Any contact between two bodies results in a friction force. In this case, it does not matter in what aggregate state of matter the bodies are, whether they move relative to each other or are at rest. In this article, we will briefly consider what types of friction exist in nature and technology.
Rest friction
For many, it may be a strange idea 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 other types. It manifests itself when we try to move any object. It can be a block of wood, a stone, or even a wheel.
The reason for the existence of the static friction force is the presence of irregularities on the contact surfaces, which mechanically interact with each other according to the peak-trough principle.
The static friction force is calculated using the following formula:
Ft1=µtN
Here N is the reaction of the support with which the surface acts on the body along the normal. The parameter µt is the coefficient of friction. It depends onthe material of the contacting surfaces, the quality of processing of these surfaces, their temperature and some other factors.
The written formula shows that the static friction force does not depend on the contact area. The expression for Ft1 allows you to calculate the so-called maximum force. In a number of practical cases, Ft1 is not the maximum. It is always equal in magnitude to the external force that seeks to bring the body out of rest.
Rest friction plays an important role in life. Thanks to this, we can move on the ground, pushing off from it with the soles of our feet, without slipping. Any bodies that are on planes inclined to the horizon do not slip off them due to the force Ft1.
Friction during sliding
Another important type of friction for a person manifests itself when one body slides over the surface of another. This friction arises for the same physical reason as the static friction. What's more, his strength is calculated using a similar formula.
Ft2=µkN
The only difference with the previous formula is the use of different coefficients for sliding friction µk. Coefficients µk are always less than similar parameters for static friction for the same pair of rubbing surfaces. In practice, this fact manifests itself as follows: a gradual increase in the external force leads to an increase in the value of Ft1 until it reaches its maximum value. After that shedrops sharply by several tens of percent to the value Ft2 and is maintained constant during the movement of the body.
Coefficient µk depends on the same factors as parameter µt for static friction. The force of sliding friction Ft2 practically does not depend on the speed of movement of bodies. Only at high speeds does it become noticeable to decrease.
The importance of sliding friction to human life can be seen in examples such as skiing or skating. In these cases, the coefficient µk is reduced by modifying the rubbing surfaces. On the contrary, sprinkling roads with s alt and sand aims to increase the values of the coefficients µk and µt.
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 rest friction, rolling friction is due to the deformation of the wheel during movement. This deformation, which occurs in the elastic region, dissipates energy as a result of hysteresis, manifesting itself as a friction force during movement.
Calculation of the maximum rolling friction force is carried out according to the formula:
Ft3=d/RN
That is, the force Ft3, as the forces Ft1 and Ft2, is directly proportional to the reaction of the support. However, it also depends on the hardness of the materials in contact and the wheel radius R. The valued is called the rolling resistance coefficient. Unlike the coefficients µk and µt, d has the dimension of length.
As a rule, the dimensionless ratio d/R turns out to be 1-2 orders of magnitude less than the value µk. This means that the movement of bodies with the help of rolling is much more energetically favorable than with the help of sliding. That is why rolling friction is used in all rubbing surfaces of mechanisms and machines.
Friction angle
All three types of friction manifestations described above are characterized by a certain friction force Ft, which is directly proportional to N. Both forces are directed at right angles relative to each other. The angle that their vector sum forms with the normal to the surface is called the angle of friction. To understand its importance, let's use this definition and write it in mathematical form, we get:
Ft=kN;
tg(θ)=Ft/N=k
Thus, the tangent of the friction angle 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 a loss of velocity of the rigid body, are the cause of friction in fluid substances.
This type of friction is highly dependent on speed. So, at relatively low speeds, the friction forceturns out to be directly proportional to the speed v, while at high speeds we are talking about proportionality v2.
There are many examples of this friction, from the movement of boats and ships to the flight of aircraft.
