Why is the sin(60) term doubled in the truss equilibrium equation?

[canicon25]

cannot understand something about this. here are the equations:

-T1-T3-T2cos(60)=0 x direction
2T2sin(60)-981N=0 y directionI don't understand why the sin(60) term is doubled in the y direction equation. Can anyone explain?

 

[Studiot]

I guess you are using the "Method of Sections"?

However your diagram as you have drawn it is incomplete and it is impossible to complete the calculation as you need some geometrical information for the third (moment ) equation.

I agree that the vertical equilibrium, as shown, should not contain the factor of 2.
Is this a cantilever?

 

[canicon25]

Thanks for reply. I got the question from here:

http://en.wikibooks.org/wiki/Statics/Method_of_Sections

As I was reading through the double sine term seemed odd and wanted some clarification. As you can see on the website no moment calculations were done. Is that correct or is something in act missing? The diagram shown at the link looks fixed at the LHS.

 

[Studiot]

Well obviously you can't trust WikiXXX all the time!

If you think about it you have 2 equations and 3 unknowns, T₁,T₂ and T₃.

If you take moments about the joint where T₂ and T₃ meet then you can solve for T₁.

That is why it is normally recommended to only cut 3 members in the section.