How to use coefficient of friction for connected boxes with applied force

[frownifdown]

1. Here is the problem http://i.imgur.com/WH0Z4ko.png


So I've set up a Free Body Diagram and I think I have it right. I have the two Forces of Weight (8.82N and 21.27N), the force of 12.8 that is pulling the .9kg box, and then the Force Normal (just the opposites of the two Forces of Weight). I didn't really understand how the prof. was using the coefficient of friction in the problem and I'm not sure how to start with this problem.

 

[462chevelle]

I would start by finding the Ff
then you can see how much force is actually being applied horizontally at any given direction

 

[frownifdown]

I would start by finding the Ff
then you can see how much force is actually being applied horizontally at any given direction

Thanks!

 

[462chevelle]

did you figure it out?