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Drawing Free Body Diagrams

  1. Sep 12, 2012 #1
    1. The problem statement, all variables and given/known data
    tUv0v.png
    Assume the system is massless.

    3. The attempt at a solution
    Here is my quick attempt at drawing a free body diagram for this system:
    LSe0p.jpg

    What I am not sure about is the direction of the force at F. Should the force be A or B?
     
  2. jcsd
  3. Sep 12, 2012 #2

    cepheid

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    Can't you just assume a direction, and if the force comes out negative, it's in the opposite direction from what you assumed?
     
  4. Sep 13, 2012 #3
    The prof posted answers online and it says that F is a roller joint and there is no force in the x direction. The reason why it is a roller joint is because if it wasn't it would be over constrained. Does anyone understand what is meant by this?
     
  5. Sep 13, 2012 #4

    cepheid

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    Hmm...try looking at this:

    http://en.wikipedia.org/wiki/Overdetermined_system
     
  6. Sep 13, 2012 #5

    cepheid

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    You didn't actually write down a well-defined problem statement. What exactly is it that you're being asked to solve for?
     
  7. Sep 13, 2012 #6
    The reason there is no force in the x-direction at F is because the joint is a roller, it resists no forces in the x-direction. A force in the x-direction on a roller would cause it to move.

    The idea your professor is mentioning is that if the joint at F was not a roller and rather a pin or some other type of constraint which imposed more than one resistance on the system, the system would be over constrained or statically indeterminate. These types of problems are solvable only through some sort of continuity condition such as material property relations that go beyond just performing force analysis.
     
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