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Homework Help: How do I set up the equations for this?

  1. Mar 9, 2010 #1
    I am to find the reactions at A. How would I set up the equations for Ay and Ax in this problem? It seems pretty confusing. Also, will there be a momentum in this problem?

  2. jcsd
  3. Mar 9, 2010 #2


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    This system is supposed to be in equilibrium, right? In that case, forces in both x and y directions must sum to 0 and torques must sum to 0. Write out those conditions in 3 equations, plug in the numbers, and you'll get the answer.
  4. Mar 10, 2010 #3
    Well for forces in the X direction, it's just A (in the X direction) is 0, right?

    For y direction, this is the equation I wrote down.

    Ay + 300 - 100 - 200 = 0

    Solving for the unknown, I got Ay = 0.

    As for the momentum of A, here's what I wrote down. It doesn't exactly make sense to me.

    -100(12) + 300(15) - 200(22) = 0

    but...that comes out to -1100 = 0 which makes no sense? What's going on? Is the momentum of A just -1100?
  5. Mar 10, 2010 #4


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    You are using the wrong terminology;it's the moments (or torques) of all forces about A that must equal 0.
    Since the support at A is fixed, it is capable of providing both forces and moment (or torque, or couple; these terms are sometimes used synonymously). So in your equation, you are missing the moment provided by the support at A.
  6. Mar 10, 2010 #5
    So the moment at A would be written like this?

    A - 100(12) + 300(15) - 200(22) = 0

    A = 1100

    Also, was I right about the forces in the x and y directions?
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