Any contact between two bodies leads to the appearance of friction. It doesn’t matter in what aggregate state of the substance the bodies are, they move relative to each other or are at rest. This article will briefly consider what types of friction exist in nature and technology.
Friction of rest
For many, it may be strange to think that the friction of bodies exists even when they are at rest relative to each other. In addition, this friction force is the largest force among the other types. It manifests itself when we try to move an object from its place. It can be a wooden block, stone and even a wheel.
The reason for the existence of the rest friction force is the presence of irregularities on the contacting surfaces, which mechanically interact with each other according to the peak-trough principle.
The rest friction force is calculated by the following formula:
F t1 = µ t * N
Here N is the reaction of the support with which the surface acts along the normal along the body. The parameter µ t is the coefficient of friction. It depends on the material of the contacting surfaces, the quality of processing of these surfaces, their temperature and some other factors.
The written formula indicates that the rest friction force does not depend on the contact area. The expression for F t1 allows you to calculate the so-called maximum force. In a number of practical cases, F t1 is not maximum. It is always equal in magnitude to an external force that seeks to bring the body out of rest.
Friction of rest plays an important role in life. Thanks to this, we can move on the ground, pushing away from it with the soles of our feet, without slipping. Any bodies that are on planes inclined to the horizon do not slide off them due to the force F t1 .
Sliding friction
Another important type of friction for a person manifests itself when one body glides over the surface of another. This friction arises for the same physical reason as the static friction. Moreover, his strength is calculated by a similar formula.
F t2 = µ k * N
The only difference with the previous formula is the use of other coefficients µ k for sliding friction. Coefficients µ k are always less than similar parameters for rest friction for the same pair of friction surfaces. In practice, this fact manifests itself as follows: a gradual increase in external force leads to an increase in the value of F t1 until it reaches its maximum value. After that, it drops sharply by several tens of percent to the value of F t2 and is maintained constant during the movement of the body.
Coefficient µ k depends on the same factors as parameter µ t for rest friction. The sliding friction force F t2 is practically independent of the velocity of the bodies. Only at high speeds does its reduction become noticeable.
The importance of sliding friction for human life can be traced to examples such as skiing or ice skating. In these cases, the coefficient µ k is reduced by modifying the rubbing surfaces. On the contrary, sprinkling roads with salt and sand is aimed at increasing the coefficients µ k and µ t .
Rolling friction
This is one of the important types of friction for the functioning of modern technology. It is present during the rotation of bearings and the movement of the wheels of vehicles. Unlike sliding and resting friction, rolling friction is caused by deformation of the wheel during movement. This deformation, which occurs in the elastic region, dissipates energy as a result of hysteresis, manifesting itself in the form of friction during movement.
The calculation of the maximum rolling friction force is carried out according to the formula:
F t3 = d / R * N
That is, the force F t3 , as the forces F t1 and F t2 , is directly proportional to the reaction of the support. However, it also depends on the hardness of the materials in contact and the radius of the wheel R. The value of d is called the coefficient of rolling resistance. In contrast to the coefficients µ k and µ t , the quantity d has a dimension of length.
As a rule, the dimensionless d / R ratio is 1-2 orders of magnitude smaller than the value of µ k . This means that moving bodies using rolling is energetically much more beneficial than using sliding. That is why in all the rubbing surfaces of mechanisms and machines they try to use rolling friction.
Friction angle
All three types of manifestations of friction described above are characterized by a certain friction force F t , which is directly proportional to the value of N. Both forces are directed at right angles to each other. The angle that their vector sum with the normal to the surface forms is called the angle of friction. To understand its importance, we use this definition and write it in mathematical form, we get:
F t = k * N;
tg (θ) = F t / N = k
Thus, the tangent of the angle of friction θ is equal to the coefficient of friction k for a given type of force. This means that the larger the angle θ, the greater the friction force itself.
Friction in liquids and gases
When a solid body moves in a gaseous or liquid medium, it constantly collides with particles of this medium. These collisions, accompanied by the loss of speed of the solid, cause friction in fluid substances.
This type of friction is highly dependent on speed. So, at relatively low speeds, the friction force is directly proportional to the speed v, while at high speeds we are talking about proportionality v 2 .
A lot of examples of the manifestation of this friction can be given, starting from the movement of boats and ships and ending with the flight of aircraft.