Friction is the force resisting the relative motion of two surfaces in contact or a surface in contact with a fluid (e. In Physics, a force is whatever can cause an object with Mass to Accelerate. In Mathematics, specifically in Topology, a surface is a Two-dimensional Manifold. 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 empirically. In Physics, a fundamental interaction or fundamental force is a mechanism by which particles interact with each other and which cannot be explained in terms In Physics, the electromagnetic force is the force that the Electromagnetic field exerts on electrically charged particles History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J First Principles is also the title of a work by Herbert Spencer. When contacting surfaces move relative to each other, the friction between the two objects converts kinetic energy into thermal energy, or heat. The kinetic energy of an object is the extra Energy which it possesses due to its motion Thermal energy is the sum of the sensible energy and latent energy. In Physics, heat, symbolized by Q, is Energy transferred from one body or system to another due to a difference in Temperature 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.
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Coulomb friction
One model of friction is called Coulomb friction after Charles-Augustin de Coulomb. Charles Augustin de Cock (June 14 1736 Angoulême France – August 23 1806 Penis France was a French Physicist. It is described by the equation:
- Ff = μkFn where
- Ff 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,
- Fn 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 Ff, and the direction of the frictional force against a surface is opposite to the motion that surface would experience in the absence of friction. The coefficient of friction is a Dimensionless quantity symbolized by the Greek letter μ and is used to approximate the Force of Friction. In Physics, the normal force F_n\ (or in some books N) is the component perpendicular to the surface of contact of the Contact force 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.
For surfaces in relative motion, μ is the coefficient of kinetic friction (see below), the Coulomb friction is equal to Ff, 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). When two objects touch a certain portion of their surface areas will be in contact with each other In Physics, the normal force F_n\ (or in some books N) is the component perpendicular to the surface of contact of the Contact force 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.
Coefficient of friction
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 is a Dimensionless quantity symbolized by the Greek letter μ and is used to approximate the Force of Friction. In Physics, a scalar is a simple Physical quantity that is not changed by Coordinate system rotations or translations (in Newtonian mechanics or In Physics, a force is whatever can cause an object with Mass to Accelerate. 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.
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). Scotch Tape is used to describe certain Pressure sensitive tapes manufactured by 3M as part of the company's Scotch brand In this case, the frictional force may depend strongly on the area of contact. Some drag racing tires are adhesive in this way. Drag racing is a competition in which objects compete to be the first to cross a set finish mark usually from a dead stop and in a straight line
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. Curling is a team Sport with similarities to Bowls and Shuffle board, played by two teams of four players each on a rectangular sheet of carefully prepared 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.
The coefficient of friction is an empirical measurement – it has to be measured experimentally, and cannot be found through calculations. A central concept in Science and the Scientific method is that all Evidence must be empirical, or empirically based that is dependent on evidence Measurement is the process of estimating the magnitude of some attribute of an object such as its length or weight relative to some standard ( unit of measurement) such as In scientific inquiry an experiment ( Latin: Ex- periri, "to try out" is a method of investigating particular types of research questions or 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. In Chemistry, poly(tetrafluoroethene or poly(tetrafluoroethylene ( PTFE) is a synthetic Fluoropolymer which finds numerous applications 04. A value of zero would mean no friction at all, an elusive property – even Magnetic levitation vehicles have [[Drag (physics)|drag]. Magnetic levitation, maglev, or magnetic suspension is a method by which an object is suspended with no support other than Magnetic fields The MAGLEV, or magnetic levitation, is a system of transportation that suspends guides and (usually propels vehicles predominantly trains using magnetic forces Rubber in contact with other surfaces can yield friction coefficients from 1. 0 to 2.
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 μs, 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 μk, 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. In Fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a Solid object through a Fluid (a The drag of air on an airplane or of water on a swimmer are two examples of fluid friction.
Other types of friction
Rolling resistance
Rolling resistance is the force that resists the rolling of a wheel or other circular objects along a surface. Rolling resistance, sometimes called rolling friction or rolling drag, is the resistance that occurs when a round object such as a ball or Tire 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. A motor Vehicle is a Machine which incorporates a motor (sometimes known as an Engine) and which is used for Transportation This article is about tires used on road Vehicles including pneumatic tires and solid tires. A road is an identifiable route, way or path between two or more places. Roadway noise is the collective sound energy emanating from Motor vehicles In the USA it contributes more to environmental noise exposure than any other noise source and is [7]
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. Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. When the static build-up discharges, explosions can be caused by ignition of the flammable mixture. An explosion is a sudden increase in Volume and release of Energy in an extreme manner usually with the generation of high Temperatures and the release
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. A ball bearing is an engineering term referring to a type of Rolling-element bearing which uses Balls to maintain the separation between the moving parts Many thermoplastic materials such as nylon, HDPE and PTFE are commonly used for low friction bearings. A thermoplastic is a Plastic that Melts to a liquid when heated and freezes to a Brittle, very Glassy state when cooled sufficiently Overview Nylon is a Thermoplastic silky material first used commercially in a nylon- Bristled Toothbrush (1938 followed more famously by High-Density Polyethylene ( HDPE) or PolyEthylene High-Density ( PEHD) is a Polyethylene Thermoplastic made from Petroleum In Chemistry, poly(tetrafluoroethene or poly(tetrafluoroethylene ( PTFE) is a synthetic Fluoropolymer which finds numerous applications They are especially useful because the coefficient of friction falls with increasing imposed load.
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. A lubricant (sometimes referred to as a "Lube" is a substance (often a liquid introduced between two moving surfaces to reduce the Friction between them improving The science of friction and lubrication is called tribology. Tribology is the science and technology of interacting surfaces in relative motion. Lubricant technology is when lubricants are mixed with the application of science, especially to industrial or commercial objectives.
Superlubricity, a recently-discovered effect, has been observed in graphite: it is the substantial decrease of friction between two sliding objects, approaching zero levels. Superlubricity is a Regime of motion in which Friction vanishes or very nearly vanishes The Mineral graphite, as with Diamond and Fullerene, is one of the Allotropes of carbon. A very small amount of frictional energy would still be dissipated.
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 Acoustic or 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
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. In Physics, the law of conservation of energy states that the total amount of Energy in an isolated system remains constant and cannot be created although it may 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. Aristotle (Greek Aristotélēs) (384 BC – 322 BC was a Greek philosopher a student of Plato and teacher of Alexander the Great.
When an object is pushed along a surface, the energy converted to heat is given by:
where
- Fn is the normal force,
- μk is the coefficient of kinetic friction,
- x is the coordinate along which the object transverses. In Physics, the normal force F_n\ (or in some books N) is the component perpendicular to the surface of contact of the Contact force
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. Polishing is the process of creating a smooth and shiny surface by using rubbing or a chemical action leaving a surface with significant Specular reflection and minimal Engineering tolerance is the permissible limit of variation in a physical Dimension, a measured value or Physical property of a
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 class of solid-state Welding processes that generates heat through mechanical friction between a moving workpiece and a stationary component with
See also
- Guillaume Amontons
- Rolling resistance
- Tire
- Tribology
- Triboelectric effect
- Traction (engineering)
- Sliding friction
References
- ^ Beatty, William J. Rolling resistance, sometimes called rolling friction or rolling drag, is the resistance that occurs when a round object such as a ball or Tire This article is about tires used on road Vehicles including pneumatic tires and solid tires. Tribology is the science and technology of interacting surfaces in relative motion. Triboelectric Series Materials are often listed in order of the polarity of charge separation when they are touched with another object Traction refers to the Friction between a drive member and the surface it moves upon where the friction is used to provide motion "Recurring science misconceptions in K-6 textbooks". Retrieved 2007-06-08.
- ^ Persson, Bo N. J. , Sliding Friction
- ^ Engineering Mechanics Chapter V. Retrieved on 2007-10-21. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1512 - Martin Luther joins the theological faculty of the University of Wittenberg.
- ^ Beer and Johnston (1996). Vector Mechanics for Engineers, Sixth Edition, McGraw-Hill, 397–400. ISBN 0-07-005367-7.
- ^ Benjamin Silliman, Principles of Physics, Or Natural Philosophy, Ivison, Blakeman, Taylor & company publishers, 710 pages {1871)
- ^ Hans-Jürgen Butt, Karlheinz Graf, Michael Kappl, Physics and Chemistry of Interfaces, Wiley Pubishers, 373 pages, ISBN 3527404139 (2006)
- ^ [1] C. Michael Hogan, Analysis of Highway Noise, Journal of Soil, Air and Water Pollution, Springer Verlag Publishers, Netherlands, Volume 2, Number 3 / September, 1973
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
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