Friction is a force that resists the relative motion of two bodies in contact with each other. The magnitude of the frictional force, or simply friction, depends on the nature of the surfaces in contact and their roughness, as well as on the normal force pressing them together. The direction of the frictional force is always opposite to that of the relative motion. In dry (solid-solid) friction, such as rubbing your hands together to generate heat, both surfaces are pressed firmly together so that intermolecular attractive forces can act between them. The coefficient of friction is a measure of how strongly these forces resist relative motion and is independent of the area over which they act.
The static coefficient of friction (µs) is larger than the kinetic coefficient of friction (µk). This makes intuitive sense because it takes more effort to start sliding than it does to keep sliding once you have started. It also takes more effort to slide one object across another than it does to slide both objects together in the same direction. The difference between µs and µk reflects this fact: when starting from rest (“static”), there must be enough force applied so that Ff = µsN; but once moving (“kinetic”), only enough force must be applied so that Ff = µkN .
In addition, there are different types of tribological contacts: rolling contact, sliding contact or impact. Different types will result in different Coefficients Of Friction (COF), even if material and surface roughness remain constant. For example: brass on glass has a lower COF in rolling contact than in either sliding or impact.
Static coefficients typically range from 0.1 for lubricated steel on steelto around 0.8 for unlubricated cast iron on cast iron. Kinetic coefficients are usually lower than static coefficients by a factor ranging from 2 to 10, although values close to 1 are not unheard of – e.g., PTFE against austenitic stainless steel, or polymers against themselves.