Setup: Block A (mass m) on top of Block B (mass M) on a surface. Force F applied on one block.

Step 1: Check if blocks move together. Assume they move together: a = F/(m+M). Required friction on A = ma (if F is on B) or = Ma (if F is on A... depends on which block). Compare with f_max = mu_s * m * g (between A and B).

Step 2: If f_required <= f_max: Blocks move together at a = F/(m+M).

Step 3: If f_required > f_max: Blocks slide relative to each other.

• Write separate FBDs with f_k = mu_k * m * g between them
• Solve for individual accelerations

Common error: Using static friction when blocks are sliding, or kinetic when they're not.