1. The problem statement, all variables and given/known data A block of mass m1 is on top of a block of mass m2. Block 2 is connected by an ideal rope passing through a pulley to a block of unknown mass m3 as shown. The pulley is massless and frictionless. There is friction between block 1 and 2 and between the horizontal surface and block 2. Assume that the coefficient of kinetic friction between block 2 and the surface, μ, is equal to the coefficient of static friction between blocks 1 and 2. Q1 : The mass of block 3 has been changed such that block 1 and block 2 are moving together with a given acceleration of magnitude a. What is the magnitude and the direction of the force of friction exerted by block 2 on block 1? Q2: What is the minimum value of m3 for which block 1 will start to move relative to block 2? 2. Relevant equations 3. The attempt at a solution For part 1, I thought that the force needed to move both block 1 and block 2 would be F= (m_1+m_2)a, but I couldn't think of how I should compute the friction force between block 1 and block 2. I know that the maximum friction force for block 1 is fs=m_1*g*mu For part 2, I still couldn't get my head around it... Please help! I'm generally confused with this problem.