Statics- Resultant couple moment

AI Thread Summary
The discussion focuses on calculating the resultant couple moment from three forces, F1, F2, and F3, with given magnitudes. The participant initially struggles with determining the moment for an additional force, F4, and miscalculates its contribution. After clarification that F4 has both x and y components and is not solely in the x-direction, the participant successfully recalculates the resultant moment. The correct resultant couple moment is confirmed to be 71.9 Nm. The exchange highlights the importance of understanding force direction and components in static equilibrium problems.
rico22
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Homework Statement


If F_1 = 100N , F_2 = 120N and F_3 = 80N , determine the magnitude and coordinate direction angles of the resultant couple moment.

Homework Equations


M=Fd
M=r x F
MR=Ʃ(r x F)


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 I am 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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  • Hibbler_ch4_p95.jpg
    Hibbler_ch4_p95.jpg
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hi rico22! :smile:
rico22 said:
The correct answer should be (MC)R=71.9 Nm...

i don't get 71.9 unless i use .33 instead of .3 :redface:
 
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.
 
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TSny said:
Note that the F4 couple is not in the x-direction.

ooh, i didn't notice that! :redface:

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

yes, now i get 71.9 … thanks, Tsny :smile:​
 
thanks for the replies guys...
 

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