Presentation in physics on the topic "Friction Forces between Contiguous Surfaces of Solids" (Grade 10). Theoretical introduction. Friction occurs at the contact surfaces of two solid bodies The dependence of the friction force on the force of gravity

Friction occurs at the contact surfaces of two solid bodies. It plays an important role both in technology and in everyday life. There are three types of external friction: static friction, sliding friction, rolling friction. The magnitude of the friction forces and the nature of their dependence on the speed are significantly affected by the state of the surfaces, their processing, the presence of contamination, etc. However, the magnitude of these forces depends on the magnitude of the normal pressure between the surfaces. The force of friction between solid bodies in contact has a characteristic feature: it does not vanish along with the speed. The force of friction that exists between bodies in contact but not in motion is called static friction. The magnitude and direction of the static friction force are determined by the magnitude and direction of the external force that should have caused the slip. The static friction force is equal in magnitude and opposite in direction to the external force that caused the motion. The static friction force cannot exceed a certain value, which is called the maximum static friction force (or static friction force). As long as the external force does not exceed this value, slip does not occur (Fig. 6.1). The maximum value is followed by a steep decline and a constant sliding friction force remains.

Friction of rest and sliding friction do not depend on the size of the area of ​​contact of solid bodies. For these bodies, the static and sliding friction forces are directly proportional to the pressure force N, which simultaneously compresses both bodies:

, , (6.1)

where and are the coefficients of static and sliding friction. The value in most cases varies from 0.2 to 0.7; – from 0.2 to 0.5.

Friction at rest plays an essential role in technology. It determines the greatest amount of necessary driving force for the driving wheels of cars, as well as for the soles of pedestrians. At the point of contact with the ground, the rolling wheel and the sole of the foot of the moving person are at rest relative to the ground. Therefore, friction is at work here. Sliding friction, on the contrary, almost always interferes, therefore, in machines and apparatuses, they strive to exclude, if possible, external friction between rubbing parts. It is replaced by internal friction of thin layers of liquid between mutually moving parts - this is called lubrication.

Lecture 4. Friction of solids

External friction, mechanical resistance that occurs in the plane of contact of two contacting bodies during their relative movement. The resistance force F tr directed opposite to the movement of a given body is called the friction force acting on this body. External friction is a dissipative process, accompanied by heat release, electrization of bodies, their destruction, etc.

Distinguish between external sliding and rolling friction. Sliding friction- the force arising from the translational movement of one of the contacting bodies relative to the other and acting on this body in the direction opposite to the direction of sliding. rolling friction - the moment of forces arising from the rolling of one of the two contacting bodies relative to the other, preventing rolling.

Characteristic sliding friction- coefficient of sliding friction f c - dimensionless value equal to the ratio of the friction force to the normal load; characteristic of rolling friction is the coefficient of rolling friction f k - a value that has the dimension of length, is the ratio of the moment of rolling friction to the normal load. External conditions (load, speed, roughness, temperature, lubrication) affect the value of external friction no less than the nature of rubbing bodies, changing it several times.

The mechanism of friction occurrence is explained by the molecular-mechanical theory of friction, the development of which was greatly contributed by Russian scientists (B.V. Deryagin, I.V. Kragelsky, etc.) and foreign scientists (Bowden, Tabor, Tomlinson, etc.). According to this theory, friction has a dual molecular-mechanical nature. The friction force F tr can be represented as the sum of the molecular (adhesive) F a and mechanical (deformation) F σ components:

The molecular component is due to the resistance to rupture of molecular or interatomic bonds that occur between contacting bodies. The mechanism of this process is similar to the destruction of the crystal lattice during shear. The dissipation of the work of friction into heat is associated with the elastic deformation of crystal lattices. The work of the external force is converted into the potential energy of the lattices. After

In the first case, non-lubricated surfaces are in contact, covered with oxide films and the thinnest layers of gas and water molecules adsorbed from the environment. In this case, the friction force is the sum of the adhesive and cohesive components. Dry and boundary friction are similar in their

nature and have common patterns. The reason is the fact that in the case of boundary friction, the monomolecular layers of the lubricant are firmly bound to the solid surface, have solid-like properties, and, as it were, serve as a continuation of the solid phase. Therefore, as in dry friction, in fact, there is a contact between two solid surfaces. The difference is manifested in different values ​​of the friction coefficient.

In the second case, in addition to the above films, there are molecules of lubricants in the form of a thin layer several molecules thick, which are firmly bonded to the surface. Characteristic in this case is a decrease in both one and the other component.

In the third case, a layer of liquid lubricant completely separates the mating surfaces. The adhesive component decreases to zero.

Numerous studies have shown that for metals the deformation component of the friction coefficient is about 100 times less than the adhesion component. Therefore, the friction coefficient in the first approximation is equal to the adhesive component. The situation is somewhat different for plastics and rubbers. In the latter case, the difference is reduced by more than an order of magnitude, and if the rubber slides over a rough surface, the deformation component should not be neglected.

lubricants and many other factors.

Rice. 4.2. Tribometer scheme

Determination of forces and coefficients of external friction. With elastic deformations in the contact zones, the interaction of solid bodies can be carried out at unsaturated and saturated contact.

With elastic unsaturated contact the distances between the individual contact zones are large enough so that the influence of the zones on each other can be neglected. The total friction force during sliding of an absolutely rigid body with a rough surface relative to a softer body with an absolutely even surface will be equal to

where F i is the friction force arising on a single arbitrary microroughness; n r is the number of microroughnesses having the same penetration.

To determine the force F i, consider the processes occurring in the contact zone of a single microroughness (Fig. 4.3). The deformation component of the friction force arises due to the imperfect elasticity of the material of the deformable layers. It is due to hysteresis losses. In accordance with the research of the English scientist D. Tabor

deformation component friction force is

where E is the modulus of elasticity of the deformable material; μ is the Poisson's ratio of this material; α hist is the coefficient of hysteresis losses of the material under conditions of a complex stress state.

Rice. 4.3. distribution of stresses during elastic deformations in the contact zone of the ball with a flat surface of a deformable body

Molecular component friction force is due to interatomic and intermolecular interaction and is equal to

Then the total friction force arising from the sliding of an arbitrary microroughness can be expressed as follows

The friction force F tr is calculated from expression (4.4), in which all i-th parameters are determined through known values. If we define

normal load P depending on the approach, then it is possible to calculate the coefficient of friction depending on the approach f =

F tr /P . Calculations show that as the approach between the surfaces of solids increases, the molecular component

coefficient of friction (containing the frictional parameters τ 0 and β ) decreases, while the deformation coefficient increases. The dependence of the friction coefficient on the parameter h/R is shown in fig. 4.4.

Rice. 4.4 Dependence of friction coefficient on approach

Experimental results. The behavior of the material during friction is determined by the depth of propagation of plastic deformation into the sample. With an increase in normal pressure on the spots of actual contact, first elastic and then plastic deformations develop. Some form change associated with the creep of the material also occurs after, under conditions of a constant load. The final equilibrium is established after the actual contact area is sufficient to provide the required bearing capacity. Thus, after running in the surface, a stationary friction mode is established, in which the wear of the surface is in equilibrium with the growth of new deformed layers. On fig. Figures 4.5 and 4.6 show the dependences of the friction coefficient on pressure in the steady state mode of boundary lubrication when sliding specimens of steel 36NKhTYu in the hardened and aged states over hardened steel 45. Austenitic steel 36NKhTYu

structure and tribological properties of materials in each specific case, in relation to a particular friction unit.

Rice. 4.6. The dependence of the coefficient of friction

(k) pressure for 36NKhTYu alloy quenched from 9700 C (1) and aged after quenching at 7500 C for 1 hour (2)

Fig.4.7. Dependence of the coefficient of friction of a sample made of steel 36NKhTYu (a) and copper (b) on the sliding speed and load

On fig. 4.7 shows the surfaces formed by the values ​​of the coefficient of friction of copper and 36NKhTYu alloy, depending on the sliding speed and load. The coefficient of friction of copper varies along a curve with a maximum depending on the load at all speeds. For the 36NKhTYu alloy, the friction coefficient at low speeds is practically independent of the applied force. An increase in load at high speeds leads to a drop in the coefficient of friction. This indicates that the contribution to the friction force due to the plastic flow of the surface layer decreases. This is possible with a decrease

viscosity of the material associated with an increase in excitation during friction. Apparently, the process of fragmentation of the surface layers, which leads to an increase in the mobility of the elements constituting the structure, is of importance in this case.

Rice. 4.8. Dependence of the moment of friction force of the TiC-NiCr composite material (a) on the load in a pair with various alloys (b - TiC-NiCr; c - 3V16K; d - composition based on KAM bronze)

An analysis of the friction parameters (Fig. 4.8) shows that the heat released on the surface and in the near-surface layer plays an important role in the process of contacting two materials with their relative slip.

Indeed, an example of the effect of contact temperature on the friction process can be the behavior of the TiC-NiCr composite material during friction in tandem with materials, among which were TiC-NiCr CM, stellite, and the “hard alloy-bronze” composition, which differ in thermal conductivity. In these tests, when the interface was in the form of a mechanical seal, heat removal from the friction zone can be carried out mainly due to the thermal conductivity of the contacting materials. Since the thermal conductivity of CM TiCNiCr and stellite (3V16K) is much lower than that of the CM composite developed for highly loaded friction units, the nature of friction should be different. Indeed, from Fig. 4.8b it can be seen that the friction of a pair of identical TiC-NiCr CMs becomes unstable after several minutes of operation at a load of 1 t. An increase in the load to 2 t is accompanied by jumps in the friction torque, which indicates

Rice. 4.9. Schematic representation of the temperature distribution in the surface layer in the case of plastic deformation during friction

Friction force. Types of dry friction forces

Friction forces appear when contacting bodies or their parts move relative to each other. Friction arising from the relative movement of two bodies in contact is called external; friction between parts of the same solid body (for example, liquid or gas) is called internal friction .

The friction force that occurs when a solid body moves relative to a liquid or gaseous medium should be classified as a force internal friction, since in this case the layers of the medium that are in direct contact with the body are drawn into motion by it at the same speed as the body has, and the motion of the body is influenced by friction between these layers of the medium external to them.

Definition 1

Friction between the surfaces of two solids in the absence of any interlayer, such as lubricant between them, is called dry . Friction between a solid body and a liquid or gaseous medium, as well as between layers of such a medium is called viscous (or liquid). With regard to dry friction, there are sliding friction, rolling friction and static friction.

sliding friction force

Sliding friction occurs when one body moves over the surface of another. The greater the weight of the body, and the greater the coefficient of friction between these surfaces (the coefficient depends on the material from which the surfaces are made), the greater the force of sliding friction.

The force of sliding friction does not depend on the area of ​​the contacting surfaces. When moving, a block lying on its largest face will have the same sliding friction force as if it is placed on the smallest face.

Causes of the sliding friction force:

The smallest irregularities of the surfaces of two bodies - with them the bodies cling to each other when moving. If there were no sliding friction force, then the body, set in motion by a short-term action of a force on it, would continue to move uniformly. However, since the sliding friction force exists, and it is directed against the movement of the body, the body gradually stops.

Intermolecular interactions on the contact surfaces of two bodies. This interaction can only occur on very smooth, well polished surfaces. Molecules of different bodies are very close to each other and are attracted. Because of this, the movement of the body is inhibited.

The sliding friction force vector $\overline(F)_(mp) $ is always directed oppositely to the velocity vector of the body relative to the body in contact with it. Therefore, the action of the sliding friction force always leads to a decrease in the modulus of the relative velocity of bodies.

Rolling friction force

The rolling friction force occurs when another, usually round, rolls over the surface of one body. For example, the wheels of vehicles are rolling on the road, a barrel turned on its side from a hillock, a ball on the floor. The rolling friction force is much less than the sliding friction force. Remember, it is easier to carry a large bag on wheels than to drag it along the ground. The reason lies in the different way of contact between the moving body and the surface. When rolling, the wheel, as it were, presses, crushes the surface under itself, repels from it. A rolling wheel does not have to catch many small surface irregularities, as when sliding bodies.

Remark 1

The harder the surface, the lower the rolling friction force. For example, it is more difficult to ride a bicycle on sand than on asphalt, since on sand you have to overcome a large rolling friction force. This is due to the fact that it is easier to push off from hard surfaces, they are not strongly pressed. It can be said that the force that acts from the side of the wheel on a solid surface is not spent on deformation, but almost all of it returns in the form of a normal reaction force of the support.

static friction force

The force that arises at the boundary of contact between the bodies in the absence of relative motion of the bodies is called the static friction force.

The static friction force $\overline(F)_(mp) $is equal in absolute value to the external force $\overline(F)$, directed tangentially to the contact surface of the bodies, and opposite to it in direction:

The static friction force is all around us. All objects that lie on other bodies are held by the static friction force. The static friction force is even enough to hold objects on inclined surfaces. For example, a person can stand on a hillside, a block lying motionless on a slightly inclined ruler. In addition, due to the force of static friction, such forms of movement as walking and riding are possible. In these cases, there is "adhesion" with the surface due to the static friction force, as a result, it becomes possible to repel from the surface.

The causes of the static friction force are the same as those of the sliding friction force.

The static friction force arises when an attempt is made to move a standing body. As long as the force trying to move the body is less than the static friction force, the body will stay in place. As soon as this force exceeds a certain maximum static friction force for these two bodies, one body will begin to move relative to the other, and the sliding or rolling friction force will already act on it.

Remark 2

In most cases, the maximum force of static friction is slightly greater than the force of sliding friction. So, to start moving the cabinet, you must first make a little more effort than applying them when the cabinet is already moving. Often the difference between the forces of static friction and sliding friction is neglected, considering them equal.

In the simplest model of dry friction, the following laws are satisfied. They are a generalization of experimental facts and are of an approximate nature:

the maximum value of the static friction force is equal to the sliding friction force;

the absolute value of the sliding friction force is directly proportional to the reaction force of the support: $\overline(F)_(mp) =\mu N$, and the coefficient of proportionality $\mu $ is called the coefficient of friction;

the coefficient of friction does not depend on the speed of the body on a rough surface;

the coefficient of friction does not depend on the area of ​​the contacting surfaces.

Example 1

The students placed a magnet weighing $30$ g to the blackboard. The magnet is pressed against the board with a force of $6 H$. What force must be applied to slide the magnet down and move it vertically up if the coefficient of friction is $0.3$?

Given: $m=30$r, $N=6 H$, $\mu =0.3$.

Find: $F_(1) $, $F_(2) $-?

Decision:

Picture 1.

In order to move the magnet down, the sum of the gravity $mg$ and the additional applied force $F_(1) $ must be equal to the friction force $F_( [email protected]) $ (or be greater):

$mg+F=F_(mp) $ (1).

From formula (1) and from the general formula for the friction force

we find the required force necessary for the magnet to slide down:

$F_(mp) =\mu N$($N$ is the force with which the magnet is pressed against the board):

$F_(1) =\mu N-mg=1.5 H$.

For an upward force, equation (1) takes the form:

$F_(2) =\mu N+mg=2.1 H$

Answer:$F_(1) =1.5 H$, $F_(2) =2.1 H$.

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The force of friction in terrestrial conditions accompanies any movement of bodies. It occurs when two bodies come into contact, if these bodies move relative to each other. The friction force is always directed along the contact surface, in contrast to the elastic force, which is directed perpendicularly (Fig. 1, Fig. 2).

Rice. 1. The difference between the directions of the friction force and the elastic force

Rice. 2. The surface acts on the bar, and the bar acts on the surface

There are dry and non-dry types of friction. Dry type of friction occurs when solids come into contact.

Consider a bar lying on a horizontal surface (Fig. 3). It is affected by the force of gravity and the reaction force of the support. Let's act on the bar with a small force , directed along the surface. If the bar does not move, then the applied force is balanced by another force, which is called the static friction force.

Rice. 3. Force of static friction

The static friction force () opposite in direction and equal in magnitude to the force tending to move the body parallel to the surface of its contact with another body.

With an increase in the “shearing” force, the bar remains at rest, therefore, the static friction force also increases. With some, sufficiently large, force, the bar will begin to move. This means that the static friction force cannot increase to infinity - there is an upper limit, more than which it cannot be. The value of this limit is the maximum static friction force.

Let's act on the bar with a dynamometer.

Rice. 4. Measuring the friction force with a dynamometer

If the dynamometer acts on it with a force, then it can be seen that the maximum static friction force becomes greater with an increase in the mass of the bar, that is, with an increase in the force of gravity and the reaction force of the support. If accurate measurements are taken, they will show that the maximum static friction force is directly proportional to the reaction force of the support:

where is the modulus of the maximum static friction force; N– support reaction force (normal pressure); - coefficient of static friction (proportionality). Therefore, the maximum static friction force is directly proportional to the force of normal pressure.

If we conduct an experiment with a dynamometer and a bar of constant mass, while turning the bar on different sides (changing the area of ​​​​contact with the table), we can see that the maximum static friction force does not change (Fig. 5). Therefore, the maximum static friction force does not depend on the contact area.

Rice. 5. The maximum value of the static friction force does not depend on the contact area

More accurate studies show that static friction is completely determined by the force applied to the body and the formula.

The static friction force does not always prevent the body from moving. For example, the static friction force acts on the sole of the shoe, while imparting acceleration and allowing you to walk on the ground without slipping (Fig. 6).

Rice. 6. Force of static friction acting on the sole of the shoe

Another example: the static friction force acting on the wheel of a car allows you to start moving without slipping (Fig. 7).

Rice. 7. The static friction force acting on the car wheel

In belt drives, the static friction force also acts (Fig. 8).

Rice. 8. Force of static friction in belt drives

If the body is moving, then the friction force acting on it from the side of the surface does not disappear, this type of friction is called sliding friction. Measurements show that the force of sliding friction is practically equal in magnitude to the maximum force of static friction (Fig. 9).

Rice. 9. Force of sliding friction

The force of sliding friction is always directed against the speed of the body, that is, it prevents movement. Consequently, when the body moves only under the action of the friction force, it imparts negative acceleration to it, that is, the speed of the body is constantly decreasing.

The magnitude of the sliding friction force is also proportional to the force of normal pressure.

where is the modulus of the sliding friction force; N– support reaction force (normal pressure); – coefficient of sliding friction (proportionality).

Figure 10 shows a graph of the dependence of the friction force on the applied force. It shows two different areas. The first section, in which the friction force increases with an increase in the applied force, corresponds to static friction. The second section, where the friction force does not depend on the external force, corresponds to sliding friction.

Rice. 10. Graph of the dependence of the friction force on the applied force

The coefficient of sliding friction is approximately equal to the coefficient of static friction. Typically, the coefficient of sliding friction is less than unity. This means that the sliding friction force is less than the normal pressure force.

The coefficient of sliding friction is a characteristic of two bodies rubbing against each other, it depends on what materials the bodies are made of and how well the surfaces are processed (smooth or rough).

The origin of static and sliding friction forces is due to the fact that any surface at the microscopic level is not flat, there are always microscopic inhomogeneities on any surface (Fig. 11).

Rice. 11. Surfaces of bodies at the microscopic level

When two bodies in contact are attempting to move relative to each other, these inhomogeneities are caught and prevent this movement. With a small amount of applied force, this engagement is sufficient to prevent the bodies from moving, so static friction arises. When the external force exceeds the maximum static friction, then the engagement of the roughness is not enough to hold the bodies, and they begin to shift relative to each other, while the sliding friction force acts between the bodies.

This type of friction occurs when bodies roll over each other or when one body rolls on the surface of another. Rolling friction, like sliding friction, imparts negative acceleration to the body.

The occurrence of the rolling friction force is due to the deformation of the rolling body and the supporting surface. So, a wheel located on a horizontal surface deforms the latter. When the wheel moves, the deformations do not have time to recover, so the wheel has to climb a small hill all the time, which causes a moment of forces that slows down the rolling.

Rice. 12. Occurrence of rolling friction force

The magnitude of the rolling friction force, as a rule, is many times less than the sliding friction force, all other things being equal. Due to this, rolling is a common type of movement in engineering.

When a solid body moves in a liquid or gas, a resistance force acts on it from the side of the medium. This force is directed against the speed of the body and slows down the movement (Fig. 13).

The main feature of the resistance force is that it occurs only in the presence of relative motion of the body and its environment. That is, the static friction force in liquids and gases does not exist. This leads to the fact that a person can move even a heavy barge that is on the water.

Rice. 13. Resistance force acting on a body when moving in a liquid or gas

The resistance force modulus depends on:

From the size of the body and its geometric shape (Fig. 14);

Conditions of the body surface (Fig. 15);

Properties of a liquid or gas (Fig. 16);

The relative speed of the body and its environment (Fig. 17).

Rice. 14. Dependences of the modulus of resistance force on the geometric shape

Rice. 15. Dependences of the resistance force modulus on the state of the body surface

Rice. 16. Dependences of the resistance force modulus on the properties of a liquid or gas

Rice. 17. Dependences of the resistance force modulus on the relative velocity of the body and its environment

Figure 18 shows a graph of the dependence of the resistance force on the speed of the body. At a relative velocity equal to zero, the drag force does not act on the body. With an increase in the relative velocity, the drag force first grows slowly, and then the growth rate increases.

Rice. 18. Graph of the dependence of the resistance force on the speed of the body

At low values ​​of the relative speed, the drag force is directly proportional to the value of this speed:

where is the value of the relative velocity; - resistance coefficient, which depends on the type of viscous medium, the shape and size of the body.

If the relative speed is large enough, then the drag force becomes proportional to the square of this speed.

where is the value of the relative velocity; is the drag coefficient.

The choice of formula for each specific case is determined empirically.

A body of mass 600 g moves uniformly along a horizontal surface (Fig. 19). In this case, a force is applied to it, the value of which is 1.2 N. Determine the value of the coefficient of friction between the body and the surface.