# Homework Help: Moment about a point in trusses

1. Aug 1, 2016

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

for moment about C, the author gave [-F_(FE)](5) +3.2(4) +6(5) = 0
i dont understand the working...Why there are moment 3.2(4) and also moment 6(5)?

2. Relevant equations

3. The attempt at a solution
for moment 3.2(4) , it's centered at F, so ,it's not moment about C ,am i right?
for moment 6(5) , it's moment about A, am i right?

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2. Aug 1, 2016

### PhanthomJay

The author has isolated the bot left section in a free body diagram. In order to determine the force in FE, you need to sum moments about C because summing about A or B wil not allow solution of the FE force because the force action line passes through those joints. The 6(5) moment you talk about is the moment of the horizontal reaction force at A about C.

3. Aug 1, 2016

how about moment 3.2(4) ? it's moment about F,am i right? why the author included it into calculation?

4. Aug 1, 2016

### PhanthomJay

Well it is a moment about F, but it also is a moment about C, which is the point of interest . Remenber that moment is force times perpendicular distance from line of action of that force to the point of interest.

5. Aug 1, 2016

i still dont understand...
the perpendicular distance is the 4m(see red labelled part), it's span from F to A, so moment is about F, not C, am i right?

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6. Aug 1, 2016

### PhanthomJay

The perpendicular distance from the line of action of the 3.2 kN force to C is also 4 m. Just extend the 3.2 kN force arrow straight up and measure the perp distance from that line to C. This is one of several methods to calculate moments.

7. Aug 1, 2016

but,the force 3.2kN is being applied at A, not 4m away from C, how can we dothat?

8. Aug 1, 2016

### PhanthomJay

why not?

9. Aug 1, 2016

The force is not applied at C, but at A

10. Aug 1, 2016

### PhanthomJay

you are not understanding how to calculate moments using perpendicular distances from the force line of action.

Now I just noticed the reactions are incorrectly calculated by the author, based on the dimensions and loads given. . You might want to find the correct vertical reaction at A by looking at the entire truss and summing moments about B and report your result.