PFC supply professional and honest service.
Friction between two objects. The blue one has more friction against the sloped surface than the green one. Simulated blocks with fractal rough surfaces, exhibiting static frictional interactions[1]
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.[2] Types of friction include dry, fluid, lubricated, skin, and internal.
Friction can have dramatic consequences, as illustrated by the use of friction created by rubbing pieces of wood together to start a fire. Kinetic energy is converted to thermal energy whenever motion with friction occurs, for example when a viscous fluid is stirred. Another important consequence of many types of friction can be wear, which may lead to performance degradation or damage to components.
Friction is a non-conservative force &#; work done against friction is path dependent. In the presence of friction, some kinetic energy is always transformed to thermal energy, so mechanical energy is not conserved. Friction is not itself a fundamental force. Dry friction arises from a combination of inter-surface adhesion, surface roughness, surface deformation, and surface contamination. The complexity of these interactions 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.
There are several types of friction:
Many ancient authors including Aristotle, Vitruvius, and Pliny the Elder, were interested in the cause and mitigation of friction.[8] They were aware of differences between static and kinetic friction with Themistius stating in 350 A.D. that "it is easier to further the motion of a moving body than to move a body at rest".[8][9][10][11]
The classic laws of sliding friction were discovered by Leonardo da Vinci in , a pioneer in tribology, but the laws documented in his notebooks were not published and remained unknown.[12][13][14][15][16][17] These laws were rediscovered by Guillaume Amontons in [18] 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[19] and Leonhard Euler (), who derived the angle of repose of a weight on an inclined plane and first distinguished between static and kinetic friction.[20] John Theophilus Desaguliers () first recognized the role of adhesion in friction.[21] 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 ().[18] 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 (or load); and the length of time that the surfaces remained in contact (time of repose).[12] 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 (see below), although this distinction was already drawn by Johann Andreas von Segner in .[12] 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, why then isn't it 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.[12] 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.
Heat by friction captured by a thermal cameraIn 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 , Benjamin Thompson reported on cannon boring experiments.[22]
Arthur Jules Morin () developed the concept of sliding versus rolling friction.
In , Julius Robert Mayer frictionally generated heat in paper pulp and measured the temperature rise.[23] In , 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.[24]
Osborne Reynolds () derived the equation of viscous flow. This completed the classic empirical model of friction (static, kinetic, and fluid) commonly used today in engineering.[13] In , Fleeming Jenkin and J. A. Ewing investigated the continuity between static and kinetic friction.[25]
In , 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 driven hard surface sliding on a body driven by it, the work done by the driver exceeds the work received by the body. The difference is accounted for by heat generated by friction.[26] Over the years, for example in his thesis, but particularly in , 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.[27]
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.[14] This actual area of contact, caused by asperities increases with pressure. The development of the atomic force microscope (ca. ) enabled scientists to study friction at the atomic scale,[13] 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 (below); the macroscopic proportionality between normal force and static frictional force between dry surfaces.
The elementary property of sliding (kinetic) friction were discovered by experiment in the 15th to 18th centuries and were expressed as three empirical laws:
Dry friction resists relative lateral motion of two solid surfaces in contact. 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.
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: F f &#; μ F n , {\displaystyle F_{\mathrm {f} }\leq \mu F_{\mathrm {n} },} where
F f {\displaystyle F_{\mathrm {f} }}
μ {\displaystyle \mu }
F n {\displaystyle F_{\mathrm {n} }}
normal force exerted by each surface on the other, directed perpendicular (normal) to the surface.The Coulomb friction F f {\displaystyle F_{\mathrm {f} }} may take any value from zero up to μ F n {\displaystyle \mu F_{\mathrm {n} }} , 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 (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.
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 N = m g {\displaystyle N=mg\,} . In this case, conditions of equilibrium tell us that the magnitude of the friction force is zero, F f = 0 {\displaystyle F_{f}=0} . In fact, the friction force always satisfies F f &#; μ N {\displaystyle F_{f}\leq \mu N} , with equality reached only at a critical ramp angle (given by tan &#; 1 &#; μ {\displaystyle \tan ^{-1}\mu } ) that is steep enough to initiate sliding.
The friction coefficient is an empirical (experimentally measured) 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 N {\displaystyle N} might include forces other than gravity. If an object is on a level surface and subjected to an external force P {\displaystyle P} tending to cause it to slide, then the normal force between the object and the surface is just N = m g + P y {\displaystyle N=mg+P_{y}} , where m g {\displaystyle mg} is the block's weight and P y {\displaystyle P_{y}} is the downward component of the external force. Prior to sliding, this friction force is F f = &#; P x {\displaystyle F_{f}=-P_{x}} , where P x {\displaystyle P_{x}} is the horizontal component of the external force. Thus, F f &#; μ N {\displaystyle F_{f}\leq \mu N} in general. Sliding commences only after this frictional force reaches the value F f = μ N {\displaystyle F_{f}=\mu N} . 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 tilted surface such as an inclined plane, the normal force from gravity is smaller than m g {\displaystyle mg} , 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 F f &#; μ N {\displaystyle F_{f}\leq \mu N} , then the tentative assumption was correct, and it is the actual frictional force. Otherwise, the friction force must be set equal to F f = μ N {\displaystyle F_{f}=\mu N} , and then the resulting force imbalance would then determine the acceleration associated with slipping.
The coefficient of friction (COF), often symbolized by the Greek letter μ, is a dimensionless scalar value which equals the ratio of the force of friction between two bodies and the force pressing them together, either during or at the onset of slipping. The coefficient of friction depends on the materials used; for example, ice on steel has a low coefficient of friction, while rubber on pavement has a high coefficient of friction. Coefficients of friction range from near zero to greater than one. The coefficient of friction between two surfaces of similar metals is greater than that between two surfaces of different metals; for example, brass has a higher coefficient of friction when moved against brass, but less if moved against steel or aluminum.[28]
For surfaces at rest relative to each other, μ = μ s {\displaystyle \mu =\mu _{\mathrm {s} }} , where μ s {\displaystyle \mu _{\mathrm {s} }} is the coefficient of static friction. This is usually larger than its kinetic counterpart. 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. The fractality of surfaces, a parameter describing the scaling behavior of surface asperities, is known to play an important role in determining the magnitude of the static friction.[1]
For surfaces in relative motion μ = μ k {\displaystyle \mu =\mu _{\mathrm {k} }} , where μ k {\displaystyle \mu _{\mathrm {k} }} is the coefficient of kinetic friction. The Coulomb friction is equal to F f {\displaystyle F_{\mathrm {f} }} , and the frictional force on each surface is exerted in the direction opposite to its motion relative to the other surface.
Arthur Morin introduced the term and demonstrated the utility of the coefficient of friction.[12] The coefficient of friction is an empirical measurement&#;&#;&#;it has to be measured experimentally, and cannot be found through calculations.[29] Rougher surfaces tend to have higher effective values. Both static and kinetic coefficients of friction depend on the pair of surfaces in contact; for a given pair of surfaces, the coefficient of static friction is usually larger than that of kinetic friction; in some sets the two coefficients are equal, such as teflon-on-teflon.
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. Rubber in contact with other surfaces can yield friction coefficients from 1 to 2. Occasionally it is maintained that μ is always < 1, but this is not true. While in most relevant applications μ < 1, a value above 1 merely implies that the force required to slide an object along the surface is greater than the normal force of the surface on the object. For example, silicone rubber or acrylic rubber-coated surfaces have a coefficient of friction that can be substantially larger than 1.
While it is often stated that the COF is a "material property," it is better categorized as a "system property." Unlike true material properties (such as conductivity, dielectric constant, yield strength), the COF for any two materials depends on system variables like temperature, velocity, atmosphere and also what are now popularly described as aging and deaging times; as well as on geometric properties of the interface between the materials, namely surface structure.[1] For example, a copper pin sliding against a thick copper plate can have a COF that varies from 0.6 at low speeds (metal sliding against metal) to below 0.2 at high speeds when the copper surface begins to melt due to frictional heating. The latter speed, of course, does not determine the COF uniquely; if the pin diameter is increased so that the frictional heating is removed rapidly, the temperature drops, the pin remains solid and the COF rises to that of a 'low speed' test.[citation needed]
In systems with significant non-uniform stress fields, because local slip occurs before the system slides, the macroscopic coefficient of static friction depends on the applied load, system size, or shape; Amontons' law is not satisfied macroscopically.[30]
Under certain conditions some materials have very low friction coefficients. An example is (highly ordered pyrolytic) graphite which can have a friction coefficient below 0.01.[40] This ultralow-friction regime is called superlubricity.
Static friction is friction between two or more solid 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. Static friction is considered to arise as the result of surface roughness features across multiple length scales at solid surfaces. These features, known as asperities are present down to nano-scale dimensions and result in true solid to solid contact existing only at a limited number of points accounting for only a fraction of the apparent or nominal contact area.[41] The linearity between applied load and true contact area, arising from asperity deformation, gives rise to the linearity between static frictional force and normal force, found for typical Amonton&#;Coulomb type friction.[42]
The static friction force must be overcome by an applied force before an object can move. The maximum possible friction force between two surfaces before sliding begins is the product of the coefficient of static friction and the normal force: F max = μ s F n {\displaystyle F_{\text{max}}=\mu _{\mathrm {s} }F_{\text{n}}} . When there is no sliding occurring, the friction force can have any value from zero up to F max {\displaystyle F_{\text{max}}} . Any force smaller than F max {\displaystyle F_{\text{max}}} attempting to slide one surface over the other is opposed by a frictional force of equal magnitude and opposite direction. Any force larger than F max {\displaystyle F_{\text{max}}} overcomes the force of static friction and causes sliding to occur. The instant sliding occurs, static friction is no longer applicable&#;the friction between the two surfaces is then called kinetic friction. However, an apparent static friction can be observed even in the case when the true static friction is zero.[43]
An 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. Upon slipping, the wheel friction changes to kinetic friction. An anti-lock braking system operates on the principle of allowing a locked wheel to resume rotating so that the car maintains static friction.
The maximum value of static friction, when motion is impending, is sometimes referred to as limiting friction,[44] although this term is not used universally.[3]
Kinetic friction, also known as dynamic friction or sliding 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 for the same materials.[45][46] However, Richard Feynman comments that "with dry metals it is very hard to show any difference."[47] The friction force between two surfaces after sliding begins is the product of the coefficient of kinetic friction and the normal force: F k = μ k F n {\displaystyle F_{k}=\mu _{\mathrm {k} }F_{n}} . This is responsible for the Coulomb damping of an oscillating or vibrating system.
New models are beginning to show how kinetic friction can be greater than static friction.[48] In many other cases roughness effects are dominant, for example in rubber to road friction.[48] Surface roughness and contact area affect kinetic friction for micro- and nano-scale objects where surface area forces dominate inertial forces.[49]
The origin of kinetic friction at nanoscale can be rationalized by an energy model.[50] During sliding, a new surface forms at the back of a sliding true contact, and existing surface disappears at the front of it. Since all surfaces involve the thermodynamic surface energy, work must be spent in creating the new surface, and energy is released as heat in removing the surface. Thus, a force is required to move the back of the contact, and frictional heat is released at the front.
Angle of friction, θ, when block just starts to slide.For the maximum angle of static friction between granular materials, see Angle of repose
For certain applications, it is more useful to define static friction in terms of the maximum angle before which one of the items will begin sliding. This is called the angle of friction or friction angle. It is defined as: tan &#; θ = μ s {\displaystyle \tan {\theta }=\mu _{\mathrm {s} }} and thus: θ = arctan &#; μ s {\displaystyle \theta =\arctan {\mu _{\mathrm {s} }}} where θ {\displaystyle \theta } is the angle from horizontal and μs is the static coefficient of friction between the objects.[51] This formula can also be used to calculate μs from empirical measurements of the friction angle.
Determining the forces required to move atoms past each other is a challenge in designing nanomachines. In scientists for the first time were able to move a single atom across a surface, and measure the forces required. Using ultrahigh vacuum and nearly zero temperature (5 K), a modified atomic force microscope was used to drag a cobalt atom, and a carbon monoxide molecule, across surfaces of copper and platinum.[52]
The Coulomb approximation 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 the frictional force is proportional to the applied normal force, independently of the contact area. The Coulomb approximation is fundamentally an empirical construct. 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 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.
When the surfaces are conjoined, Coulomb friction becomes a very poor approximation (for example, adhesive 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 for this reason. However, despite the complexity of the fundamental physics behind friction, the relationships are accurate enough to be useful in many applications.
"Negative" coefficient of frictionAs of , a single study has demonstrated the potential for an effectively negative coefficient of friction in the low-load regime, meaning that a decrease in normal force leads to an increase in friction. This contradicts everyday experience in which an increase in normal force leads to an increase in friction.[53] This was reported in the journal Nature in October and involved the friction encountered by an atomic force microscope stylus when dragged across a graphene sheet in the presence of graphene-adsorbed oxygen.[53]
Despite being a simplified model of friction, the Coulomb model is useful in many numerical simulation applications such as multibody systems and granular material. Even its most simple expression encapsulates the fundamental effects of sticking and sliding which are required in many applied cases, although specific algorithms have to be designed in order to efficiently numerically integrate mechanical systems with Coulomb friction and bilateral or unilateral contact.[54][55][56][57][58] Some quite nonlinear effects, such as the so-called Painlevé paradoxes, may be encountered with Coulomb friction.[59]
Dry friction can induce several types of instabilities in mechanical systems which display a stable behaviour in the absence of friction.[60] These instabilities may be caused by the decrease of the friction force with an increasing velocity of sliding, by material expansion due to heat generation during friction (the thermo-elastic instabilities), or by pure dynamic effects of sliding of two elastic materials (the Adams&#;Martins instabilities). The latter were originally discovered in by George G. Adams and João Arménio Correia Martins for smooth surfaces[61][62] and were later found in periodic rough surfaces.[63] In particular, friction-related dynamical instabilities are thought to be responsible for brake squeal and the 'song' of a glass harp,[64][65] phenomena which involve stick and slip, modelled as a drop of friction coefficient with velocity.[66]
A practically important case is the self-oscillation of the strings of bowed instruments such as the violin, cello, hurdy-gurdy, erhu, etc.
A connection between dry friction and flutter instability in a simple mechanical system has been discovered,[67] watch the movie Archived -01-10 at the Wayback Machine for more details.
Frictional instabilities can lead to the formation of new self-organized patterns (or "secondary structures") at the sliding interface, such as in-situ formed tribofilms which are utilized for the reduction of friction and wear in so-called self-lubricating materials.[68]
Fluid friction occurs between fluid layers that are moving relative to each other. This internal resistance to flow is named viscosity. In everyday terms, the viscosity of a fluid is described as its "thickness". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. The less viscous the fluid, the greater its ease of deformation or movement.
All real fluids (except superfluids) offer some resistance to shearing and therefore are viscous. For teaching and explanatory purposes it is helpful to use the concept of an inviscid fluid or an ideal fluid which offers no resistance to shearing and so is not viscous.
Lubricated friction is a case of fluid friction where a fluid separates two solid surfaces. Lubrication is a technique employed to reduce wear of one or both surfaces in close proximity moving relative to each another by interposing a substance called a lubricant between the surfaces.
In most cases the applied load is carried by pressure generated within the fluid due to the frictional viscous resistance to motion of the lubricating fluid between the surfaces. Adequate lubrication allows smooth continuous operation of equipment, with only mild wear, and without excessive stresses or seizures at bearings. When lubrication breaks down, metal or other components can rub destructively over each other, causing heat and possibly damage or failure.
Skin friction arises from the interaction between the fluid and the skin of the body, and is directly related to the area of the surface of the body that is in contact with the fluid. Skin friction follows the drag equation and rises with the square of the velocity.
Skin friction is caused by viscous drag in the boundary layer around the object. There are two ways to decrease skin friction: the first is to shape the moving body so that smooth flow is possible, like an airfoil. The second method is to decrease the length and cross-section of the moving object as much as is practicable.
If you are looking for more details, kindly visit constant friction.
Internal friction is the force resisting motion between the elements making up a solid material while it undergoes deformation.
Plastic deformation in solids is an irreversible change in the internal molecular structure of an object. This change may be due to either (or both) an applied force or a change in temperature. The change of an object's shape is called strain. The force causing it is called stress.
Elastic deformation in solids is reversible change in the internal molecular structure of an object. Stress does not necessarily cause permanent change. As deformation occurs, internal forces oppose the applied force. If the applied stress is not too large these opposing forces may completely resist the applied force, allowing the object to assume a new equilibrium state and to return to its original shape when the force is removed. This is known as elastic deformation or elasticity.
As a consequence of light pressure, Einstein[69] in predicted the existence of "radiation friction" which would oppose the movement of matter. He wrote, "radiation will exert pressure on both sides of the plate. The forces of pressure exerted on the two sides are equal if the plate is at rest. However, if it is in motion, more radiation will be reflected on the surface that is ahead during the motion (front surface) than on the back surface. The backward-acting force of pressure exerted on the front surface is thus larger than the force of pressure acting on the back. Hence, as the resultant of the two forces, there remains a force that counteracts the motion of the plate and that increases with the velocity of the plate. We will call this resultant 'radiation friction' in brief."
Rolling resistance is the force that resists the rolling of a wheel or other circular object along a surface caused by deformations in the object or surface. Generally the force of rolling resistance is less than that associated with kinetic friction.[70] Typical values for the coefficient of rolling resistance are 0.001.[71] 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.[72]
Any wheel equipped with a brake is capable of generating a large retarding force, usually for the purpose of slowing and stopping a vehicle or piece of rotating machinery. Braking friction differs from rolling friction because the coefficient of friction for rolling friction is small whereas the coefficient of friction for braking friction is designed to be large by choice of materials for brake pads.
Rubbing two materials against each other can lead to charge transfer, either electrons or ions. The energy required for this contributes to the friction. In addition, sliding 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.
Belt friction is a physical property observed from the forces acting on a belt wrapped around a pulley, when one end is being pulled. The resulting tension, which acts on both ends of the belt, can be modeled by the belt friction equation.
In practice, the theoretical tension acting on the belt or rope calculated by the belt friction equation can be compared to the maximum tension the belt can support. This helps a designer of such a rig to know how many times the belt or rope must be wrapped around the pulley to prevent it from slipping. Mountain climbers and sailing crews demonstrate a standard knowledge of belt friction when accomplishing basic tasks.
Devices such as wheels, ball bearings, roller bearings, and air cushion or other types of fluid bearings can change sliding friction into a much smaller type of rolling friction.
Many thermoplastic materials such as nylon, HDPE and PTFE are commonly used in low friction bearings. They are especially useful because the coefficient of friction falls with increasing imposed load.[73] For improved wear resistance, very high molecular weight grades are usually specified for heavy duty or critical bearings.
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.
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.
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.
Another way to reduce friction between two parts is to superimpose micro-scale vibration to one of the parts. This can be sinusoidal vibration as used in ultrasound-assisted cutting or vibration noise, known as dither.
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 thermal energy. A sliding hockey puck comes to rest because friction converts its kinetic energy into heat which raises the thermal energy of the puck and the ice surface. Since heat quickly dissipates, many early philosophers, including Aristotle, wrongly concluded that moving objects lose energy without a driving force.[citation needed]
When an object is pushed along a surface along a path C, the energy converted to heat is given by a line integral, in accordance with the definition of work
E t h = &#; C F f r i c ( x ) &#; d x = &#; C μ k F n ( x ) &#; d x , {\displaystyle E_{th}=\int _{C}\mathbf {F} _{\mathrm {fric} }(\mathbf {x} )\cdot d\mathbf {x} \ =\int _{C}\mu _{\mathrm {k} }\ \mathbf {F} _{\mathrm {n} }(\mathbf {x} )\cdot d\mathbf {x} ,}
where
F f r i c {\displaystyle \mathbf {F} _{\mathrm {fric} }}
F n {\displaystyle \mathbf {F} _{\mathrm {n} }}
against the object's motion,μ k {\displaystyle \mu _{\mathrm {k} }}
x {\displaystyle \mathbf {x} }
Dissipation of energy by friction in a process is a classic example of thermodynamic irreversibility.[27]
The work done by friction can translate into deformation, wear, and heat that can affect the contact surface properties (even the coefficient of friction between the surfaces). This can be beneficial as in polishing. The work of friction is used to mix and join materials such as in the process of friction welding. Excessive erosion or wear of mating sliding surfaces occurs when work due to frictional forces rise to unacceptable levels. Harder corrosion particles caught between mating surfaces in relative motion (fretting) exacerbates wear of frictional forces. As surfaces are worn by work due to friction, fit and surface finish of an object may degrade until it no longer functions properly.[74] For example, bearing seizure or failure may result from excessive wear due to work of friction.
In the reference frame of the interface between two surfaces, static friction does no work, because there is never displacement between the surfaces. In the same reference frame, kinetic friction is always in the direction opposite the motion, and does negative work.[75] However, friction can do positive work in certain frames of reference. 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 frame of reference in which the floor is stationary. Thus, the kinetic friction between the box and rug accelerates the box in the same direction that the box moves, doing positive work.[76]
When sliding takes place between two rough bodies in contact, the algebraic sum of the works done is different from zero, and the algebraic sum of the quantities of heat gained by the two bodies is equal to the quantity of work lost by friction, and the total quantity of heat gained is positive.[77][78] In a natural thermodynamic process, the work done by an agency in the surroundings of a thermodynamic system or working body is greater than the work received by the body, because of friction. Thermodynamic work is measured by changes in a body's state variables, sometimes called work-like variables, other than temperature and entropy. Examples of work-like variables, which are ordinary macroscopic physical variables and which occur in conjugate pairs, are pressure &#; volume, and electric field &#; electric polarization. Temperature and entropy are a specifically thermodynamic conjugate pair of state variables. They can be affected microscopically at an atomic level, by mechanisms such as friction, thermal conduction, and radiation. The part of the work done by an agency in the surroundings that does not change the volume of the working body but is dissipated in friction, is called isochoric work. It is received as heat, by the working body and sometimes partly by a body in the surroundings. It is not counted as thermodynamic work received by the working body.
Friction is an important factor in many engineering disciplines.
Sachin Rekhi
·
Follow
10 min read
·
Jul 6,
--
As product designers we spend a lot of time trying to understand user friction and solve for it in the products we build. Doing so is absolutely critical to delivering delightful experiences for our users. I find though that sometimes teams are only perceiving and solving the most basic forms of user friction and aren&#;t taking on some of the harder to perceive yet incredibly important higher level forms of friction that users are experiencing. So I wanted to share how I think about the hierarchy of user friction and provide examples and best practices for solving for each.
User friction is really anything that prevents a user from accomplishing a goal in your product. I categorize user friction into a hierarchy of three levels: interaction friction, cognitive friction, and emotional friction. Interaction friction is what I hear talked about most often amongst product designers, but the higher levels of cognitive friction and emotional friction are equally important to solve for to build a great user experience.
Interaction Friction
Interaction friction refers to friction a user experiences when interacting with your product&#;s interface. It covers all aspects of the UI that may be hindering your users from accomplishing their goal. We strive to build intuitive and consistent interfaces to prevent interaction friction. We ensure our call-to-actions are prominent, we reduce the number of steps or fields in our forms, we leverage style guide lines to ensure we have a consistent UX across all of our experiences, and we try to do the work for the user so they don&#;t have to whenever we can. These are just a few of the techniques that are commonly used to address interaction friction.
The canonical example of significantly reducing interaction friction remains the iPhone. Everyone knows that smartphones existed before the iPhone. We remember the days of the Blackberry or Windows Mobile devices that were technically and web capable devices with a keyboard. And yet the iPhone changed everything. It was the first device to have a highly accurate on-screen keyboard thanks to their innovation in capacitive multi-touch screens. This freed up significant real estate on the screen when you weren&#;t typing. They packed enough CPU power on the device to support a full web browser instead of the extremely limited web experiences that existed on smart phones before it. And most importantly, they made the entire software experience extremely intuitive, beautiful, and fast. And these improvements to the overall UX significantly reduced the interaction friction of every prior generation of smartphones and laid the groundwork for this new class of smartphone to become the predominant mobile device that everyone now carries around in their pockets.
While the iPhone example illustrates how an entire industry can be redefined by solving for interaction friction, Amazon provides an example of how even the simplest of interaction friction can be a huge detriment to user experience. Amazon illustrates the incredible importance of site speed to online experiences. They found that they could increase revenue by 1% for every 100 ms of load time improvement. Think about that for a minute: such a small change in load time, 100 ms, could have such a dramatic impact on revenue for Amazon. It&#;s a strong reminder that any type of interaction friction, especially page load time time, is important to address.
The rise of messaging apps like Facebook Messenger and as well as Slack and HipChat in the enterprise can also be attributed to a significant reduction in interaction friction compared to the alternatives at the time, including and text messaging. You no longer had to worry about whether you had someone&#;s most recent address or number. You didn&#;t have to think about coming up with a subject line. You didn&#;t need to use many of the formalities that people had become accustomed to with , including greetings and signatures. All of that was taken away and reduced to the simplest form a communication: find the person you want to message and simply start typing a message to them. We&#;ve taken this even further now with the use of emoji, which communicate far more in a single image than the alternative of having to type it all out.
Usability testing remains one of the most important techniques for identifying sources of interaction friction in your product and addressing it. When you watch end users using your product you&#;ll undoubtedly discover friction points that you hadn&#;t anticipated yourself. It&#;s important to continue to leverage this technique not just the first time you launch a product, but continually as you iterate as well.
Cognitive Friction
Cognitive load refers to the total amount of mental effort being used in working memory. When cognitive load is high when performing a task, it means there is significant cognitive friction. The product designers goal then becomes minimizing cognitive friction. Cognitive friction is broader than just interaction friction as it encompasses all aspects of the experience that result in mental effort. This is best illustrated through a set of examples.
Uber has been incredibly successful not only because it delivers rides at a lower price, but it significantly reduces the cognitive friction previously associated with taking a taxi, bus, or really any form of public transportation. Previously when you wanted to take a taxi, for example, you&#;d have to find a cab company and their number. Then you&#;d call to schedule a ride and often be stuck calling a few times until the dispatcher took your call. Then they&#;d tell you they are sending someone but you could never really trust their time estimate, so you&#;d often call the dispatcher back just to make sure the taxi driver is coming. This resulted in significant anxiety around whether you would get to your destination on time. You&#;d then get in the taxi and be wondering how much the ride was even going to cost you and at the end of the ride have to deal with paying with either cash or credit and tipping the driver. Uber solved all of the cognitive friction throughout this user experience by creating an incredibly simple app to request a ride, get an accurate estimate and real-time details of how far your driver was, provide a detailed price quote, and make payment completely frictionless so you can just walk out of the car.
Nuzzel is another great example of an app that has removed significant cognitive friction from my use of Twitter. Twitter can be an incredible resource for professionals to stay on top of recent news & articles around their specific industry. You can accomplish this by following a set of thought leaders and peers in your industry and then read your feed to come across such industry news. However the Twitter experience has significant cognitive friction in accomplishing this goal. Since Twitter provides a real-time feed, you need to either check it constantly or spend a lot of time reading through older tweets to stay on top of it. And then you have to remember if you&#;ve seen a previous article several times to get a sense of popularity. The whole experience is pretty painful if your goal is staying on top of industry news. That&#;s where Nuzzel comes in. It reads your Twitter feed for you and provides you an always up-to-date summary of your Twitter feed, showcasing all the articles that have been shared on Twitter sorted by the number of people who shared the articles and what each of the people you are following said about the article. Now I can spend just 5 minutes reading Nuzzel instead of tons of wasted time on Twitter to accomplish my goal. Nuzzel saw all the cognitive friction that existed to accomplish the goal and created a solution specifically around it.
Wealthfront has done the same in terms of reducing cognitive friction when it comes to investing. Investing can be an incredibly daunting task. Even if you decide on a passive index fund approach, you still need to decide which indexes to invest in. S&P 500? Total market? Emerging markets? Bonds? Once you decide on the specific funds, you need to pick your actual allocation across asset classes. How much exposure do you want to domestic vs international stocks? What percentage equity vs bonds do you want to have? And ideally you should be thinking about adjusting this allocation based on your appropriate level of risk, which may change over time as your circumstances change. And you know your supposed to rebalance these allocations as your exposure to each asset class changes with actual returns, but you always forget to. Wealthfront solves for every part of this cognitive friction, reducing it to a simple questionnaire to understand your risk tolerance and then taking care of absolutely everything else: index selection, asset allocation, rebalancing, and more, automatically for you. This not only makes it so investing is easier for those participating, but frankly gets more people to invest in the first place instead of just leaving their money in cash because it&#;s become so much more accessible with all of the usual cognitive friction removed.
An effective tool for thinking through cognitive friction is building detailed user journeys of the entire current experience the user goes through to accomplish their goal, including all aspects of that journey, regardless of whether it&#;s currently serviced by your product or not. When you are able to detail each and every step and understand what&#;s painful, frustrating, or just time-consuming about it, then you can start to brainstorm opportunities for your product to directly address the cognitive friction you have now identified.
Emotional Friction
The final category of user friction is emotional friction, which refers to emotions your users feel that prevent them from accomplishing their goal. These are often the most difficult to perceive and equally the most difficult to address. Let&#;s take a look at a few of these in practice and how product designers ultimately solved them.
Patreon is a platform that enables creators of anything (music, blog posts, journalists, comics, etc) to run a membership business for their fans. They have now reached over 1 million monthly active Patrons and pay over $150 million per year to creators. When they were optimizing their overall on-boarding experience for creators, they initially solved for much of the typical interaction friction in the flow, simplifying the on-boarding, providing more detailed explanations, making their pricing far clearer, and more. But when they really dug into understanding their customers, they finally uncovered that one of the main reasons creators don&#;t actually go on to create a page is they are hesitant to look like they are begging their fans for money. They absolutely love what they do and they love their fans and it&#;s a very real emotional fear they have that they will alienate their audience by simply asking them for money. Once Patreon realized this emotional friction, they were able to get to work on solving it. And the tactic that worked best for them was showing the user other respected creators in their field who are successfully using Patreon with their fans. Once they see others asking for money and getting their fans to do it successfully, they usually lose their hangup over it.
Tinder is another great example of an app that solved for emotional friction in the world of online dating. With the previous generation of online dating experiences like Match.com, it was an incredibly intimidating experience that required you to put a ton of effort into an online profile, spend significant time searching for potential matches based on pics, interests, age, location, and more, and finally reach out to them. Reaching out to someone when you have no idea whether they have any mutual interest is as nerve-wracking of an experience as it gets. And the constant rejection can be incredibly discouraging. Tinder simplified the whole experience by reducing it to a series of profile cards that you simply swiped right if you were interested. If two people swiped right on each other, then their is mutual interest and then you have the ability to start a conversation. Tinder co-founder Sean Rad&#;s core insight was that no matter who you are, you feel more comfortable approaching somebody if you know they want you to approach them. Tinder took on the emotional friction of online dating head on and made it far more approachable by developing a light-weight mechanic for quickly determining mutual interest.
Over the last few years Facebook has struggled with it&#;s own challenges of emotional friction resulting in the percentage of unique sharers on it&#;s platform declining over time, especially amongst younger audiences, including teens and college students. These folks started sharing less for a variety of reasons, but largely because the bar for sharing on Facebook felt to them like it had gotten so much higher. They needed to showcase the very best of themselves to their friends and family. They were far more self-conscious about their sharing, especially since they had more and more friends on Facebook, including family, and the ability for anyone to easily find their profile. This emotional friction to sharing became a core focus for the team to solve. They launched a ton of new experiences attempting to address this, including Poke, Slingshot, Camera, Moments, and more. But none of them got any meaningful traction. All the while Snapchat grew its traction amongst the specific users Facebook was losing with a new core mechanic around Stories, which were short ephemeral photos and videos that you could share with your friends. And the crucial aspect was that they were self-deleting, which significantly reduced the bar to share and solved for the emotional friction associated with Facebook sharing. Ultimately Facebook realized that this was a far better approach to solve for their own emotional friction and adopted the same Stories mechanic across many of their apps.
Truly understanding the emotions of your users takes in-depth user interviews often out in the field where your users are and spending time getting to really know them, their aspirations, their goals, their frustrations, and encouraging them to share their emotions associated with your product experience. This is not a simple user research task. But necessary if your ultimately going to solve for the emotional friction that might ultimately be holding your product back from it&#;s full potential.
I hope this provides you an overview of the hierarchy of user friction across interaction friction, cognitive friction, and emotional friction, and gives you the tools to understand and solve for each type of user friction in your own product experience.
Sachin Rekhi is the Founder & CEO of Notejoy, a collaborative notes app for your entire team. It helps you get your most important work out of the noise of & Slack and into a fast and focused workspace. He&#;s also written over 125+ essays on product management and entrepreneurship. Subscribe to new essays at sachinrekhi.com.
For more information, please visit what is sach foot.