# Mechanics - Truss Equilibrium

1. May 20, 2010

### canicon25

cannot understand something about this. here are the equations:

-T1-T3-T2cos(60)=0 x direction
2T2sin(60)-981N=0 y direction

I don't understand why the sin(60) term is doubled in the y direction equation. Can anyone explain?

Last edited: May 20, 2010
2. May 20, 2010

### 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?

3. May 20, 2010

### 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.

4. May 20, 2010

### 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.

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