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Lecture number 4.
Classification of friction theories
Friction is always accompanied by the transition of mechanical energy into its other forms. This leads to heating, electrification, oxidation of rubbing bodies and other changes in the contact zone. These transitions have a significant effect on the laws of friction.
4.1 Terminology
According to the kinematic features, sliding and rolling friction are distinguished.
Sliding friction occurs when the velocities of contacting bodies at the points of contact are different.
Rolling friction occurs when the velocities of moving solid bodies in contact at the points of contact are the same in value and direction.
Both types of friction appear together if rolling is accompanied by slippage.
In GOST 276774 - 88 there is the term "static friction", which sounds strange, because the concept of "friction" implies the mutual movement of bodies.
Friction of rest - friction of two bodies during microdisplacements preceding the transition to relative motion.
The dependence of the force required to move a rigid body located on the surface of another rigid body on the sliding path is shown in Fig. 4.1. The static friction force F _n is greater than the sliding friction force F _c . The application of force F _i < F _n causes the movement of S _i bodies. It is due to the deformation of the surface layers of the contacting bodies.
Pre-displacement - the relative movement of two solid bodies during friction within the transition from a state of rest to relative motion. It is small in magnitude (of the order of 0.1–5 µm) and partially reversible, i.e. as the shear force decreases, S also decreases.



Fig 4.1 Dependence of friction force on displacement. Explanations in the text

The phenomenon of displacement significantly affects the rigidity of machine tools.

In the 18th century, two laws of friction were established: Amonton's law F= fN and Coulomb's law F= fN + A , where F is the friction force, f is the coefficient of friction, N is the normal load, A is the friction force independent of N.
Coulomb's law is applicable in cases where significant forces of intermolecular interaction of contacting bodies arise during friction and Amonton's law is not observed. So, when squeezing samples of plastic materials (lead, tin), the area of actual contact and, accordingly, the adhesion force are large. The latter significantly affect the friction during the movement of powders over the surface of solids. In the Coulomb formula, the intermolecular interaction of contacting bodies is taken into account using the empirical coefficient A.