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Homework Help: The problem with Statics

  1. Oct 13, 2015 #1
    I really don't understand what is the logic in the direction of forces S1,S2 and S3.This is an example and here are given directions of the forces,but in the exam I will have to find them on my own.
    Could someone explain to me how should I find them and what kind of thinking I have to apply here,maybe action equals minus reaction ?
    I really have a bad professor from Mechanics and she just doesn't care to explain very much.

    Thank you in advance I hope pictures are clear.

    Attached Files:

  2. jcsd
  3. Oct 13, 2015 #2
    The last picture...don't know why it wasnt uploaded in the first post
  4. Oct 13, 2015 #3

    Attached Files:

  5. Oct 13, 2015 #4


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    I think the direction of the forces as shown on the diagram is arbitrary. You can assume that the direction indicates, for example, a compressive force in each rod.

    After you work out the forces in each rod which keep this system in statics equilibrium, there may indeed be forces which are in tension, that is, which will tend to pull a rod out of the wall.

    It's hard to figure out what is the true case here just by inspection. The rods appear to be symmetrical with respect to the y-axis.

    As always, use the equations of statics to find the unknown force magnitudes in each of the three rods.
  6. Oct 13, 2015 #5


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    In general if a problem statement doesn't define which direction is +ve then you have to do so. You can make an arbitrary choice but you must be consistent. eg you have to take that into account when you are writing your equations (eg when summing components of forces or torques). The answer force(s) or torque(s) will turn out to be +ve or -ve then you refer to your definition of +ve to work out which direction it points and calculate any angles appropriately.
  7. Oct 13, 2015 #6
    So I could basically take directions as I've taken on this picture and it should be fine?

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  8. Oct 13, 2015 #7
    Sorry for the 2nd picture being reverse uploaded tried to upload it again normally but failed.

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