A block is accelerated with a, take a look at the picture. There is a friction between the red block and the other one with mass m1. The friction is given by the coefficient u1. I have to determine a such that the blue block is not moving relative to the red one. a has to bi in depenence of angle alpha. 2. Relevant equations The relevant equations are basically newton's equations. 3. The attempt at a solution What I have done so far is the following: One force acting on the block is coming from the gravitional field, so F_g=m1*g. I have splitted this one in components: F||=sin(alpha)*F_g and (already added friction) F_s=cos(alpha)*F_g*(1-u1). I don't know how I can go on. The basic problem is I don't really know how a acts one the block. And how I've to balance the forces.