In terrestrial conditions, any moving bodies (or coming into motion) come into contact with the environment or with other bodies. In this case, forces arise that resist their movement. These forces are called friction forces, they translate part of the mechanical energy of motion into internal energy, which is accompanied by heating of bodies and the environment.
Friction is external and internal. Internal (also called viscosity) is the occurrence of a tangential force between moving layers of liquid or gas, which interferes with this movement.
In contrast, external friction arises at the contact points of solids in the form of a force tangential to their surface and hindering their mutual displacement. It, in turn, is divided into static ( static friction) and kinematic. Static friction is manifested when trying to move one motionless body relative to another. Kinematic exists between moving bodies in contact with each other. External friction can be divided into sliding and rolling friction.
What is the physical meaning of friction? Is it useful or harmful? At first glance, friction only bothers us: the details of mechanisms, car tires wear out, the soles of shoes are erased, and so the creation of a perpetual motion machine is impossible only for this reason. But take a closer look. Friction will disappear - we can neither walk, nor flip through a book, nor move a car, nor stop a moving one. A huge number of physical phenomena in the world are based on friction. The two main achievements of mankind that determined the development of civilization - the production of fire and the invention of the wheel - would not have been possible without it.
This phenomenon is based on the unevenness of any bodies: upon contact, the notches of one always cling to the roughness of the other. For perfectly smooth (for example, carefully sanded) surfaces that are tightly adjacent to each other, the laws of molecular friction based on the mutual attraction of molecules apply.
Studies friction science tribology. In 1781, the French physicist S. Coulomb formulated the basic laws of dry friction. Empirically, the scientist found that the friction force F arising during sliding is directly proportional to the force N of normal pressure acting on the body. This dependency is as follows:
N: F = k ∙ N;
where k is the coefficient of friction (proportionality coefficient). Its value was calculated as follows: the body was placed on an inclined plane and its uniform movement was achieved by changing the angle of inclination . In this case, the friction force F was equal to the driving force P:
F = P ∙ sin a;
The magnitude of the force N (normal pressure force) is equal to P ∙ cos a; hence k = tan a. The friction coefficient from here is the tangent of the angle of inclination of the surface over which the body glides uniformly, i.e., at a constant speed.
In practice, its value can only be calculated approximately. The surfaces of bodies, as a rule, are more or less contaminated, have oxides, rust and other inclusions. The friction coefficient, determined in pairs for combinations of various materials through experiments, is entered in special look-up tables.
When rolling, friction arises because the moving wheel is slightly pressed into the road surface, that is, it is forced to overcome a small tubercle. The harder the road, the less this tubercle and less friction. Its value is calculated in this case by the formula: F = k ∙ N / r, in which r is the value of the radius of the wheel. Therefore, the rolling friction coefficient has a dimension of extent. Usually it is expressed in centimeters, in contrast to the coefficient of sliding friction, which is a dimensionless quantity.
As mentioned above, the coefficient of internal friction exists not only for solids, but also for liquids. In hydraulics, it is often necessary to calculate the specific energy losses of hydraulic systems that occur in pipelines. They are of two types: length losses that occur in straight pipes with a uniform flow, and local losses, the cause of which is a deformation of the flow due to a change in the shape of the channel (narrowing, expansion, turns). Hydraulic losses are calculated using a similar value called the “hydraulic friction coefficient”.