# Homework Help: Engineering Mechanics Question

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1. Mar 2, 2016

### sophiebeth100

1. The problem statement, all variables and given/known data

The right triangles of the plane truss are all the same. What are the forces in members BH and HG?

Diagram for question is below, or a link:

2. Relevant equations

n/a

3. The attempt at a solution

Reaction force at A + reaction force at E are equal, therefore both = 15kN.

It was after this point I became stuck.

I calculated the forces in the x axis:
ABcos60 + AHcos30 = 0
Then in the y axis:
ABcos30 + AHcos30 = 15kN

Not sure if the above is correct, and not sure how to continue with the question. Any help would be great!!!

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2. Mar 2, 2016

### haruspex

You can get more equations by considering the balance of forces at each node. But that could generate a lot of equations and a lot of unknowns.
Using the symmetry, there are 7 unknowns.
It will help if you can find an easy one to start with. I suggest G. What do you notice there?

3. Mar 3, 2016

### sophiebeth100

Perhaps that GC is equal to 12kN? Not entirely sure if that's right but if so I could work from there.

4. Mar 3, 2016

### haruspex

No, that isn't right.
What forces act at G? What are their directions?

5. Mar 3, 2016

### sophiebeth100

I wouldn't think there are any forces in the x axis since the total force in the x axis is zero.

Does the force at GC act upwards? Equal to the total downward force, so 30kN?

6. Mar 3, 2016

### haruspex

In analysing forces in such interconnected systems, you need to focus on one node at a time, just thinking about what that node experiences directly. The only forces node G feels are from the three members that meet there, it doesn't 'know' anything about the externally applied loads. (Only nodes B, C, D feel those.)
The forces from the three members meeting at G must balance both vertically and horizontally. That allows you to write two equations involving the forces in those members. What equations do you get?