# Homework Help: Axial force at X

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1. Nov 14, 2017

### DanRow93

1. The problem statement, all variables and given/known data
I have to find axial force at X for the following structure:

https://i.imgur.com/8o01fkC.jpg

3. The attempt at a solution
These are my workings
https://imgur.com/JfSfk9Z
https://imgur.com/3HuDN4d

I didn't work out moments because the question only asks for axial force, is that alright? I think I have gone wrong somewhere though, because I haven't used x=1.54m anywhere. Thanks!

2. Nov 14, 2017

### PhanthomJay

You went through more work than necessary, you could have isolated a section of the right hand part with a cut at x to find the axial force. Your answer looks numerically correct, but is the axial force compression or tension in the member?

3. Nov 14, 2017

### DanRow93

Which right hand part do you mean I could have isolated, from point X to C? It would be compression I think, so should be -5.08?

4. Nov 14, 2017

### PhanthomJay

Isolate from point x to D.
Compression is correct, but why do you say that?

5. Nov 14, 2017

### DanRow93

I actually don't know, I've confused myself. At point B the axial force along BC is downwards, so axial force in X is opposite which means it should be tension doesn't it?

I put the answer 5.08 into the online test that I had to do and I got it wrong...

6. Nov 14, 2017

### PhanthomJay

when you isolated the section AB, the vertical force at B acts downward on AB. (Don't rotate the members as you have done, this confuses the issue). Anyway, if the vertical force at B acts downward on AB, then what does Newton's third law tell you about the direction of the vertical force at B on BC?
the problem statement tells you that a positive value for the axial force in BC indicates tension, implying that compression is entered as a negative value.