1. The problem statement, all variables and given/known data Small block 1 with mass m = 2.19 kg sits on top of large block 2 of mass M = 7.0 kg, and the pair sit on a frictionless, horizontal table. Between the blocks: the coefficient of kinetic friction is μk = 0.247, and the coefficient of static friction is μs = 0.319. Find the magnitude of the maximum force applied horizontally to the upper block (block 1) that will cause the two blocks to slide together. 2. Relevant equations Frictional force = μ*Normal F=ma 3. The attempt at a solution If we want to keep the two blocks together, then we need to use static friction. The frictional force should equal the force that is pushing the upper box so that it doesn't slip off the bottom box. Therefore: Ff = μs * N = .319 * 2.19 * 9.81 = 6.85 This is obviously wrong, but that is what I began with originally. The only other idea that comes to mind is if we add the sums of the masses instead of only the top box. Ff = μs * N = .319 * (2.19+7.0) * 9.81 = 28.76 I don't know which is correct, or if both are wrong.