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In physics, friction is the non-conservative resistive force that occurs when two surfaces travel along each other when forced together. It causes physical deformation and heat buildup.
The frictional force is a function of the force pressing the surfaces together and the coefficient of friction between the materials. In particular:
where
= the force of friction
= the force perpendicular to the contact surface (also called the normal force, 
)
= the coefficient of friction, either static (
) or kinetic/dynamic (
/
)
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Coefficient of Friction
The coefficient of friction (also known as the frictional coefficient or the friction coefficient) is a scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used -- for example, ice on metal has a low coefficient of friction (they slide past each other easily), while rubber on pavement has a high coefficient of friction (they do not slide past each other easily).
It is also important to discriminate between sliding (dynamic) friction and static friction. For sliding friction, the force of friction does not vary with the area of contact between the two objects. This means that sliding friction does not depend on the size of the contact area. However, for static friction where there is an element of adhesion, the contact area does matter. For a race car, wide wheels are used to increase the static friction with the road. However, once adhesion is lost, the size of the contact area is no longer relevant.
The force of friction is always exerted in a direction that opposes movement. For example, a chair sliding to the right across a floor experiences the force of friction in the left direction.
The coefficient of friction is an empirical measurement -- it has to be measured experimentally, and cannot be found through calculations. Most dry materials in combination give friction coefficient values from 0.3 to 0.6. It is difficult to maintain values outside this range. A value of 0.0 would mean there is no friction. Note that a system with "interlocking teeth" between surfaces isn't really the same thing as surface friction. Rougher surfaces tend to have higher values.
Saying that rougher surfaces experience more friction sounds safe enough - two pieces of coarse sandpaper will obviously be harder to move relative to each other than two pieces of fine sandpaper. However, if two pieces of flat metal are made progressively smoother, you will reach a point where the resistance to relative movement increases.
If you make them very flat and smooth, and remove all surface contaminants in a vacuum, the smooth flat surfaces will actually adhere to each other, making what is called a cold weld. Once you reach a certain degree of mechanical smoothness, the frictional resistance is found to depend on the nature of the molecular forces in the area of contact, so that substances of comparable "smoothness" can have significantly different coefficients of friction.
Types of Friction
Static Friction
Static friction occurs when the two objects are not moving relative to each other (like a desk on the ground). The coefficient of static friction is typically denoted as μs. The initial force to get an object moving is often dominated by static friction.
- Rolling friction occurs moving relative to each other and one "rolls" on the other (like a car's wheels on the ground). The coefficient of rolling friction is typically denoted as μr.
Kinetic Friction
Kinetic friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as μk, and is usually less than the coefficient of static friction.
Examples of kinetic friction:
- Sliding friction is when two objects are rubbing against each other. Putting a book flat on a desk and moving it around is an example of sliding friction.
- Fluid friction is the friction between a solid object as it moves through a liquid or a gas. The drag of air on an airplane or of water on a swimmer are two examples of fluid friction.
When an object is pushed along a surface with coefficient of friction μk and a perpendicular (normal) force acting on that object directed towards the surface of magnitude N, then the energy loss of the object is given by:
Where d is the distance travelled by the object whilst in contact with the surface. This equation is identical to Energy Loss = Force x Distance as the frictional force is a non-conservative force. Note, this equation only applies to kinetic friction, not rolling friction.
Physical deformation is associated with friction. While this can be beneficial, as in polishing, it is often a problem, as the materials are worn away, and may no longer hold the specified tolerances.
The work done by friction can translate into deformation and heat that in the long run may affect the surface's specification and the coefficient of friction itself. Friction can in some cases cause solid materials to melt.
Friction may occur between solids, gases and fluids or any combination thereof. See aerodynamics and hydrodynamics.
Reducing Friction
Devices
Devices, such as ball bearings, can change sliding friction into the less significant rolling friction.
Techniques
One technique used by railroad engineers is to back up the train to create slack in the linkages between cars. This allows the train to pull forward and only take on the static friction of one car at a time, instead of all cars at once, thus spreading the static frictional force out over time.
Lubricants
A common way to reduce friction is by using a lubricant, such as oil, that is placed between the two surfaces, often dramatically lessening the coefficient of friction. The science of friction and lubrication is called tribology. Superlubricity, a recently-discovered effect, has been observed in graphite: it is the substantial decrease of friction between two sliding objects, approaching zero levels - a very small amount of frictional energy would be dissipated due to electronic and/or atomic vibrations.
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.
Lubricant Technology
AF coatings (anti-friction coatings) have been successfully used for years as an element of heavy-duty lubrication. Typically used for applications where a permanent lubricating film is needed for metal-to-plastic or plastic-to-plastic lubrication, AF coating technology offers an economic solution to a wide range of engineering problems.
The usage of AF coatings, such as Molykote® brand or other prominent anti-friction coating brand, is most successful when requirements for wear and corrosion protection and optimal coefficient of friction are properly met. A low, high, or even constant coefficient of friction is achievable, if the appropriate application and type of AF coating is utilized.
A firm, completely dry, and non-contaminating lubricating film results once it is properly prepared and applied. The AF coating generally consists of the resin (epoxy, phenolic, and silicone) - a base material, which adheres well to the surface. Solid lubricants such as MoS2, PTFE, polyamide, polyethylene, and graphite are set in this base material, passing on the anti-friction properties of an AF coating.
Water-dilutable AF coatings, coatings low in solvents, as well as non-combustible or electrostactically sprayable AF coatings, are now being offered to help save energy and meet environmental protection regulations.
Many products using AF technology offer corrosion protection in excess of normal industrial requirements, while some are unaffected by fuels, solvents, or oils.
Application is typically simple: preferably by spraying, dipping, or brushing on thoroughly degreased metal surfaces. The drying and curing times are short (between three minutes for air-drying and sixty minutes for oven cured coatings).
There are also lubricants used for sexual stimulation, including KY jelly and Astroglide.
Products of friction
According to the law of conservation of energy, no energy should be lost due to friction. The kinetic energy lost is transformed primarily into heat and/or motion of other objects and fluids. An airplane will heat and accelerate the air as it passes. A submarine will do the same to the water. In some cases, the "other object" to be accelerated may be the Earth. A sliding hockey puck will come to rest due to friction both by changing it's energy into heat and accelerating the Earth in it's direction of travel (by an immeasurable amount). Since heat and fluid motion quickly dissipate and the change in velocity of the Earth can't be seen, many early philosophers, such as Aristotle, concluded that moving objects lose energy without an opposing force.
References
- Tipler, Paul (1998). Physics for Scientists and Engineers: Vol. 1 (4th ed.), W. H. Freeman. ISBN 1572594926