# Equilibrium in roof truss

#### exequor

The diagram for this quesion is at http://www.drewtemp.com/helios/physics.gif [Broken]

The first thing that i did was that i made the free body diagram. From my knowledge i would say that AB and BC are in compression and AC is in tension (i think that is correct). Now the question asked to take moments at B for finding the force in AC whether it be tension or compression, however when i do that it seems like the forces just cancel off each other, and that is where i am stuck

Here is the link for my free body diagram: http://www.drewtemp.com/helios/physics_freebody.gif [Broken]

1. What i want to find is which members are in tension and compression (AB, BC, AC)?

2. What is the force at AC? The weight of AC is negligible.

3. What force from the left (the wind) is required to make the truss pivot at C?

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#### NateTG

Homework Helper
How many times are you going to post this?

Your FBD has some problems:

What is the downward force is that you've got at B?
If AB and BC are under compression, why don't you have the compression inthe FBD?

It's pretty clear to me that AC is under 1800 N of tension. You'll have to solve for a net force of zero at A to get that answer.

For part 3: Since you're dealing with Torque, it matters where the force is going to apply. You can get your answer by eliminating the normal force at A, and solving for zero net torque about C.

#### enigma

Staff Emeritus
Gold Member
Hi cipher, welcome to the forums.

People typically read all or most subforums, so you'll be fine posting it in one place. It will also be easier for you to keep track of.

I moved one of your other posts into homework help. Please continue the discussion there:

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