Friction

Objective: Distinguish friction before and during sliding.

Static friction acts between surfaces without relative sliding at their contact and can adjust up to a maximum set by conditions. Kinetic friction acts while surfaces slide relative to each other. Friction direction opposes relative motion or its tendency at the contact, not always the object’s overall motion. A walking foot can receive forward static friction; a sliding box typically receives kinetic friction opposing sliding. Friction magnitude depends on contact conditions and normal force in simple models, but textbook coefficients are approximations rather than universal properties of materials independent of surface state. A force diagram should identify which surfaces interact.

Objective: Apply friction to calculations, evidence, and specified situations.

Kinetic friction is modeled by f_k=μ_kN during sliding, while static friction adjusts from zero up to f_s,max=μ_sN. It is incorrect to set all static friction equal to its maximum. Friction opposes relative sliding or the tendency to slide at the contact, not necessarily every object’s absolute velocity. Coefficients describe the contact pair under stated conditions and are approximations. Use the actual normal force; on an incline it is not simply mg without resolving components.