# Friction: Wikis

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.
Force diagram for block on ground.
^ We draw a free body diagram for the block, labeling reaction forces at the contact in the usual way.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.Arrows are vectors indicating directions and magnitudes of forces.^ Force and accleration are vectors, meaning that they have direction, as well as magnitude.

^ The tangential forces can act in any direction, but their magnitude is limited.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.W is the force of weight, N is the normal force, F is an applied force, and Ff is the force of kinetic friction which is equal to the coefficient of kinetic friction times the normal force.^ Thus, the friction force is proportional to the normal force.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ What factors affect the kinetic coefficient of friction ?

^ COEFFICIENT OF FRICTION A dimensionless number representing the ratio of the friction force to normal force.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

Since the magnitude of the applied force is greater than the magnitude of the force of kinetic friction opposing it, the block is accelerating to the left.
.Friction is the force resisting the relative lateral (tangential) motion of solid surfaces, fluid layers, or material elements in contact.^ Friction forces at 2D contacts .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The contacting surfaces will experience both a normal and tangential force.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ If the tangential forces get too large, the two contacting surfaces will slip relative to each other.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

It is usually subdivided into several varieties:
.
• Dry friction resists relative lateral motion of two solid surfaces in contact.^ DYNAMIC FRICTION Resistance to relative movement of two bodies that are already in motion.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ The relative velocity can be computed from the velocities and of the two contacting solids, using the equation .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The area of contact between the two surfaces .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.Dry friction is also subdivided into static friction between non-moving surfaces, and kinetic friction (sometimes called sliding friction or dynamic friction) between moving surfaces.
• Lubricated friction[1] or fluid friction[2][3] resists relative lateral motion of two solid surfaces separated by a layer of gas or liquid.
• Fluid friction is also used to describe the friction between layers within a fluid that are moving relative to each other.^ DYNAMIC FRICTION Resistance to relative movement of two bodies that are already in motion.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ FRICTION Mechanical resistance to the relative movement of two surfaces.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ On the other hand, the y-direction component of the external force in the downward direction increases the normal force, which increases the maximum static friction force.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

[4][5]
• .
• Skin friction is a component of drag, the force resisting the motion of a solid body through a fluid.
• Internal friction is the force resisting motion between the elements making up a solid material while it undergoes deformation.^ On the other hand, the y-direction component of the external force in the downward direction increases the normal force, which increases the maximum static friction force.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

^ Direction of friction is opposite to the component of net external force parallel to the contact surface.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

^ The angle of friction for two surfaces in contact is defined as the angle that the maximum contact force makes with the direction of normal force as shown in the figure.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

[5]
.Friction is not a fundamental force, as it is derived from electromagnetic force between charged particles, including electrons, protons, atoms, and molecules, and so cannot be calculated from first principles, but instead must be found empirically.^ Friction The coefficient of friction (also known as the frictional coefficient) is a dimensionless physics value which describes the ratio of the force of friction between two bodies and the force pressing them together.

^ Four Wheel Driving System The coefficient of friction - This factor relates the amount of friction force between two surfaces to the force holding the two surfaces together.

^ At first consideration, we may reason that force would be always least when it is applied exactly opposite to the friction.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

.When contacting surfaces move relative to each other, the friction between the two surfaces converts kinetic energy into thermal energy, or heat.^ You try, if you can, to avoid contact between two surfaces.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ FRICTION Mechanical resistance to the relative movement of two surfaces.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ The book starts with introductory concepts about friction and progressively delves into the subtler fundamentals of surface contact, use of various lubricants, and specific applications like brakes, piston rings, and machine components.
• CRC Press Online - Book: Friction Science and Technology 12 January 2010 9:14 UTC www.crcpress.com [Source type: FILTERED WITH BAYES]

.Contrary to earlier explanations, kinetic friction is now understood not to be caused by surface roughness but by chemical bonding between the surfaces.^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Cleanliness of the surfaces is paramount for success in such a demonstration, which speaks to another issue--a connection between internal friction and surface physics.

^ I know that this is the maximum friction possible which now becomes mu kinetic-- .
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

[6] .Surface roughness and contact area, however, do affect kinetic friction for micro- and nano-scale objects where surface area forces dominate inertial forces.^ What factors affect the kinetic coefficient of friction ?

^ Moving to the right, I apply a force to the right and the object moves to the right: F hand and ds are both positive: I do positive work (shown in the histogram) and friction does negative work.

^ I am doing a science experiment and I've found that the coefficient of friction decreases when the object I am pulling increases in weight (and therefore the pulling force increases as well).

[7]

## History

.Several famous scientists and engineers contributed to our understanding of friction.^ The concept of friction acknowledges this duality and puts it at the heart of our understanding of "modern" global interconnections, that is, those that have developed under the aegis of Enlightenment universals.
• Sample Chapter for Tsing, A.L.: Friction: An Ethnography of Global Connection. 12 January 2010 9:14 UTC press.princeton.edu [Source type: FILTERED WITH BAYES]

.^ Leonardo da Vinci was apparently the first person to do this – his experiments were repeated by Amontons and Coulomb about 100 years later.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Interestingly, the author's model has similarities to ordinary sliding friction, as given to us by Charles Augustin Coulomb.

^ The friction force of the mass against the table is of the type first described quantitatively by Charles Augustin Coulomb (1736-1806), although Leonardo da Vinci is probably the first to consider it scientifically.

Their findings are codified into these laws:[8]
.
1. The force of friction is directly proportional to the applied load.^ Thus, the friction force is proportional to the normal force.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ For example, we can not push an object unless external force is applied outside a region called "cone of friction".
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

^ BEARING STRESS The surface pressure acting on a joint face directly as a result of the force applied by a fastener.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

.(Amontons' 1st Law)
2. The force of friction is independent of the apparent area of contact.^ Friction forces at 2D contacts .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The contact forces are normal and friction forces.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

^ Friction forces at a 2D contact are described by the following laws: .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.(Amontons' 2nd Law) (Amontons' 2nd Law does not work for elastic, deformable materials.^ Internal friction arises even at the smallest stress levels if Hooke's law does not properly describe the static stress-strain curve of the material.

^ Usually in engineering practice the stress is reckoned with respect to the external force (negative of the spring F), so that the equivalent to Hookes law is s = E e, where E is an elastic modulus descriptive of the material from which the spring is fabricated.

.For example, wider tires on cars provide more traction than narrow tires for a given vehicle mass because of surface deformation of the tire)
3. Kinetic friction is independent of the sliding velocity.^ Friction of tires and road surfaces .
• CRC Press Online - Book: Friction Science and Technology 12 January 2010 9:14 UTC www.crcpress.com [Source type: FILTERED WITH BAYES]

^ Plane on a Treadmill The speed of the tire will match the speed of the treadmill because of the force of friction.

^ The energy dissipation in real structures is far more complicated and tends to be proportional to displacements rather than proportional to the velocity.

(Coulomb's Law of Friction)

## Coulomb friction

.Coulomb friction, named after Charles-Augustin de Coulomb, is a model used to calculate the force of dry friction.^ Reaction forces are calculated using the standard procedure – free body diagram; force and moment balance; solution of equilibrium equations.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Efforts to model internal friction inluence on long-period pendula uncovered something surprising to most--that the foundation for physics laid by Charles Augustin Coulomb may be much broader than had been realized.

^ Interestingly, the author's model has similarities to ordinary sliding friction, as given to us by Charles Augustin Coulomb.

It is governed by the equation:
$F_\mathrm{f} \leq \mu F_\mathrm{n}$
where
• $F_\mathrm{f}\,$ is the force exerted by friction (in the case of equality, the maximum possible magnitude of this force).
• $\mu\,$ is the coefficient of friction, which is an empirical property of the contacting materials,
• $F_\mathrm{n}\,$ is the normal force exerted between the surfaces.
.For surfaces at rest relative to each other $\mu = \mu_\mathrm{s}\,$, where $\mu_\mathrm{s}\,$ is the coefficient of static friction.^ What is the static coefficient of friction between an object and ...

^ On the other hand, the y-direction component of the external force in the downward direction increases the normal force, which increases the maximum static friction force.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.This is usually larger than its kinetic counterpart.^ The static is always larger than the kinetic for reasons that are quite obvious.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.The Coulomb friction $F_\mathrm{f}\,$ may take any value from zero up to $\mu F_\mathrm{n}\,$, and the direction of the frictional force against a surface is opposite to the motion that surface would experience in the absence of friction.^ It is moving, so sure, it's going to be accelerated so the frictional force is in this direction, has this value.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Newton's law of motion says that a force is required to get an object moving (an acceleration in the direction of its motion), or to stop it moving (a negative acceleration in that direction).

^ A large flat piece of plastic was attached to the bottom of the pendulum, so that its movement (normal vector to the surface in the direction of motion) would disturb a great amount of air in turbulent manner.

.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.^ What is the static coefficient of friction between an object and ...

^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ A body in motion will tend to stay in motion at a constant velocity unless acted upon by an outside force.
• Space Friction - Television Tropes & Idioms 12 January 2010 9:14 UTC tvtropes.org [Source type: General]

.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.^ In this case friction forces will be smaller.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ It is moving, so sure, it's going to be accelerated so the frictional force is in this direction, has this value.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Notice how the Q rises initially, peaks at a value less than what would be true for hysteretic damping alone (constant Q case), and then later declines.

.For surfaces in relative motion $\mu = \mu_\mathrm{k}\,$, where $\mu_\mathrm{k}\,$ is the coefficient of kinetic friction.^ What factors affect the kinetic coefficient of friction ?

^ Ductile iron has the highest coefficient of friction of any rotor surface.

^ Four Wheel Driving System The coefficient of friction - This factor relates the amount of friction force between two surfaces to the force holding the two surfaces together.

.The Coulomb friction is equal to $F_\mathrm{f}\,$, and the frictional force on each surface is exerted in the direction opposite to its motion relative to the other surface.^ This simple argument also explains why friction force is independent of contact area; why it is so sensitive to surface films, and why it can be influenced (albeit only slightly) by surface roughness.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Newton's law of motion says that a force is required to get an object moving (an acceleration in the direction of its motion), or to stop it moving (a negative acceleration in that direction).

^ We could measure the critical tangential force as a function of N , the area of contact A , the materials and lubricants involved, the surface finish, and other variables such as temperature.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.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).^ It's also independent of the surface area that is in contact with this incline which is equally nonintuitive.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ The critical force required to initiate sliding between surfaces is independent of the area of contact.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Friction forces at 2D contacts .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.Such reasoning aside, however, the approximation is fundamentally an empirical construction.^ The fundamental mechanisms responsible for damping are in most cases nonlinear; however , the oscillator's motion can itself be approximated in many cases by a linear 2nd order differential equation.

^ Whatever the reason for a pure exponential, whether fundamentally linear (viscous) or nonlinear (hysteretic present model), the quality factor Q for such a pure exponential decay is constant.

^ However, it is such an excellent approximation that Newtonian gravity is what we use to calculate in almost all circumstances, while recognising that it is just a very convenient approximation to more exact theories.

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.^ It's also independent of the surface area that is in contact with this incline which is equally nonintuitive.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ The critical force required to initiate sliding between surfaces is independent of the area of contact.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The force of friction is dependent on the normal force and the nature of the surfaces.

### Coefficient of friction

.The coefficient of friction (COF), also known as a frictional coefficient or friction coefficient and symbolized by the Greek letter μ, is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together.^ This is a very high value of friction coefficient.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Let denote the coefficient of friction between the two surfaces.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Friction The coefficient of friction (also known as the frictional coefficient) is a dimensionless physics 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, while rubber on pavement has a high coefficient of friction.^ The friction coefficient only depends on the materials that you have the materials that are rubbing over each other.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ FRICTION STABILIZERS Coating materials used on fasteners with the intention of reducing the scatter in the thread and bearing surface friction coefficients.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ A rigid non-asbestos aramid material with medium to high friction exhibiting excellent integrity, good stability and good wear characteristics.
• Great Lakes Friction Products 12 January 2010 9:14 UTC www.glfp.com [Source type: Reference]

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, adhesive tape resists sliding even when there is no normal force, or a negative normal force).^ The force of friction is dependent on the normal force and the nature of the surfaces.

^ Assume that there is no sliding friction between the piston and the cylinder.

^ Coulomb friction is empirically simple, at least as a first approximation; since it depends only on the normal force between surfaces and the algebraic sign of the velocity, when there is relative motion.

.In this case, the frictional force may depend strongly on the area of contact.^ The contact forces are normal and friction forces.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

^ Direction of friction is opposite to the component of net external force parallel to the contact surface.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

^ The angle of friction for two surfaces in contact is defined as the angle that the maximum contact force makes with the direction of normal force as shown in the figure.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

Some drag racing tires are adhesive in this way. .However, despite the complexity of the fundamental physics behind friction, the relationships are accurate enough to be useful in many applications.^ Friction stir welding: from basics to applications reviews the fundamentals of the process and how it is used in industrial applications.

^ In recent years, however, physics has been circumstantially forced to reconsider damping fundamentals .

^ The fundamental mechanisms responsible for damping are in most cases nonlinear; however , the oscillator's motion can itself be approximated in many cases by a linear 2nd order differential equation.

.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.^ Static and kinetic friction .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The area of contact between the two surfaces .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Friction forces depend quite strongly on what the two surfaces are made from.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.For example, a curling stone sliding along the ice experiences a kinetic force slowing it down.^ The kinetic friction force usually (but not always) decreases slightly as the sliding speed increases.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The force required to maintain steady sliding is referred to as the kinetic friction force.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ A second value, known as the coefficient of kinetic friction , and denoted by , is used in the equation for the force required to maintain steady sliding between two surfaces .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.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.^ Mine belt conveyor to run the new experience Roller conveyor and transmission coefficient of friction between the determination of principle: use of critical sliding friction test method were determined.

^ And as I increase the tilt this frictional force will get larger and larger and larger and then there comes a time that the object will start to slide.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Worked out to book length the constant references to friction would no doubt grate, but in this story they slid easily by.
• Escape Pod » EP144: Friction 12 January 2010 9:14 UTC escapepod.org [Source type: General]

.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.^ If it starts raining and there is a little bit of dust on the road the friction coefficient between your tires and road can become so low that you begin to hydroplane and that you literally...
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Here it means the amount of traction between the tires and the road to the weight ...

^ They would both start to slide at the same angle given the fact, of course the same road and the same kind of rubber.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.The coefficient of friction is an empirical measurement – it has to be measured experimentally, and cannot be found through calculations.^ Experimental measurement of friction forces .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Experimental values for friction coefficient .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Friction coefficient can even vary significantly during a measurement.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

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.^ This is a very high value of friction coefficient.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ What is the coefficient of friction between the ladder and the floor?

^ This means that this value of the coefficient will be used only to ...

.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.^ This simple argument also explains why friction force is independent of contact area; why it is so sensitive to surface films, and why it can be influenced (albeit only slightly) by surface roughness.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ We could measure the critical tangential force as a function of N , the area of contact A , the materials and lubricants involved, the surface finish, and other variables such as temperature.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The actual grip that a tyre can generate is dictated by the coefficient of friction of the rubber compound used in the tyre.

.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.^ The critical force required to initiate sliding between two surfaces is proportional to the normal force.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The force required to produce this acceleration is hundreds of times smaller than the usual forces required to support our weight.

^ The force required to maintain steady sliding is referred to as the kinetic friction force.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

For example, silicone rubber or acrylic rubber-coated surfaces have a coefficient of friction that can be substantially larger than 1.
.Both static and kinetic coefficients of friction depend on the pair of surfaces in contact; their values are usually approximately determined experimentally.^ This is a very high value of friction coefficient.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Static and kinetic friction .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ What factors affect the kinetic coefficient of friction ?

.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.^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Static and kinetic friction .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Let denote the coefficient of friction between the two surfaces.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.In the case of kinetic friction, the direction of the friction force may or may not match the direction of motion: a block sliding atop a table with rectilinear motion is subject to friction directed along the line of motion; an automobile making a turn is subject to friction acting perpendicular to the line of motion (in which case it is said to be 'normal' to it).^ The force required to maintain steady sliding is referred to as the kinetic friction force.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Thus, the friction force is proportional to the normal force.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The direction that the individual bolt force acts is perpendicular to a line joining that bolt to the instantaneous centre.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

.The direction of the static friction force can be visualized as directly opposed to the force that would otherwise cause motion, were it not for the static friction preventing motion.^ Friction prevents it from doing so so there's going to be a frictional force in this direction.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Otherwise there would be an acceleration in the y direction.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ It is moving, so sure, it's going to be accelerated so the frictional force is in this direction, has this value.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.In this case, the friction force exactly cancels the applied force, so the net force given by the vector sum, equals zero.^ DECOMPRESSION POINT The point at which there is zero pressure at the joint interface as a result of forces applied to the joint.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ The surface friction case is not necessarily the trivial textbook presentation involving a coefficient of kinetic friction and a normal force.

^ Black Hole, The That's because the coefficient of friction in the cup is sufficiently high enough to cause the force of friction to equal or surpass the force of gravity.

.It is important to note that in all cases, Newton's first law of motion holds.^ This is Newton's first law of motion.

^ Forces, acceleration and Newton's laws of motion .

^ Newton's law of motion says that a force is required to get an object moving (an acceleration in the direction of its motion), or to stop it moving (a negative acceleration in that direction).

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.^ Coefficient and wear rate can vary significantly depending on the environment, such as finish and hardness of mating part, unit load, lubricant and operating temperature.
• Great Lakes Friction Products 12 January 2010 9:14 UTC www.glfp.com [Source type: Reference]

^ The process must depart somewhat from adiabatic, however, since there is damping, which equation (25) shows to derive from temperature variations yielding hysteresis .

^ The temperature sensitivity is due to the difference of thermal coefficients of the materials from which the instrument is constructed, and the pressure variation is a buoyancy effect.

.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.^ This is quite a high value – friction coefficient could easily drop below 0.36 if the surfaces are greasy or wet.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ When used in this way the thin nut should be adjacent to the joint surface and tightened against the thick nut.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ Finally, to understand the cause of the Coulomb/Amonton friction law, we need to visualize what happens when two rough surfaces slide against each other.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.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.^ The Red Alto® lining has a unique dual saturation which imparts high frictional properties and high heat resistance.

^ Because of the difficulties in attaining a wide range of eigenmodes , internal friction is typically determined with a specimen that does not oscillate.

^ Increasing sliding speed by a factor of 10 might drop the friction force by a few percent.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

#### The normal force

Block on a ramp (top) and corresponding free body diagram of just the block (bottom).
.The normal force is defined as the net force compressing two parallel surfaces together; and its direction is perpendicular to the surfaces.^ And I will call this direction y, as I always do perpendicular to the surface.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ We put two solids in contact, and push them together with a normal force N .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ There is no acceleration in the y direction, so I can be sure that the surface pushes back with a normal force, N and that normal force N must be exactly mg cosine alpha because those are the only two forces in the y direction.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.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 $N=mg\,$.^ Here, the normal force and the gravitational force acting on a mass at the surface of the water must also add to give a centrip, , otherwise the water would flow out of the bowl.

^ It is difficult to measure G simply because gravity is so weak: the forces between masses than can be manipulated are small.

^ When we measure the weight m g of a mass m, we typically do this by putting it on a balance and measuring the normal force N required to keep it stationary with respect to the earth – i.e.

.In this case, the magnitude of the friction force is the product of the mass of the object, the acceleration due to gravity, and the coefficient of friction.^ The object has a mass, m, gravitational force, mg.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Friction force = mass of the trolley * acceleration and if you want to go further as in determining the coefficient of friction , the formula is coeff of friction = acceleration / acceleration due to gravity.

^ Magnitude of force on the moon due to earth: F g = m m a m = m m ω 2 r m and Magnitude of force on the earth due to moon: F g = m e a e = m e ω 2 r e .

.However, the coefficient of friction is not a function of mass or volume; it depends only on the material.^ Only the dynamic friction coefficient is ...

^ The friction coefficient only depends on the materials that you have the materials that are rubbing over each other.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ These are rough guides only – friction coefficients for a given material can by highly variable.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.For instance, a large aluminum block has the same coefficient of friction as a small aluminum block.^ If the coefficient of friction between the block ...

^ The coefficient of friction for that same tire in ...

^ Nissan GT-R The Nissan GT-R has a coefficient of friction of 0.27, with a large front and rear load, which provides the GT-R more grip in all weather conditions, while improving fuel efficiency.

.However, the magnitude of the friction force itself depends on the normal force, and hence the mass of the block.^ Thus, the friction force is proportional to the normal force.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The magnitude of the normal force acting at the contact .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The normal force must be repulsive, but can have an arbitrary magnitude.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.If an object is on a level surface and the force tending to cause it to slide is horizontal, the normal force $N\,$ between the object and the surface is just its weight, which is equal to its mass multiplied by the acceleration due to earth's gravity, g.^ Magnitude of force on the moon due to earth: F g = m m a m = m m ω 2 r m and Magnitude of force on the earth due to moon: F g = m e a e = m e ω 2 r e .

^ Forces cause accelerations, and also decelerations, which are just negative accelerations.

^ A force that causes a centripetal acceleration is called a centripetal force.

.If the object is on a tilted surface such as an inclined plane, the normal force is less, because less of the force of gravity is perpendicular to the face of the plane.^ We normally call that the "normal force" because it's normal to this surface and it must be the same as mg.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Plane on a Treadmill The speed of the tire will match the speed of the treadmill because of the force of friction.

^ A simple equilibrium calculation shows that, as long as the weight of the components can be neglected, the contacting surfaces must be subject to a normal force N and a tangential force T .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.Therefore, the normal force, and ultimately the frictional force, is determined using vector analysis, usually via a free body diagram.^ Thus, the friction force is proportional to the normal force.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The new free body diagram is shown .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Reaction forces are calculated using the standard procedure – free body diagram; force and moment balance; solution of equilibrium equations.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.Depending on the situation, the calculation of the normal force may include forces other than gravity.^ A simple equilibrium calculation shows that, as long as the weight of the components can be neglected, the contacting surfaces must be subject to a normal force N and a tangential force T .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ The force of friction is dependent on the normal force and the nature of the surfaces.

^ It is difficult to measure G simply because gravity is so weak: the forces between masses than can be manipulated are small.

#### Approximate coefficients of friction

Materials Static friction, $\mu_s\,$
Dry & clean Lubricated
Aluminum Steel 0.61
Copper Steel 0.53
Brass Steel 0.51
Cast iron Copper 1.05
Cast iron Zinc 0.85
Concrete (wet) Rubber 0.30
Concrete (dry) Rubber 1.0
Concrete Wood 0.62[9]
Copper Glass 0.68
Glass Glass 0.94
Metal Wood 0.2-0.6[9] 0.2 (wet)[9]
Polythene Steel 0.2[10] 0.2[10]
Steel Steel 0.80[10] 0.16[10]
Steel Teflon 0.04[10] 0.04[10]
Teflon Teflon 0.04[10] 0.04[10]
Wood Wood 0.25-0.5[9] 0.2 (wet)[9]
The slipperiest solid known, discovered in 1999, dubbed BAM (for the elements boron, aluminum, and magnesium), has an approximate coefficient of friction of 0.02, about half that of Teflon.[11]

### Static friction

.Static friction is friction between two solid objects that are not moving relative to each other.^ What is the static coefficient of friction between an object and ...

^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ In other words , internal friction from defects at the mesoscale cause behavior that is in some ways similar to ordinary sliding friction, where the static coefficient is greater than the kinetic coefficient.

.For example, static friction can prevent an object from sliding down a sloped surface.^ What is the static coefficient of friction between an object and ...

^ It is probably closely related to the aforementioned microdynamics , except that the latter seems to have focused on surfaces (sliding friction), whereas mesodynamics is concerned with internal friction.

^ And as I increase the tilt this frictional force will get larger and larger and larger and then there comes a time that the object will start to slide.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.The coefficient of static friction, typically denoted as μs, is usually higher than the coefficient of kinetic friction.^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ What factors affect the kinetic coefficient of friction ?

^ As with many non-lubricated systems, the static coefficient of friction is higher than the dynamic coefficient of friction .

.The static friction force must be overcome by an applied force before an object can move.^ It is moving, so sure, it's going to be accelerated so the frictional force is in this direction, has this value.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Newton's law of motion says that a force is required to get an object moving (an acceleration in the direction of its motion), or to stop it moving (a negative acceleration in that direction).

^ And as I increase the tilt this frictional force will get larger and larger and larger and then there comes a time that the object will start to slide.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.The maximum possible friction force between two surfaces before sliding begins is the product of the coefficient of static friction and the normal force: $f = \mu_s F_{n}\,$.^ The force of friction is dependent on the normal force and the nature of the surfaces.

^ What is the coefficient of friction between the ladder and the floor?

^ What is the coefficient of friction between the cars and the road?

.When there is no sliding occurring, the friction force can have any value from zero up to $F_{max}\,$.^ Assume that there is no sliding friction between the piston and the cylinder.

^ And I touched the thing, and there was no friction.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ And as I increase the tilt this frictional force will get larger and larger and larger and then there comes a time that the object will start to slide.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.Any force smaller than $F_{max}\,$ attempting to slide one surface over the other is opposed by a frictional force of equal magnitude and opposite direction.^ An SDC sensor placed near one boom end is connected in electrical phase opposition to a second SDC sensor placed near the other end of the boom.

^ It is moving, so sure, it's going to be accelerated so the frictional force is in this direction, has this value.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ It is probably closely related to the aforementioned microdynamics , except that the latter seems to have focused on surfaces (sliding friction), whereas mesodynamics is concerned with internal friction.

.Any force larger than $F_{max}\,$ overcomes the force of static friction and causes sliding to occur.^ And as I increase the tilt this frictional force will get larger and larger and larger and then there comes a time that the object will start to slide.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ In other words , internal friction from defects at the mesoscale cause behavior that is in some ways similar to ordinary sliding friction, where the static coefficient is greater than the kinetic coefficient.

^ Kinetic friction occurs when an object is in motion, and static friction occurs when an object is at rest.

.The instant sliding occurs, static friction is no longer applicable and kinetic friction becomes applicable.^ Kinetic friction occurs when an object is in motion, and static friction occurs when an object is at rest.

^ Kinetic friction occurs when the object is moving or sliding.

^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.An example of static friction is the force that prevents a car wheel from slipping as it rolls on the ground.^ STATIC FRICTION Friction at rest; a force is required to initiate relative movement between two bodies - static friction is the force that resists such relative movement.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ Train of the Week: General Electric ES44AC Theoretical maximum tractive effort (a force) of a locomotive is the vector component of the product of the coefficient of friction (static or kinetic) between the wheels and rails (a dimensionless number) and its weight (a force which ...

^ Four Wheel Driving System The coefficient of friction - This factor relates the amount of friction force between two surfaces to the force holding the two surfaces together.

.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.^ Kinetic friction occurs when an object is in motion, and static friction occurs when an object is at rest.

^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ The bolts are tightened to a specified minimum shank tension so that transverse loads are transferred across the joint by friction between the plates rather than by shear across the bolt shank.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

.The maximum value of static friction, when motion is impending, is sometimes referred to as limiting friction,[12] although this term is not used universally.^ And that maximum value that the friction can achieve-- .
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ The friction grows and grows and grows fights an heroic battle to not make it go uphill loses the battle at one point, reaches the maximum value.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ The word 'friction' is used in a generic sense, meaning any process responsible for conversion of the oscillator's coherent motion into incoherent thermal activity.

[4]

### 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).^ Kinetic friction occurs when an object is in motion, and static friction occurs when an object is at rest.

^ Kinetic friction occurs when the object is moving or sliding.

^ In other words , internal friction from defects at the mesoscale cause behavior that is in some ways similar to ordinary sliding friction, where the static coefficient is greater than the kinetic coefficient.

.The coefficient of kinetic friction is typically denoted as μk, and is usually less than the coefficient of static friction for the same materials.^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ I take a static friction coefficient which is 0.5 and I take a kinetic friction coefficient which is a little less, which is 0.4.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Internal friction arises even at the smallest stress levels if Hooke's law does not properly describe the static stress-strain curve of the material.

[13][14] In fact, Richard Feynman reports that "with dry metals it is very hard to show any difference."[15] .Finally, new models are beginning to show how kinetic friction can be greater than static friction.^ How many non-lubricated systems, the static friction is greater than the coefficient of friction .

^ N2L and friction reflection The final thing I learned about was the force of friction, I learned how to calculate friction and the coefficient of friction (expressed in mu).

^ Send2Press (press release) The company guarantees all treated floors have a Static Coefficient of Friction (SCOF) reading of 0.06 or greater, the minimum measurement required for ...

[16]
Examples of kinetic friction:
.
• Kinetic friction is when two objects are rubbing against each other.^ He is far less known for two other significant contributions: ( i ) kinetic theory of gases and (ii) viscoelasticity both of which are important to theories of oscillator damping.

^ In other words , internal friction from defects at the mesoscale cause behavior that is in some ways similar to ordinary sliding friction, where the static coefficient is greater than the kinetic coefficient.

^ Newton's Second Law There are two types of friction, kinetic and static.

.Putting a book flat on a desk and moving it around is an example of kinetic friction.
• Fluid friction is the interaction between a solid object and a fluid (liquid or gas), as the object moves through the fluid.^ Kinetic friction occurs when the object is moving or sliding.

^ What is the static coefficient of friction between an object and ...

^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.The skin friction of air on an airplane or of water on a swimmer are two examples of fluid friction.^ It's above the water, so if it moves now all it has to overcome is the air friction and that's it and that helps tremendously.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Since the index of friction of air is known from pendulum experiments, we may easily calculate the terminal velocity of a globule [water] of given size.

.This kind of friction is not only due to rubbing, which generates a force tangent to the surface of the object (such as sliding friction).^ It is probably closely related to the aforementioned microdynamics , except that the latter seems to have focused on surfaces (sliding friction), whereas mesodynamics is concerned with internal friction.

^ Cleanliness of the surfaces is paramount for success in such a demonstration, which speaks to another issue--a connection between internal friction and surface physics.

^ The external force is associated respectively with ( i ) laminar fluid flow, (ii) induced currents, and (iii) surface friction.

.It is also due to forces that are orthogonal to the surface of the object.^ We note in passing that an object at rest on the earth's surface already has a speed of ωr e cos (latitude), due to the rotation of the earth.

.These orthogonal forces significantly (and mainly, if relative velocity is high enough) contribute to fluid friction.^ STATIC FRICTION Friction at rest; a force is required to initiate relative movement between two bodies - static friction is the force that resists such relative movement.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ For mechanical oscillators, it is not generally appropriate, since viscous damping amounts to some part of the system moving in an external Newtonian fluid that removes energy because of a friction force that is proportional to velocity.

^ Maxwell's model of viscoelasticity combines a purely elastic spring with a purely viscous dashpot (fluid damper in which the friction force is proportional to the velocity).

.Fluid friction is the classic name of this force.^ The external force is associated respectively with ( i ) laminar fluid flow, (ii) induced currents, and (iii) surface friction.

^ Maxwell's model of viscoelasticity combines a purely elastic spring with a purely viscous dashpot (fluid damper in which the friction force is proportional to the velocity).

.This name is no longer used in modern fluid dynamics.^ Stokes is best known for his equations of fluid dynamics that also include the name Navier .

^ People use hovercrafts so they blow air out from below so the craft lifts itself up and now it's no longer in contact with the water.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.Since rubbing is not its only cause, in modern fluid dynamics the same force is typically referred to as drag or fluid resistance, while the force component due to rubbing is called skin friction.^ As also noted in [Nelson 1986], this form of fluid friction caused a significant amplitude dependent damping.

^ The external force is associated respectively with ( i ) laminar fluid flow, (ii) induced currents, and (iii) surface friction.

^ STATIC FRICTION Friction at rest; a force is required to initiate relative movement between two bodies - static friction is the force that resists such relative movement.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

.Notice that a fluid can in some cases exert, together with drag, a force orthogonal to the direction of the relative motion of the object (lift).^ Newton's law of motion says that a force is required to get an object moving (an acceleration in the direction of its motion), or to stop it moving (a negative acceleration in that direction).

^ And the frictional force, in this case will adjust itself just the right way so that Newton's Second Law in the x direction will give you, for the force, a zero.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ A force is also required to change the direction of its motion because, as we shall see, that is also an acceleration.

The net force exerted by a fluid (drag + lift) is called fluidodynamic force (aerodynamic if the fluid is a gas, or hydrodynamic if the fluid is a liquid).

### Angle of friction

.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.^ The friction grows and grows and grows fights an heroic battle to not make it go uphill loses the battle at one point, reaches the maximum value.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ More frequently in physics, the term is used to describe pressure broadened linewidths [ Milonni 1988].

^ One of the simplest friction models is that in which a Hookes law spring is connected on one end to a mass that slides on a level table.

.This is called the angle of friction or friction angle.^ Perhaps the measured angle, which is an indicator of the internal friction, has been called Q -1 because k = m w 0 2 for an oscillator of frequency w 0 , and Q = m w 0 2 / z for the freely-decaying oscillator.

It is defined as:
$an{ heta} = \mu\,$
where .$heta\,$ is the angle from horizontal and $\mu\,$ is the static coefficient of friction between the objects.^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ STATIC FRICTION Friction at rest; a force is required to initiate relative movement between two bodies - static friction is the force that resists such relative movement.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ I want to remind you I have used the static friction coefficient.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

[17] .This formula can also be used to calculate $\mu\,$ from empirical measurements of the friction angle.^ So you should be able to calculate what the friction coefficient is using the equation tangent alpha equals mu s.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ For large values of Q, the lag angle (radian measure) is given by d = 1/Q. Researchers usually measure d and specify the magnitude of the internal friction as Q -1 .

^ It is fairly easy to measure a friction coefficient by putting an object on an incline and by changing the angle of the incline, increasing it.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

### Friction at the atomic level

.Determining the forces required to move atoms past each other is a challenge in designing nanomachines.^ Newton's law of motion says that a force is required to get an object moving (an acceleration in the direction of its motion), or to stop it moving (a negative acceleration in that direction).

^ A change in motion requires a force You're standing in the bus when it starts moving forwards (when it accelerates forwards).

^ This can be calibrated: one can determine the forces required at each end of the rod to twist the wire by a given angle (for instance, this can be determined by rotating the rod, letting it go, and measuring the frequency of its oscillation).

.In 2008 scientists for the first time were able to move a single atom across a surface, and measure the forces required.^ Newton's law of motion says that a force is required to get an object moving (an acceleration in the direction of its motion), or to stop it moving (a negative acceleration in that direction).

^ A change in motion requires a force You're standing in the bus when it starts moving forwards (when it accelerates forwards).

^ This can be calibrated: one can determine the forces required at each end of the rod to twist the wire by a given angle (for instance, this can be determined by rotating the rod, letting it go, and measuring the frequency of its oscillation).

.Using ultrahigh vacuum and nearly-zero temperature (5 K), they used a modified atomic force microscope to drag a cobalt atom, and a carbon monoxide molecule, across surfaces of copper and platinum.^ Even they are complicated and do not strictly obey Stokes law of drag force proportional to the velocity.

^ Gaseous atoms and molecules constantly adsorb and desorb from device surfaces.

^ To fit their theory with experiment, they used a "modified" Maxwell model with a distribution of time constants that ranged from 30 s to more than 4000 s.

[18]

## Other types of friction

### Rolling resistance

.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 and/or surface.^ If there are no forces -- no friction, no air resistance etc -- then an object at rest remains at rest, and a moving object continues moving at a constant speed in a straight line.

^ The object will not be accelerated and the frictional force is going to adjust along the x direction so that the acceleration indeed is zero.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.Generally the force of rolling resistance is less than that associated with kinetic friction.^ In other words , internal friction from defects at the mesoscale cause behavior that is in some ways similar to ordinary sliding friction, where the static coefficient is greater than the kinetic coefficient.

^ The external force is associated respectively with ( i ) laminar fluid flow, (ii) induced currents, and (iii) surface friction.

^ But even better it would be if you could separate the object completely and only have air in between because air has much less friction than a liquid.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

[19] .Typical values for the coefficient of rolling resistance are 0.001.[20] 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.^ The mass-spring oscillator is the textbook example of harmonic motion, for which one of the most sophisticated mechanical oscillators ever built is the LaCoste version of vertical seismometer.

^ One of the most dramatic examples of a memory effect is the demonstration mentioned above by Gross in the 1950's, concerning a twisted wire.

^ If it starts raining and there is a little bit of dust on the road the friction coefficient between your tires and road can become so low that you begin to hydroplane and that you literally...
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

[21]

### 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.

## Reducing friction

### Devices

.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.^ Great Lakes Friction can offer several different types of cones, cups, pads, brackets or any other configuration required.
• Great Lakes Friction Products 12 January 2010 9:14 UTC www.glfp.com [Source type: Reference]

.Many thermoplastic materials such as nylon, HDPE and PTFE are commonly used in low friction bearings.^ The same pendulum was used to demonstrate some counterintuitive features of internal friction damping, by replacing the Si wafers with various materials.

^ The filler material used is typically graphite or PTFE. The metal strip and retaining rings being typically made from stainless steel.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ Hysteresis and creep are common to many systems, such as electromechanical actuators, especially when used at high drive levels.

.They are especially useful because the coefficient of friction falls with increasing imposed load.^ Thermoelastic damping is important for high frequency compressional oscillations in materials with significant thermal coefficients and especially for metals, because of their large thermal conductivity.

^ So you should be able to calculate what the friction coefficient is using the equation tangent alpha equals mu s.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ It is fairly easy to measure a friction coefficient by putting an object on an incline and by changing the angle of the incline, increasing it.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

### 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.^ Coefficient and wear rate can vary significantly depending on the environment, such as finish and hardness of mating part, unit load, lubricant and operating temperature.
• Great Lakes Friction Products 12 January 2010 9:14 UTC www.glfp.com [Source type: Reference]

^ STATIC FRICTION Friction at rest; a force is required to initiate relative movement between two bodies - static friction is the force that resists such relative movement.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ Concerning the latter, the Mohr criterion, applied to the Coulomb failure envelope, defines a 'coefficient of internal friction' which is used to predict brittle failure [ Gere 1996].

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.^ Liberal universals mix and meld with the universals of science, world religions (especially Christianity and Islam), and emancipatory philosophies including Marxism and feminism.
• Sample Chapter for Tsing, A.L.: Friction: An Ethnography of Global Connection. 12 January 2010 9:14 UTC press.princeton.edu [Source type: FILTERED WITH BAYES]

.Superlubricity, a recently-discovered effect, has been observed in graphite: it is the substantial decrease of friction between two sliding objects, approaching zero levels.^ As we shall see, to produce substantial gravitational force, one of the objects has to approach planetary size.

^ And as I increase the tilt this frictional force will get larger and larger and larger and then there comes a time that the object will start to slide.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Recent theory shows that the Granato model is not always adequate; that " dislocation interactions may alter substantially the dislocation component of the spectrum observed during internal friction experiments".

.A very small amount of frictional energy would still be dissipated.^ Your mass would still be 70 kg -- you would still be made of the same amount of material -- but your weight would be only 120 newtons.

^ In the typical torsional pendulum used to measure internal friction, the sample is of very small mass.

.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.^ The adjective, "equivalent", draws attention to the fact that internal friction in a solid can't really result from fluid effects.

^ Ltd of Woodgate, Leicester, United Kingdom, LE3 5GJ. ANTI-FRICTION COATING AF coatings are dry lubricants consisting of suspensions of solid lubricants, such as graphite, PTFE or molydbenum disulphide of small particle size in a binder.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ Can be used by itself as a dry lubricant as well as in with other solid lubricants and in oils and greases.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

.Another way to reduce friction between two parts is to superimpose micro-scale vibration to one of the parts.^ Cleanliness of the surfaces is paramount for success in such a demonstration, which speaks to another issue--a connection between internal friction and surface physics.

^ The larger density of states of the surface (reduced order) is probably an important factor in the difference between surface friction and the modified internal friction model which follows.

^ There is another way that one can measure the friction coefficient which is way more complicated and really, that's not the reason why I want you to see it.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

This can be sinusoidal vibration as used in ultrasound-assisted cutting or vibration noise, known as dither.

## 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 the following damping model for internal friction, Coulombs law of sliding friction is modified by assuming that the coefficient of friction is not constant, but rather involves the energy of oscillation E in a power law; i.e., .

^ By plotting the internal friction versus reciprocal temperature, one may estimate the activation energy of the process responsible for the damping.

^ The quality factor Q is in general defined as 2 p times the ratio of the energy of the oscillator to the energy lost to friction per cycle.

Energy is transformed from other forms into heat. .A sliding hockey puck comes to rest because friction converts its kinetic energy into heat.^ And as I increase the tilt this frictional force will get larger and larger and larger and then there comes a time that the object will start to slide.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ In other words , internal friction from defects at the mesoscale cause behavior that is in some ways similar to ordinary sliding friction, where the static coefficient is greater than the kinetic coefficient.

^ In the following damping model for internal friction, Coulombs law of sliding friction is modified by assuming that the coefficient of friction is not constant, but rather involves the energy of oscillation E in a power law; i.e., .

.Since heat quickly dissipates, many early philosophers, including Aristotle, wrongly concluded that moving objects lose energy without a driving force.^ Newton's law of motion says that a force is required to get an object moving (an acceleration in the direction of its motion), or to stop it moving (a negative acceleration in that direction).

^ If there are no forces -- no friction, no air resistance etc -- then an object at rest remains at rest, and a moving object continues moving at a constant speed in a straight line.

When an object is pushed along a surface, the energy converted to heat is given by:
$E_{th} = \mu_\mathrm{k} \int F_\mathrm{n}(x) dx\,$
where
$F_{n}\,$ is the normal force,
$\mu_\mathrm{k}\,$ is the coefficient of kinetic friction,
$x\,$ is the coordinate along which the object transverses.

### 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.^ Assume that there is no sliding friction between the piston and the cylinder.

^ And I touched the thing, and there was no friction.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.In the same reference frame, kinetic friction is always in the direction opposite the motion, and does negative work.^ Newton's law of motion says that a force is required to get an object moving (an acceleration in the direction of its motion), or to stop it moving (a negative acceleration in that direction).

^ Roads are a good image for conceptualizing how friction works: Roads create pathways that make motion easier and more efficient, but in doing so they limit where we go.
• Sample Chapter for Tsing, A.L.: Friction: An Ethnography of Global Connection. 12 January 2010 9:14 UTC press.princeton.edu [Source type: FILTERED WITH BAYES]

[22] 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.^ It is moving, so sure, it's going to be accelerated so the frictional force is in this direction, has this value.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Newton's law of motion says that a force is required to get an object moving (an acceleration in the direction of its motion), or to stop it moving (a negative acceleration in that direction).

[23]
.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).^ The Red Alto® lining has a unique dual saturation which imparts high frictional properties and high heat resistance.

^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ You try, if you can, to avoid contact between two surfaces.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

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.^ The word 'friction' is used in a generic sense, meaning any process responsible for conversion of the oscillator's coherent motion into incoherent thermal activity.

^ Many statistical processes, such as the removal of "noise" from data and computing correlations, are also based on working with Fourier transforms.

^ The same pendulum was used to demonstrate some counterintuitive features of internal friction damping, by replacing the Si wafers with various materials.

.Excessive erosion or wear of mating surfaces occur when work due frictional forces rise to unacceptable levels.^ The external force is associated respectively with ( i ) laminar fluid flow, (ii) induced currents, and (iii) surface friction.

^ The surface friction case is not necessarily the trivial textbook presentation involving a coefficient of kinetic friction and a normal force.

^ With friction, dE/dt is determined by the rate of doing work against the friction force; i.e., dE/dt is proportional to w y f, where f is the friction force.

.Harder corrosion particles caught between mating surfaces (fretting) exacerbates wear of frictional forces.^ Cleanliness of the surfaces is paramount for success in such a demonstration, which speaks to another issue--a connection between internal friction and surface physics.

^ The external force is associated respectively with ( i ) laminar fluid flow, (ii) induced currents, and (iii) surface friction.

^ The larger density of states of the surface (reduced order) is probably an important factor in the difference between surface friction and the modified internal friction model which follows.

Bearing seizure or failure may result from excessive wear due to work of friction. .As surfaces are worn by work due to friction, fit and surface finish of an object may degrade until it no longer functions properly.^ So you have seen difference in friction coefficients and you have seen there's almost no effect on surface area and there's no effect on the mass.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ We note in passing that an object at rest on the earth's surface already has a speed of ωr e cos (latitude), due to the rotation of the earth.

[24]

## References

1. ^ Ruina, Andy; Rudra Pratap (2002) (PDF). Introduction to Statics and Dynamics. Oxford University Press. p. 713. Retrieved 2006-08-04.
2. ^ Hibbeler, R. C. (2007). "Engineering Mechanics" (Eleventh ed.). Pearson, Prentice Hall. p. 393.
3. ^ Soutas-Little, Robert W.; Inman, Balint (2008). "Engineering Mechanics". Thomson. p. 329.
4. ^ a b Beer, Ferdinand P.; E. Russel Johnston, Jr. (1996). "Vector Mechanics for Engineers" (Sixth ed.). McGraw-Hill. p. 397.
5. ^ a b Meriam, J. L.; L. G. Kraige (2002). "Engineering Mechanics" (fifth ed.). John Wiley & Sons. p. 328.
6. ^ Beatty, William J.. "Recurring science misconceptions in K-6 textbooks". Retrieved 2007-06-08.
7. ^ Persson, Bo N. J., Sliding Friction
8. ^
9. ^ a b c d e Engineers: Handbook Friction Coefficients
10. ^ a b c d e f g h
11. ^ Kurt Kleiner (2008-11-21). "Material slicker than Teflon discovered by accident". Retrieved 2008-12-25.
12. ^ Bhavikatti, S. S.; K. G. Rajashekarappa. Engineering Mechanics. p. 112. Retrieved 2007-10-21.
13. ^ Sheppard and Tongue (2005). Statics: Analysis and Design of Sytems in Equilibrium. Wiley and Sons. pp. 618. "In general, for given contacing surfaces, μk < μs"
14. ^ Meriam and Kraige (2002). Engineering Mechanics: Statics. Wiley and Sons. pp. 330. "Kinetic friction force is usually somewhat less than the maximum static friction force."
15. ^ Feynman, Leighton, and Sands (1964). "The Feynman Lectures on Physics, Vol. I, p. 12-5". Addison-Wesley. Retrieved 2009-10-16.
16. ^ Persson and Volokitin (2002). "Theory of rubber friction: Nonstationary sliding". PHYSICAL REVIEW B, VOLUME 65, 134106. Retrieved 2009-10-16.
17. ^ Nichols, Edward Leamington; Franklin, William Suddards (1898). The Elements of Physics. 1. Macmillan. p. 101.
18. ^ Ternes, Markus; Lutz, Christopher P.; Hirjibehedin, Cyrus F.; Giessibl, Franz J.; Heinrich, Andreas J. (2008-02-22), "The Force Needed to Move an Atom on a Surface", Science 319 (5866): 1066–1069, doi:10.1126/science.1150288.
19. ^ Benjamin Silliman, Principles of Physics, Or Natural Philosophy, Ivison, Blakeman, Taylor & company publishers, 710 pages (1871)
20. ^ Hans-Jürgen Butt, Karlheinz Graf, Michael Kappl, Physics and Chemistry of Interfaces, Wiley Publishers, 373 pages, ISBN 3527404139 (2006)
21. ^ [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
22. ^ Den Hartog, J. P. (1961), Mechanics, Courier Dover Publications, p. 142, ISBN 0486607542
23. ^ Leonard, William J (2000). Minds-on Physics. Kendall/Hunt. pp. 603. ISBN 0-7872-3932-1.
24. ^ Bayer, Raymond George (2004). Mechanical wear. CRC Press. pp. 1,2. ISBN 0824746201. Retrieved 2008-07-07.

# Study guide

Up to date as of January 14, 2010

### From Wikiversity

 Run a search on Friction at Wikipedia.
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.Consider a system of two blocks on a rough inclined plane touching each other, making an angle alpha with the horizontal.^ To illustrate the restriction, we consider a block lying on a rough horizontal surface.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

^ The angle of friction for two surfaces in contact is defined as the angle that the maximum contact force makes with the direction of normal force as shown in the figure.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

^ Figure 1: The blocks slide together on a smooth horizontal plane.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

.Let the masses of the blocks be m1 and m2 and let the coefficients or friction between the blocks and the plane be k1 and k2 respectively,where k1>k2. (The block with mass m2 is behind m1 on the inclined plane).^ If the coefficient of friction between the block ...

^ Finding the coefficient of friction on an inclined plane (Urgent ...

^ What is the coefficient of friction between the ladder and the floor?

Find the forces of interaction of the blocks in the process of motion.

# 1911 encyclopedia

Up to date as of January 14, 2010

### From LoveToKnow 1911

FRICTION (from Lat. .fricare, to rub), in physical and mechanical science, the term given to the resistance which every material surface presents to the sliding of any other such surface upon it.^ 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.
• Friction encyclopedia topics | Reference.com 12 January 2010 9:14 UTC www.reference.com [Source type: Reference]

^ We could measure the critical tangential force as a function of N , the area of contact A , the materials and lubricants involved, the surface finish, and other variables such as temperature.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Rolling resistance is the force that resists the rolling of a wheel or other circular objects along a surface caused by deformations in the object and/or surface.
• Friction encyclopedia topics | Reference.com 12 January 2010 9:14 UTC www.reference.com [Source type: Reference]

.This resistance is due to the roughness of the surfaces; the minute projections upon each enter more or less into the minute depressions on the other, and when motion occurs these roughnesses must either be worn off, or continually lifted out of the hollows into which they have fallen, or both, the resistance to motion being in either case quite perceptible and measurable.^ One wonders if the brass may be forced into the steel valleys a good deal more vigorously by the cartridge case internal pressure and the real friction under firing dynamics may well be quite a bit higher.
• Friction Coefficient Tests of 416 Stainless Steel on Cartridge Brass 12 January 2010 9:14 UTC www.varmintal.com [Source type: FILTERED WITH BAYES]

^ Yet they must make these rhetorics work within the compromises and collaborations of their particular situations.
• Sample Chapter for Tsing, A.L.: Friction: An Ethnography of Global Connection. 12 January 2010 9:14 UTC press.princeton.edu [Source type: FILTERED WITH BAYES]

^ Other factors contributing to fiction force are surface roughness and electrostatic effects.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

.Friction is preferably spoken of as "resistance" rather than "force," for a reason exactly the same as that which induces us to treat stress rather as molecular resistance (to change of form) than as force, and which may be stated thus: although friction can be utilized as a moving force at will, and is continually so used, yet it cannot be a primary moving force; it can transmit or modify motion already existing, but cannot in the first instance cause it.^ Friction is the force that resists motion between two bodies.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

^ Rolling Friction When a cylinder rolls on a surface the force resisting motion is termed rolling friction.
• Coefficients Of Friction 12 January 2010 9:14 UTC www.roymech.co.uk [Source type: Academic]

^ Thus, the friction force is proportional to the normal force.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

.For this some external force, not friction, is required.^ External force is required to initiate motion against friction force.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

^ Thus if a box weighs 100 pounds and the coefficient of friction between it and the ground is 0.7, then the force required to push the box along the floor is 70 pounds.
• Standard Friction Equation - Succeed in Understanding Physics: School for Champions 12 January 2010 9:14 UTC www.school-for-champions.com [Source type: FILTERED WITH BAYES]

^ It is generally believed that friction only moderates the acceleration and work of other external forces negatively.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

.The analogy with stress appears complete; the motion of the "driving link" of a machine is communicated to all the other parts, modified or unchanged as the case may be, by the stresses in those parts; but the actual setting in motion of the driving link itself cannot come about by stress, but must have for its production force obtained directly from the expenditure of some form of energy.^ Our production facility can mold most parts net shape and for those that we can not secondary operations are no problem.
• Knott Brake Company, complete brake-product line solutions 330.948.0144 12 January 2010 9:14 UTC www.knottbrake.com [Source type: Academic]

• Sliding friction - encyclopedia article - Citizendium 12 January 2010 9:14 UTC en.citizendium.org [Source type: FILTERED WITH BAYES]

^ Kink On Tap is more than just a netcast about sexuality; it’s also a community of people for whom intelligent conversations about sexuality and how sexuality relates to other aspects of their lives is a motivating force for Doing Good.

.It is important, however, that the use of the term "resistance" should not be allowed to mislead.^ INTEGRAL FASTENER A term used to describe types of fasteners which are highly resistant to vibration loosening and/or removal.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ THREADLOCKER Can be a term used for a number of vibration resistant products but is now usually reserved for threadlocking adhesives.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ It is easy to show how resistance in an electric network is responsible for damping; however, it is a challenge to understand anelastic processes of mechanical damping in terms of viscosity.

.Friction resists the motion of one surface upon another, but it may and frequently does confer the motion of the one upon the other, and in this way causes, instead of resists, the motion of the latter.^ Can friction cause motion?
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

^ Rolling Friction When a cylinder rolls on a surface the force resisting motion is termed rolling friction.
• Coefficients Of Friction 12 January 2010 9:14 UTC www.roymech.co.uk [Source type: Academic]

^ In other words, static friction can never cause relative motion of the surfaces in contact.
• Sliding friction - encyclopedia article - Citizendium 12 January 2010 9:14 UTC en.citizendium.org [Source type: FILTERED WITH BAYES]

This may be made more clear, perhaps, by an illustration. .Suppose we have a leather strap A passing over a fixed cylindrical drum B, and let a pulling force or effort be applied to the strap.^ So let's consider the case – still one dimensional – in which the force is applied over a short distance dx, and that the mass m increases in velocity from v to v+dv.

^ Let's apply a constant force F to a mass m as it moves, in one dimension, a distance x.

^ An 18-kg suitcase is pulled at constant speed by a 43-N force applied to an attached strap..
• Physics Homework Help: Friction and Weight 12 January 2010 9:14 UTC www.physics247.com [Source type: Academic]

.The force applied to A can act on B only at the surfaces of contact between them.^ This expression represents the maximum contact force between a pair of surfaces.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

^ The frictional force acts between surfaces.

^ 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).
• Friction encyclopedia topics | Reference.com 12 January 2010 9:14 UTC www.reference.com [Source type: Reference]

.There it becomes an effort tending either to move A upon B, or to move the body B itself, according to the frictional conditions.^ Draw a free-body diagram showing all the forces (neglecting air resistance) acting upon a rightward-moving car which is skidding to a stop.
• Sliding Friction - Makeup Lab 20 September 2009 7:07 UTC www.glenbrook.k12.il.us [Source type: FILTERED WITH BAYES]

^ The friction will adjust itself so that there is equilibrium if nothing is going.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ We draw the free body diagram superimposed on the body diagram for the condition of maximum static friction as shown in the figure.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

.In the absence of friction it would simply cause A to slide on B, so that we may call it an effort tending to make A slide on B. The friction is the resistance offered by the surface of B to any such motion.^ When it is measured when the surfaces are in relative motion, it is called the "dynamic coefficient of friction".
• Decorating Studio Home Decor Article on Flooring - What is Coefficient of Friction? 20 September 2009 7:07 UTC www.decoratingstudio.com [Source type: FILTERED WITH BAYES]

^ Force to cause sliding of film surfaces ( g f )   .
• Coefficient of Friction 12 January 2010 9:14 UTC www.polyprint.com [Source type: FILTERED WITH BAYES]

^ All you have to do is slide a striker across the friction call's surface.
• Turkey Friction Calls From Knight & Hale Dramatically Improve Turkey Hunting Success 12 January 2010 9:14 UTC www.knightandhale.com [Source type: General]

.But the value of this resistance is not in any way a function of the effort itself, - it depends chiefly upon the pressure normal to the surfaces and the nature of the surfaces.^ The force of friction is dependent on the normal force and the nature of the surfaces.

^ This depends on the nature of surfaces in contact.
• friction@Everything2.com 12 January 2010 9:14 UTC www.everything2.com [Source type: Original source]

^ This means that it depends on the nature of surfaces and is unique for the two surfaces in contact.
• Features of friction force 12 January 2010 9:14 UTC cnx.org [Source type: FILTERED WITH BAYES]

.It may therefore be either less or greater than the effort.^ 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.
• Friction encyclopedia topics | Reference.com 12 January 2010 9:14 UTC www.reference.com [Source type: Reference]

^ You may recall that at the end of Part 1 it was mentioned that static friction is greater than kinetic friction.
• Coefficient of Friction and the Effect of the Normal Force 20 September 2009 7:07 UTC www.cdli.ca [Source type: FILTERED WITH BAYES]

^ For low mass objects, such as electrons, and especially at lower temperatures, momentum*size may be much less than one, and quantum effects may dominate.

.If less, A slides over B, the rate of motion being determined by the excess of the effort over the resistance (friction).^ Sliding friction is less than rolling friction .
• Resistive Force of Friction - Succeed in Understanding Physics: School for Champions 12 January 2010 9:14 UTC www.school-for-champions.com [Source type: FILTERED WITH BAYES]

^ Types of Friction Static Friction The Resistance TO motion when an object is not moving Lets think about the table again, When you first try and move it, The heavy table seems to be "glued in place" and much effort is required to overcome the inertia of this heavy object.

^ Rolling friction is generally considerably less than sliding friction.
• Coefficients Of Friction 12 January 2010 9:14 UTC www.roymech.co.uk [Source type: Academic]

.But if the latter be greater no sliding can occur, i.e. A cannot, under the action of the supposed force, move upon B. The effort between the surfaces exists, however, exactly as before, - and it must now tend to cause the motion of B. But the body B is fixed, - or, in other words, we suppose its resistance to motion greater than any effort which can tend to move it, - hence no motion takes place.^ Friction is the force that resists motion between two bodies.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

^ The amount of travel before any action takes place.
• Automotive Dictionary - "FR" 12 January 2010 9:14 UTC www.motorera.com [Source type: FILTERED WITH BAYES]

^ The friction force between two bodies that are moving relative to each other is “kinetic friction”.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

.It must be specially noticed, however, that it is not the friction between A and B that has prevented motion, this only prevented A moving on B, - it is the force which keeps B stationary, whatever that may be, which has finally prevented any motion taking place.^ When a force is applied to the part that is to be moved, this force must overcome the static friction.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

^ Friction is the force that resists motion between two bodies.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

^ Calculate the frictional force required to keep the block moving uniformly.
• Coefficient of Friction and the Effect of the Normal Force 20 September 2009 7:07 UTC www.cdli.ca [Source type: FILTERED WITH BAYES]

This can be easily seen. .Suppose B not to be fixed, but to be capable of moving against some third body C (which might, e.g., contain cylindrical bearings, if B were a drum with its shaft), itself fixed, - and further, suppose the frictional resistance between B and C to be the only resistance to B's motion.^ Friction is the force that resists motion between two bodies.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

^ Kinetic friction, the resisting force that occurs between moving bodies, is independent of velocity.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

^ The limiting friction between second and third is : .
• Friction (application) 12 January 2010 9:14 UTC cnx.org [Source type: General]

.Then if this be less than the effort of A upon B, as it of course may be, this effort will cause the motion of B. Thus friction causes motion, for had there been no frictional resistance between the surfaces of A and of B, the latter body would have remained stationary, and A only would have moved.^ Friction is the force that resists motion between two bodies.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

^ The friction resistance is almost independent of the specific pressure between the surfaces.
• Coefficients Of Friction 12 January 2010 9:14 UTC www.roymech.co.uk [Source type: Academic]

^ Rolling friction is friction between a wheel and the surface on which it is moving.
• friction Facts, information, pictures | Encyclopedia.com articles about friction 12 January 2010 9:14 UTC www.encyclopedia.com [Source type: Academic]

In the case supposed, therefore, the friction between A and B is a necessary condition of B receiving any motion from the external force applied to A.
.Without entering here on the mathematical treatment of the subject of friction, some general conclusions may be pointed out which have been arrived at as the results of experiment.^ With its distinguished editor and contributions from some of the world’s leading authorities in the subject, Friction in textile materials will be a standard reference for the textile industry and those researching this important topic.

^ It uses a rotating tool to generate frictional heat that causes material of the components to be welded to soften without reaching the melting point and allows the tool to move along the weld line.

^ Saj P. from Zero Friction Marketing has discovered some new ways to get a lot of visitors to your CPA offers without the need of Google.

.The "laws" first enunciated by C. A. Coulomb (1781), and afterwards confirmed by A. J. Morin (1830-1834), have been found to hold good within very wide limits.^ Exceptions to Coulomb's laws: First Law: The frictional properties of some very hard materials such as diamonds and certain very soft materials such as Dupont's Teflon do not obey the first law.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

.These are: (1) that the friction is proportional to the normal pressure between the surfaces of contact, and therefore independent of the area of those surfaces, and (2) that it is independent of the velocity with which the surfaces slide one on the other.^ For low surface pressures the friction is directly proportional to the pressure between the surfaces.
• Coefficients Of Friction 12 January 2010 9:14 UTC www.roymech.co.uk [Source type: Academic]

^ It's also independent of the surface area that is in contact with this incline which is equally nonintuitive.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ The friction resistance is almost independent of the specific pressure between the surfaces.
• Coefficients Of Friction 12 January 2010 9:14 UTC www.roymech.co.uk [Source type: Academic]

.For many practical purposes these statements are sufficiently accurate, and they do in fact sensibly represent the results of experiment for the pressures and at the velocities most commonly occurring.^ Commonly occurs as a result of embedment but can also be due to gasket creep, metal creep (at elevated temperatures), differential thermal expansion and stress relaxation.
• Glossary of Terminology Related to Nuts and Bolts 12 January 2010 9:14 UTC www.boltscience.com [Source type: Reference]

^ In the range from 100 mTorr to 8 Torr, the pressure could not be accurately measured with either of these gauges.
• Internal Friction Damping of a Harmonic Oscillator 12 January 2010 9:14 UTC arxiv.org [Source type: Academic]

^ Instead of integrating the second-order equation of motion twicefirst the acceleration, followed by the resulting velocity; more accurate results are obtained by integrating the equivalent pair of first order equations.

.Assuming the correctness of these, friction is generally measured in terms simply of the total pressure between the surfaces, by multiplying it by a "coefficient of friction" depending on the material of the surfaces and their state as to smoothness and lubrication.^ This instrument is used to measure friction between polymeric surfaces.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ For low surface pressures the friction is directly proportional to the pressure between the surfaces.
• Coefficients Of Friction 12 January 2010 9:14 UTC www.roymech.co.uk [Source type: Academic]

^ The friction resistance is almost independent of the specific pressure between the surfaces.
• Coefficients Of Friction 12 January 2010 9:14 UTC www.roymech.co.uk [Source type: Academic]

.But beyond certain limits the "laws" stated are certainly incorrect, and are to be regarded as mere practical rules, of extensive application certainly, but without any pretension to be looked at as really general laws.^ Practice operating the controls without looking at them.
• Motorcycle friction zone,Motorcycle Safety Foundation 12 January 2010 9:14 UTC www.womenridersnow.com [Source type: General]

^ To move beyond this it is important to see generalization to the universal as an aspiration, an always unfinished achievement, rather than the confirmation of a pre-formed law.
• Sample Chapter for Tsing, A.L.: Friction: An Ethnography of Global Connection. 12 January 2010 9:14 UTC press.princeton.edu [Source type: FILTERED WITH BAYES]

^ Internet Marketing Strategy , Lead Generation , Practical Application friction , Lead Generation Rate , quality leads , two dials analogy .

.Both at very high and very low pressures the coefficient of friction is affected by the intensity of pressure, and, just as with velocity, it can only be regarded as independent of the intensity and proportional simply to the total load within more or less definite limits.^ This is a very high value of friction coefficient.
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Frictional force is proportional to the load.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

^ It is very nonintuitive that this friction coefficient is completely independent of the mass.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

.Coulomb pointed out long ago that the resistance of a body to be set in motion was in many cases much greater than the resistance which it offered to continued motion; and since his time writers have always distinguished the "friction of rest," or static friction, from the "friction of motion," or kinetic friction.^ This is because the frictional force for a stationary object (the static friction ) is greater than the frictional force for an object that is moving (the kinetic friction ).
• Coefficient of Friction and the Effect of the Normal Force 20 September 2009 7:07 UTC www.cdli.ca [Source type: FILTERED WITH BAYES]

^ Kinetic friction is greater than the static friction .
• Resistive Force of Friction - Succeed in Understanding Physics: School for Champions 12 January 2010 9:14 UTC www.school-for-champions.com [Source type: FILTERED WITH BAYES]

^ Stick-slip occurs because static friction is usually greater than kinetic friction.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

.He showed also that the value of the former depended often both upon the intensity of the pressure and upon the length of time during which contact had lasted, both of which facts quite agree with what we should expect from our knowledge of the physical nature, already mentioned, of the causes of friction.^ Static friction depends on the length of contact time.
• http://www.stle.org/resources/lubelearn/friction/default.aspx 12 January 2010 9:14 UTC www.stle.org [Source type: FILTERED WITH BAYES]

^ Therefore, the dependence upon any one or combination of these physiological properties to provide or to enhance lubricity of a tampon is quite risky.
• Method of reducing the coefficient of friction of absorbent products and wax coated products produced thereby - Patent EP0692263 20 September 2009 7:07 UTC www.freepatentsonline.com [Source type: Reference]

^ As also noted in [Nelson 1986], this form of fluid friction caused a significant amplitude dependent damping.

It seems not unreasonable to expect that the influence of time upon friction should show itself in a comparison of very slow with very rapid motion, as well as in a comparison of starting (i.e. motion after a long time of rest) with continued motion. That the friction at the higher velocities occurring in engineering practice is much less than at common velocites has been shown by several modern experiments, such as those of Sir Douglas Galton (see Report Brit. Assoc., 1878, and Proc. Inst. Mech. .Eng., 1878, 1879) on the friction between brake-blocks and wheels, and between wheels and rails.^ The limiting friction between first block and ground is : .
• Friction (application) 12 January 2010 9:14 UTC cnx.org [Source type: General]

^ Context: Bicycle brakes use friction to stop the wheels from turning.

^ Rolling friction is friction between a wheel and the surface on which it is moving.
• friction Facts, information, pictures | Encyclopedia.com articles about friction 12 January 2010 9:14 UTC www.encyclopedia.com [Source type: Academic]

But no increase in the coefficient of friction had been detected at slow speeds, until the experiments of Prof. Fleeming Jenkin (Phil. Trans., 1877, pt. 2) showed conclusively that at extremely low velocities (the lowest measured was about 0002 ft. per second) there is a sensible increase of frictional resistance in many cases, most notably in those in which there is the most marked difference between the friction of rest and that of motion. .These experiments distinctly point to the conclusion, although without absolutely proving it, that in such cases the coefficient of kinetic friction gradually increases as the velocity becomes extremely small, and passes without discontinuity into that of static friction.^ We make a distinction between static friction coefficients and kinetic.
• MIT OpenCourseWare | Physics | 8.01 Physics I: Classical Mechanics, Fall 1999 | Video Lectures | detail 12 January 2010 9:14 UTC ocw.mit.edu [Source type: Original source]

^ Static and kinetic friction .
• Introduction to Statics: Friction 12 January 2010 9:14 UTC www.engin.brown.edu [Source type: FILTERED WITH BAYES]

^ Typically the value for the coefficient of kinetic friction is less than that for static friction.
• Friction an introduction and example | Dot Physics 12 January 2010 9:14 UTC blog.dotphys.net [Source type: FILTERED WITH BAYES]

(A. B. W. K.; W. E. D.)

# Simple English

Friction is a force that acts to stop the movement of two touching things. The energy lost to friction is turned into sound and heat. Two kinds of friction are static and kinetic. Friction is static friction is when the friction is strong enough to stop movement between two objects. Kinetic friction is when the frictional force is not strong enough to stop all motion.

• Friction can be thought to be caused by bumps on two touching surfaces. These bumps are called asperities. As these two rough surfaces move against each other they get stuck and prevent motion until the asperities are broken or slid over. Even the smoothest materials have these asperities.
• As the force that is pushing on an object increases, the static friction increases too. However, if the force gets bigger than the maximum static friction, it makes the object accelerate (begin to move). The thing will begin to accelerate as soon as the static friction is less than the force pushing on it. After that point kinetic friction will act on the object.
• The coefficient of friction is the ratio between the force slowing something down and the force pushing them together. The larger it is, the stronger friction is ( and the more the object is slowed down). For example, a person's skates on ice have a small coefficient of friction, but rubber on concrete has a big one.
• Internal friction in a fluid is called viscosity. Viscosity is how hard it is for the fluid to move or flow. For example, honey is more "viscous" than water.
• Friction is really caused by electromagnetism. When the atoms that make the objects come close they are drawn to each other and may connect. Effort is needed to break this connection in objects that are not moving and effort is needed to keep an object moving against this attraction. Things with high friction have many points of contact or have atoms that strongly attract others.

# Citable sentences

Up to date as of December 17, 2010

Here are sentences from other pages on Friction, which are similar to those in the above article.