Force needed to keep block B stationary on inclined surface

[Tesla11]

Only girl on electro engineering :(

Hello! This is my first post but not the last! :D
I'm a student at norwegian universety and I really have difficulty with phisics

Here's my question (look at attacment)


How big force F has to be so that B doestn't change position (doesn't flys over and doesn't fells down)?

Answer is this:

Mg(tanø*y) Mg(tanø+ y)
------------ < F< -----------
1+y*tanø 1-y*tanø

ø=angle
y=friction ( between A and B)


I'm not very proud to say that I used 10 hours in the row on this, so any help will be very appreciated!

 

[LowlyPion]

Welcome to PF.

Consider then the forces acting on the block A, with mass call it m, along the plane of the incline.
Gravity wants it to slide down at m*g*sinθ
Friction wants to hold it at μ*m*g*cosθ

Now what about your force F ?

How much is it accelerating the system of A and B?

Then what components of that force are increasing friction? and what component is wanting to push it up the incline?

 

[ShadowDatsas]

Hi! I am a newcomer too... So welcome:P
Where is the Force apllied? To the A body? To the B? To none of these?