Another moment question, completely lost

[pyroknife]

1. Homework Statement
I attached the drawing.
The force of magnitude F acts along the edge of the triangular plate. Determine the moment of F about point O.

2. Homework Equations

M=rxF

3. The Attempt at a Solution
What I did was I just picked the top left point of the triangle and labeled that (0,h) which give me my r vector 0i+hj. After that I really don't know how to proceed except to find the angle by doing inverse tangent of h/b, but I just keep getting variable after variable. What other way is there to do this?

 

[PeterO]

1. Homework Statement
I attached the drawing.
The force of magnitude F acts along the edge of the triangular plate. Determine the moment of F about point O.

2. Homework Equations

M=rxF

3. The Attempt at a Solution
What I did was I just picked the top left point of the triangle and labeled that (0,h) which give me my r vector 0i+hj. After that I really don't know how to proceed except to find the angle by doing inverse tangent of h/b, but I just keep getting variable after variable. What other way is there to do this?

First thing, you didn't attach a drawing?

 

[pyroknife]

hmm it's not letting me attach the image. I uploaded it on imageshack
http://imageshack.us/photo/my-images/714/48631281.png/

 

[PeterO]

hmm it's not letting me attach the image. I uploaded it on imageshack
http://imageshack.us/photo/my-images/714/48631281.png/

You had that M = r * F

you need to find r, which is the distance from O to the closest point on the hypotenuse [lets call that point D.

Similar triangle work comparing the original triangle to the triangle ODX where X is either the point you decided to call (0,h) or the other corner of the triangle.

 

[pyroknife]

i'm a little confused by r. is it always the "closest" distance from the point you're revolving around or can it be any point in the line of action of the force? Like can you pick any point on the hypotenuse in this case, even though it'll be easier to make 1 of the 2 corners?