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Friction
Friction is the force that opposes the relative motion or tendency of such motion of two surfaces in contact. It is also the contact of two objects creating static electricity. more...
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It is not, however, a fundamental force, as it originates from the electromagnetic forces and exchange force between atoms. In situations where the surfaces in contact are moving relative to each other, the friction between the two objects converts kinetic energy into sensitive energy, or heat (atomic vibrations). Friction between solid objects and fluids (gases or liquids) is called fluid friction. See also aerodynamics and hydrodynamics.
Equations
The classical approximation of the force of friction known as Coulomb friction (named after Charles-Augustin de Coulomb) is expressed as:
 where-
- μ is the coefficient of friction,
- N is the normal force to the contact surface,
- Ff is the maximum possible force exerted by friction.
This force is exerted in the direction opposite the object's motion, thus it is an opposing force.
This law mathematically follows from the fact that contacting surfaces have atomically close contacts only over extremely small fraction of their overall surface area, and this contact area is proportional to load (until saturation which takes place when all area is in atomic contact thus no further increase of friction force takes place).
This simple (although incomplete) representation of friction is adequate for the analysis of many physical systems.
Coefficient of friction
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The coefficient of friction (also known as the frictional coefficient) is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction 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). Coefficients of friction need not be less than 1 - under good conditions, a tire on concrete may have a coefficient of friction of 1.7. Magnetically attractive surfaces can have very large friction coefficients, and, theoretically, glued or welded together surfaces have infinite friction coefficients.
Sliding (kinetic) friction and static friction are distinct concepts. 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.
Read more at Wikipedia.org
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