Friction
F ≤ μR. Static friction matches applied force up to a maximum; kinetic = μR when sliding.
Y13 Mechanics. Friction opposes motion or tendency to motion. Coefficient μ depends on surfaces; Fmax = μR where R is the normal reaction.
Worked examples
Block 5 kg on rough horizontal, μ = 0.4: max friction = 0.4×5g = 19.6 N.
Block on slope; θ just steep enough for slipping: tanθ = μ.
Once moving, kinetic friction = μkR (typically < static).
Frequently asked questions
Static vs kinetic?
A-Level often uses one coefficient. Strictly, μs > μk in real surfaces.
Direction of friction?
Opposite to motion (or attempted motion).