# Find the net couple

1. Aug 1, 2014

### utkarshakash

1. The problem statement, all variables and given/known data
If P=0, replace the two remaining couples with a single equivalent couple, specifying its magnitude and the direction of its axis.

3. The attempt at a solution

Position vector of B(rb)=0.3i
Similarly, rd=0.16j+0.17k

∠GCF=atan(16/30)=28.07°
∠DEF=atan(DF/FE)=45°
Then I tried to resolve the forces into its individual components. For 34N force applied at B, in vector form,

$34 \cos 28.07 \hat{i}-34 \sin 28.07 \hat{j}-34 \cos 45 \hat{k}$
For the other force at B, I can write -18k.
At D, $-34 \cos 28.07 \hat{i}+34 \sin 28.07 \hat{j}+34 \cos 45 \hat{k}$

Now I take moments of all the above forces about the origin which gives me
1.13i + 7.51j

the magnitude of which is given by $\sqrt{1.13^2+7.51^2} = 7.59$

which is incorrect according to answer key.

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2. Aug 1, 2014

### SteamKing

Staff Emeritus
Are you sure angle DEF is 45 degrees? It doesn't look it from the figure.

3. Aug 1, 2014

### ehild

You calculated the components of the 34 N force incorrectly. The 34 N forces are parallel to the vector connecting D and E or F and B. Find the direction of those vectors from the coordinates of the points. Multiply the magnitude of the force with the unit vector in the appropriate direction.

ehild