Definition 1

The friction force represents the force that appears at the moment of contact of two bodies and impedes their relative motion.

The main reason that provokes friction lies in the roughness of the rubbing surfaces and the molecular interaction of these surfaces. The friction force depends on the material of the contacting surfaces and on the force of their mutual pressing.

Concept of friction force

Based on simple models of friction (based on Coulomb's law), the friction force will be considered directly proportional to the degree of normal reaction of the contacting and rubbing surfaces. If we consider it as a whole, the friction force processes cannot be described only by simple models of classical mechanics, which is explained by the complexity of reactions in the interaction zone of rubbing bodies.

Friction forces, like elastic forces, are electromagnetic in nature. Their occurrence becomes possible thanks to the interaction between molecules and atoms of bodies that come into contact.

Note 1

Friction forces are different from elastic and gravitational forces due to the fact that they depend not only on the configuration of bodies (on their relative position), but also on the relative speeds of their interaction.

Types of friction force

Subject to availability relative motion two bodies in contact with each other, the friction forces arising in this process are divided into the following types:

1. Sliding friction (represents a force that arises as a consequence of the translational movement of one of the interacting bodies relative to the second and acts on this body in a direction that is opposite to the direction of sliding).
2. Rolling friction (represents the moment of force that can arise under the conditions of the rolling process of one of two bodies in contact with the other).
3. Static friction (considered a force that arises between two interacting bodies, and it becomes a serious obstacle to the occurrence of relative motion. Such a force is overcome in order to bring these contacting bodies into motion relative to each other. This type of friction appears during micro-movements (for example, during deformation ) of the contacting bodies. As the forces increase, the friction force will also increase.
4. Rotational friction (is a moment of force that arises between contacting bodies under conditions of rotation of one of them in relation to the other and directed against the rotation). Determined by the formula: $M=pN$, where $N$ is normal pressure, $p$ is the rotational friction coefficient, which has the dimension of length.

The independence of the friction force from the surface area along which contact of the bodies is observed, and the proportionality of the normal pressure force with which one body will act on the second was established experimentally.

Definition 2

A constant value represents the friction coefficient, which depends on the nature and condition of the rubbing surfaces.

In certain situations, friction is beneficial. Examples can be given of the impossibility of human walking (in the absence of friction) and the movement of vehicles. At the same time, friction can also have a harmful effect. Thus, it provokes wear of the contacting parts of the mechanisms, additional fuel consumption for vehicles. Various lubricants (air or liquid cushions) serve as a means of counteracting this. One more in an efficient way is considered to be the replacement of sliding by rolling.

Basic calculation formulas for determining friction force

The calculation formula for the friction force during sliding will look like this:

• $m$-proportionality coefficient (sliding friction),
• $Р$ – vertical (normal) pressure force.

The sliding friction force represents one of the forces controlling movement, and its formula is written using the support reaction force. Based on the action of Newton's third law, the normal pressure forces, as well as the support reaction, turn out to be equal in magnitude and opposite in direction:

Before determining the friction force, the formula of which will be written as follows: $F=mN$, the reaction force is determined.

Note 2

The resistance coefficient during the sliding process is introduced experimentally for rubbing surfaces, and it will depend on the material and the quality of processing.

The maximum static friction force is determined similarly to the sliding friction force. This is important for solving problems of determining the force of driving resistance. An example can be given of a book being moved by a hand pressed to it. Thus, the sliding of this book will be carried out under the influence of the static resistance force between the book and the hand. In this case, the amount of resistance will depend on the force of vertical pressure on the book.

An interesting fact will be that the friction force is proportional to the square of the corresponding speed, and its formula will change depending on the speed of movement of the interacting bodies. This force includes the force of viscous resistance in a liquid.

Depending on the speed of movement, the resistance force will be determined by the speed of movement, the shape of the moving body or the viscosity of the liquid. The movement of the same body in oil and water is accompanied by resistance of different magnitudes. For low speeds it looks like this:

• $k$ – proportionality coefficient, depending on the linear dimensions of the body and the properties of the environment,
• $v$ is the speed of the body.

The part of mechanics in which motion is studied without considering the reasons causing this or that character of motion is called kinematics.
Mechanical movement called a change in the position of a body relative to other bodies
Reference system called the body of reference, the coordinate system associated with it and the clock.
Body of reference name the body relative to which the position of other bodies is considered.
Material point is a body whose dimensions can be neglected in this problem.
Trajectory called a mental line that, when moving, describes material point.

According to the shape of the trajectory, the movement is divided into:
A) rectilinear- the trajectory is a straight line segment;
b) curvilinear- the trajectory is a segment of a curve.

Path is the length of the trajectory that a material point describes over a given period of time. This is a scalar quantity.
Moving is a vector connecting the initial position of a material point with its final position (see figure).

It is very important to understand how a path differs from a movement. The most important difference is that movement is a vector with a beginning at the point of departure and an end at the destination (it does not matter at all what route this movement took). And the path is, on the contrary, a scalar quantity that reflects the length of the trajectory traveled.

Uniform linear movement called a movement in which a material point makes the same movements over any equal periods of time
Speed ​​of uniform linear motion is called the ratio of movement to the time during which this movement occurred:

For uneven movement use the concept average speed. Often administered average speed as a scalar quantity. This is the speed of such uniform motion in which the body covers the same path in the same time as when not uniform motion:

Instant speed call the speed of a body at a given point in the trajectory or at a given moment in time.
Uniformly accelerated linear motion- this is a rectilinear movement in which the instantaneous speed for any equal periods of time changes by the same amount

Acceleration is the ratio of the change in the instantaneous speed of a body to the time during which this change occurred:

The dependence of the body coordinates on time in uniform rectilinear motion has the form: x = x 0 + V x t, where x 0 is the initial coordinate of the body, V x is the speed of movement.
Free fall called uniformly accelerated motion with constant acceleration g = 9.8 m/s 2, independent of the mass of the falling body. It occurs only under the influence of gravity.

Free fall speed is calculated using the formula:

Vertical movement is calculated using the formula:

One type of motion of a material point is motion in a circle. With such movement, the speed of the body is directed along a tangent drawn to the circle at the point where the body is located (linear speed). You can describe the position of a body on a circle using a radius drawn from the center of the circle to the body. The displacement of a body when moving in a circle is described by turning the radius of the circle connecting the center of the circle with the body. The ratio of the angle of rotation of the radius to the period of time during which this rotation occurred characterizes the speed of movement of the body in a circle and is called angular velocity ω:

Angular velocity is related to linear speed ratio

where r is the radius of the circle.
The time it takes the body to describe full turn, called circulation period. The reciprocal of the period is the circulation frequency - ν

Since during uniform motion in a circle the velocity module does not change, but the direction of the velocity changes, with such motion there is acceleration. They call him centripetal acceleration, it is directed radially towards the center of the circle:

Basic concepts and laws of dynamics

The part of mechanics that studies the reasons that caused the acceleration of bodies is called dynamics

Newton's first law:
There are reference systems relative to which a body maintains its speed constant or is at rest if other bodies do not act on it or the action of other bodies is compensated.
The property of a body to maintain a state of rest or uniform linear motion with balanced external forces acting on it is called inertia. The phenomenon of maintaining the speed of a body under balanced external forces is called inertia. Inertial reference systems are systems in which Newton's first law is satisfied.

Galileo's principle of relativity:
in all inertial reference systems under the same initial conditions, all mechanical phenomena proceed in the same way, i.e. subject to the same laws
Weight is a measure of body inertia
Strength is a quantitative measure of the interaction of bodies.

Newton's second law:
The force acting on a body is equal to the product of the mass of the body and the acceleration imparted by this force:
$F↖(→) = m⋅a↖(→)$

The addition of forces consists of finding the resultant of several forces, which produces the same effect as several simultaneously acting forces.

Newton's third law:
The forces with which two bodies act on each other are located on the same straight line, equal in magnitude and opposite in direction:
$F_1↖(→) = -F_2↖(→) $

Newton's III law emphasizes that the action of bodies on each other is in the nature of interaction. If body A acts on body B, then body B acts on body A (see figure).

Or in short, the force of action is equal to the force of reaction. The question often arises: why does a horse pull a sled if these bodies interact with equal forces? This is possible only through interaction with the third body - the Earth. The force with which the hooves press into the ground must be greater than the frictional force of the sled on the ground. Otherwise, the hooves will slip and the horse will not move.
If a body is subjected to deformation, forces arise that prevent this deformation. Such forces are called elastic forces.

Hooke's law written in the form

where k is the spring stiffness, x is the deformation of the body. The “−” sign indicates that the force and deformation are directed in different directions.

When bodies move relative to each other, forces arise that impede the movement. These forces are called friction forces. A distinction is made between static friction and sliding friction. Sliding friction force calculated by the formula

where N is the support reaction force, µ is the friction coefficient.
This force does not depend on the area of ​​the rubbing bodies. The friction coefficient depends on the material from which the bodies are made and the quality of their surface treatment.

Static friction occurs if the bodies do not move relative to each other. The static friction force can vary from zero to a certain maximum value

By gravitational forces are the forces with which any two bodies are attracted to each other.

Law universal gravity:
any two bodies are attracted to each other with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Here R is the distance between the bodies. The law of universal gravitation in this form is valid either for material points or for spherical bodies.

Body weight called the force with which the body presses on a horizontal support or stretches the suspension.

Gravity- this is the force with which all bodies are attracted to the Earth:

With a stationary support, the weight of the body is equal in magnitude to the force of gravity:

If a body moves vertically with acceleration, its weight will change.
When a body moves with upward acceleration, its weight

It can be seen that the weight of the body is greater than the weight of the body at rest.

When a body moves with downward acceleration, its weight

In this case, the weight of the body is less than the weight of the body at rest.

Weightlessness is the movement of a body in which its acceleration is equal to the acceleration free fall, i.e. a = g. This is possible if only one force acts on the body - gravity.
Artificial Earth satellite- this is a body that has a speed V1 sufficient to move in a circle around the Earth
There is only one force acting on the Earth's satellite - the force of gravity directed towards the center of the Earth
First escape velocity- this is the speed that must be imparted to the body so that it revolves around the planet in a circular orbit.

where R is the distance from the center of the planet to the satellite.
For the Earth, near its surface, the first escape velocity is equal to

1.3. Basic concepts and laws of statics and hydrostatics

A body (material point) is in a state of equilibrium if vector sum the forces acting on it are zero. There are 3 types of equilibrium: stable, unstable and indifferent. If, when a body is removed from an equilibrium position, forces arise that tend to bring this body back, this stable balance. If forces arise that tend to move the body further from the equilibrium position, this unstable position; if no forces arise - indifferent(see Fig. 3).


When we are not talking about a material point, but about a body that can have an axis of rotation, then in order to achieve an equilibrium position, in addition to the equality of the sum of forces acting on the body to zero, it is necessary that the algebraic sum of the moments of all forces acting on the body be equal to zero.

Here d is the force arm. Shoulder of strength d is the distance from the axis of rotation to the line of action of the force.

Lever equilibrium condition:
the algebraic sum of the moments of all forces rotating the body is equal to zero.
Pressure is a physical quantity equal to the ratio of the force acting on a platform perpendicular to this force to the area of ​​the platform:

Valid for liquids and gases Pascal's law:
pressure spreads in all directions without changes.
If a liquid or gas is in a gravity field, then each layer above presses on the layers below, and as the liquid or gas is immersed inside, the pressure increases. For liquids

where ρ is the density of the liquid, h is the depth of penetration into the liquid.

A homogeneous liquid in communicating vessels is established at the same level. If liquid with different densities is poured into the elbows of communicating vessels, then the liquid with a higher density is installed at a lower height. In this case

The heights of liquid columns are inversely proportional to densities:

Hydraulic press is a vessel filled with oil or other liquid, in which two holes are cut, closed by pistons. The pistons have different areas. If a certain force is applied to one piston, then the force applied to the second piston turns out to be different.
Thus, the hydraulic press serves to convert the magnitude of the force. Since the pressure under the pistons must be the same, then

Then A1 = A2.
A body immersed in a liquid or gas is acted upon by an upward buoyant force from the side of this liquid or gas, which is called by the power of Archimedes
The magnitude of the buoyancy force is determined by Archimedes' law: a body immersed in a liquid or gas is acted upon by a buoyant force directed vertically upward and equal to weight liquid or gas displaced by a body:

where ρ liquid is the density of the liquid in which the body is immersed; V submergence is the volume of the submerged part of the body.

Body floating condition- a body floats in a liquid or gas when the buoyant force acting on the body is equal to the force of gravity acting on the body.

1.4. Conservation laws

Body impulse is a physical quantity equal to the product of a body’s mass and its speed:

Momentum is a vector quantity. [p] = kg m/s. Along with body impulse, they often use impulse of power. This is the product of force and the duration of its action
The change in the momentum of a body is equal to the momentum of the force acting on this body. For an isolated system of bodies (a system whose bodies interact only with each other) law of conservation of momentum: the sum of the impulses of the bodies of an isolated system before interaction is equal to the sum of the impulses of the same bodies after the interaction.
Mechanical work called a physical quantity that is equal to the product of the force acting on the body, the displacement of the body and the cosine of the angle between the direction of the force and the displacement:

Power is the work done per unit of time:

The ability of a body to do work is characterized by a quantity called energy. Mechanical energy is divided into kinetic and potential. If a body can do work due to its motion, it is said to have kinetic energy. Kinetic energy forward movement material point is calculated by the formula

If a body can do work by changing its position relative to other bodies or by changing the position of parts of the body, it has potential energy. An example of potential energy: a body raised above the ground, its energy is calculated using the formula

where h is the lift height

Compressed spring energy:

where k is the spring stiffness coefficient, x is the absolute deformation of the spring.

The amount of potential and kinetic energy amounts to mechanical energy. For an isolated system of bodies in mechanics, law of conservation of mechanical energy: if there are no frictional forces between the bodies of an isolated system (or other forces leading to energy dissipation), then the sum of the mechanical energies of the bodies of this system does not change (the law of conservation of energy in mechanics). If there are friction forces between the bodies of an isolated system, then during interaction part of the mechanical energy of the bodies turns into internal energy.

1.5. Mechanical vibrations and waves

Oscillations movements that have varying degrees of repeatability over time are called. Oscillations are called periodic if the values ​​of physical quantities that change during the oscillation process are repeated at regular intervals.
Harmonic vibrations are called such oscillations in which the oscillating physical quantity x changes according to the law of sine or cosine, i.e.

The quantity A equal to the largest absolute value of the fluctuating physical quantity x is called amplitude of oscillations. The expression α = ωt + ϕ determines the value of x at a given time and is called the oscillation phase. Period T is the time it takes for an oscillating body to complete one complete oscillation. Frequency of periodic oscillations The number of complete oscillations completed per unit of time is called:

Frequency is measured in s -1. This unit is called hertz (Hz).

Mathematical pendulum is a material point of mass m suspended on a weightless inextensible thread and oscillating in a vertical plane.
If one end of the spring is fixed motionless, and a body of mass m is attached to its other end, then when the body is removed from the equilibrium position, the spring will stretch and oscillations of the body on the spring will occur in the horizontal or vertical plane. Such a pendulum is called a spring pendulum.

Period of oscillation of a mathematical pendulum determined by the formula

where l is the length of the pendulum.

Period of oscillation of a load on a spring determined by the formula

where k is the spring stiffness, m is the mass of the load.

Propagation of vibrations in elastic media.
A medium is called elastic if there are interaction forces between its particles. Waves are the process of propagation of vibrations in elastic media.
The wave is called transverse, if the particles of the medium oscillate in directions perpendicular to the direction of propagation of the wave. The wave is called longitudinal, if the vibrations of the particles of the medium occur in the direction of wave propagation.
Wavelength is the distance between two closest points oscillating in the same phase:

where v is the speed of wave propagation.

Sound waves are called waves in which oscillations occur with frequencies from 20 to 20,000 Hz.
The speed of sound is different in different environments. The speed of sound in air is 340 m/s.
Ultrasonic waves are called waves whose oscillation frequency exceeds 20,000 Hz. Ultrasonic waves are not perceived by the human ear.

Called dry. Otherwise, friction is called "fluid". A characteristic feature of dry friction is the presence of static friction.

It has been experimentally established that the friction force depends on the force of pressure of bodies on each other (support reaction force), on the materials of the rubbing surfaces, on the speed of relative movement and Not depends on the area of ​​contact. (This can be explained by the fact that no body is absolutely flat. Therefore, the true contact area is much smaller than the observed one. In addition, by increasing the area, we reduce the specific pressure of the bodies on each other.) The quantity characterizing the rubbing surfaces is called friction coefficient, and is most often denoted by the Latin letter “k” or Greek letter"μ". It depends on the nature and quality of processing of the rubbing surfaces. In addition, the coefficient of friction depends on speed. However, most often this dependence is weakly expressed, and if greater measurement accuracy is not required, then “k” can be considered constant.

To a first approximation, the magnitude of the sliding friction force can be calculated using the formula:

Where

Sliding friction coefficient,

Normal ground reaction force.

According to the physics of interaction, friction is usually divided into:

• Dry when interacting solids are not separated by any additional layers/lubricants - a very rare case in practice. Characteristic distinguishing feature dry friction - the presence of a significant static friction force.
• Dry with dry lubricant (graphite powder)
• Liquid, during the interaction of bodies separated by a layer of liquid or gas (lubricant) of varying thickness - as a rule, it occurs during rolling friction, when solid bodies are immersed in a liquid;
• Mixed, when the contact area contains areas of dry and liquid friction;
• Boundary, when the contact area may contain layers and areas of different nature (oxide films, liquid, etc.) is the most common case of sliding friction.

Due to the complexity of the physicochemical processes occurring in the zone of frictional interaction, friction processes fundamentally cannot be described using the methods of classical mechanics.

During mechanical processes, there is always, to a greater or lesser extent, a transformation of mechanical motion into other forms of motion of matter (most often into a thermal form of motion). In the latter case, interactions between bodies are called friction forces.

Experiments with the movement of various bodies in contact (solids on solids, solids in liquid or gas, liquid in gas, etc.) with different states of the contact surfaces show that friction forces appear during the relative movement of the contacting bodies and are directed against the relative velocity vector tangentially to contact surfaces. In this case, heating of the interacting bodies always occurs.

Friction forces are the tangential interactions between contacting bodies that arise during their relative movement. The frictional forces that arise during the relative movement of various bodies are called external friction forces.

Friction forces also arise during relative movement of parts of the same body. The friction between layers of the same body is called internal friction.

In real movements, friction forces of greater or lesser magnitude always arise. Therefore, when drawing up equations of motion, strictly speaking, we must always introduce the friction force F tr into the number of forces acting on the body.

A body moves uniformly and rectilinearly when an external force balances the friction force that arises during movement.

To measure the frictional force acting on a body, it is enough to measure the force that must be applied to the body so that it moves without acceleration.

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• The Force of Destiny (opera)
• Elastic force

See what “Sliding friction force” is in other dictionaries:

static friction force- limiting friction The force of static friction at the moment the sliding begins. IFToMM code: 3.5.48 Section: DYNAMICS OF MECHANISMS... Theory of mechanisms and machines

FRICTION COEFFICIENT- quantity characterizing external friction. Depending on the type of movement of one body over another, T... Physical encyclopedia

Friction coefficient- the ratio of the friction force F to the reaction T, directed normal to the touching surface, which occurs when a load is applied pressing one body to another: f = F/T. Because the characteristic used when performing technical calculations is... ...

friction- Friction, friction. It is curious to see the combination of three types of nominative meanings in the word friction. The mechanics term friction has been used to characterize public relations. This happened in literary language the last third of the 19th century, not earlier... ... History of words

POWER- vector quantity is a measure of the mechanical impact on the body from other bodies, as well as the intensity of other physical forces. processes and fields. Forces are different: (1) C. Ampere, the force with which (see) acts on a conductor carrying current; direction of the force vector... ... Big Polytechnic Encyclopedia

Friction coefficient- Friction is the process of interaction of solid bodies during their relative motion (displacement) or during the movement of a solid body in a liquid or gaseous medium. Otherwise called frictional interaction. Studying friction processes... ... Wikipedia

Law of Friction- Sliding friction forces are forces that arise between contacting bodies during their relative motion. If there is no liquid or gaseous layer (lubricant) between the bodies, then such friction is called dry. Otherwise, friction... ... Wikipedia

external sliding friction- contact friction is mechanical resistance to the movement of one body on the surface of another; in the deformation zone occurs during the interaction of the tool and the material being processed. Features of contact friction during processing... ... Encyclopedic Dictionary in metallurgy

Sleeve bearing- Rolling bearing with a fixed outer ring A bearing is a technical device that is part of a support that supports a shaft, axis or other structure, fixes a position in space, provides rotation, swing or linear... ... Wikipedia

Sleeve bearing- a support or guide of a Mechanism or machine (See Machine), in which friction occurs when mating surfaces slide. Based on the direction of load perception, a distinction is made between radial and axial (thrust) load bearings. Depending on the lubrication mode... Great Soviet Encyclopedia

Target: Consolidate acquired knowledge about friction and types of friction.

Work progress:

1. Study the theoretical part
2. Fill out table 1.
3. Solve the problem using the option from Table 2.
4. Answer security questions.

Table 1

Table 2

	A skater skates along a smooth horizontal ice surface by inertia of 80 m. Determine the friction force and initial speed, if the mass of the skater is 60 kg and the coefficient of friction is 0.015
	A body weighing 4.9 kg lies on a horizontal plane. What force must be applied to the body in the horizontal direction to impart to it an acceleration of 0.5 m/s 2 with a friction coefficient of 0.1?
	A wooden block with a mass of 500 g lies on a horizontal table, which is driven by a load with a mass of 300 g suspended from the vertical end of a thread thrown through a block attached to the end of the table. The coefficient of friction when the block moves is 0.2. With what acceleration will the block move?

Friction force- this is the force that arises between the surfaces of contacting bodies. If there is no lubrication between the surfaces, then friction is called dry. The force of dry friction is directly proportional to the force pressing the surfaces against each other and is directed in the direction opposite to the possible movement. The proportionality coefficient is called the friction coefficient. The pressing force is perpendicular to the surface. It is called normal ground reaction.

The laws of friction in liquids and gases differ from the laws of dry friction. Friction in liquids and gases depends on the speed of movement: at low speeds it is proportional to the square, and at high speeds it is proportional to the cube of the speed.

Formulas for solution:

Where "k" is the friction coefficient, "N" is the normal reaction of the support.

Newton's second law and equations of motion in vector form. F = ma

According to Newton's third law N = - mg

expression for speed

Equations of motion for uniformly accelerated kinematic motion

; 0 - V = a t where 0 – final speed V – initial speed

Algorithm for solving a typical problem:

1. Briefly write down the conditions of the problem.

2. We depict the condition graphically in an arbitrary reference system, indicating the forces acting on the body (point), including the normal reaction of the support and the friction force, the speed and acceleration of the body.

3. We correct and designate the reference system in the figure, introducing the origin of time and specifying the coordinate axes for forces and acceleration. It is better to direct one of the axes along the normal reaction of the support, and start counting the time at the moment the body (point) is at coordinate zero.

4. We write Newton’s second law and equations of motion in vector form. The equations of motion and speed are the dependences of movement (path) and speed on time.

5. We write these equations in scalar form: in projections on the coordinate axes. We write down the expression for the friction force.

6. Solve the equations in general form.

7. Substitute the values ​​into general solution, we calculate.

8. Write down the answer.

Theoretical part
Friction is the resistance of bodies in contact to movement relative to each other. Friction is accompanied by every mechanical movement, and this circumstance has a significant consequence in modern technical progress.
The force of friction is the force of resistance to the movement of bodies in contact relative to each other. Friction is explained by two reasons: the unevenness of the rubbing surfaces of the bodies and molecular interaction between them. If we go beyond mechanics, we should say that friction forces are of electromagnetic origin, like elastic forces. Each of the above two causes of friction manifests itself to varying degrees in different cases. For example, if the contacting surfaces of solid rubbing bodies have significant unevenness, then the main term in the friction force arising here will be due precisely to this circumstance, i.e. unevenness, roughness of the surfaces of rubbing bodies. Bodies moving with friction relative to each other must touch the surfaces or move one in the environment of the other. The movement of bodies relative to each other may not occur due to the presence of friction if driving force less than the maximum static friction force. If the contacting surfaces of solid rubbing bodies are perfectly polished and smooth, then the main component of the friction force arising in this case will be determined by the molecular adhesion between the rubbing surfaces of the bodies.

Let us consider in more detail the process of the emergence of sliding and static friction forces at the junction of two contacting bodies. If you look at the surfaces of bodies under a microscope, you will see micro-irregularities, which we will depict in an enlarged form (Fig. 1, a). Let us consider the interaction of contacting bodies using the example of one pair of irregularities (ridge and trough) (Fig. 3, b). In the case when there is no force trying to cause movement, the nature of the interaction on both slopes of microroughness is similar. With this type of interaction, all horizontal components of the interaction force balance each other, and all vertical ones are summed up and make up the force N (support reaction) (Fig. 2, a).

A different picture of the interaction of bodies is obtained when a force begins to act on one of the bodies. In this case, the contact points will be predominantly on the “slopes” to the left of the pattern. The first body will press on the second. The intensity of this pressure is characterized by force R". The second body, in accordance with Newton's third law, will act on the first body. The intensity of this action is characterized by force R (support reaction). Force R

can be decomposed into components: force N, directed perpendicular to the surface of contact of the bodies, and force Fsc, directed against the action of force F (Fig. 2, b).

After considering the interaction of bodies, two points should be noted.
1) When two bodies interact in accordance with Newton’s third law, two forces R and R arise; for the convenience of taking it into account when solving problems, we decompose the force R into components N and Fst (Ftr in the case of motion).
2) Forces N and F Tp have the same nature (electromagnetic interaction); It couldn’t be otherwise, since these are components of the same force R.
Very important in modern technology To reduce the harmful effects of friction forces, sliding friction is replaced by rolling friction. The rolling friction force is defined as the force required for uniform rectilinear rolling of a body along a horizontal plane. Experience has established that the rolling friction force is calculated by the formula:

where F is the rolling friction force; k-coefficient of rolling friction; P is the pressure force of a rolling body on the support and R is the radius of the rolling body.

From practice it is obvious, from the formula it is clear that the larger the radius of a rolling body, the less obstacle the uneven surface of the support poses to it.
Note that the rolling friction coefficient, in contrast to the sliding friction coefficient, is a named value and is expressed in units of length - meters.
Sliding friction is replaced by rolling friction, in necessary and possible cases, by replacing plain bearings with rolling bearings.

There is external and internal friction (otherwise known as viscosity). External friction is a type of friction in which forces arise at the points of contact of solid bodies that impede the mutual movement of the bodies and are directed tangentially to their surfaces.

Internal friction (viscosity) is the type of friction that occurs during mutual movement. Layers of liquid or gas between them generate tangential forces that prevent such movement.

External friction is divided into static friction (static friction) and kinematic friction. Static friction occurs between fixed solid bodies when they try to move one of them. Kinematic friction exists between mutually touching moving solid bodies. Kinematic friction, in turn, is divided into sliding friction and rolling friction.

Friction forces play an important role in human life. In some cases he uses them, and in others he fights them. Friction forces are electromagnetic in nature.
Types of friction forces.
Friction forces are of an electromagnetic nature, i.e. Friction forces are based on the electrical forces of interaction between molecules. They depend on the speed of movement of bodies relative to each other.
There are 2 types of friction: dry and liquid.
1. Fluid friction is a force that arises when a solid body moves in a liquid or gas or when one layer of liquid (gas) moves relative to another and slows down this movement.

In liquids and gases there is no static friction force.
At low speeds of movement in liquid (gas):
Ftr= k1v,
where k1 is the resistance coefficient, depending on the shape, size of the body and the environment. Determined empirically.

At high speeds:
Ftr= k2v,
where k2 is the resistance coefficient.
2. Dry friction is a force that arises when bodies come into direct contact, and is always directed along the contact surfaces of electromagnetic bodies precisely by breaking molecular bonds.
Rest friction.
Let's consider the interaction of a bar with the surface of a table. The surface of the contacting bodies is not absolutely flat. The greatest force of attraction occurs between atoms of substances located at a minimum distance from each other, that is, on microscopic protrusions. The total force of attraction of atoms and bodies in contact is so significant that even under the influence of an external force applied to the block parallel to the surface of its contact with the table, the block remains at rest. This means that a force equal in magnitude acts on the block external force, but in the opposite direction. This force is the static friction force. When the applied force reaches a maximum critical value sufficient to break the bonds between the protrusions, the block begins to slide along the table. The maximum static friction force does not depend on the surface contact area. According to Newton’s third law, the normal pressure force is equal in magnitude to the support reaction force N.
The maximum static friction force is proportional to the normal pressure force:

where μ is the static friction coefficient.

The coefficient of static friction depends on the nature of the surface treatment and on the combination of materials that make up the contacting bodies. High-quality processing of smooth contact surfaces leads to an increase in the number of attracted atoms and, accordingly, to an increase in the coefficient of static friction.

The maximum value of the static friction force is proportional to the modulus of the force F d of the pressure produced by the body on the support.
The value of the static friction coefficient can be determined as follows. Let the body (flat block) lie on the inclined plane AB (Fig. 3). Three forces act on it: the force of gravity F, the static friction force Fп and the support reaction force N. The normal component Fп of the force of gravity is the pressure force Fд produced by the body on the support, i.e.
FН=Fд. The tangential component Ft of gravity is a force tending to move a body down an inclined plane.
At small angles of inclination a, the force Ft is balanced by the static friction force Fp and the body is at rest on the inclined plane (the support reaction force N, according to Newton’s third law, is equal in magnitude and opposite in direction to the force Fd, i.e., it balances it).
We will increase the angle of inclination a until the body begins to slide down the inclined plane. At this moment
Fт=FпmaxFrom Fig. 3 it is clear that Ft = Fsin = mgsin; Fн=Fcos = mgcos.
we get
fн=sin/cos=tg.
By measuring the angle at which the body begins to slide, you can use the formula to calculate the value of the coefficient of static friction fп.

Rice. 3. Static friction.
Sliding friction

Sliding friction occurs when relative movement of contacting bodies occurs.
The sliding friction force is always directed in the direction opposite to the relative speed of the contacting bodies.
When one body begins to slide over the surface of another body, the bonds between the atoms (molecules) of the initially motionless bodies are broken, and friction decreases. With further relative motion of bodies, new connections between atoms are constantly formed. In this case, the sliding friction force remains constant, slightly less than the static friction force. Like the maximum static friction force, the sliding friction force is proportional to the normal pressure force and, therefore, to the support reaction force:
,where is the coefficient of sliding friction (), depending on the properties of the contacting surfaces.

Rice. 3. Sliding friction

Security questions

1. What is external and internal friction?
2. What kind of friction is called static friction?
3. What is dry and liquid friction?
4. What is the maximum static friction force?
5. How to determine the value of the coefficient of static friction?

Friction force

Species

In the presence of relative motion of two contacting bodies, the friction forces arising during their interaction can be divided into:

• Sliding friction- a force that arises during the translational movement of one of the contacting/interacting bodies relative to another and acts on this body in the direction opposite to the direction of sliding.
• Rolling friction- moment of force that occurs when one of two contacting/interacting bodies rolls relative to the other.
• Static friction- a force that arises between two contacting bodies and prevents the occurrence of relative motion. This force must be overcome in order to set two contacting bodies in motion relative to each other. Occurs during micromovements (for example, during deformation) of contacting bodies. It acts in the direction opposite to the direction of possible relative motion.

In the physics of interaction, friction is usually divided into:

• dry when interacting solids are not separated by any additional layers/lubricants (including solid lubricants) - a very rare case in practice. A characteristic feature of dry friction is the presence of a significant static friction force;
• boundary when the contact area may contain layers and areas of different nature (oxide films, liquid, etc.) - the most common case of sliding friction.
• mixed when the contact area contains areas of dry and liquid friction;
• liquid (viscous), during the interaction of bodies separated by a layer of solid (graphite powder), liquid or gas (lubricant) of varying thickness - as a rule, it occurs during rolling friction, when solid bodies are immersed in a liquid, the amount of viscous friction is characterized by the viscosity of the medium;
• elastohydrodynamic, When crucial has internal friction in the lubricant. Occurs when relative movement speeds increase.

Due to the complexity of the physical and chemical processes occurring in the frictional interaction zone, friction processes fundamentally cannot be described using the methods of classical mechanics.

Amonton-Coulomb law

The main characteristic of friction is friction coefficient, which is determined by the materials from which the surfaces of interacting bodies are made.

In the simplest cases, the friction force and normal load (or force normal reactions) are related by inequality

turning into equality only in the presence of relative movement. This relationship is called the Amonton-Coulomb law.

Amonton-Coulomb law taking into account adhesion

For most pairs of materials, the friction coefficient value does not exceed 1 and is in the range of 0.1 - 0.5. If the friction coefficient exceeds 1, this means that there is a force between the contacting bodies adhesion and the formula for calculating the friction coefficient changes to

.

Application value

Friction in mechanisms and machines

In most traditional mechanisms (internal combustion engines, cars, gears, etc.), friction plays a role negative role, reducing the efficiency of the mechanism. To reduce the friction force, various natural and synthetic oils and lubricants are used. In modern mechanisms, spraying of coatings (thin films) onto parts is also used for this purpose. With the miniaturization of mechanisms and the creation of microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS), the amount of friction compared to the forces acting in the mechanism increases and becomes very significant, and at the same time cannot be reduced using conventional lubricants, which is of significant theoretical and practical interest engineers and scientists in this field. To solve the problem of friction, new methods are being created to reduce it within the framework of tribology and surface science ( English).

Surface grip

The presence of friction provides the ability to move along the surface. So, when walking, it is due to friction that the sole adheres to the floor, resulting in repulsion from the floor and forward movement. In the same way, adhesion of the wheels of a car (motorcycle) to the road surface is ensured. In particular, to increase the improvement of this grip, new shapes and special types of rubber for tires are being developed, and wings are installed on racing cars, pressing the car more firmly to the track.

See also

Magazines

• Friction, Wear, Lubrication, a magazine about friction.
• Friction and Wear, a magazine on friction is published National Academy Sciences of Belarus since 1980
• Journal of Tribology, an international journal of friction.
• Wear, an international journal on friction and wear.
• Friction coefficient tables, numerical values ​​of friction coefficients.

Literature

• Deryagin B.V. What is friction? M.: Publishing house. USSR Academy of Sciences, 1963.
• Kragelsky I. V., Shchedrov V. S. Development of the science of friction. Dry friction. M.: Publishing house. USSR Academy of Sciences, 1956.
• Frolov, K. V. (ed.) Modern tribology: Results and prospects. LKI, 2008.
• Bowden F. P., Tabor D. The Friction and Lubrication of Solids. Oxford University Press, 2001.
• Persson Bo N. J.: Sliding Friction. Physical Principles and Applications. Springer, 2002.
• Popov V. L. Kontaktmechanik und Reibung. Ein Lehr- und Anwendungsbuch von der Nanotribologie bis zur numerischen Simulation, Springer, 2009.
• Rabinowicz E. Friction and Wear of Materials. Wiley-Interscience, 1995.

Links

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Synonyms:

See what “Friction” is in other dictionaries:

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