Seriously? Moment of Force about a specified Axis

[Saladsamurai]

[SOLVED] Seriously? Moment of Force about a specified Axis

So I really thought I understood this. I have attached the problem and attempt at solution. It should be easy to follow. Problem

Attempt at Solution
This was my process:

1. Write each F and each r as cartesian vectors
2. Find each moment by cross product.
3. Add the moments to get resultant moment M_R. Find unit vector along Oa axis=U_oa
4. Use resultant Moment and Unit vector along Oa to find
M_R dot U_oa=|M_oa|
5. Write as Cartesian vector-->|M_oa|*U_oa

The answer is supposed to be 26.1i-15.1j lb*ft. As you can see, my answer isn't close (WTF else is new!)

What is wrong with my method?
Thanks,
Casey

 

[Saladsamurai]

I can't believe I keep getting the same wrong answer!

 

[tongpu]

I think for Moment 2 you suppose to only have (i) term

 

[dontdisturbmycircles]

Look at your second cross product again. Don't get discouraged, :-).

Imagine attaching a flexible pipe from the origin to the a point on the line of action of F2 and pulling on it with a rope in the direction of f2, the pipe would begin to trace out a disk and the direction of the moment caused by F2 about the origin would be defined as being perpendicular to this disk and hence must only have an i component.

 

[Saladsamurai]

I think for Moment 2 you suppose to only have (i) term

Look at your second cross product again. Don't get discouraged, :-).

Thanks guys! I was worried that my method was wrong. I wonder if I will EVER stop making tiny stupid errors. I just can't seem to stop...no matter how careful and aware I think I am being.

Oh well...siggghhh.
Thanks again!