Finding minimum angle of friction.

[pooface]

Homework Statement

A uniform ladder of mass m and length L leans at an angle of theta against a frictionless wall. If the coefficient of static friction between the ladder and the ground is mu, determine a formula for the minimum angle at which the ladder will not slip.

I have added my own additional information in red.

Homework Equations

various equations perhaps.

mu= Ffr/FN
Ffr is friction force.
FW is force by the wall

The Attempt at a Solution

http://img171.imageshack.us/img171/1830/s8sv4.jpg
this is what I have done. I took my ref point at the top of the ladder.

(FN)(Lcostheta) + (Ffr)(Lsintheta) = (mass*g)(L/2 costheta)
I don't know if this approach is correct. I doubt it.

Can someone give me head start here?...I have been stuck on this problem for a while now. How do I set this up?

Thanks.

 

[Doc Al]

this is what I have done. I took my ref point at the top of the ladder.

(FN)(Lcostheta) + (Ffr)(Lsintheta) = (mass*g)(L/2 costheta)
I don't know if this approach is correct. I doubt it.

Nothing wrong with this approach. Except for a sign error: the second term is a counter-clockwise torque.

Fix that error and keep going. Combine this with the friction equation. And with the fact that vertical and horizontal forces must add to zero.