Friction Forces: Maximum Intensity & Acceleration

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http://www.digitaluploader.com/1/2a578af8cd660754014ea75dbe966588.GIF

Hi, I need some help with this exercise because I can't figure out where the friction forces point to.Find the maximum intensity possible for F before any of the blocks start moving, and the acceleration that they acquire once they start to move, if F keeps the calculated intensity.There is no friction in the pulley and the rope can not stretch nor shrink. The coefficients of friction between the blocks and with the floor are in the picture.

 

[Avodyne]

Friction forces always resist (act opposite to) other forces that are parallel to the plane of friction.

 

[Doc Al]

Friction opposes slipping between surfaces. So figure out which way the surfaces would slip (with respect to each other) if there were no friction--that will tell you which way the friction must act.