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Shear force

  1. Oct 4, 2016 #1
    1. The problem statement, all variables and given/known data
    in this diagram , we could see two forces acted to left , one force acted to right , total of two forces acting to left = force acting to right .
    2. Relevant equations


    3. The attempt at a solution
    i'm wondering if the total of two forces on the left are not the same with the force on the right , then , how to do the question ? We know that tau = P/ A , tau = stress , P = force . let's say one of the force is 30kN to left , another one is 40kN to left , total is 70kN to the left , but on the right , the force is 60kN to right ... so , the shear stress = 10kN to right divided by area ?
     

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  3. Oct 4, 2016 #2
    Why my post is deleted?
     
  4. Oct 4, 2016 #3

    PhanthomJay

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    Not deleted from my end, check again.

    The system is in equilibrium, so the applied force of 60kN to the right must be balanced by the two 30 kN forces on the left. You can't have it any other way for equilibrium And if you thus have 60 kN to the right and 60 kN to the left, the shear force on the bolts is not 0, is it? Follow the solution explanation.
     
  5. Oct 4, 2016 #4
    Do you mean if the system is not in equilibrium, then shear force cant exist?
     
  6. Oct 4, 2016 #5

    PhanthomJay

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    If the system is not in equilibrium, it is accelerating, and you don't even want to think about that when studying engineering statics and mechanics for bodies at rest.
     
  7. Oct 4, 2016 #6
    Can anyone try to answer?
     
  8. Oct 4, 2016 #7
    Can you explain further? If the body is not in equilibrium, why there's no shear force?
     
  9. Oct 4, 2016 #8
    it's in equlibrium , so shear force = 0
     
  10. Oct 4, 2016 #9

    Nidum

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    One of the first things that an engineer looks at when analysing stresses in a mechanical assembly is the load path .

    Load path describes how forces interact sequentially between individual components of an assembly . The complete load path starts with the applied forces , goes through the assembly and terminates with the balancing forces .

    Load path can be a very simple linear chain with simple forces at each end or very complex with multiple sub paths and multiple forces .

    What do you think that the load path is for the forces acting in your assembly ?
     
  11. Oct 4, 2016 #10
    Load path can be start from the 2 30m forces or the single 60m forces
     
  12. Oct 4, 2016 #11

    PhanthomJay

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    It is in equilibrium, so the net external force acting on the system is zero. But that does not mean that the shear force or stress on the bolts is zero. Follow the solution explanation.
     
  13. Oct 4, 2016 #12
    I don't understand why the shear force not =0 when the whole structure in equilibrium? When it's in equilibrium, there's no net force acting on it, right?
     
  14. Oct 4, 2016 #13

    Nidum

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    Pin2.jpg

    Think about a slightly simpler problem with just one pin .

    What forces are acting on the pin ?
     
    Last edited: Oct 4, 2016
  15. Oct 4, 2016 #14

    PhanthomJay

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    systems in equilibrium have no net external force acting on them, per Newton's first law. You apply a 60 N force on the right plate, and the end plate reactions are 30 kN apiece or 60 kN to the left.. But you are not looking at the internal forces acting within the system. Imagine the right plate is your head and the left plates are your legs, and someone pulled on your head with a force of 400 N and another person was pulling on your legs and you didn't move. Then the person pulling on your legs also exerts a force of 400 N in the other direction, since net external force must be zero. But if you think there is no force in your body, think again, because you will the pain as your body is being stressed. 400N force internal. If you draw a FBD of the right part of your body (cutting you in half in an imaginary sense), then the internal force of 400N is pulling you leftward and the applied force of 400 N is pulling ' you rightward, no net force, but you sure feel the 400 N force, it is quite real and certainly non-zero.
     
  16. Oct 4, 2016 #15
    Why when the structure is accelerating , there can't be shear force acting?
     
  17. Oct 4, 2016 #16
    2 F/2 act to left, and one F act to right?
     
  18. Oct 4, 2016 #17

    Nidum

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    So what does the shear force diagram for the pin look like ?
     
  19. Oct 4, 2016 #18

    PhanthomJay

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    I did not say that . I ask you to focus instead on systems in equilibrium where net forces are zero.
     
  20. Oct 4, 2016 #19

    David Lewis

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    Then you multiply the mass of the right portion by the acceleration of the body, and add it to the 60kN force. For example, if the mass of the right portion is negligible, shear force would be 60 kN.
     
  21. Oct 4, 2016 #20
    My apology for the 1st deleted post.

    I don't know Whether Force imbalance is Possible unless there is Deformation of the Bolt.

    But when the whole system is in Acceleration, Force P in both Segments of Bolted Joint is still the Same.

    See the Figure below.
     

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