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. This is a simple empirical model, which happens to do a reasonably good job in many situations--typically static cases or low velocity cases involving contact between two dry, solid surfaces.
The coefficient of friction depends on both of the materials used; for example, ice on steel has a low coefficient of friction, while rubber on pavement has a high coefficient of friction. Steel on brass will not necessarily have the same COF as steel on polished wood, etc. Coefficients of friction range from near zero to greater than one. A negative coefficient of friction would not make sense as it would generally lead to violations of energy conservation.
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