applied force P →friction fμs·N (peak, just before it moves)μk·N (while sliding)static: f = Pkinetic: f = μk·Nnot movingsliding
Push gently on a heavy box and nothing happens: friction pushes back exactly as hard as you push, whatever that is, up to a limit of μs·N. Push past that limit and the box breaks free. Friction then drops to a lower, roughly constant μk·N, which is why getting something moving is harder than keeping it moving. Typical values: rubber on dry concrete μ ≈ 0.7–0.9, steel on steel ≈ 0.6 dry and 0.1 oiled, ice on ice ≈ 0.03.