1. The problem statement, all variables and given/known data Two blocks of masses M and m are oriented as shown in the diagram. The block M moves on a surface with coefficient of kinetic friction μ_1, and the coefficient of static friction between two blocks is μ_2. What is the minimum force F which must be applied to M such that m remains stationary relative to M? The diagram essentially looks like Utah rotated 90 degrees clockwise. 2. Relevant equations ∑F=ma F_f = F_N * μ 3. The attempt at a solution I drew force diagrams for both objects and summed up the forces, as is standard with these problems. My answer was: g * (μ_1 * M + (m / μ_2)) The book tells you the correct answer and this is not it. I have triple checked my diagrams but I can't find my error. Any help is greatly appreciated!