Laws of Friction

These laws were stated by Coulomb in the year 1781. They are following :
1.    The frictional force is proportional to normal force between surfaces of contact.
2.    The force of friction is independent of apparent area of contact.

Clearly the two laws together make us to conclude that the force of friction shall not depend upon the area of contact however only upon mutual normal reaction. From first statement if

F = force of friction
RN = normal reaction

Then    F ∝ RN
or    F = μ RN

Here the constant of proportionality μ is called as coefficient of friction which based upon the nature of contacting surfaces in case of dry friction and upon nature of lubricant in case of film or fluid lubrication. However, it may be pointed out that in case of film lubrication it is effective coefficient of friction that determines the force of friction. The effective coefficient of friction shall depend upon the viscosity of lubricant that is understood as force required causing shearing of layers of lubricant.