Determining the magnitude of the resultant couple

[Pascal1p]

Homework Statement

Calculate the magnitude and coordinate angles of the resultant couple moment, caused by the forces.

Homework Equations

M= r x F

The Attempt at a Solution

M3= -0.2* 80k= -16k
M2= 0.2* 120i= 24i
M1= -0.2*100j= -20j
M4= r x F= ?

I just can't seem to be able to expres r into a vector, I tried to first determine the coordinates of the ends of the bar on which the tips of the arrows of F4 and -F4 lie. And then use this to calculate the vector that specifies the distance between them. Sadly without succes. Using the geometry to find the coordinates won't work for me.

 

[SteamKing]

Homework Statement

Calculate the magnitude and coordinate angles of the resultant couple moment, caused by the forces.

Homework Equations

M= r x F

The Attempt at a Solution

M3= -0.2* 80k= -16k
M2= 0.2* 120i= 24i
M1= -0.2*100j= -20j
M4= r x F= ?

I just can't seem to be able to expres r into a vector, I tried to first determine the coordinates of the ends of the bar on which the tips of the arrows of F4 and -F4 lie. And then use this to calculate the vector that specifies the distance between them. Sadly without succes. Using the geometry to find the coordinates won't work for me.

For M4, it's not important to find the coordinates of r. What is important is to find the component of F4 which acts at right angles to r in order to calculate the couple created there.

Still, given the orientation of r with respect to the three coordinate planes, you should have been able to find its coordinates. Did you make a sketch before doing your calculations?

 

[TSny]

Pascal1p, the magnitude of a couple is easily found using the distance between the lines of actions of the two forces. How is the direction of the couple related to the plane containing the lines of action of the forces?