coefficient of frictionstatic frictionfriction coefficientsliding frictioninternal frictionkinetic frictionresistancefrictional forceCoulomb frictioncoefficient of static friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.wikipedia
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lubricantslubricating oillube oil
*Lubricated friction is a case of fluid friction where a lubricant fluid separates two solid surfaces.
A lubricant is a substance, usually organic, introduced to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move.


forcesattractiveelastic force
In part this was due to an incomplete understanding of the sometimes non-obvious force of friction, and a consequently inadequate view of the nature of natural motion.


Friction is a component of the science of tribology.
It includes the study and application of the principles of friction, lubrication and wear.


static coefficient of frictionBreakaway friction
The two regimes of dry friction are 'static friction' ("stiction") between non-moving surfaces, and kinetic friction (sometimes called sliding friction or dynamic friction) between moving surfaces.
Stiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact.

Drag (physics)

dragaerodynamic dragair resistance
In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid.

Sliding (motion)

slidingslideSlide (motion)
The relative motion or tendency toward such motion between two surfaces is resisted by friction.

Traction (engineering)

tractionsurface tractiongrip
Friction is desirable and important in supplying traction to facilitate motion on land.
Traction, or tractive force, is the force used to generate motion between a body and a tangential surface, through the use of dry friction, though the use of shear force of the surface is also commonly used.

Guillaume Amontons

AmontonAmontonsAmontons' laws of friction
These laws were rediscovered by Guillaume Amontons in 1699 and became known as Amonton's three laws of dry friction (below).
He was one of the pioneers in studying the problem of friction, that is the resistance to motion where bodies are in contact.

Mechanical energy

mechanicalconservation of mechanical energyenergy
In the presence of friction, some kinetic energy is always transformed to thermal energy, so mechanical energy is not conserved.
In all real systems, however, nonconservative forces, such as frictional forces, will be present, but if they are of negligible magnitude, the mechanical energy changes little and its conservation is a useful approximation.

Kinetic energy

kinetickinetic energiesenergy
When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energy into thermal energy (that is, it converts work to heat).
On a level surface, this speed can be maintained without further work, except to overcome air resistance and friction.

Angle of repose

the maximum possible sand slopeCritical angle of reposean engineering term
This view was further elaborated by Bernard Forest de Bélidor and Leonhard Euler (1750), who derived the angle of repose of a weight on an inclined plane and first distinguished between static and kinetic friction.
The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose and is related to the density, surface area and shapes of the particles, and the coefficient of friction of the material.

Conservative force

conservativenon-conservativenon-conservative force
Friction is a non-conservative force - work done against friction is path dependent.
Gravitational force is an example of a conservative force, while frictional force is an example of a non-conservative force.

Charles-Augustin de Coulomb

CoulombCharles CoulombCharles Augustin de Coulomb
The understanding of friction was further developed by Charles-Augustin de Coulomb (1785).
He is best known as the eponymous discoverer of what is now called Coulomb's law, the description of the electrostatic force of attraction and repulsion, though he also did important work on friction.


The Greeks, including Aristotle, Vitruvius, and Pliny the Elder, were interested in the cause and mitigation of friction.
In violent motion, as soon as the agent stops causing it, the motion stops also; in other words, the natural state of an object is to be at rest, since Aristotle does not address friction.


wear resistanceabrasiondeterioration
Another important consequence of many types of friction can be wear, which may lead to performance degradation or damage to components.

Mu (letter)

The coefficient of friction (COF), often symbolized by the Greek letter µ, is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together.
It is also used as coefficient of friction.

Surface roughness

roughnessroughrough surface
The coefficient of static friction exhibited by a pair of contacting surfaces depends upon the combined effects of material deformation characteristics and surface roughness, both of which have their origins in the chemical bonding between atoms in each of the bulk materials and between the material surfaces and any adsorbed material.
In tribology, rough surfaces usually wear more quickly and have higher friction coefficients than smooth surfaces.


This ultralow-friction regime is called superlubricity.
Superlubricity is a regime of motion in which friction vanishes or very nearly vanishes.

Fundamental interaction

fundamental forcesfundamental forcefundamental interactions
Friction is not itself a fundamental force.
Electromagnetism has infinite range like gravity, but is vastly stronger than it, and therefore describes a number of macroscopic phenomena of everyday experience such as friction, rainbows, lightning, and all human-made devices using electric current, such as television, lasers, and computers.


Values outside this range are rarer, but teflon, for example, can have a coefficient as low as 0.04.
PTFE has one of the lowest coefficients of friction of any solid.

Asperity (materials science)

asperitiesasperityasperity geometry
John Leslie (1766–1832) noted a weakness in the views of Amontons and Coulomb: If friction arises from a weight being drawn up the inclined plane of successive asperities, why then isn't it balanced through descending the opposite slope?
The fractal dimension of these structures has been correlated with the contact mechanics exhibited at an interface in terms of friction and contact stiffness.


inertial massgravitational massweight
If an object is on a level surface and the force tending to cause it to slide is horizontal, the normal force N\, between the object and the surface is just its weight, which is equal to its mass multiplied by the acceleration due to earth's gravity, g.
All other forces, especially friction and air resistance, must be absent or at least negligible.

Free body diagram

free-body diagramforce diagramfree body
Therefore, the normal force, and ultimately the frictional force, is determined using vector analysis, usually via a free body diagram.
The external forces acting on the object include friction, gravity, normal force, drag, tension, or a human force due to pushing or pulling.


vehiclesvehicularroad vehicle
Most land vehicles rely on friction for acceleration, deceleration and changing direction.
With no power applied, most vehicles come to a stop due to friction.

Granular material

granulesgranulargranular flow
Despite being a simplified model of friction, the Coulomb model is useful in many numerical simulation applications such as multibody systems and granular material.
A granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact (the most common example would be friction when grains collide).