# Friction force - definition, formula, species, how to find?

## Friction force: quantity, direction

With the force of friction, you literally come across every second. Each time you interact with any surface - go on asphalt, sit on a chair, drink tea from a cup - the friction force acts on you.

 Friction - This is the interaction in the plane of contacting two surfaces. To translate friction into the language of mathematics, the concept is introduced friction force .Friction force - This is a value that characterizes the process of friction in magnitude and direction.

The friction force is measured, as well as any power - in Newton.

The friction force arises for two reasons:

• Various roughness, scratches and other "imperfections" of surfaces. These defects hurt each other when contacting and is created by the force that slows down movement.
• When the contacting surfaces are practically smooth (before the ideal it is impossible to bring, but strive for it - it means to strive the force of friction to zero), then the distance between them becomes minimal. In this case, the mutual attraction of the molecules of the substance of these surfaces occurs. The attraction is due to the interaction between the electrical charges of atoms. In this regard, you can often hear the wording "The force of friction is the power of electromagnetic nature"

The friction force is always directed against the body velocity. In this regard, everything is simple, but there is always a question:

In tasks, something like something like: "Table the surface is perfectly smooth." This means that there is no friction force in this task. Yes, in real life it is impossible, but in the name of a beautiful mathematical model, friction is often neglected.

Do not worry because of this injustice, and simply solve the tasks without friction, if we saw the phrase "smooth surface".

## Dry and viscous friction

There is a very big difference between your contact with water in the pool during swimming and contact between the asphalt and the wheels of your bike.

In the case of swimming, we are dealing with viscous friction - resistance phenomenon when driving a solid in a liquid or air. The plane is also viscous friction and won the arrogant pigeon from your yard.

But dry friction is the phenomenon of resistance in contacting two solids. For example, if a schoolboy is lying on a chair or a villain from the film rubs his hands - it will be dry friction.

And if the villain is clean and rubs his hands, pinning an antiseptic on them?

Then this viscous friction, despite the fact that the hands are solid. In this case, there is a wet layer.

A viscous friction in the school course of physics is not considered in detail, but dry - disassemble along and across. Dry friction also has varieties, let's talk about them.

## Friction of rest

If you decide to move a truck from the spot, it is unlikely that you will succeed. Not that we do not believe in you - it's easy to do this due to the fact that the mass of a person is many times less than the mass of the truck, and the friction force prevents it. The world is cruel, what to do here.

In the case when the friction force is, but the body does not move from the place, we are dealing with The strength of rest friction.

The strength of friction of peace is equal to the power of thrust. For example, if you are trying to move off the scene, acting on them with the force of traction 10 n, then the friction force will be equal to 10 N.

 The strength of friction rest FTR = FLYAGI FTR - Slip friction force [H] FLYAGES - PRICE STRIUM [H]

Slightly practice!

Find the power of friction of rest for the body to which the force of traction in 4 N.

Decision:

The body rests, it means that

FTR = FLATYA = 4N

Answer: The friction force is 4 N.

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## Sliding friction

And now let's slide on ice skating. The rink is smooth enough, but, as we have already found out, the friction force will still be present and will be calculated by the formula:

 Slip friction force FTR = μN. FTR - Slip friction force [H] μ - friction coefficient [-] N - Power of the reaction support [H]

The friction force that we get on this formula will be the maximum possible - that is, there is no longer anywhere.

Power reaction support - This is the force with which the support acts on the body. It is numerically equal to the strength of normal pressure and is opposite to direction.

The power of normal pressure is the same as the body weight?

Not really. The power of normal pressure is always aimed perpendicular to the surface (normal - perpendicular to the surface). The weight is not necessarily directed perpendicular to the surface.

As part of the school year, weight is always directed perpendicular to the surface Therefore, the power of the reaction of the support can be equally equal to the weight.

Also, if the body is on the horizontal surface, the reaction force of the support will be equal to the force of gravity: n = mg.

Friction coefficient - This is a surface characteristic. It is determined experimentally, does not have dimension and shows how much the surface is smooth - the more the coefficient, the more cooler surface. The friction coefficient is positive and most often less than one.

We will be vigilant!

From the formula, the dependence of the friction force from the area of ​​contact is not followed. For example, if you put a bar one side and drag on the table, and then turn over to another, not equal in the area, and make the same thing - the friction force will not change.

The mass of the cat lying on the table is 5 kg. Friction coefficient μ = 0.2. The cat is attached to the external force equal to 2.5n. What kind of friction does it occur?

Decision:

By the condition of this task it is impossible to understand, our cat moves or not. The decision on whether we equal to the strength of the thrust force, it is impossible to accept immediately. In such cases, you still need to calculate according to the formula:

F = μN.

Since the cat lies on the horizontal surface, the reaction force of the support in this case is equal to the force of gravity: n = mg.

F = μmg = 0.2 * 5 * 10 = 10n

We got the highest possible friction force. External force under the condition of the task is less than the maximum. This means that the cat is alone. The friction force balances the external force. Consequently, it equals 2.5n.

Answer: There is a friction force of 2.5 n

Badger slides along the horizontal plane. Find the friction coefficient if the friction force is 5 H, and the power of the body pressure on the plane - 20 N.

Decision:

In this task, we know that Barcuchk slides. So you need to use the formula:

FTR = μN.

Since the badger is located on the horizontal surface, the support force of the support in this case is equal to the pressure of the pressure on the plane: n = FD.

FTR = μFD

Express the friction coefficient:

μ = FTR / FD = 5/20 = 0.25

Answer: The friction coefficient is 0.25

Your grandmother's poodle weighing 5 kilograms slides horizontal surface. Slip friction force is 20 N. Find the force of friction, if the poodle will lose greatly, and its mass will decrease twice, and the friction coefficient will remain unchanged.

Decision:

In this task, we know that the poodle slips. So you need to use the formula:

FTR = μN.

Since the poodle is on the horizontal surface, the reaction force of the support in this case is equal to the force of gravity: n = mg.

FTR = μmg.

Express the friction coefficient:

μ = FTR / Mg = 20/5 * 10 = 0.4

Now we calculate the force of friction for mass, less than twice:

Answer: The friction force will be equal to 10 N.

The student conducted an experiment to study the sliding friction force by moving a bar with loads evenly along horizontal surfaces using a dynamometer.

The results of experimental measurements of the mass of the bar with loads M, the area of ​​contact of the bar and the surface S and the applied force F are presented in the table.

## What is the force of friction in physics

Definition

The friction force is called the force that appears during the movement when there is a touch of two bodies of each other and which has resistance to this movement (always directed towards the opposite movement).

The greater the value of the friction force between the two bodies, the more difficult to move them relative to each other. Example

An example may be a cardboard box weighing 20 kilograms, which move around the apartment. The friction force between the box and laminate in the same room will be much smaller than between a box and a carpet with a long pile to another. In the first case, moving the box is quite easy, in the second - difficult.

Caution! If the teacher detects plagiarism in work, not to avoid major problems (up to deductions). If you do not have the opportunity to write yourself, order here.

Two bodies with friction about each other, they are experiencing the action of the third law of Newton. The friction force acting on the first object is equal to the value of the force of friction affecting the second object. But the two of these strengths have the exact opposite direction.

### What is measured

The physical nature of friction consists in the interaction of atoms and molecules of bodies that come into contact with each other.

In physics, it is customary to sign the strength of the friction of the big Latin letter F with the mark of TR.: F Tr. .

Measuring this physical value is carried out in Newton (H).

## Types of friction force with examples

Depending on the nature of the movement and the type of surfaces interacting with each other, several classifications of friction concepts are distinguished.

### In the direction of action

By the nature of the movement of bodies, friction forces are distinguished:

1. Rest . It always has a zero value and occurs when the two bodies are touched, which are at rest relative to each other.
2. Slip . This is a classic type of friction that occurs when two bodies are slide relative to each other. The value of this value depends on body weight (the greater the mass, the greater the friction force) and the nature of the surface (gliding on ice / sliding on the ground).
3. Rolling . This force appears when one object is rolling over the surface of another (car on the road). Thanks to the discovery that the friction force when rolling is at times less compared with the force of friction during slide, our ancestors and invented one of the main inventions of humanity - the wheel.
4. Version . This type of force appears when one body rotates over the surface of another.

### By type of interacting surfaces

Friction itself is also a few species:

1. Dry. It occurs when there is a touch of a friend of solid surfaces.
2. Viscous (liquid). It occurs when the solid is touched with liquid or gas. As a rule, the strength of viscous friction is much less dry friction.
3. Mixed. It appears between the rigorous surfaces of solids, between which the lubricant layer is located.

### Internal and external friction

Friction happens:

External occurs when the solid bodies are coming. The internal manifests itself in the interaction of gas or liquids. Inside a single body, the layers displacement relative to each other.

## Friction coefficient

In order to make the calculation of the friction force, it is necessary to know the friction coefficient (K), which depends on the surface material and does not have units of measurement in the SI system.

The friction coefficient is a constant physical quantity, whose value for all sorts of bodies can be recognized from the table.

## Formulas for calculating friction force

For the body located on a horizontal surface The calculation of the friction force is made by the formula:

\ (F_Tr = K \ Times N \)

Where k is the friction coefficient, and N is the power of the support reaction.

From the formula for calculating the friction force, it is clear that in addition to the friction coefficient, it is necessary to know the force of the support reaction (N), which is equal to the power of gravity and depends on the body weight (M) and accelerating the free fall (G):

\ (N = M \ Times G \)

When driving a body By inclined surface The formula for finding friction force is complicated:

\ (F = k \ Times M \ Times G \ Times \ COS \ Alpha \)

Where \ (\ cos \ alpha \) - This is the ratio of the corner of the category for the hypotenneus of the resulting triangle.

Depending on the conditions of the task of finding the friction force, it is necessary to choose to calculate one of the above formulas.

## Schedule of the dependence of the injection force from the force of traction

• Determination of friction force
• Types of friction force
• How to find friction power?
• Formula of friction force
• Recommended literature and useful links
• Friction force, video
• The friction force arises from the contact of the surfaces of two physical bodies who are in motion towards each other. The theory of friction since the mind was worried by the minds of humanity, ancient engineers: builders of the Egyptian pyramids, Stonehenge in England or mysterious stonestorms on Easter Island, they all (as well, and their modern colleagues) solved the pressing problem associated with friction and how to minimize it. After all, the friction force makes it difficult to move heavy loads on the ground (the same stones for pyramids or Stonehenge), and to facilitate this task, our distant ancestors have been invented such a useful invention as a wheel and many other important discoveries have been made. In our article we will look at the strength of friction in the physical aspect, we will analyze how it acts on those or other bodies, which there are its types and formulas for calculating.

## Determination of friction force

What is friction force? The classic definition sounds like this: the friction force is the force appearing when the two bodies are contacted during the movement and preventing this very movement. In other words, the more friction force between the bodies, the harder it is to move them relative to each other. As for the physical nature of the friction, it appears as a result of the interaction between atoms and body molecules in contact with each other.

It is also worth noting that with the friction of the two bodies, the third Newton law acts on them: the friction force acting on the first body (body A) is equal to the power of friction acting on the second body (b), only the module is the opposite direction.

In this picture, the friction force acting on the refrigerator is equal to the power of friction acting on the floor, but these forces are directed in opposite sides.

## Types of friction force

Depending on the nature of the motion of the bodies, such types of friction forces are distinguished as:

• Rest. The friction force of rest occurs when the two bodies are coming, which, however, do not move relative to each other, and is zero.
• Slip. Slip friction force is the most classic illustration of friction action, occurs when sliding bodies relative to each other. It affects its magnitude (than it is more, the more friction force), the nature of the surface (of course, when the strength of the friction will be many times less than when gliding on the ground).
• Rolling. Rolling friction force appears when one body rolls over the surface of another, for example, when riding a bike or car. When rolling, the friction force is much less than when sliding. This is an experienced, empirically established yet those distant ancestors who invented the wheel - the greatest invention in the history of science and technology.
• Digging. The friction force of the turn is manifested when the single body is rotated over the other surface.

As for the friction itself, it also happens a few species:

• Dry - manifests itself in contact of solid surfaces.
• A viscous, also similar friction is called liquid, appears when contacting a solid with liquid or gas. For example, a viscous (liquid) friction is applied to the ship floating on water, as well as on the water surface. The force of viscous friction is usually much less dry friction.
• Mixed, occurs when there is a lubricant layer between surfaces that come into contact.

An interesting fact: in the siege of Constantinople in 1453, the Turks in order to bypass the special chain, blocking the path to the Turkish ships in the Golden Horn Bay pulled them on land. And in order to reduce the strength of friction when moving large heavy warships made flooring from wooden rails, which was abundantly smeared with lard. Thus, due to the lubricant and mixed friction, the strength of which is much less than with friction dry, the Turks successfully embodied their intention, bringing the defenders of Constantinople into genuine confusion.

Sultan Mehmed II observes the carriage of his ships.

As you can see, knowledge of the laws of physics and mechanics more than once and not two found their practical embodiment in real life.

But back from history again to physics, friction is also divided into external and internal. External friction is characteristic of the interaction of exclusively solid bodies. Internal friction is characterized by viscosity and occurs in the interaction of liquids or gases, and such interaction can occur inside the conditionally single body. For example, in the waters of the World Ocean, there are different currents, with a colder or warmer water, in the interaction of these flows between them and inner friction occurs.

## How to find friction power?

To calculate the friction force, you need to know the friction coefficient K, which depends on the nature of the surface. The friction coefficient is a constant value and its value can be obtained from a special table.

In addition to the friction coefficient, it is necessary to know the force of the support reaction N, which, in essence, is equal to the power of gravity (gravity) depending on the mass of the body (M) and accelerate the free fall. Its formula will have the following form:

N = m * g

Where M is the mass of the body, and G is the acceleration of the free fall, this is a constant value of 9.8 m / s 2.

## Formula of friction force

The friction force is calculated by means of the reaction of Support N and the friction coefficient K. The formula of the friction force will have the following form:

F. Tr. = K * N.

In some formulas, the friction coefficient K is indicated by the symbol μ.

The calculations written above are valid in the simplest case when the body lies on a strictly horizontal surface.

If the movement occurs on the inclined plane, then the calculations of the friction force are somewhat more complicated. The body, as before, acts the force of gravity and the reaction of the surface support, but not in one direction.

Thus, the friction force formula for the body, which moves along the inclined surface, will have the following form:

F. Tr. = K * M * G * COSα.

Where k is the friction coefficient, M - body weight, G gravitational constant (remember that it is 9.8 m / s 2), COSα is the ratio of the category adjacent to the corner, to the triangle hypothenuze (cosine).

In determining the friction force on inclined surfaces, the link between physics and geometry is clearly manifested.

## Recommended literature and useful links

• Friction force. ZFTSH, MIPT. Candle date February 14, 2019.
• Yenohovich A. S. Physics Handbook. - Enlightenment, 1978. - P. 85. - 416 p.
• Zaitsev A. K. Basics of teaching on friction, wear and lubrication of cars. Part 1. Friction in the machines. Theory, calculation and design of bearings and sliding spikes. Mashgiz. M.-L. - 1947. 256 p.
• Bowden F. P., Tabor D. The Friction and Lubrication of Solids. Oxford University Press, 2001.person BO N. J. 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.

## Friction force, video

And at the end of the educational video on our article.

### Author: Pavel Chaika, Editor-in-Chief of the Journal

When writing the article tried to make it the most interesting, useful and high-quality. I would be grateful for any feedback and constructive criticism in the form of comments on the article. Also your wish / question / offer can write to my mail [email protected] or in Facebook, with respect, author.

Friction - The process of interaction between their relative movement (displacement) or when the body moves in a gaseous or liquid medium. Differently called friction interaction (eng. Friction. ). The study of friction processes is engaged in a section of physics, which is called mechanics of friction interaction, or tribology.

Frequency mainly has an electronic nature, provided that the substance is in normal condition. In the superconducting state away from the critical temperature, the main "source" of friction is phonons, and the friction coefficient can decrease several times. [Link 1] .

## Friction force

### Views

In the presence of the relative movement of the two contacting bodies of friction force arising from their interaction, can be divided into:

• Sliding friction - The force arising in the proper movement of one of the contacting / interacting bodies relative to the other and acting on this body in the direction opposite to the direction of the slide.
• Rolling friction - the moment of forces arising from rolling one of two contacting / interacting bodies relative to the other.
• Friction of rest - force arising between two contacting bodies and preventing the occurrence of relative movement. This force must be overcome in order to bring two contacting bodies in motion relative to each other. It occurs in microswits (for example, during deformation) in contact bodies. It acts in the direction opposite to the direction of a possible relative movement.

In physics of interaction, friction is made to divide on:

• Dry When interacting solid bodies are not separated by any additional layers / lubricants (including solid lubricants) - a very rare case occurred in practice. The characteristic distinctive feature of dry friction is the presence of a significant force of friction of rest;
• Boundary When the contact area may contain layers and sections of different nature (oxide films, liquid, and so on) - the most common case by friction of sliding.
• Mixed when the contact area contains sections of dry and liquid friction;
• liquid (viscous) , with the interaction of bodies separated by a layer of solid body (graphite powder), liquid or gas (lubrication) of various thicknesses - as a rule, occurs during rolling friction, when hard-wing bodies are immersed in liquid, the viscous friction value is characterized by viscosity of the medium;
• Elastohydrodynamic When internal friction in the lubricant material is crucial. It occurs with increasing the relative speeds of movement.

Due to the complexity of physico-chemical processes occurring in the frictional interaction area, friction processes are not fundamentally a description with the help of methods of classical mechanics.

## Amonton Law - Cool

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

In the simplest cases, the force of friction and normal load (or power Normal reaction) associated inequality

Applying to equality only in the presence of relative movement. This ratio is called Amonton's Law - Coulomb.

### Amonton's law - Coulomb with adhesion

For most pairs of materials, the value of the friction coefficient 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 power between contacting bodies. Adhesion and the formula for calculating the friction coefficient changes to

.

## Application value

### Friction in the mechanisms and machines

In most traditional mechanisms (DVS, cars, gear gears, etc.) friction plays a negative role, reducing the efficiency of the mechanism. To reduce friction force, various natural and synthetic oils and lubricants are used. In modern mechanisms, this goal also uses coatings (thin films) to the details. With miniaturization of mechanisms and the creation of microelectromechanical systems (MEMS) and nanoelectromechanical systems (NAMS) the magnitude of the friction compared with the forces operating in the mechanism increases and becomes very significant And at the same time cannot be reduced by conventional lubricants, which causes significant theoretical and practical interest of engineers and scientists to this area. To solve the friction problem, new methods of its decline in the framework of the tribology and surface science are created ( English ).

### Clutch with a surface

The presence of friction provides the ability to move on the surface. So, when walking precisely, precisely due to friction, the soles with a floor occurs, as a result of which the floor repulsion and movement forward. The clutch of the wheel of the car (motorcycle) with the surface of the road is ensured. In particular, to increase the improvement of this clutch, new forms and special types of rubber for tires are developed, and anti-grooves are installed on racing cars, stronger the pressing machine to the track.

## Literature

• Dreagin B. V. What is friction? M.: Ed. Academy of Sciences of the USSR, 1963.
• Kravelsky I. V., Shcheders V. S. Development of friction science. Dry friction. M.: Ed. Academy of Sciences of the USSR, 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.

The friction force appears when two surfaces come into contact and move relative to each other. The process studies physics, in particular mechanics. It considers the basic laws that are amenable to the body when they are moved and interaction, finds out the causes affecting the change in the position of the subjects.

## Definition and nature of friction force

Friction force F. Tr. It occurs when you touch two bodies. It creates obstacles to their further movement.

This occurs when the interaction of atoms and molecules from which items consist. Therefore, the nature of its appearance is electromagnetic waves. It acts in two directions, is directed to both bodies.

In this case, its value in the module does not change. If the strength acts on one of the two contacting bodies, then it affects the other.

On the subject remaining without movement, the strength of rest friction is influenced. While its value will exceed the external intervention, trying to shift the subject, it will not change the position.

When its magnitude increases to a certain limit, there will be a move in a new place. Then the sliding friction force appears, its direction is opposite to the displacement of the subject.

Due to the action of friction it is impossible to move forever. The movement will end after a certain time. If the external force again exceeds the value of the friction of rest, then the movement will resume.

## Types of friction force

Main types of friction:

1. Rest. It resists external factors trying to shift the body. With their absence, its value is equal to zero.

2. Slip. It is directly dependent on the friction coefficient and the values ​​of the force with which the surface is putting pressure on the body. Its direction of action is always perpendicular to the surface. It is usually lower than the maximum coach friction force.

3. Rolling. It occurs when one body rolls over the surface of another. For example, when contacting the wheel of a riding bike with an expensive or when the bearing mechanism is running. It has a much smaller effect than the friction of slip, if the remaining conditions are considered unchanged. Its discovery has become indispensable for technology. Wheels and round parts, rotating and changing position, are the basis of many mechanisms and work of vehicles.

4. Digging. It appears when one item begins to rotate over the surface of another.

Friction itself may be several types:

1. Dry. Manifests in contact of solid surfaces. They do not observe other materials and layers. Such in nature and life is extremely rare.

2. Viscous. It is also called liquid. It occurs when the solid body interacts with liquid or gas. They can flow past a stationary subject. Or it moves in a liquid or gaseous substance. For example, the boat is drawn on the rope rope. The body makes it navigate the top layer of fluid or gas. As if it pulls him. In turn, it acts on another layer below. The farther from the body, the lower the speed of the layers. This is due to a decrease in the influence of a solid subject. A friction force arises between the layers, since the bodies move relative to each other. It leads to their braking, which means it acts on a solid, stopping it. The temperature determines the degree of viscosity of substances. For example, it decreases when the oil is heated. It is clearly visible at the work of the car motor. When the car was on the cold for a long time, the engine must first warm up to increase the speed of its rotation. Gas feed addiction. Viscosity grows with increasing temperature.

3. Mixed. It is observed when there is a lubricant layer between bodies in contact with surfaces.

Also friction is divided into inner and external. The latter occurs when the interaction of solids. So you can attribute dry friction.

The inner is characterized by viscosity. It is in the interaction of liquids or gas that the displacement occurs inside one body when the layers move relative to each other.

## How to find friction power

To find the power of friction, you need to know the friction coefficient K, depending on the properties of the surface. This is a constant value, the value of which is taken from the tables.

The reaction force of the support is also needed. The desired value is determined by the work of two values:

F. Tr. = K * N

The letter K is indicated by the coefficient. You can also meet the symbol μ. It usually is in the range from 0.1 to 1.

For example, for rubber moving along dry asphalt, when driving, it ranges from 0.5 to 0.8. With a metal slide on a tree - 0.4, iron for cast iron - 0.18.

Power reaction support It does not differ from the amount of gravity, depending on the weight of the body. Therefore, its value is equal to the product of body weight (M) to accelerate the free fall (G).

N = m * g

This is a constant value of 9.8 m / c². This rule acts when you have to deal with a horizontal surface. The strength of gravity and the support reaction balance each other. Therefore, they are considered equal values.

If there is a movement on the inclined plane, the course of reasoning changes somewhat. For the subject, the strengths of gravity and the reaction of support are still acting, but not in one direction.

When knowledge of the angle of inclination of the plane to the horizon, the formula is transformed and becomes the following form:

N = k * m * · g * · cosα

Here it is necessary to be guided by the fact that the cosine is the ratio of the category adjacent to the corner to the triangle hypotenneus. This is one of those cases proving the close relationship of physics and trigonometry.

## An example of solving the problem

The task, to apply the knowledge related to the force of friction, will help consolidate the material.

The task. On the floor there is a box weighing 7 kg. The friction coefficient between it and the floor is 0.3. To the box apply power equal to 14 N. Will it move from the spot?

Decision.

The box is on the horizontal plane. It is subject to the action of gravity, which equates the reaction of the support. They are directed perpendicular to the box and the floor. It means that to determine the force of the reaction of the support, you need to multiply the mass of the box to accelerate:

N = m * g;

N = 10 kg * 9.8 m / s² = 98 kg * m / s² = 98 n;

FTr. = k * n;

FTr. = 0.3 · * 98H = 29.4 N.

Answer: The value obtained exceeds the efforts attached to the box from the side, since 29.4 N> 14 N. It means that it will remain at the initial place.

The friction force is present in life constantly. It prevents objects to move and oppose their long-term slip and moving. Its value depends on the surfaces with which they have to touch, their properties and characteristics.

The area of ​​contact is not taken into account, but it matters the position of the body. For example, the force arising when the car moves along a flat surface differs from the value when moving through the mountainous area, located at an angle to the horizon. And if the car has to move on the wet road, the value changes again.

## What is the force of friction

Bodies interact with each other in different ways. One of the types of interaction is friction. Before dealing with the subtleties of dry and viscous friction, they will respond to two questions. What is friction force, and when does it occur?

As for any other interaction, the third Newton law is fair for friction. If the friction force acts on one of the two interactive bodies, then the same force in the module acts on another body in the opposite direction.

## Rain friction force and sliding friction force

Different dry and viscous friction, peace friction force, sliding friction force, rolling friction force.

Dry friction is friction that occurs between solid bodies in the absence of a liquid or gaseous layer between them. Friction forces are aimed at tangent to contacting surfaces.

Imagine that on the body, for example, a bar lying on the table, there is some external force. This strength seeks to move the bar from the scene. While the body is resting, the strength of friction of peace and, actually, the external force act on the bar. The strength of friction of rest is equal to the external strength and balances it.

When the external force exceeds some limit value ${\mathrm{F.}}_{тр.max}$, the bar is shifted from the place. It also acts the strength of friction, but this is no longer the strength of friction of rest, but the power of friction of slip. Slip friction force is directed towards opposite to movement, and depends on the speed of the body.

When solving physical problems, the force of slip friction is often taken equal to the maximum strength of rest friction, and the dependence on the force of friction on the relative speed of the movement of the bodies neglected.

The figure above shows the actual and idealized characteristics of dry friction. As we see, in fact, the grip friction force varies depending on the speed, but the changes are not so great so that they cannot be neglected.

Need help teacher?

The friction force is proportional to the strength of the normal reaction of the support.

${\mathrm{F.}}_{тр}={\mathrm{F.}}_{тр.max}=\mu N$.

What is the coefficient of friction slip?

$\mu$- The coefficient of proportionality, which is called the sliding friction coefficient. It depends on the materials of the contact bodies and their properties. The sliding friction coefficient is a dimensionless value that does not exceed the unit.

Rolling friction forces arise when rolling tel. Usually, when solving the tasks they neglect.

## Viscous friction in liquids and gases

Viscous friction occurs when the bodies in fluids and gases occurs. The strength of viscous friction is also aimed in the direction opposite to the movement of the body, but much less smaller friction force is much less. Freeding rest is absent when viscous friction.

The calculation of the forces of viscous friction is more complicated than the calculation of the force of friction of sliding. At low speeds of the body movement in the liquid, the force of viscous friction is proportional to the body velocity, and at high speeds - the square of speed. The proportionality coefficients are dependent on the form of bodies, it is also necessary to take into account the properties of the medium itself, in which movement occurs.

For example, viscous friction forces in water and oil will differ, since these liquids have different viscosity.

Friction force - This is the force arising from the movement of one body along the surface of another body and directed opposite to the direction of movement.

The friction force slows down.

The friction force always appears when the surfaces of the bodies in motion come into contact.

Reasons for friction:

one. irregularities of surfaces of contact; 2. With perfectly smooth surfaces - the action of the attraction of molecules.

The magnitude of the friction force depends on:

one. properties of contacting surfaces;

2. Forces that presses on the surface. The more power that presses the body to the surface, the more friction force.

The turn of friction between different surfaces characterizes the friction coefficient, for example,

Steel steel - \ (0.24 \);

Rubber on asphalt - \ (0.45 \);

Rubber on wet asphalt - \ (0.2 \).

Friction is Useful , for example, without friction, we could not make a step (friction less on ice, so it's harder to go on it), The strength of friction rest Allows subjects to remain in their places, it does not allow to slip the supports that hold on the site of screws and nails, as well as threads in the tissues. Without friction, cars and bicycles could not move in the desired direction.

Friction increases using winter tires, winter sprinkling slippery sand, using in gear mechanisms, etc.

However, friction is also harmful It affects the heating and wear of the details of the mechanisms in contact.

This phenomenon is eliminated using lubricants, replacing friction sliding friction rolling (using wheels and bearings).

In the direction opposite to the movement of the body, the resistance force is also valid.

The force arising when the body moves in the gas or in the liquid and the suspension of movement is called power resistance .

Resistance force depends on:

one. properties of the medium (in water it is harder to run than by land);

2. body shapes;

3. Movement speeds (the higher the speed, the more resistance increases).

To eliminate the power of resistance, the body is attached rounded, flat (streamlined) forms.

The streamlined form of a high-speed train.