f_s ≤ μ_s · N
f_k = μ_k · N
N = mg·cosθ
a = (F − f_k) / m
Static friction prevents motion until applied force exceeds μ_s·N. Once sliding, kinetic friction (always less) opposes motion. On an incline, the block slides when mg·sinθ > μ_s·mg·cosθ, i.e., tanθ > μ_s.