# Forces in Equilibrium (Vectors)

1. Sep 26, 2008

### Oblivion77

Hey guys, I am stuck on this question. It is first year engineering vector statics. Here it is.

Thanks for any help!

2. Sep 27, 2008

### tiny-tim

Hi Oblivion77!

Show us what you've tried, and where you're stuck, and then we'll know how to help.

3. Sep 27, 2008

### Oblivion77

Well, i resolved the vectors into the components and added the sums and equaled them to zero (since its in equilibrium)

$$\Sigma Fx = -118.65 -F3Cos[\alpha] + F1 =0$$

$$\Sigma Fy = 68.5 - F3Sin[\alpha] = 0$$

I don't know how to go from here, with the "minimum"

4. Sep 28, 2008

### tiny-tim

Hi Oblivion77!

… just woken up … :zzz:
Yes, that's right.

Tip: when you have two equations with cos and sin,

put the cos and sin on their own on the left, then square and add (using cos2 + sin2 = 1, of course)

That will give you F3 in terms of F1, and then … ?

5. Sep 28, 2008

### MrEnergy

...after you solve a little bit, you get;

137/2=F3*Sin(a)

so for a=>90 degrees sin is max and F3 is min.

Min F3 is 137/2...And a is 90 degrees...

and magnitude of F1 is;

F1=Cos30*137

6. Sep 28, 2008

### tiny-tim

Hi MrEnergy!
oops! I over-complicated it!!

Yes, you're absolutely right.

(hmm … not sure how I helped there … )