Friction


Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding or grinding against each other. Types of friction include dry, fluid, lubricated, skin, and internal – an incomplete list. The study of the processes involved is called tribology, and has a history of more than 2,000 years.
Friction can have dramatic consequences, as illustrated by the use of friction created by rubbing pieces of wood together to start a fire. Another important consequence of many types of friction can be wear, which may lead to performance degradation or damage to components. It is known that frictional energy losses account for about 20% of the total energy expenditure of the world.
There are many different contributors to the retarding force in friction, ranging from asperity deformation to the generation of charges and changes in local structure. When two bodies in contact move relative to each other, due to these various contributors some mechanical energy is transformed to heat, the free energy of structural changes, and other types of dissipation. The total dissipated energy per unit distance moved is the retarding frictional force, FE/d. The complexity of the interactions involved makes the calculation of friction from first principles difficult, and it is often easier to use empirical methods for analysis and the development of theory.

Types

There are several types of friction:
  • Dry friction is a force that opposes the relative lateral motion of two solid surfaces in contact. Dry friction is subdivided into static friction between non-moving surfaces, and kinetic friction between moving surfaces. With the exception of atomic or molecular friction, dry friction generally arises from the interaction of surface features, known as asperities.
  • Fluid friction describes the friction between layers of a viscous fluid that are moving relative to each other.
  • Lubricated friction is a case of fluid friction where a lubricant fluid separates two solid surfaces.
  • Skin friction is a component of drag, the force resisting the motion of a fluid across the surface of a body.
  • Internal friction is the force resisting motion between the elements making up a solid material while it undergoes deformation.

    History

Many ancient authors including Aristotle, Vitruvius, and Pliny the Elder, were interested in the cause and mitigation of friction. They were aware of differences between static and kinetic friction with Themistius stating in 350 that "it is easier to further the motion of a moving body than to move a body at rest".
The classic laws of sliding friction were discovered by Leonardo da Vinci in 1493, a pioneer in tribology, but the laws documented in his notebooks were not published and remained unknown. These laws were rediscovered by Guillaume Amontons in 1699 and became known as Amonton's three laws of dry friction. Amontons presented the nature of friction in terms of surface irregularities and the force required to raise the weight pressing the surfaces together. This view was further elaborated by Bernard Forest de Bélidor and Leonhard Euler, who derived the angle of repose of a weight on an inclined plane and first distinguished between static and kinetic friction.
John Theophilus Desaguliers first recognized the role of adhesion in friction. Microscopic forces cause surfaces to stick together; he proposed that friction was the force necessary to tear the adhering surfaces apart.
The understanding of friction was further developed by Charles-Augustin de Coulomb. Coulomb investigated the influence of four main factors on friction: the nature of the materials in contact and their surface coatings; the extent of the surface area; the normal pressure ; and the length of time that the surfaces remained in contact. Coulomb further considered the influence of sliding velocity, temperature and humidity, in order to decide between the different explanations on the nature of friction that had been proposed. The distinction between static and dynamic friction is made in Coulomb's friction law, although this distinction was already drawn by Johann Andreas von Segner in 1758.
The effect of the time of repose was explained by Pieter van Musschenbroek by considering the surfaces of fibrous materials, with fibers meshing together, which takes a finite time in which the friction increases.
John Leslie 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, then why is it not balanced through descending the opposite slope? Leslie was equally skeptical about the role of adhesion proposed by Desaguliers, which should on the whole have the same tendency to accelerate as to retard the motion. In Leslie's view, friction should be seen as a time-dependent process of flattening, pressing down asperities, which creates new obstacles in what were cavities before.
In the long course of the development of the law of conservation of energy and of the first law of thermodynamics, friction was recognised as a mode of conversion of mechanical work into heat. In 1798, Benjamin Thompson reported on cannon boring experiments.
Arthur Jules Morin developed the concept of sliding versus rolling friction.
In 1842, Julius Robert Mayer frictionally generated heat in paper pulp and measured the temperature rise. In 1845, Joule published a paper entitled "The Mechanical Equivalent of Heat", in which he specified a numerical value for the amount of mechanical work required to "produce a unit of heat", based on the friction of an electric current passing through a resistor, and on the friction of a paddle wheel rotating in a vat of water.
Osborne Reynolds derived the equation of viscous flow. This completed the classic empirical model of friction commonly used today in engineering. In 1877, Fleeming Jenkin and J. A. Ewing investigated the continuity between static and kinetic friction.
In 1907, G.H. Bryan published an investigation of the foundations of thermodynamics, Thermodynamics: an Introductory Treatise dealing mainly with First Principles and their Direct Applications. He noted that for a rough body driven over a rough surface, the mechanical work done by the driver exceeds the mechanical work received by the surface. The lost work is accounted for by heat generated by friction.
Over the years, for example in his 1879 thesis, but particularly in 1926, Planck advocated regarding the generation of heat by rubbing as the most specific way to define heat, and the prime example of an irreversible thermodynamic process.
The focus of research during the 20th century has been to understand the physical mechanisms behind friction. Frank Philip Bowden and David Tabor showed that, at a microscopic level, the actual area of contact between surfaces is a very small fraction of the apparent area. This actual area of contact, caused by asperities increases with pressure. The development of the atomic force microscope enabled scientists to study friction at the atomic scale, showing that, on that scale, dry friction is the product of the inter-surface shear stress and the contact area. These two discoveries explain Amonton's first law ; the macroscopic proportionality between normal force and static frictional force between dry surfaces.

Laws of dry friction

The elementary property of sliding friction were discovered by experiment in the 15th to 18th century and were expressed as three empirical laws:
  • Amontons' first law: The force of friction is directly proportional to the applied load.
  • Amontons' second law: The force of friction is independent of the apparent area of contact.
  • Coulomb's law of friction: Kinetic friction is independent of the sliding velocity.

    Dry friction

Dry friction resists relative lateral motion of two solid surfaces in contact. The two regimes of dry friction are 'static friction' between non-moving surfaces, and kinetic friction between moving surfaces.
Coulomb friction, named after Charles-Augustin de Coulomb, is an approximate model used to calculate the force of dry friction. It is governed by the model:
where
  • is the force of friction exerted by each surface on the other. It is parallel to the surface, in a direction opposite to the net applied force.
  • is the coefficient of friction, which is an empirical property of the contacting materials,
  • is the normal force exerted by each surface on the other, directed perpendicular to the surface.
The Coulomb friction may take any value from zero up to, 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 motion would commence. This maximum force is known as traction.
The force of friction is always exerted in a direction that opposes movement or potential movement 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 sliding between tire and road.

Normal force

The normal force is defined as the net force compressing two parallel surfaces together, and its direction is perpendicular to the surfaces. In the simple case of a mass resting on a horizontal surface, the only component of the normal force is the force due to gravity, where. In this case, conditions of equilibrium tell us that the magnitude of the friction force is zero,. In fact, the friction force always satisfies, with equality reached only at a critical ramp angle that is steep enough to initiate sliding.
The friction coefficient is an empirical structural property that depends only on various aspects of the contacting materials, such as surface roughness. The coefficient of friction is not a function of mass or volume. For instance, a large aluminum block has the same coefficient of friction as a small aluminum block. However, the magnitude of the friction force itself depends on the normal force, and hence on the mass of the block.
Depending on the situation, the calculation of the normal force might include forces other than gravity. If an object is on a and subjected to an external force tending to cause it to slide, then the normal force between the object and the surface is just, where is the block's weight and is the downward component of the external force. Prior to sliding, this friction force is, where is the horizontal component of the external force. Thus, in general. Sliding commences only after this frictional force reaches the value. Until then, friction is whatever it needs to be to provide equilibrium, so it can be treated as simply a reaction.
If the object is on a such as an inclined plane, the normal force from gravity is smaller than, because less of the force of gravity is perpendicular to the face of the plane. The normal force and the frictional force are ultimately determined using vector analysis, usually via a free body diagram.
In general, process for solving any statics problem with friction is to treat contacting surfaces tentatively as immovable so that the corresponding tangential reaction force between them can be calculated. If this frictional reaction force satisfies, then the tentative assumption was correct, and it is the actual frictional force. Otherwise, the friction force must be set equal to, and then the resulting force imbalance would then determine the acceleration associated with slipping.