Solving 2D moments about point O

[majunee]

(a) The moment the force, F, makes about O is 15 N-m clockwise, and θ = 30˚, find the magnitude of F.
(b) If the force F = 100 N, determine the angle θ so the force develops a clockwise moment about point O
of 20 N-m. Limit your answers to a range of 0º ≤ θ ≤ 90º.

I'm a little lost can someone please help me with how to set up equations so i can figure out the answers. Thanx

 

[tiny-tim]

welcome to pf!

hi majunee! welcome to pf!

start by completing the diagram …

continue the line of force backwards, and find its perpendicular distance from O ​

 

[majunee]

thanx so is this correct so far
-15 KN-m = F[-(sin30°)-(50mm)-(300sin60°)]

 

[tiny-tim]

hmm … you're going to need another hint:

call that corner "C"

draw the line G through C parallel to F

draw the two perpendicular lines from O to G and from C to F …

how long are they? ​

 

[majunee]

Sorry I am really having trouble understanding the concept...
wouldnt 0-G be 300sin60 and c-f be 50 + xsin30

 

[tiny-tim]

ahhh, you seem to be measuring vertical distances

no, to find the moment of a force, you need to measure distance(s) perpendicular to the line of the force

start again ​

 

[majunee]

how? if force has continuing line of action shouldn't we measure along the yaxis which would be like i did?

 

[tiny-tim]

why the y axis?

(are we using the same diagram? … the force is at 30°)

 

[majunee]

yes we are
so would it be -300sin60 + 300 cos 60 - 50 + xsin30 - x cos 30

 

[tiny-tim]

why the y axis?

and what is the "x" in your equation?