Coefficient of friction
If the simple experiments we have described are repeated with various weights on the block it is found that both the static and the kinetic friction are increased roughly in simple proportion to the force, perpendicular to the surfaces, which is pressing them together.
The ratios of the static and kinetic friction to the force pressing the surfaces together are called the coefficients of static and kinetic friction respectively. Coefficient of friction is denoted by the Greek letter )1 (mu).
F where F = force of friction
R and R = force pressing surfaces together
In the seventeenth century these coefficients were thought to be constants, but we now know that this is far from being the case especially under conditions of fast slipping and high contact pressure.