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Static equilibrium questions

  1. Apr 22, 2012 #1
    I do not understand how to solve static equilibrium questions. Like those:

    Or this one

    http://www.chegg.com/homework-help/...sitionby-a-pin-and-cable-as-shown-in--q701621

    The second one with the pin is especially hard, imo.

    I don't even know what the basic equation here is. The book dosen't tell you this. It's always [itex]\Sigma[/itex][itex]\tau[/itex]=....... something else. So I do have to create my own equation each time, basically. Is this correct? Depending on whatever the problem is. I don't get it. Can someone please explain this to me?
     
  2. jcsd
  3. Apr 22, 2012 #2
    For these types of questions, essentially you make the torque provided by the cable equal to the torque caused by the weight of the beam. This must be true, because otherwise the beam would be rotating on its pivot. I can't really help you with anything until you show me an attempt at a solution.
     
  4. Apr 22, 2012 #3
    I already have the solution... that's not my question. I'm wanting to know how to solve those kind of problems/concepts.
     
  5. Apr 22, 2012 #4
    I essentially just told you in my last post. Think of torque as acting either clockwise or counterclockwise about the pivot. Take the perpendicular components and make them equal each other. Solve for missing variable. If you're looking for a basic equation, there isn't really one. All torque questions are based off the fact that the sum of torques must be equal to zero.
     
  6. Apr 22, 2012 #5
    What are the perpendicular components for the 1st example? Torque(0)=Tsin51*length of beam -weight of person*meters from the pivot - weight of beam*(I don't know). Don't know what the last component in this equation would be. But then I would solve it for T. I have no idea if this equation would get me the tension of the cable., but that's what I can come up with.
     
  7. Apr 22, 2012 #6
    You've got it mostly right. The beam's center of gravity is in the middle, so how far away from the pivot is it acting on?
     
  8. Apr 22, 2012 #7
    Well, since it's in the middle, it's 5m.
     
  9. Apr 22, 2012 #8
    Yes. With that information, you should be able to solve for T.
     
  10. Apr 22, 2012 #9
    But that is the tension of the cable. How would I do the second one? How do I know how to approach this ?
     
  11. Apr 22, 2012 #10
    For the second question, the cable is acting at an angle right? This means that the cable has a horizontal force. Since the beam is not moving, therefore the pin must be supplying a horizontal force that either pushes out against the beam or pulls it toward the wall to stop it from moving sideways.
     
  12. Apr 22, 2012 #11
    I don't think I fully understand this, but would it look like this?

    Basically the same I did above but just for this problem?
     
  13. Apr 22, 2012 #12
    Find the horizontal component of the force that the cable is pulling on the beam. The force that the pin provides will be equal to that.
     
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