Calculating Moments and Resolving Forces in Engineering Statics

[andrew.c]

Homework Statement

http://img512.imageshack.us/img512/2617/staticsquestion4.jpg

Homework Equations

- Ability to resolve forces into components
- M = Fd

The Attempt at a Solution

(a)
Resolve the force acting at an angle into components...
10kN force = -10cos50i - 10 sin 50 j kN

Therefore, resultant R becomes...

= (-10cos50)i + (15 - 3 - 10 sin 50)j kN
= -6.428i + 4.3396j kN

(with a magnitude of 7.756kN)

(b) This is where I get stuck.

I basically calculated the moments created by the individual elements of R,

F1 = 15kn force
F2 = 3kn force
F3 = vertical component 10kn force
and, because I am using the joint with the wall as an origin, the line of action of the horizontal component passes through my origin thus no moment.

M1 = 15*3 = 45kNm
M2 = -3*1.5 = -4.5kNm
M3 = (-10 sin 50)*9 = -68.944kNm

Sum of moments = 45 - 4.5 - 68.944 = -28.444kNm

and then I get stuck. Do you add the moment you've just found to the moment provide in the couple, then sub. back into M=Fd where F=R?

Help!

 

[nvn]

andrew.c: Nice work. The answer to the question in your last paragraph is yes. Also, notice a mistake in your M2 equation.

 

[andrew.c]

Also, notice a mistake in your M2 equation.

Yes, should use 4.5, not 1.5.
Thanks, I got the right answer!