The phenomenon of friction plays a huge role in modern technology. In some cases, they are struggling with him and striving to reduce, in others, on the contrary, they use different methods to increase the friction force. In this article, we consider in more detail the question of what the coefficient of friction depends on.
Friction and its types
Before proceeding to answer the question of what the coefficient of friction depends on, the phenomenon itself and its types should be considered.
Each person intuitively understands that any kind of friction involves the presence of physical contact of at least two surfaces. It can be solid, liquid and gaseous media.
Friction between solids is divided into three types. The so-called rest friction has the greatest power. Many have noticed that some force must be applied to move a cabinet or cabinet on the floor. The value that prevents this displacement is called rest friction.
The next view is sliding friction . In absolute value, it is usually 10-30% less than static friction. This force manifests itself when two bodies glide over each other. For example, the movement of a skater or skier is possible due to the low value of sliding friction. At the same time, slipping in boots on asphalt is not possible due to the considerable friction.
The rolling friction acts when a body with a circular surface rolls along a certain plane. For example, the movement of a ball or roller in a bearing or wheel along the road. In some cases, the rolling friction is one to two orders of magnitude smaller than the sliding friction.
Any movements in liquids and gases are also accompanied by the appearance of friction. Unlike previous types, friction in fluid substances depends on the speed of movement of the object in them.
It is important to understand that no matter what type of friction is considered, the corresponding force always hinders the mechanical movement.
Rest friction and coefficient µ1
To understand what the coefficient of friction depends on, you must first give it a definition. Let's start with the friction of rest. The corresponding force is mathematically calculated by the following formula:
F 1 = µ 1 * N.
Where N is the reaction of the support on which the body is located, µ 1 is the coefficient of rest friction. What determines the last value:
- Firstly, from materials of rubbing surfaces. Obviously, µ 1 will be much smaller for a tree-ice pair than for a tree-tree pair.
- Secondly, the quality of surface treatment. So, if the roughness (the magnitude of the microscopic depressions and peaks and their number on the surfaces) is significant, then the coefficient µ 1 will also be large.
- Thirdly, µ 1 depends on the temperature of the bodies. In some cases, a change in temperature can significantly change the nature of the friction itself. So, lowering the temperature of the ice leads to the fact that it ceases to slip, that is, μ 1 increases.
Note that µ 1 does not depend on the contact area of two bodies.
Sliding friction and coefficient µ2
By their physical nature, sliding friction does not differ significantly from rest friction. The formulas by which the forces for these types of phenomena are calculated also have the same form. For the sliding force we have:
F 2 = µ 2 * N.
The only difference in the formulas is that in the latter case, the value μ 2 is used - the coefficient of sliding friction. What does the quantity depend on? In short, µ 2 is determined by the same factors as µ 1 . Since the slip process occurs, the peaks and troughs on the surfaces do not have time to go into tight mechanical contact. Also, weak intermolecular interactions do not have time to form. All this is due to the fact that µ 2 <µ 1 .
As in the case of rest friction, and in the case of sliding, the main cause of their occurrence is surface roughness. If you get rid of it in any way, then you can significantly reduce the forces F 1 and F 2 . For this purpose, a large number of lubricants have now been created. The lubricant layer leads to a spatial separation of the contacts of hard surfaces, so the friction forces are significantly reduced.
Note that the coefficient μ 2 does not depend on the contact area and on the sliding velocity (at high speeds, it begins to gradually decrease).
Rolling Friction and CR Ratio
It should immediately be said that the cause of the appearance of rolling friction is completely different than for the previous types considered. The rolling friction arises due to the hysteresis of the elastic deformation of the rolling body. If this deformation were not present, then the rolling friction would be almost zero.
The rolling friction force F 3 is determined as follows:
F 3 = C R * N.
Here C R is the rolling friction coefficient. What does C R depend on? Firstly, it is inversely proportional to the radius of the rolling body. Secondly, it strongly depends on the hardness of the contacting objects, the higher this hardness, the less C R.
The values of the coefficients C R as well as the values of μ 1 and μ 2 are given in special tables.
Coefficient of friction in liquids and gases
Friction in fluid substances has a simpler nature than the same phenomenon between solids. It consists in the mechanical interaction with the particles of the substance when the body moves in it.
Nevertheless, the mathematical description of the energy losses associated with this friction is rather complicated. The corresponding equation is called the Darcy-Weisbach formula. We will not give it here, but only say that it uses the concept of the hydraulic coefficient of friction to estimate the noted losses. What determines its significance? This coefficient is determined by the flow regime (laminar or turbulent). The mode depends on the speed of movement, viscosity and density of the fluid substance, as well as on the diameter of the pipe. All these parameters allow us to calculate the so-called Reynolds number, which uniquely determines the value of the coefficient of friction.