# Homework Help: Determining the magnitude of the resultant couple

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1. Feb 13, 2016

### Pascal1p

1. The problem statement, all variables and given/known data
Calculate the magnitude and coordinate angles of the resultant couple moment, caused by the forces.

2. Relevant equations
M= r x F

3. 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.

2. Feb 13, 2016

### SteamKing

Staff Emeritus
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?

3. Feb 13, 2016

### 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?