موضوع: Coefficient of friction الخميس 29 ديسمبر 2011 - 22:32
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. The coefficient of friction depends on the materials used; for example, ice on steel has a low coefficient of friction, while rubber on pavement has a high coefficient of friction. Coefficients of friction range from near zero to greater than one – under good conditions, a tire on concrete may have a coefficient of friction of 1.7
For surfaces at rest relative to each other , where is the coefficient of static friction. This is usually larger than its kinetic counterpart.
For surfaces in relative motion , where is the coefficient of kinetic friction. The Coulomb friction is equal to , and the frictional force on each surface is exerted in the direction opposite to its motion relative to the other surface.
It was Arthur-Jules Morin who introduced the term and demonstrated the utility of the coefficient of friction. The coefficient of friction is an empirical measurement – it has to be measured experimentally, and cannot be found through calculations. Rougher surfaces tend to have higher effective values. Both static and kinetic coefficients of friction depend on the pair of surfaces in contact; for a given pair of surfaces, the coefficient of static friction is usually larger than that of kinetic friction; in some sets the two coefficients are equal, such as teflon-on-teflon.
Most dry materials in combination have friction coefficient values between 0.3 and 0.6. Values outside this range are rarer, but teflon, for example, can have a coefficient as low as 0.04. A value of zero would mean no friction at all, an elusive property – even magnetic levitation vehicles have drag. Rubber in contact with other surfaces can yield friction coefficients from 1 to 2. Occasionally it is maintained that µ is always < 1, but this is not true. While in most relevant applications µ < 1, a value above 1 merely implies that the force required to slide an object along the surface is greater than the normal force of the surface on the object. For example, silicone rubber or acrylic rubber-coated surfaces have a coefficient of friction that can be substantially larger than 1.
While it is often stated that the COF is a "material property," it is better categorized as a "system property." Unlike true material properties (such as conductivity, dielectric constant, yield strength), the COF for any two materials depends on system variables like temperature, velocity, atmosphere and also what are now popularly described as aging and deaging times; as well as on geometric properties of the interface between the materials. For example, a copper pin sliding against a thick copper plate can have a COF that varies from 0.6 at low speeds (metal sliding against metal) to below 0.2 at high speeds when the copper surface begins to melt due to frictional heating. The latter speed, of course, does not determine the COF uniquely; if the pin diameter is increased so that the frictional heating is removed rapidly, the temperature drops, the pin remains solid and the COF rises to that of a 'low speed' test
Approximate coefficients of friction
Materials
Static friction,
Dry & clean
Lubricated
Aluminium
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[10]
Copper
Glass
0.68
Glass
Glass
0.94
Metal
Wood
0.2–0.6[10]
0.2 (wet)[10]
Polyethene
Steel
0.2[11]
0.2[11]
Steel
Steel
0.80[11]
0.16[11]
Steel
PTFE
0.04[11]
0.04[11]
PTFE
PTFE
0.04[11]
0.04[11]
Wood
Wood
0.25–0.5[10]
0.2 (wet)[10]
The most slippery solid known, discovered in 1999, dubbed BAM (for the elements boron, aluminium, and magnesium), has an approximate coefficient of friction of 0.02, about half that of PTFE. Under certain special conditions some materials have even lower friction coefficients. An example is (highly ordered pyrolytic) graphite, of which the coefficient can drop below 0.01. This regime is also called superlubricity.
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الخميس 21 نوفمبر 2019 - 12:14
موضوع: Coefficient of friction 
الخميس 29 ديسمبر 2011 - 22:32
, where 
is the coefficient
, where 
is the coefficient
, and the frictional
