Calculating Moment & Horizontal Force at O for 100 lb Force

[David Donald]

Homework Statement

A 100 lb vertical force is applied to the end of a lever, which is attached to a shaft at O. Determine (a) the moment of the 100 lb force about O (b) the horizontal force applied at a that creates the same moment about O.

Homework Equations

The Attempt at a Solution

I know the solution but I'm still a bit confused as to what I'm doing for part (a) I have Mo = Fd = 24in*cos(60) * 100 lb
But I don't understand why I'm using
24in*cos(60) and not 24in*sin(60)...

 

[Doc Al]

But I don't understand why I'm using
24in*cos(60) and not 24in*sin(60)...

You need the perpendicular distance between O and the line of action of the force.