The force of friction is a physical quantity that prevents any movement of the body. It occurs, as a rule, when bodies move in solid, liquid and gaseous matter. Various types of friction forces play an important role in human life, as they prevent an excessive increase in the speed of bodies.
Classification of friction forces
In the general case, all types of friction forces are described by three types: sliding, rolling and rest friction forces. 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. Rolling friction occurs when a round object moves. Let's take an example. A striking example of the type (rolling friction force) is the movement of car wheels on asph alt.
The nature of friction forces is the existence of microscopic imperfections between the rubbing surfaces of two bodies. For this reason, the resulting force acting onan object moving or starting to move, consists of the sum of the force of the normal reaction of the support N, which is directed perpendicular to the surface of the contacting bodies, and of 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 different types of friction forces, the following patterns were observed for two solid bodies:
- The friction force is directed parallel to the support surface.
- The coefficient of friction depends on the nature of the contacting surfaces, as well as on their condition.
- Maximum frictional force is in direct proportion to the normal force or support reaction that acts between the contact surfaces.
- For the same bodies, the friction force is greater before the body starts moving and then decreases when the body starts moving.
- The coefficient of friction does not depend on the contact area, and it practically does not depend on the sliding speed.
Laws
Summarizing the experimental material on the laws of motion, we have established the following basic laws regarding friction:
- The resistance to sliding between two bodies is proportional to the normal force acting between them.
- Resistance to movement between rubbing bodies does not depend on the contact area between them.
To demonstrate the second law, we can give the following example: if you take a block and move it by sliding on the surface, then the necessary force for such a movementwill be the same when the block lies on the surface with its long side, and when it stands with its end.
The 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 is the friction force greater than that of sliding at rest?
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 static friction coefficient and the reaction force of the support. The coefficient of friction is determined experimentally for these rubbing materials and entered in the appropriate tables.
Dynamic force is calculated in the same way as static force. Only in this case, the coefficient of friction is used specifically for sliding. The friction coefficient is usually denoted by the Greek letter Μ (mu). Thus, the general formula for both friction forces is: Ftr=ΜN, 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 static 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 microfusions of individual points of the surfaces can form between their surfaces. These factors cause an increase in staticindicator.
An example of several types of friction force and their manifestation is the piston in the cylinder of a car engine, which is "soldered" to the cylinder if the engine is not running for a long time.
Horizontal sliding body
Let's get the equation of motion for a body that, under the action of an external force Fin, starts to move along the surface by sliding. In this case, the following forces act on the body:
- Fv – external force;
- Ftr – friction force that is opposite in direction to the force Fv;
- N is the reaction force of the support, which is equal in absolute value to the weight of the body P and is directed to the surface, that is, at a right angle to it.
Taking into account the directions of all forces, we write Newton's second law for this case of motion: Fv - Ftr=ma, where m - body mass, a - acceleration of movement. Knowing that Ftr=ΜN, N=P=mg, where g is the free fall acceleration, we get: Fv – Μmg=ma. Whence, expressing the acceleration with which the sliding body moves, we get: a=F in / m – Μg.
Movement of a rigid body in a liquid
When considering what types of friction forces exist, one should mention an important phenomenon in physics, which is the description of how a solid body moves in a liquid. 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:
- Whena rigid body moves at a low speed, one speaks of laminar motion. The friction force in laminar motion is proportional to the velocity. An example is Stokes' law for spherical bodies.
- When the movement of a body in a fluid occurs at a higher speed than a certain threshold value, then vortices from 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.
Nature of the rolling friction force
When talking about the types of friction forces, it is customary to call the rolling friction force the third type. It manifests itself when a body rolls over a certain surface and deformation of this body and the surface itself occurs. That is, in the case of an absolutely non-deformable body and surface, there is no point in talking about the force of rolling friction. Let's take a closer look.
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. Repeating cycles of elastic deformation lead to the transfer of part of the mechanical energy into thermal energy. In addition, due todamage, the contact of the wheel and the surface already has some finite contact area.
rolling friction force formula
If we apply the expression for the moment of force that rotates the wheel, then we can get that the rolling friction force is Ftr.k.=Μk N / R, here N is the support reaction, R is the radius of the wheel, Μк – rolling friction coefficient. Thus, the rolling friction force is inversely proportional to the radius, which explains the advantage of large wheels over 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 that have the same mass and are made of the same material, the wheel with the larger radius is easier to budge.
Rolling ratio
In accordance with the formula for this type of friction force, we obtain that the coefficient of rolling friction Μk has the dimension of length. 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, Ck=Μk / R is a dimensionless quantity.
The rolling coefficient Ck is significantly less than the coefficient of sliding friction Μtr. Therefore, when answering the question of which type of friction force is the smallest, we can safely call the rolling friction force. Thanks to this fact, the invention of the wheel is considered an important step in technological progress.humanity.
The rolling ratio is system specific and depends on the following factors:
- hardness of the wheel and surface (the smaller the deformation of 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 between the wheel and the surface;
- body temperature
