Friction is a set of physical interactions that oppose relative motion or the tendency toward relative motion between surfaces or materials in contact, or between layers within a fluid. In elementary mechanics, friction is often modelled as a force parallel to the contact surface that acts opposite the direction of relative or impending sliding. The simplest dry friction models distinguish static friction and kinetic or sliding friction.
Static friction acts when surfaces are not sliding relative to each other and can adjust in magnitude up to a limiting value. Kinetic friction acts during sliding and is often approximated as proportional to the normal force through a coefficient of friction. These simple laws are useful engineering approximations rather than universal microscopic laws. At the microscopic level, friction involves surface roughness, adhesion, deformation, molecular interactions, energy dissipation and, in some systems, chemical or electrical effects. Friction is therefore both a useful source of traction and an important cause of energy loss and wear.
Static friction prevents relative sliding when the applied tangential force remains within the available frictional limit. It is therefore incorrect to assume that static friction always equals coefficient of friction multiplied by normal force. In the common idealised Coulomb model, the magnitude of static friction adjusts as required up to a maximum approximately given by mu sub s N. If an object rests on a horizontal surface and no horizontal force acts, static friction can be zero.
If a horizontal force is applied but remains below the limiting value, static friction can match it and the object remains stationary. Once the required friction exceeds the maximum available static friction, sliding begins. Kinetic friction is then often approximated by mu sub k N, where mu sub k is the coefficient of kinetic friction. Usually, although not universally, mu sub k is smaller than mu sub s. This distinction explains why starting an object moving can require more force than maintaining its sliding motion.
Friction arises from complex interactions at contacting surfaces. Surfaces that appear smooth at ordinary scales often contain microscopic asperities. When two surfaces contact, their actual contact area is much smaller than their apparent geometric area, and local deformation and adhesion occur at these microscopic contact regions. Sliding can cause these microscopic junctions to form and break, dissipating mechanical energy as heat and sometimes producing wear particles.
The relationship between friction and normal force is therefore an empirical approximation over particular operating conditions rather than a fundamental universal law. Surface chemistry, temperature, lubrication, pressure, roughness, material composition and sliding speed can all influence friction. In some cases, deformation of soft materials dominates. In lubricated contacts, fluid films can separate surfaces and substantially alter the friction mechanism. These microscopic mechanisms explain why friction cannot always be predicted accurately using a single coefficient.
Friction is essential for many everyday and technological activities. Walking requires sufficient friction between footwear and the ground so that the foot does not simply slide backwards during propulsion. Vehicles depend on tyre road friction for acceleration, braking and steering. Brakes convert kinetic energy into thermal energy through friction or related contact mechanisms. Clutches use friction to transfer torque between rotating components. Belts, pulleys and many mechanical couplings rely on friction to transmit forces.
Writing with a pencil or pen involves friction and material transfer between the writing instrument and a surface. Without adequate friction, tools could not grip objects and many structures would be difficult to stabilise. Friction can therefore be beneficial when controlled, even though it can also cause unwanted energy loss. Engineering frequently involves choosing materials and surface treatments that provide enough friction for useful operation while limiting excessive wear and heat generation.
Friction also has significant disadvantages. Mechanical systems lose energy when surfaces slide or deform against one another, with much of the lost mechanical energy becoming thermal energy. Friction contributes to wear, which can change dimensions, damage surfaces and eventually cause component failure. Bearings, gears, engines and other moving mechanisms therefore require careful lubrication, material selection and surface engineering. Lubricants reduce direct contact and can create fluid films that lower friction and wear.
However, lubrication itself introduces constraints involving viscosity, temperature, contamination, pressure and chemical compatibility. In some applications, engineers intentionally use low friction materials or coatings to improve efficiency. In others, they use high friction surfaces to improve braking, traction or gripping. The goal is rarely simply to maximise or minimise friction; it is to obtain an appropriate frictional behaviour for the application.
Fluid friction is often called drag or viscous resistance and differs from dry contact friction. When an object moves through air or water, it experiences resistance associated with interactions between the object and the fluid. The magnitude of fluid drag depends on factors including velocity, fluid density, shape, surface characteristics and flow regime. At low Reynolds numbers, viscous effects can dominate and drag may have a different dependence on velocity from that seen in turbulent flow.
At higher speeds, pressure drag and turbulent effects can become important. This distinction matters in aerodynamics, hydrodynamics and spacecraft reentry. Atmospheric drag can remove orbital energy from satellites in low Earth orbit, while aerodynamic drag affects aircraft, cars and projectiles. Fluid friction is therefore not adequately described by the simple dry friction equation involving a coefficient multiplied by normal force.
Friction can be analysed using energy and momentum concepts. Friction does not violate conservation of energy; it transfers organised mechanical energy into internal energy and other forms. In a sliding block problem, for example, the block's kinetic energy decreases because friction performs negative work on it, while thermal energy increases in the contacting materials and surroundings.
Static friction can behave differently because no relative sliding is required and therefore its energy transfer must be analysed through the actual motion of the contact point and deformation. Rolling resistance is another distinct phenomenon involving deformation, hysteresis and other losses rather than idealised sliding friction alone. Lubrication, rolling bearings and aerodynamic design can reduce certain forms of resistance, increasing mechanical efficiency. In practical engineering, friction is therefore part of a broader study of tribology, which examines friction, wear and lubrication.
A common misconception is that friction always acts opposite to the direction in which an object is moving. More precisely, friction acts to oppose relative motion or the tendency toward relative motion at the contact interface. This distinction explains why friction can help a car accelerate forward: the tyre exerts a backward force on the road, and the road exerts a forward static friction force on the tyre. Another misconception is that friction is always caused simply by roughness.
Microscopic adhesion and deformation can be important even for apparently smooth surfaces, and extremely rough surfaces do not necessarily have proportionally greater friction under all conditions. Friction is also not a single universal force with one fixed coefficient. Different mechanisms operate under different conditions, and coefficients are empirical parameters whose values depend on material pair, environment and operating regime. Friction is therefore best understood as a family of resistance and traction mechanisms whose practical effects range from essential grip to substantial energy loss and wear.
