Friction

Friction is a force that resists relative motion or attempted motion between surfaces in contact. Its magnitude and direction depend on the surfaces and their motion.

Normal force: The perpendicular contact force exerted by a surface on an object touching it. It sets the load that often determines the maximum static friction and sliding friction.
Contact mechanics: The study of forces and deformation where bodies touch. Real contact occurs at microscopic regions rather than across perfectly smooth apparent surfaces.
Tribology: The study of friction, lubrication, and wear between interacting surfaces. This field measures and engineers friction across machines and materials.
Viscosity: A fluid's resistance to deformation and flow. It creates drag within fluids, unlike dry friction between solid surfaces.
Static friction: Friction between surfaces that are not sliding relative to each other. It adjusts to resist applied forces until its maximum value is reached.
Surface roughness: The small-scale irregularity of a material's surface. Interlocking asperities and their deformation contribute to friction between solids.
Lubrication: The use of a substance between surfaces to reduce friction and wear. Lubricants separate or modify contacting surfaces to control their resistance to motion.
Drag force: A force opposing an object's motion through a fluid. Fluid drag can resemble friction but depends on motion through a surrounding fluid.
Kinetic friction: Friction between surfaces sliding relative to each other. It usually opposes sliding with a force less than the maximum static friction.
Adhesion: The attraction that causes unlike or like surfaces to cling at their interface. Interfacial bonding can contribute to friction even when surfaces appear smooth.

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