method to do , but i get different answer , why ?

    Here's my working : , Zp = Sum of area x ( difference between centroid of particluar area and the equal area axis )

    Zp = [ (130)(20)(150-10) + (150)(20)(150/2) ] x 2 = 1178000 , but the ans provided is only 929040
  5. Jun 14, 2017 #4


    Staff: Mentor

    No, I'm not, but I am familiar with the concept of centroids.
    My answer is based on the fact that the shape in the drawing is symmetric about its horizontal midline.
    If you had a rectangular piece of some uniform, rigid material 150 mm by 300 mm, its centroid would be at the center of the piece, at a point 75 mm to the right of the left edge, and 150 mm above the lower edge. If you cut out a rectangle 130 mm by 260 mm to form a "C" shape as in your drawing, the centroid of the piece you remove would be at its center, and the centroid of the remaining C-shaped piece would be along the horizontal midline, but a bit right of where it was for the original uncut piece of material.
  6. Jun 21, 2017 #5


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    Science Advisor
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    Gold Member

    Your number looks correct for the plastic section modulus perhaps the book is looking for the elastic section modulus you'll have to run the numbers
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