Sum of the friction and normal forces mechanics problem

[luiseduardo]

Hi,

Can anyone explains me how to solve this problem ? I'm very confused what to do.

Homework Statement

"A wedge of mass M subjected to a horizontal force is on a frictionless horizontal surface. Place a block of mass "m" on the sloping surface of the wedge. If the coefficient of static friction between the surfaces of the wedge and the block is "μe", find the maximum and minimum values of force F so that the block remains at rest on the wedge."

[PLAIN]http://img411.imageshack.us/img411/4226/calso.jpg

Homework Equations

Fr = M.a

The Attempt at a Solution

Well, I tried to do, but I have two questions:

What is the force applied to the block that will make it up? It would be a Normal or force F?

 

[kjohnson]

Yes, it is the sum of the friction and normal forces that contribute to the motion.

 

[luiseduardo]

Ok, but the force F will contribute to the motion too ?
Or the force F don't acts to the motion only the Normal, is it?

 

[kjohnson]

Both of them will contribute since they both have horizontal components and the motion is horizontal...

 

[luiseduardo]

Thanks, another question: We need two Normal ?

N (down) = mgcos a - F*sen a
N (up) = F*sen a + mgcos a

 

[luiseduardo]

I found:

[mg(cos a*μe - sen a)] / (sen a*μe + cos a) =< F

F =< [mg(sen a + cos a*μe)] / (- sen a*μe + cos a)