Friction

Friction is the resistance to relative motion, or impending relative motion, between contacting surfaces. More broadly, the term includes resistance associated with deformation and fluid flow. In classical mechanics, solid-surface friction is represented by a force acting tangentially to the contact. It enables walking, gripping, and traction while also contributing to heating and wear. The study of friction, wear, and lubrication is called tribology. (openstax.org)

Static and kinetic friction

Static friction acts between surfaces that are not sliding relative to one another. Its magnitude adjusts to the forces tending to cause sliding, up to a limiting value. A stationary crate pushed lightly across a floor therefore experiences friction equal and opposite to the applied horizontal force; friction is not automatically at its maximum. Kinetic friction, also called sliding friction, acts when the surfaces slide relative to each other. Its direction opposes their relative sliding velocity. (openstax.org)

For many dry contacts, an approximate empirical model is

where $N$ is the magnitude of the normal force, perpendicular to the contact, and $\mu_s$ and $\mu_k$ are dimensionless coefficients of friction. The first expression is a bound, not an equation prescribing the actual static force. Often $\mu_s>\mu_k$, although this relationship is not universal. These expressions supplement Newton’s laws of motion rather than replace them. (openstax.org)

Friction opposes relative slip, not necessarily an object's motion relative to the ground. For example, it can accelerate a crate carried by an accelerating truck, provided the required force remains below the static-friction limit. (openstax.org)

Empirical laws and their limitations

The simple dry-friction model makes friction proportional to normal load and approximately independent of sliding speed over a restricted range. A further common approximation is independence from apparent contact area. These relationships are useful engineering rules, not fundamental laws applying to every interface. The coefficient characterizes a pair of surfaces under specified conditions, rather than either material alone. (feynmanlectures.caltech.edu)

Surface cleanliness, oxide layers, contaminants, and lubrication can substantially change friction. Large loads or high speeds may generate enough heating to invalidate a constant-coefficient model. Even a polished surface is not necessarily low-friction: sufficiently clean metal surfaces may adhere strongly. Consequently, tabulated coefficients require attention to the conditions under which they were measured. (feynmanlectures.caltech.edu)

An inclined plane provides a simple measurement method. For a block acted on only by its weight and contact forces, the load perpendicular to the plane is $N=mg\cos\theta$, while the downslope component of weight is $mg\sin\theta$. At the threshold of sliding, the model gives $\mu_s=\tan\theta$. The threshold angle measures limiting static friction, not the generally smaller force acting before that threshold. (openstax.org)

Microscopic origins

Apparently smooth surfaces contain microscopic peaks, or asperities, so their actual contact is concentrated in small regions. Resistance arises from several processes, including adhesion between contacting materials, deformation, and disruption of surface features. Friction cannot therefore be explained solely as the interlocking of roughness: attractive interactions between surface molecules also matter. (openstax.org)

At very small scales, the arrangement of contacting atoms and the structure of intervening molecular layers become important. Sliding can involve repeated trapping in local configurations followed by sudden rearrangement. This produces stick–slip motion, in which intervals of sticking alternate with rapid displacement. Simulations investigate how atomic interactions, surface alignment, and thin lubricant films influence these processes. (arxiv.org)

Work and energy

Sliding friction commonly converts organized mechanical energy into internal energy, increasing the temperature of contacting bodies. For a body sliding a distance $d$ across a stationary surface against a constant friction force $f_k$, the work done on that body by friction is

The negative sign indicates removal of mechanical energy from that body's motion. Energy is transformed rather than destroyed. Because frictional work depends on the path travelled, sliding friction is classified as a nonconservative force. (openstax.org)

Static friction does not necessarily dissipate energy. In ideal rolling motion without slipping on a stationary rigid surface, the instantaneous contact point is at rest, so the static contact force does no work. It may nevertheless redistribute the motion between translation and rotation. Real rolling systems can lose energy, so this idealization should not be confused with the claim that rolling is always resistance-free. (openstax.org)

Fluid resistance and lubrication

Fluids resist relative motion through viscosity. An object moving through a fluid experiences drag, whose magnitude generally depends on speed, shape, and flow conditions. In slow viscous flow, drag on a sphere is proportional to speed; in many faster-flow situations, a quadratic speed dependence is a useful approximation. These models differ from constant-coefficient dry sliding friction. (openstax.org)

Lubrication modifies the interface between moving surfaces. A sufficiently developed fluid film can separate them, reducing direct solid contact while still producing viscous resistance. Under boundary lubrication, thin surface films influence friction where direct contact remains. Mixed lubrication combines fluid-supported regions with contacting asperities. These distinctions are important for bearings, engines, and other machine components. (eng.cam.ac.uk)

Tribological engineering addresses both friction and wear, the alteration or removal of material at interacting surfaces. Materials selection, surface engineering, and lubricant formulation are used to control these effects. At molecular film thicknesses, ordinary continuum descriptions may become inadequate; research therefore combines mechanical testing with surface measurements and atomic-scale simulation. (eng.cam.ac.uk)