How do I determine the line of action for calculating a moment in 3D space?

[salman213]

When taking the moment in 3 space i know the formula is

M = r x F


but for example ifyour finding the moment of force F about point C


how do u know from point C to where you have to make the line.

like if u have

------------------> about point C

C

you can make it

------------------> F
-a
-
C

or 

------------------>F
                -a
           -
      -
C

Ive read that the ca is a vector from point C to ANYWHERE on the line of action. So does that mean any where from point C to the line F ANYWHERE?

 

[Pyrrhus]

Yes, that's correct.