Friction

Friction is the resistive force that opposes the relative motion or the tendency toward motion between two contacting surfaces. It arises from a combination of surface roughness, interfacial forces, and deformation of materials under load. Friction can be static, kinetic (sliding), or rolling; static friction resists the initiation of motion, kinetic friction acts during sliding, and rolling friction acts when one body rolls over another.

Friction is often described by the friction coefficient, a dimensionless parameter dependent on materials, surface finish, lubrication, and temperature. In the Coulomb model, the maximum static friction is F_s ≤ μ_s N, and kinetic friction is F_k = μ_k N, where N is the normal force. Typically μ_s ≥ μ_k. In lubricated or fluid environments, friction is governed by lubrication regimes and hydrodynamic or boundary effects, reducing the effective coefficient of friction.

Origins of friction include interlocking of microscopic surface asperities, adhesive forces at contact points, and deformation of asperities during contact, all of which dissipate energy as heat during motion. Factors influencing friction include material pair, surface roughness, contact pressure, temperature, humidity, and presence of lubricant. Speed and sliding direction can alter friction, especially in lubricated contacts where the regime may change with speed.

Applications and implications of friction are broad. It enables walking and standing, braking and clutch operation, and grip for tires and tools, while also causing wear and energy loss in machinery. Engineers manage friction by choosing materials, applying lubricants, or modifying surface textures to achieve desired friction levels.