frictions

Friction is the resistive force that opposes the relative motion or tendency of motion between two contacting surfaces. It arises from the roughness of surfaces and the adhesive forces at the contact interface, acting parallel to the surface contact.

Friction is commonly classified as static friction, which opposes the onset of motion, and kinetic (sliding) friction, which acts during relative motion. There is also rolling friction, resulting from deformation at the contact as a wheel or roller moves, and fluid or viscous friction, which involves resistance from a surrounding medium such as air or oil. In dry contact, friction is often described by Coulomb’s model and Amontons’ laws, where the frictional force is proportional to the normal force: static friction satisfies F ≤ μs N, while kinetic friction is approximately F = μk N when sliding, with μs ≥ μk.

Coefficients of friction depend on material pairing, surface finish, lubrication, temperature, and load. Friction can be reduced by lubricants, coatings, surface texturing, or material changes; it can be increased to improve traction, braking, or wear resistance.

In engineering, managing friction is central to efficiency, wear, and energy use. In everyday life, friction enables walking, gripping, and vehicle control but also causes wear and energy loss. Tribology is the interdisciplinary field that studies friction, wear, and lubrication.

The term frictions can also refer to tensions or conflicts arising from competing interests in social, organizational, or economic contexts.