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Homework Help: Finding mass with given stress, strain, and original area

  1. Apr 8, 2008 #1
    1. The problem statement, all variables and given/known data

    figure whatever represents an insect caught at the midpoint of a spider-web thread. The thread breaks under a stress of 8.2 x 10^8 N/m^2 and a strain of 2. Initially, it was horizontal and had a length of 2cm and a cross-sectional area of 8 x 10^-12 m^2. As the thread was stretched under the weight of the insect, its volume remained constant. If the weight of the insect puts the thread on the verge of breaking, what is the insect's weight?

    2. Relevant equations
    I would imagine the different variations of stress and strain

    3. The attempt at a solution
    I know the answer will be small (as the insect is supposed to be a bumble bee assuming the picture in the book isn't lying to me.

    I don't know where to begin exactly.

    I did stress = F/A to get F = .00656N, which means the bumble bee would have to apply just under that amount of force, as that is what the web will break at. But I'm unsure on what to do next.
  2. jcsd
  3. Apr 8, 2008 #2

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    Draw a diagram. Suppose A and B are points where the thread is attached. The midpoint C is now vertically displaced to D. At the point D, the forces acting are the tensions T in the thread, and the weight W.

    Write down the eqn for the equilibrium of the vertical forces.
    Find the relation between BD and BC, from geometry.
    The common thing between these eqns is the angle CBD.

    That should be enough to get you started. If you show some work based on this, perhaps we could help you more, if you need it. You can do this.
    Last edited: Apr 8, 2008
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