Friction is the force resisting the relative motion of two surfaces in contact or a surface in contact with a fluid (e.g. air on an aircraft or water in a pipe). It is not a fundamental force, as it is derived from electromagnetic forces between atoms and electrons, and so cannot be calculated from first principles, but instead must be found imperically. When contacting surfaces move relative to each other, the friction between the two objects converts kinetic energy into thermal energy, or heat. Friction between solid objects is often referred to as dry friction or sliding friction and between a solid and a gas or liquid as fluid friction. Both of these types of friction are called kinetic friction. Contrary to popular credibility, sliding friction is not caused by surface roughness, but by chemical bonding between the surfaces.^[1] Surface roughness and contact area, however, do affect sliding friction for micro- and nano-scale objects where surface area forces dominate inertial forces.^[2] Internal friction is the motion- resisting force between the surfaces of the particles making up the substance. Friction may refer to: Friction, the force that opposes the relative motion or tendency of such motion of two surfaces in contact Friction, a song by Morcheeba from their 1998 album Big Calm Category: ... For other uses, see Force (disambiguation). ... An open surface with X-, Y-, and Z-contours shown. ... A fundamental interaction is a mechanism by which particles interact with each other, and which cannot be explained by another more fundamental interaction. ... In physics, the electromagnetic force is the force that the electromagnetic field exerts on electrically charged particles. ... For other uses, see Atom (disambiguation). ... For other uses, see Electron (disambiguation). ... In a formal logical system, that is, a set of propositions that are consistent with one another, it is probable that some of the statements can be deduced from one another. ... The cars of a roller coaster reach their maximum kinetic energy when at the bottom of their path. ... In thermal physics, thermal energy is the energy portion of a system that increases with its temperature. ... For other uses, see Heat (disambiguation) In physics, heat, symbolized by Q, is energy transferred from one body or system to another due to a difference in temperature. ... Sliding friction is when two objects are rubbing against each other. ...

Coulomb friction

One model of friction is called Coulomb friction after Charles-Augustin de Coulomb. It is described by the equation: Charles Augustin de Coulomb (born June 14, 1736, Angoulême, France - died August 23, 1806, Paris, France) was a French physicist. ...

F_f = μ_kF_n where

• F_f is either the force exerted by friction, or, in the case of equality, the maximum possible magnitude of this force.
• μ is the coefficient of friction, which is an empirical property of the contacting materials,
• F_n is the normal force exerted between the surfaces, and

For surfaces at rest relative to each other, μ is the coefficient of static friction (generally larger than its kinetic counterpart), the Coulomb friction may take any value from zero up to F_f, and the direction of the frictional force against a surface is opposite to the motion that surface would experience in the absence of friction. Thus, in the static case, the frictional force is exactly what it must be in order to prevent motion between the surfaces; it balances the net force tending to cause such motion. In this case, rather than providing an estimate of the actual frictional force, the Coulomb approximation provides a threshold value for this force, above which sliding would commence. The coefficient of friction is a dimensionless quantity used to calculate the force of friction (static or kinetic). ... Fn represents the normal force. ...

For surfaces in relative motion, μ is the coefficient of kinetic friction (see below), the Coulomb friction is equal to F_f, and the frictional force on each surface is exerted in the direction opposite to its motion relative to the other surface.

This approximation mathematically follows from the assumptions that surfaces are in atomically close contact only over a small fraction of their overall area, that this contact area is proportional to the normal force (until saturation, which takes place when all area is in atomic contact), and that frictional force is proportional to the applied normal force, independently of the contact area (you can see the experiments on friction from Leonardo Da Vinci). Such reasoning aside, however, the approximation is fundamentally an empirical construction. It is a rule of thumb describing the approximate outcome of an extremely complicated physical interaction. The strength of the approximation is its simplicity and versatility – though in general the relationship between normal force and frictional force is not exactly linear (and so the frictional force is not entirely independent of the contact area of the surfaces), the Coulomb approximation is an adequate representation of friction for the analysis of many physical systems. Please wikify (format) this article as suggested in the Guide to layout and the Manual of Style. ... Fn represents the normal force. ... Look up Saturation in Wiktionary, the free dictionary. ...

Coefficient of friction

Main article: Coefficient of friction

This section does not cite any references or sources. (February 2008) Please improve this section by adding citations to reliable sources. Unverifiable material may be challenged and removed.

The coefficient of friction (also known as the frictional coefficient) is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used; for example, ice on steel has a low coefficient of friction (the two materials slide past each other easily), while rubber on pavement has a high coefficient of friction (the materials do not slide past each other easily). Coefficients of friction range from near zero to greater than one–under good conditions, a tire on concrete may have a coefficient of friction of 1.7. The coefficient of friction is a dimensionless quantity used to calculate the force of friction (static or kinetic). ... Image File history File links Question_book-3. ... See scalar for an account of the broader concept also used in mathematics and computer science. ... For other uses, see Force (disambiguation). ...

When the surfaces are conjoined, Coulomb friction becomes a very poor approximation (for example, Scotch tape resists sliding even when there is no normal force, or a negative normal force). In this case, the frictional force may depend strongly on the area of contact. Some drag racing tires are adhesive in this way. Scotch Tape denotes the line of adhesive tapes manufactured by 3M as part of the companys Scotch brand. ... Top Fuel dragster Drag racing is a sport in which cars race down a track with a set distance as fast as possible. ...

The force of friction is always exerted in a direction that opposes movement (for kinetic friction) or potential movement (for static friction) between the two surfaces. For example, a curling stone sliding along the ice experiences a kinetic force slowing it down. For an example of potential movement, the drive wheels of an accelerating car experience a frictional force pointing forward; if they did not, the wheels would spin, and the rubber would slide backwards along the pavement. Note that it is not the direction of movement of the vehicle they oppose, it is the direction of (potential) sliding between tire and road. For other uses, see Curling (disambiguation). ...

The coefficient of friction is an empirical measurement–it has to be measured experimentally, and cannot be found through calculations. Rougher surfaces tend to have higher effective values. Most dry materials in combination have friction coefficient values between 0.3 and 0.6. Values outside this range are rarer, but Teflon, for example, can have a coefficient as low as 0.04. A value of zero would mean no friction at all, an elusive property–even Magnetic levitation vehicles have drag. Rubber in contact with other surfaces can yield friction coefficients from 1.0 to 2. A central concept in science and the scientific method is that all evidence must be empirical, or empirically based, that is, dependent on evidence or consequences that are observable by the senses. ... Measurement is the estimation of the magnitude of some attribute of an object, such as its length or weight, relative to a unit of measurement. ... In the scientific method, an experiment (Latin: ex- periri, of (or from) trying) is a set of observations performed in the context of solving a particular problem or question, to retain or falsify a hypothesis or research concerning phenomena. ... In chemistry, polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer which finds numerous applications. ... This article is about magnetic levitation. ... Transrapid Shanghai Maglev Train stopping at terminus Longyang Road station Transrapid Shanghai Maglev Train Inside the Shanghai Transrapid maglev Inside the Shanghai Transrapid maglev VIP section Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels vehicles (especially trains) using electromagnetic force. ... An object moving through a gas or liquid experiences a force in direction opposite to its motion. ...

Static friction

Static friction is the force between two objects that are not moving relative to each other. For example, static friction can prevent an object from sliding down a sloped surface. The coefficient of static friction, typically denoted as μ_s, is usually higher than the coefficient of kinetic friction. The initial force to get an object moving is often dominated by static friction.

Another important example of static friction is the force that prevents a car wheel from slipping as it rolls on the ground. Even though the wheel is in motion, the patch of the tire in contact with the ground is stationary relative to the ground, so it is static rather than kinetic friction.

The maximum value of static friction, when motion is impending, is sometimes referred to as limiting friction,^[3] although this term is not used universally.^[4] The value is given by the product of the normal force and coefficient of static friction.

Kinetic friction

Kinetic (or dynamic) friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as μ_k, and is usually less than the coefficient of static friction. Since friction is exerted in a direction that opposes movement, kinetic friction usually does negative work, typically slowing something down. There are exceptions, for instance if the surface itself is under acceleration. One can see this by placing a heavy box on a rug, then pulling on the rug quickly. In this case, the box slides backwards relative to the rug, but moves forward relative to the floor. Thus, the kinetic friction between the box and rug accelerates the box in the same direction that the box moves, doing positive work.

Examples of kinetic friction:

• Sliding friction (also called dry friction) is when two objects are rubbing against each other. Putting a book flat on a desk and moving it around is an example of sliding friction

• Fluid friction is the friction between a solid object as it moves through a liquid or a gas. The drag of air on an airplane or of water on a swimmer are two examples of fluid friction.

Sliding friction is when two objects are rubbing against each other. ... Fluid friction is the friction between a solid object and a fluid (a liquid or gas) it is moving through. ...

Other types of friction

Rolling resistance

Main article: Rolling resistance

Rolling resistance is the force that resists the rolling of a wheel or other circular objects along a surface. Generally the force of rolling resistance is less than that associated with kinetic friction.^[5] Typical values for the coefficient of rolling resistance are 0.001.^[6] One of the most common examples of rolling resistance is the movement of motor vehicle tires on a road, a process which generates heat and sound as by-products.^[7] Rolling resistance, sometimes called rolling friction, is the resistance that occurs when an object (e. ... Firestone tire This article is about pneumatic tires. ... For other uses, see Road (disambiguation). ... Roadway noise is the most prevalent form of environmental noise. ...

Triboelectric effect

Rubbing dissimilar materials against one another can cause a build-up of electrostatic charge, which can be hazardous if flammable gases or vapours are present. When the static build-up discharges, explosions can be caused by ignition of the flammable mixture. Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interactions. ...

Reducing friction

Devices

Devices such as tires, ball bearings, air cushion or roller bearing can change sliding friction into a much smaller type of rolling friction. Many thermoplastic materials such as nylon, HDPE and PTFE are commonly used for low friction bearings. They are especially useful because the coefficient of friction falls with increasing imposed load. Working principle for a ball bearing. ... For other uses, see Plastic (disambiguation). ... For other uses of this word, see nylon (disambiguation). ... Polyethylene or polyethene is one of the simplest and most inexpensive polymers. ... Teflon is the brand name of a polymer compound discovered by Roy J. Plunkett (1910-1994) of DuPont in 1938 and introduced as a commercial product in 1946. ...

Techniques

One technique used by railroad engineers is to back up the train to create slack in the linkages between cars. This allows the train engine to pull forward and only take on the static friction of one car at a time, instead of all cars at once, thus spreading the static frictional force out over time.

Lubricants

A common way to reduce friction is by using a lubricant, such as oil, water, or grease, which is placed between the two surfaces, often dramatically lessening the coefficient of friction. The science of friction and lubrication is called tribology. Lubricant technology is when lubricants are mixed with the application of science, especially to industrial or commercial objectives. A lubricant (colloquially, lube) is a substance (often a liquid) introduced between two moving surfaces to reduce the friction and wear between them. ... Tribology is the science and technology of interacting surfaces in relative motion. ...

Superlubricity, a recently-discovered effect, has been observed in graphite: it is the substantial decrease of friction between two sliding objects, approaching zero levels. A very small amount of frictional energy would still be dissipated. Superlubricity is a certain phenomenon in which friction can vanish almost completely. ... For other uses, see Graphite (disambiguation). ...

Lubricants to overcome friction need not always be thin, turbulent fluids or powdery solids such as graphite and talc; acoustic lubrication actually uses sound as a lubricant. Talc (derived from the Persian via Arabic talq) is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg3(SiO3)4 or Mg3Si4O10(OH)2. ... // Definition Acoustic/sonic lubrication occurs when sound (measurable in a vacuum by placing a microphone on one element of the sliding system) permits vibration to introduce separation between the sliding faces. ...

Energy of friction

According to the law of conservation of energy, no energy is destroyed due to friction, though it may be lost to the system of concern. Energy is transformed from other forms into heat. A sliding hockey puck comes to rest because friction converts its kinetic energy into heat. Since heat quickly dissipates, many early philosophers, including Aristotle, wrongly concluded that moving objects lose energy without a driving force. This article is about the law of conservation of energy in physics. ... For other uses, see Aristotle (disambiguation). ...

When an object is pushed along a surface, the energy converted to heat is given by:

where

F_n is the normal force,

μ_k is the coefficient of kinetic friction,

x is the coordinate along which the object transverses.

Physical deformation is associated with friction. While this can be beneficial, as in polishing, it is often a problem, as the materials are worn away, and may no longer hold the specified tolerances. Fn represents the normal force. ... Tolerance in Final Fantasy is an allowance, given as a permissible range, in the nominal dimension or value specification of a manufactured object. ...

The work done by friction can translate into deformation, wear, and heat that can affect the contact surface's material properties (and even the coefficient of friction itself). The work done by friction can also be used to mix materials such as in the technique of friction welding. Friction Welding (FW) is a group of solid-state welding processes using heat generated through mechanical friction between a moving workpiece, with the addition of an upsetting force to plastically displace material. ...

See also

• Guillaume Amontons
• Rolling resistance
• Tire
• Tribology
• Triboelectric effect
• Traction (engineering)
• Sliding friction

Guillaume Amontons (August 31, 1663 - October 11, 1705) was a French instrument inventor and physicist. ... Rolling resistance, sometimes called rolling friction, is the resistance that occurs when an object (e. ... Firestone tire This article is about pneumatic tires. ... Tribology is the science and technology of interacting surfaces in relative motion. ... The triboelectric effect is a type of contact electrification in which certain materials become electrically charged after they come into contact with another different material and are then separated (such as through rubbing). ... Traction usually refers to friction between a drive member and the surface it runs on, where friction is used to provide motion. ... Sliding friction is when two objects are rubbing against each other. ...

References

Year 2007 (MMVII) was a common year starting on Monday of the Gregorian calendar in the 21st century. ... is the 294th day of the year (295th in leap years) in the Gregorian calendar. ...

External links

• Coefficients of Friction - tables of coefficients, plus many links
• Physclips: Mechanics with animations and video clips from the University of New South Wales