Why is there a moment generated by the 0.8NB force at Point A?

[fonseh]

Homework Statement

for the moment about A , why there 's moment generated by the forces of 0.8NB (circled part) ?

Homework Equations

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 ...

 

[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 N_B
2. the 10 feet is the horizontal distance etc.

 

[CivilSigma]

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

 

[fonseh]

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

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 ?

 

[BvU]

So , for the moment 4NBx , it's not moment about A , am i right

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

 

[fonseh]

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

why ?

 

[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 N_B itself to the point A and compare with the total torque from x and y components.