The friction force is a certain physical quantity that prevents any movement of the body. It arises, as a rule, during the movement of bodies in solid, liquid and gaseous matter. Different types of friction forces play an important role in human life, since they prevent an excessive increase in the speed of movement of bodies.
Classification of friction forces
In the general case, all types of friction forces are described by three types: sliding friction force, rolling force and rest. The first is static, the other two are dynamic. Friction at rest prevents the body from starting to move; in turn, when sliding, friction exists when the body rubs against the surface of another body during its movement. Friction during rolling occurs when moving a circular object. We give an example. A striking example of the form (rolling friction) is the movement of the wheels of a car on asphalt.
The nature of the occurrence of friction forces lies in the existence of microscopic imperfections between the rubbing surfaces of two bodies. For this reason, the resulting force acting on an object moving or starting to move consists of the sum of the normal reaction force of the support N, which is directed perpendicular to the surface of the contacting bodies, and the friction force F. The latter is directed parallel to the contact surface and is opposite to the movement of the body.
Friction between two solids
When considering the issue of various types of friction forces, the following laws were observed for two solids:
- The friction force is directed parallel to the surface of the support.
- The friction coefficient depends on the nature of the contacting surfaces, as well as on their condition.
- The maximum friction force is in direct proportion to the normal force or to the reaction of the support that acts between the contact surfaces.
- For the same bodies, the friction force is greater before the body begins to move, and then decreases when the body begins to move.
- The friction coefficient does not depend on the contact area, and it is also practically independent of the sliding speed.
The laws
Summarizing the experimental material on the laws of motion, the following basic laws concerning friction were established:
- Resistance to the sliding movement between two bodies is proportional to the normal force acting between them.
- Resistance to movement between rubbing bodies does not depend on the area of contact between them.
To demonstrate the second law, one can give an example: if you take a block and move it by sliding on the surface, then the necessary force for such a movement will be the same, and when the block lies on the surface with its long side, and when it faces.
Laws concerning various types of friction forces in physics were discovered at the end of the 15th century by Leonard da Vinci. Then they were forgotten for a long time, and only in 1699 they were rediscovered by the French engineer Amonton. Since then, the laws of friction bear his name.
Why alone is the friction force greater than that when sliding?
When considering several types of friction forces (rest and sliding), it should be noted that the static friction force is always less than or equal to the product of the rest friction coefficient and the support reaction force. The friction coefficient is determined experimentally for these rubbing materials and is entered in the corresponding tables.
Dynamic force is calculated in the same way as static. Only in this case, the coefficient of friction is used precisely for sliding. The friction coefficient is usually denoted by the Greek letter μ (mu). Thus, the general formula for both friction forces has the form: F Tr = μ * N, where N is the support reaction force.
The nature of the difference between these types of friction forces has not been precisely established. However, most scientists believe that the rest friction force is greater than that for sliding, because when the bodies are at rest relative to each other for some time, ionic bonds or micro-fusions of individual points of the surfaces can form between their surfaces. These factors cause an increase in the static index.
An example of several types of friction force and their manifestation is a piston in the cylinder of a car engine, which is “soldered” to the cylinder if the engine does not work for a long time.
Horizontal sliding body
We obtain the equation of motion for the body, which, under the action of an external force F , begins to move along the surface by sliding. In this case, the following forces act on the body:
- F in - external force;
- F fr - friction force, which is opposite in direction to the force F in ;
- N is the reaction force of the support, which is equal in magnitude to the body weight P and is directed toward the surface, that is, at right angles to it.
Given the directions of all forces, we write down Newton’s second law for this case of motion: F in - F Tr = m * a, where m is the mass of the body, a is the acceleration of motion. Knowing that F Tr = μ * N, N = P = m * g, where g is the acceleration of gravity, we get: F - μ * m * g = m * a. From where, expressing the acceleration with which the moving body moves, we get: a = F in / m - μ * g.
The motion of a solid in a liquid
When considering the question of what types of friction forces exist, an important phenomenon in physics should be mentioned, which is a description of how a solid moves in a fluid. In this case, we are talking about aerodynamic friction, which is determined depending on the speed of the body in the fluid. There are two types of movement:
- When a solid body moves at low speed, they speak of laminar motion. The friction force during laminar motion is proportional to the speed. An example is the Stokes law for spherical bodies.
- When a body moves in a liquid at a faster rate than a certain threshold value, turbulences from the fluid flows begin to appear around the body. These vortices create an additional force that impedes movement, and as a result, the friction force is proportional to the square of the speed.
The nature of rolling friction
When talking about the types of friction forces, it is customary to call the rolling friction force the third type. It manifests itself when the body rolls over a certain surface and the deformation of this body and the surface itself occurs. That is, in the case of an absolutely undeformable body and surface, there is no point in talking about the rolling friction force. Let's consider in more detail.
The concept of rolling friction coefficient is similar to that for sliding. Since there is no slippage between the surfaces of bodies during rolling, the coefficient of rolling friction is much less than for sliding.
The main factor that affects the coefficient is the hysteresis of mechanical energy for the type of rolling friction force. In particular, the wheel, depending on the material from which it is made, as well as on the load it carries, is elastically deformed during movement. Repeated cycles of elastic deformation lead to the transition of part of the mechanical energy into thermal energy. In addition, due to damage, the contact of the wheel and the surface already has a certain finite contact area.
rolling friction force formula
If we apply the expression for the moment of force that rotates the wheel, then it can be obtained that the rolling friction force is F TR = μ k * N / R, here N is the reaction of the support, R is the radius of the wheel, μ k is the rolling friction coefficient. Thus, the rolling friction force is inversely proportional to the radius, which explains the advantage of large wheels compared to small ones.
The inverse proportionality of this force to the radius of the wheel suggests that in the case of two wheels of different radii, which have the same mass and are made of the same material, a wheel with a large radius is easier to move.
Rolling coefficient
In accordance with the formula for this type of friction force, we obtain that the rolling friction coefficient μ k has a length dimension. It mainly depends on the nature of the contacting bodies. The value, which is determined by the ratio of the rolling friction coefficient to the radius, is called the rolling coefficient, that is, C = μ / R is a dimensionless quantity.
The rolling coefficient C k is significantly less than the sliding friction coefficient μ tr . Therefore, when answering the question of what form of friction is the smallest force, the rolling friction force can be safely called. Thanks to this fact, the invention of the wheel is considered an important step in the technological progress of mankind.
The rolling coefficient is a characteristic of a particular system and depends on the following factors:
- hardness of the wheel and surface (the less the deformation of the bodies that occurs during movement, the lower the rolling coefficient);
- wheel radius;
- weight that acts on the wheel;
- contact surface area and its shape;
- viscosity in the area of contact of the wheel and the surface;
- body temperature