<P> If an object is on a level surface and the force tending to cause it to slide is horizontal, the normal force N (\ displaystyle 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 . 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 . Therefore, the normal force, and ultimately the frictional force, is determined using vector analysis, usually via a free body diagram . Depending on the situation, the calculation of the normal force may include forces other than gravity . </P> <P> The coefficient of friction (COF), often 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 . It is an axiom of the nature of friction between metal surfaces that it is greater between two surfaces of similar metals than between two surfaces of different metals--hence, brass will have a higher coefficient of friction when moved against brass, but less if moved against steel or aluminum . </P> <P> For surfaces at rest relative to each other μ = μ s (\ displaystyle \ mu = \ mu _ (\ mathrm (s)) \,), where μ s (\ displaystyle \ mu _ (\ mathrm (s)) \,) is the coefficient of static friction . This is usually larger than its kinetic counterpart . The coefficient of static friction exhibited by a pair of contacting surfaces depends upon the combined effects of material deformation characteristics and surface roughness, both of which have their origins in the chemical bonding between atoms in each of the bulk materials and between the material surfaces and any adsorbed material . The fractality of surfaces, a parameter describing the scaling behavior of surface asperities, is known to play an important role in determining the magnitude of the static friction . </P> <P> For surfaces in relative motion μ = μ k (\ displaystyle \ mu = \ mu _ (\ mathrm (k)) \,), where μ k (\ displaystyle \ mu _ (\ mathrm (k)) \,) is the coefficient of kinetic friction . The Coulomb friction is equal to F f (\ displaystyle F_ (\ mathrm (f)) \,), and the frictional force on each surface is exerted in the direction opposite to its motion relative to the other surface . </P>

The coefficient of static friction between two surfaces depends upon
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