Friction — Static & Kinetic

Friction resists sliding. It depends on the pair of surfaces, through the coefficient μ, and on how hard they are pressed together, through the normal force N — here simply the weight, mg. Notice what it does not depend on: the contact area. A block on its side and the same block on its end need the same force to move.

While the push is small, static friction matches it exactly and nothing moves; friction is a reaction, not a fixed number. It can only grow up to a limit of μ_sN. Push past that limit and the block breaks free, and friction immediately drops to the lower kinetic value μ_kN — which is why a stuck component lurches once it finally moves. This simulation takes μ_k as 0.8 μ_s, a typical ratio.

In maintenance, friction is wanted in some places and unwanted in others. It is what makes a torqued bolt hold, what makes brakes and clutches work, and what tyres need on a wet runway. It is also what wears bearings and wastes engine power as heat, which is why lubrication lowers μ wherever motion is meant to be free.

• Try this: Raise the applied force slowly until the block breaks free. What force did it take, and what does the friction do the instant it moves?
• Try this: Double the mass. Why does it now need twice the force to start sliding?
• Try this: Set the push just below the break-free value. What is the friction force, and why is it not μN?