Question about tension and deflection (Mechanics of Material)

[shinizaki]

Hello everyone, I have some difficulties to solve this question in my homework.

Homework Statement

Picture
http://img714.imageshack.us/img714/7953/picz.png

Rigid bar ACE connected to rod AB, CD, EF which have same material and surface area A. Calculate tension caused by force P in three rods.

2. Homework Equations
Equation :
sigma Fy = 0
sigma M =0
comparison between deflection in A, C, and E

The Attempt at a Solution

I try to solve this question but I can't find sufficient equation to solve it. It have 4 equations and I only find 3. Please help me solve this.

 

[Mapes]

Have you tried summing the moments around different points?

 

[pongo38]

You haven't shown your working, but have you considered the property of the bar that it is rigid? What does that imply for the displacements of A, C and E?

 

[PhanthomJay]

. Relevant equations
Equation :
sigma Fy = 0
sigma M =0
comparison between deflection in A, C, and E

these are your 3 relevant 'equations'

It has 4 equations and I only find 3.

there are 3 relevant equations (as you note; the last is concerned with geometric compatability) and 3 unknowns (the tensions in each rod). What do you mean by four ?(the x direction is irrelevant here). Note that the bar is rigid, but that the rods can extend or compress (which should have been more clearly stated in the problem). Hint: This problem is best solved by replacing the force P with an equivalent force-couple system at midpoint.

 

[shinizaki]

When I try a few times again, I can solve it at last ! The unknown P I change to one so I have 3 unknowns and 3 equation then revert it back to P. Thanks all^^