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Homework Help: Need to find 2nd moment of area

  1. Mar 30, 2012 #1
    Hi all, i m new here, been given a task to find bending stress σ when force F is applied, the figure can be model as a cantilever, the support is on the left.,but i dont know how to find 2nd moment of area (I) for the figure below, pls help
    thanks in advance.

    1. The problem statement, all variables and given/known data


    2. Relevant equations


    3. The attempt at a solution
    Last edited: Mar 30, 2012
  2. jcsd
  3. Mar 30, 2012 #2
    First, you need to give us a clue as to how far you can get. Second, the figure is not totally clear. On the top left figure, which is presumably a plan, where exactly is F applied? Is it a point load or a knife edge load along two edges? Thirdly, can you identify the cross-section whose second moment of area you want? Finally, what definition do you know for I?
  4. Apr 1, 2012 #3
    i m not sure the load is a point load or knife edge. i assume it should be knife edge. i forgot which cross-section area i should be finding for. i know i mus use (bh^3)/12 but i dunno what is my b and what is my h.

    based on the forumla σ=(M(T/2))/I, my h should be T and b should be A but the cross-section area is not uniform. i m confused now. pls point me in the right direction.. thanks
  5. Apr 1, 2012 #4
    It's not your fault that it is a badly worded question. If the support is at the left end, then it looks like A is the span. At the root of the cantilever, b is the C on the drawing. Further towards the load, it is C-D. You are correct that T is the h you are looking for. Perhaps you have to look at both sections to find which stress is maximum. It would also depend on whether F was shared between the two prongs of the fork. You are correct that I = use (bh^3)/12 where b is parallel to the neutral axis in the cross section and h perpendicular to the neutral plane...
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