Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other, the force that makes it difficult for one object to slide along the surface of another or to move through a liquid or gas.
[Merriam-Webster Dictionary].
There are several types of friction: Dry friction is a force that opposes the relative lateral motion of two solid surfaces in contact. Dry friction is subdivided into static friction ("stiction") between non-moving surfaces and kinetic friction between moving surfaces. Unlike atomic or molecular friction, dry friction generally arises from the interaction of surface features, known as asperities. Fluid friction describes the friction between layers of a viscous fluid that are moving relative to each other [Ferdin, p. 397].
Frictional forces, such as the traction needed to walk without slipping, may be beneficial, but they also present a great measure of opposition to motion. About 20 percent of the engine power of automobiles is consumed in overcoming frictional forces in the moving parts.
Lubricated friction is a case of fluid friction where a lubricant fluid separates two solid surfaces. Skin friction is a component of drag, the force resisting a fluid's motion across a body's surface. Internal friction is the force resisting motion between the elements making up a solid material while it undergoes deformation [Meriam, p. 328].
Friction is desirable and essential in supplying traction to facilitate motion on land. Most land vehicles rely on friction for acceleration, deceleration, and changing direction. Sudden reductions in traction can cause loss of control and accidents. Friction is not itself a fundamental force. Dry friction arises from inter-surface adhesion, surface roughness, surface deformation, and surface contamination. The complexity of these interactions calculates friction from first principles impractical and necessitates empirical methods for analysis and theory development. Friction is a non-conservative force – work done against friction is path dependent. In friction, some kinetic energy is always transformed into thermal energy, so mechanical energy is not conserved [Wikipedia].
Friction is a force that resists the sliding or rolling of one solid object over another. Frictional forces, such as the traction needed to walk without slipping, may be beneficial but also present great opposition to the motion. About 20 percent of the engine power of automobiles is consumed in overcoming frictional forces in the moving parts. The primary cause of friction between metals appears to be the forces of attraction, known as adhesion, between the contact regions of the surfaces, which are always microscopically irregular. Friction arises from shearing these “welded” junctions and from the action of the irregularities of the more complex surface plowing across, the softer surface [Encyclopædia Britannica].
Merriam-Webster's Collegiate® Dictionary, Eleventh Edition.
Cambridge Advanced Learner's Dictionary & Thesaurus.
Adam Augustyn.The Editors of Encyclopaedia Britannica/1768
Ferdin B. and Johnston P., Russell E. (1996). Vector Mechanics for Engineers. McGraw-Hill.
Meriam, J. L., Kraige, L. G. (2002). Engineering Mechanics (fifth ed.). John Wiley & Sons.
Friction. Encyclopædia Britannica. Retrieved from: https://www.britannica.com/science/friction
Friction. Merriam-Webster Dictionary.