1. The problem statement, all variables and given/known data http://i.imgur.com/K2hYHgq.png Picture here with question. 2. Relevant equations F=ma 3. The attempt at a solution For the system to begin moving, a>0 therefore Fnet>0 I drew some FBDs and came up with four relevant forces total; the force of gravity along the plane for each of the two blocks, and the force of friction on the plane by m1 and m2, and the force of friction on m1 by m2. I ended up with something like this (g is cancelled): m2sinx - m1 sinx - um1cosx - u(m1+m2)cosx > 0 tan x > (2um1+um2)/(m2-m1) And solved for x from there. However, my answer is wrong. I think I'm getting stuck with the forces of friction, the denominator in my expression (m2-m1) seems very reasonable considering the system should act similarly to a block of mass m2-m1 with the extra force of friction.