Solving 2D moments about point O

1. Jun 2, 2012

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

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2. Jun 3, 2012

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

3. Jun 3, 2012

majunee

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

4. Jun 3, 2012

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?

5. Jun 3, 2012

majunee

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

6. Jun 3, 2012

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

7. Jun 3, 2012

majunee

how? if force has continuing line of action shouldnt we measure along the yaxis which would be like i did?

8. Jun 3, 2012

tiny-tim

why the y axis?

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

9. Jun 3, 2012

majunee

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

10. Jun 3, 2012

tiny-tim

why the y axis?

and what is the "x" in your equation?