Friction plays a huge role in technology and in nature. Thanks to its action, pedestrians and cars move, and objects located on inclined surfaces do not slide off them. In this article, we consider the physical causes of the occurrence of friction forces.
The causes of the appearance of rest friction
In physics, several types of friction forces are considered . We begin by discussing the friction of rest. It is a contact interaction between two solid surfaces, which prevents any relative displacement. For example, in order to move a cabinet or boxes out of place, it is necessary to overcome this force.
The causes of the rest friction force are the roughness of the contacting surfaces and weak electrochemical interactions between the atoms and molecules that form the bodies in question.
Any hard surface, however smooth it may be, has microscopic peaks and troughs of irregular shape. When one body rests on the surface of another, then some peaks of the first body fall into the troughs of the second, and vice versa. There is a more or less effective mechanical adhesion of the surfaces. Obviously: in order to move the objects under consideration, it will be necessary to overcome this cohesion.
In addition to the mechanical reason, the electrochemical interaction of atoms and molecules of different bodies also contributes to the static friction . This interaction can be dipole or ionic (Coulomb) in nature.
We note that the stated reasons and the mechanism of the appearance of rest friction explain why the corresponding force does not depend on the contact area of the bodies. When this area is large, the deformation of the peaks and troughs is negligible, which reduces the efficiency of one microscopic contact. However, there are many such contacts. If the body is turned over so that the contact area becomes smaller, then the deformation of the peak-trough pair becomes larger, the contact efficiency increases, but the contacts themselves become smaller.
What are the causes of the sliding friction force
The next force we will consider is the sliding friction force. It occurs in the contact area of two solid surfaces when the bodies glide relative to each other. This force is always smaller in magnitude than the rest friction force, however, it also does not depend on the contact area.
The causes of the sliding friction force are similar to those for rest friction. The slip feature consists only in the fact that the microscopic peaks and troughs of the surfaces do not have time to deform strongly during movement and make close mechanical contact with each other. Therefore, the sliding force is less than the frictional force at rest for the same pair of bodies.
The knowledge of the named cause of the occurrence of the sliding friction force is used in technology to reduce or increase it. For example, the use of grease for skis reduces the roughness of their surface, which leads to a decrease in friction. On the contrary, the use of salt and sand on slippery roads is designed to modify the surface in the direction of increasing its roughness.
Why does rolling friction appear?
Another type of friction between solids is rolling friction. It manifests itself when any body rolls on the surface, for example, a car wheel on asphalt.
What is the cause of the rolling friction force? Naturally, the roughness of the contacting surfaces plays a role in slowing down the movement. However, this is not the main reason. The main reason is deformation hysteresis. At the point of contact of the wheel with the surface, some slight elastic deformation occurs. When the wheel rolls, each zone of its surface experiences a cyclic deformation load. Incomplete restoration of the original wheel shape after deformation is called hysteresis. It is he who is responsible for the conversion of mechanical energy into thermal energy, which in practice is manifested in the form of rolling friction.
Cause of friction when moving in gases and liquids
Everyone knows what the wind and sea currents are. From the point of view of physics, these processes are the directed movement of a huge number of atoms and molecules of which fluid substances are composed. The reason for the appearance of the friction force in these substances lies in the numerous collisions with the molecules of gases and liquids of a solid when it moves in fluid substances. Each molecule in a collision with a body gives it an impulse directed against the displacement vector. The magnitude of this impulse is small, but the number of molecules is huge, so the resulting friction force is macroscopic in nature.