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. What is the minimum value of m3 for which block 1 will start to move relative to block 2? 2. Relevant equations F=ma f= mu*N 3. The attempt at a solution I've taken a couple of approaches. The one I think is most valid is this: The point just before block 2 slips relative to block 1 is where the static friction between m1 and m2 reaches its maximum value. At this point the magnitude of acceleration between all the blocks is the same. I'm then solving for: m1*a = mu*m1*g m2*a = T-mu*m1*g-mu*(m1+m2)*g m3*a=T-m3*g Which gives me: m3=2*mu*(m1+m2)/(1+mu) Which is wrong. Am I making a math error or taking the wrong approach entirely? Thanks in advance for any help!