# Statics- Resultant couple moment

1. Jun 8, 2013

### rico22

1. The problem statement, all variables and given/known data
If F_1 = 100N , F_2 = 120N and F_3 = 80N , determine the magnitude and coordinate direction angles of the resultant couple moment.

2. Relevant equations
M=Fd
M=r x F
MR=Ʃ(r x F)

3. The attempt at a solution
I found the couple moment for each force:
M1=F1d=100(0.2) or (-20j using right hand rule)
M2=F2d=120(0.2) or +24i
M3=F3d=80(0.2) or -16k
M4=this is what im having problems with...not sure how to get the moment for F4... if do 150(.3)(cos30)=38.97 i get the wrong resultant.

The correct answer should be (MC)R=71.9 Nm...
Any help would be greatly appreciated.

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2. Jun 8, 2013

### tiny-tim

hi rico22!
i don't get 71.9 unless i use .33 instead of .3

3. Jun 8, 2013

### TSny

Note that the F4 couple is not in the x-direction. It is parallel to the xy plane but it has both x and y components which you will need to work out.

4. Jun 9, 2013

### tiny-tim

ooh, i didn't notice that!

i read the axis of that pipe, and the short line above the positive x-axis, as one continuous straight line!

yes, now i get 71.9 … thanks, Tsny

5. Jun 11, 2013

### rico22

thanks for the replies guys...