Friction

📋 Teacher notes

• •Static friction is NOT fixed — it exactly matches the applied force, up to a maximum of μsN. Push harder within that limit and friction grows to match; nothing moves.
• •Once the applied force exceeds μsN, the block breaks free and KINETIC friction takes over — μk is always a little LESS than μs, which is why things "jerk" into motion.
• •Friction is independent of the contact area and (to a good approximation) of speed — but always proportional to the normal reaction N.
• •N = mg only holds here because the surface is flat and the push is horizontal — on a slope, or with an angled push, N changes.
• •Real applications: brake pads (want HIGH μ), ice skates and ball bearings (want LOW μ), why worn tyres skid more easily.