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Homework Help: Constructing forces (mechanics)

  1. Mar 16, 2007 #1
    1. The problem statement, all variables and given/known data
    I've attached a picture of my problem (with diagram): 4.70 .

    "For the frame and loading show, determine the reactions at A and C."

    From the diagram, you can see that the metal object is made up of 2 parts:
    1) A to B
    2) B to D

    2. Relevant equations

    3. The attempt at a solution
    how do you arrive at the correct direction of the forces (ie: angle of the force; direction of its "application"). I have 2 interpretations and they will give different angles with which the force is applied (same magnitudes though). Which one is the correct interpretation? Because this is a statics problem (bodies at equilibrium), the sum of the forces = 0 (illustrated by the triangles).

    How do you logically determine which interpretation (of force directions) is correct when you do these problems?

    Thank you.

    Attached Files:

  2. jcsd
  3. Mar 16, 2007 #2


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    Homework Helper

    The simple minded thing to do is just assume there is a horizontal and vertical compoent of force at A B and C. There are no moments at A B or C because the joints are all pinned.

    Then write the equations of equilibrium for each two components. There are 3 equations for each component, that's 6 equations for the 6 unknown forces.

    If you want top do it by a "neater" method, start by looking at the equlilbrium of AB. Take moments about A (or B) and it should be clear what is the direction the forces at A and B.

    Hint: one of the options you drew is right.
  4. Mar 17, 2007 #3


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    It is worth also mentioning, the member AB is the special case of a two forces body, and the member BCD of a 3 forces (no parallel) body of statics.
  5. Mar 17, 2007 #4
    Thanks guys. Even though something is pinned though, there can still be a moment of the reaction forces at that point. I have a test tomorrow so when I'm done with it (may take a day or 2 for me to get remotivated, but I'll post something that shows that). I'll also try your way to figure out the prob. Hopefully this won't be asked before I figure it out :)

    I'm appending what I wrote: above... the moment of the forces where something in "pinned" = 0.
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