Calculating Moment Arm for a Force: Explained and Solved

[mathdude88]

Homework Statement

Homework Equations

T=Fl

The Attempt at a Solution

I'm not sure what the moment arm of this problem is. I think it is suppose to be a line connecting 0 to F1 at a right angle. To draw this moment arm one would have to extend F1 past where it meets A. However I am not sure is that is the right moment arm, how to find that distance, or how the other forces come into play for this problem.

 

[PhanthomJay]

Homework Statement

Homework Equations

T=Fl

The Attempt at a Solution

I'm not sure what the moment arm of this problem is. I think it is suppose to be a line connecting 0 to F1 at a right angle. To draw this moment arm one would have to extend F1 past where it meets A. However I am not sure is that is the right moment arm, how to find that distance, or how the other forces come into play for this problem.

The problem is asking for the moment about O of F1, so don't worry about the other forces. You are correct in that you can extend the line of action of force F1 and determine the perpendicular distance from that extension to point O, but that's too much geometry and trig. Alternatively, you can break up F1 into its x and y components, and calculate the moment arm and moment of each component about O (the moment from its y component should be 0 (Why?), so you just have to deal with the x component moment.). Or you can just use the cross product rule for torques, if you can determine the correct angle and position vector to use.