1. The problem statement, all variables and given/known data http://imgur.com/C0XYEKw 2. Relevant equations Fnet = m*a FG = m*g fstatic = μ*N 3. The attempt at a solution If both Block 1 and Block 2 have acceleration a in the x-direction and have an acceleration of 0 in the y-direction, then the force of static friction on Block 1 due to Block 2 doesn't seem to depend at all on what Block 2 is doing. This is because the normal force on Block 1 due to Block 2 is still the same as it is at rest and the force of static friction on Block 1 due to Block 2 is the only force acting in the x-direction on Block 1. I tried this by considering only Block 1 as a system. In the y-direction, the forces acting on Block 1 are the normal force on Block 1 due to Block 2, N12, and the gravitational force on Block 1 due to the Earth, F1E. The acceleration, ay, is equal to 0. Thus: Σ Fnet,y = 0 = N12 - F1E = N12 - m1*g Thus: N12 = m1*g In the x-direction, the only force acting on Block 1 is the force of static friction, which is acting in the right, positive x-direction. The acceleration, ax, is equal to a. Thus: Σ Fnet, x = m1*a = fstatic = μ*N12 = μ*m1*g Thus the force and direction of the force of static friction on Block 1 due to Block 2 is: fstatic = μ*m1*g This, however, is the wrong answer. I don't understand why though. Where am I going wrong? Thank you.