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posted by  Pat Thunyakij on 7/20/2008 3:44:18 AM  |  status: Live  

Determining the initial and end points of a vector

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 How do you know which direction of a tension force going by just looking at the problem?
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posted by rooya on 7/20/2008 6:29:31 AM  |  status: Live
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hi pal the question is not that clear, but i hope my answer would help u.

always the direction of a vector is represented by the initial and end points.

example AB=OB-OA

 
where OA and OB are the position vectors of the vector AB.
the position vectors decides the starting and ending of the vector
 
Tension and comperssion forces
 
 
hope u have understood if u still find it difficult please post a example diagram so that i can clarify ur difficulty
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posted by Pat Thunyakij on 7/20/2008 7:18:00 AM  |  status: Live
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Hey thanks for responding, but im still confused. Let me try to clear up the question asked. If a tension is giving in a problem without a force acting on it, how do you determine which direction the tension is pointing? For example in the picture below, how do you know the direction of the tension force is going from B to A and not A to B?
 
or the cables in the picture below
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posted by rooya on 7/20/2008 8:07:24 AM  |  status: Live
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Hi pat ur proplem is very very simple , if u can understand this concept u will never go wrong from.
 
In the first figure since a man is standing on the flat defnitly the rop will be subjected to tension.
But ur doubt is which direction?
that depends on the free body diagram
 soppose if u cut the rope at the middle then u have to represent the diagram like this
Now if u free the horizontal surface and draw FBD
Now from the above diagram it is evident that tension is from BtoA in thgis FBD
NOw watch this FBD below which is intresting
Now watch the tension in this FBD it is from A to B. u can also find that Cx and CY had reversed their direction.
 
so my dear pal it is clear that the direction of the tension in the rope depends upon the object for which u have drawn the FBD
i hope that it clear now. if u rstill unclear plz specify  
 
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posted by Pat Thunyakij on 8/6/2008 12:08:44 AM  |  status: Live
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Hey thanks I think i got it. I have another question, How do you determine the directioin of the reaction at C? I do you know that Cy is going up and Cx is going right?
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