# Homework Help: Moment in truss

1. Jan 11, 2017

### fonseh

1. The problem statement, all variables and given/known data
for the moment about A , why there 's moment generated by the forces of 0.8NB (circled part) ?

2. Relevant equations

3. The attempt at a solution
I think it's not necessary to include the moment 0.8NB(4) because the d = 0.4m is not measured directly from point A ....it's measured from the red circled part ...

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2. Jan 11, 2017

### BvU

1. It's not 0.4 m but 4 feet and it's the vertical distance between B and A
and that is multiplied with the horizontal component of NB
2. the 10 feet is the horizontal distance etc.

3. Jan 11, 2017

### CivilSigma

btw, this is not a truss, but a frame. Trusses are designed only under tensile and compression forces, no bending moments

4. Jan 11, 2017

### fonseh

In the diagram attached below , we can see that the d (prepedicular distance) for NBy is measured directly from A , but for the NBx ( blue part) , it's measured directly from a point which is 10 m from A . So , for the moment 4NBx , it's not moment about A , am i right ? Why the author consider it in the calculation ?

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5. Jan 12, 2017

### BvU

No. The line of action of NBxis at a perpendicular distance of 4 feet from A

6. Jan 12, 2017

### fonseh

why ?

7. Jan 12, 2017

### BvU

A force acts along a line of action. You can always move it along that line without changing anything.
Only when you move a force perpendicular to its line of action you have to add a torque.

To reassure yourself: calculate the perpendicular distance of the line of action of NB itself to the point A and compare with the total torque from x and y components.