Everyone knows how difficult it is to move heavy objects on any surface. This is due to the fact that the surface of a solid is not perfectly smooth and contains many notches (they have different sizes, which decrease during grinding). When two bodies come into contact, the teeth interlock. Let a small force (F) be applied to one of the bodies, directed tangentially to the contacting surfaces. Under the influence of this force, the notches will deform (bend). Therefore, an elastic force will appear directed along the contacting surfaces. The elastic force acting on the body to which the force F is applied compensates for it and the body remains at rest.

Static friction force – force arising at the boundary of contacting bodies in the absence of their relative motion.

The static friction force is directed tangentially to the surface of the contacting bodies (Fig. 10) in the direction opposite to the force F, and is equal to it in magnitude: Ftr = - F.

As the force modulus F increases, the bending of the hooked notches will increase and, eventually, they will begin to break and the body will begin to move.

Sliding friction force – this is the force that arises at the boundary of contacting bodies during their relative motion.

The sliding friction force vector is directed opposite to the velocity vector of the body relative to the surface on which it is sliding.

A body sliding on a solid surface is pressed against it by the force of gravity P, directed along the normal. As a result, the surface bends and an elastic force N appears (normal pressure force or support reaction), which compensates for the pressing force P (N = - P).

The greater the force N, the deeper the grip of the notches and the more difficult it is to break them. Experience shows that the modulus of the sliding friction force is proportional to the force of normal pressure:

The dimensionless coefficient μ is called the sliding friction coefficient. It depends on the materials of the contacting surfaces and the degree of their grinding. For example, when traveling on skis, the coefficient of friction depends on the quality of the lubricant (modern expensive lubricants), the surface of the ski track (soft, loose, compacted, icy), the particular state of the snow, depending on the temperature and humidity of the air, etc. A large number of variable factors are determined by the the coefficient is not constant. If the friction coefficient lies in the range of 0.045 - 0.055, sliding is considered good.

The table shows the values ​​of the sliding friction coefficient for various contacting bodies.

Sliding friction coefficients for various cases

The role of friction force is positive in many cases. It is thanks to this force that the movement of humans, animals and ground vehicles is possible. So, when walking, a person, tensing the muscles of the supporting leg, pushes off the ground, trying to move the sole back. This is prevented by the static friction force directed in the opposite direction - forward (Fig. 11).

There are three types of friction forces: sliding friction, rolling friction and static friction.

Sliding friction force 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 sliding friction force.

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

The main reason for the occurrence of sliding friction is the smallest irregularities in the surfaces of two bodies. Their bodies cling to each other when moving. If there were no sliding friction force, then a 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.

The second reason for the occurrence of sliding friction force is intermolecular interactions on the contacting 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 attract. Because of this, the movement of the body is slowed down.

Rolling friction force occurs when another, usually round in shape, rolls over the surface of one body. For example, the wheels of vehicles rolling on the road, a barrel turned on its side on a hill, 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 method of contact between the moving body and the surface. When rolling, the wheel seems to press, crush the surface under itself, and push off from it. A rolling wheel does not have to catch many small surface irregularities, as when sliding bodies.

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 greater rolling friction force. This is due to the fact that it is easier to push off from hard surfaces; they are not pressed in too much. We can say that the force that acts from the wheel on a solid surface is not spent on deformation, but almost all is returned in the form of a normal support reaction force.

Static friction force surrounds us everywhere. All objects that lie on other bodies are held by the force of static friction. The static friction force is even enough to hold objects on inclined surfaces. For example, a person may be standing on a hillside with a block lying motionless on a slightly inclined ruler. In addition, thanks to the force of static friction, forms of movement such as walking and riding are possible. In these cases, “adhesion” to the surface occurs due to the force of static friction, as a result it becomes possible to push off from the surface.

The reasons for the static friction force are the same as for the sliding friction force.

The force of static friction occurs 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 remain 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 force of sliding or rolling friction will already act on it.

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

There are many physical phenomena in the world around us: thunder and lightning, rain and hail, electric current, friction... Our report today is dedicated to friction. Why does friction occur, what does it affect, what does the force of friction depend on? And finally, is friction friend or foe?

What is friction force?

Having a little run up, you can dash along the icy path. But try doing it on regular asphalt. However, it’s not worth trying. Nothing will work out. The culprit of your failure will be a very large friction force. For the same reason, it is difficult to move a massive table or, say, a piano.

At the point of contact of two bodies, interaction always occurs, which prevents the movement of one body on the surface of another. It's called friction. And the magnitude of this interaction is the force of friction.

Types of friction forces

Let's imagine that you need to move a heavy cabinet. Your strength is clearly not enough. Let's increase the “shearing” force. At the same time, the friction force increases peace. And it is directed in the direction opposite to the movement of the cabinet. Finally, the “shearing” force “wins” and the cabinet moves away. Now the friction force comes into its own slip. But it is less than the static friction force and it is much easier to move the cabinet further.

You, of course, have had to watch how 2-3 people roll away a heavy car with a suddenly stalled engine. The people pushing the car are not strongmen, the friction force is just acting on the wheels of the car rolling. This type of friction occurs when one body rolls over the surface of another. A ball, a round or faceted pencil, the wheels of a train, etc. can roll. This type of friction is much less than the sliding friction force. Therefore, it is very easy to move heavy furniture if it is equipped with wheels.

But, in this case, the friction force is directed against the movement of the body, therefore, it reduces the speed of the body. If it were not for its “harmful nature,” having accelerated on a bicycle or roller skates, you could enjoy the ride indefinitely. For the same reason, a car with the engine turned off will move by inertia for some time and then stop.

So, remember, there are 3 types of friction forces:

• sliding friction;
• rolling friction;
• static friction.

The rate at which speed changes is called acceleration. But, since the friction force slows down the movement, this acceleration will have a minus sign. It would be correct to say Under the influence of friction, a body moves with deceleration.

What is the nature of friction

If you examine the smooth surface of a polished table or ice through a magnifying glass, you will see tiny roughnesses to which a body sliding or rolling along its surface clings. After all, a body moving along these surfaces also has similar protrusions.

At the points of contact, the molecules come so close that they begin to attract each other. But the body continues to move, the atoms move away from each other, the bonds between them break. This causes the atoms freed from attraction to vibrate. Approximately the way a spring freed from tension oscillates. We perceive these vibrations of molecules as heating. That's why friction is always accompanied by an increase in the temperature of the contacting surfaces.

This means that there are two reasons causing this phenomenon:

• irregularities on the surface of contacting bodies;
• forces of intermolecular attraction.

What does friction force depend on?

You've probably noticed the sudden braking of a sled when it slides onto a sandy area. And one more interesting observation: when there is one person on the sled, they will go one way down the hill. And if two friends slide together, the sled will stop faster. Therefore, the friction force is:

• depends on the material of the contacting surfaces;
• in addition, friction increases with increasing body weight;
• acts in the direction opposite to the movement.

The wonderful science of physics is also good because many dependencies can be expressed not only in words, but also in the form of special signs (formulas). For the friction force it looks like this:

Ftr = kN Where:

Ftr - friction force.

k - friction coefficient, which reflects the dependence of the friction force on the material and the cleanliness of its processing. Let's say, if metal rolls on metal k=0.18, if you skate on ice k=0.02 (friction coefficient is always less than one);

N is the force acting on the support. If the body is on a horizontal surface, this force is equal to the weight of the body. For an inclined plane it is less weight and depends on the angle of inclination. The steeper the slide, the easier it is to slide down and the longer you can ride.

And, by calculating the static friction force of the cabinet using this formula, we will find out what force needs to be applied to move it from its place.

Work of friction force

If a force acts on a body, under the influence of which the body moves, then work is always done. The work of the friction force has its own characteristics: after all, it does not cause movement, but impedes it. Therefore, the work it does is will always be negative, i.e. with a minus sign, no matter which direction the body moves.

Friction is friend or foe

Friction forces accompany us everywhere, bringing tangible harm and... enormous benefit. Let's imagine that friction has disappeared. An astonished observer would see how mountains collapse, trees are uprooted from the ground by themselves, hurricane winds and sea waves endlessly dominate the earth. All the bodies are sliding down somewhere, the transport is falling apart into separate parts, since the bolts do not fulfill their role without friction, an invisible monster would have untied all the laces and knots, the furniture, not held by friction forces, has slid into the lowest corner of the room.

Let's try to escape, to escape from this chaos, but without friction We won’t be able to take a single step. After all, it is friction that helps us push off the ground when walking. Now it’s clear why slippery roads are covered with sand in winter...

And at the same time, sometimes friction causes significant harm. People have learned to reduce and increase friction, deriving enormous benefits from it. For example, wheels were invented to drag heavy loads, replacing sliding friction with rolling, which is significantly less than sliding friction.

Because a rolling body does not have to catch many small surface irregularities, as when bodies slide. Then the wheels were equipped with tires with a deep pattern (treads).

Have you noticed that all the tires are rubber and black?

It turns out that rubber holds the wheels well on the road, and the coal added to the rubber gives it a black color and the necessary rigidity and strength. In addition, in case of accidents on the road, it allows you to measure the braking distance. After all, when braking, the tires leave a clear black mark.

If necessary, reduce friction, use lubricating oils and dry graphite lubricant. A remarkable invention was the creation of different types of ball bearings. They are used in a wide variety of mechanisms, from bicycles to the latest aircraft.

Is there friction in liquids?

When a body is stationary in water, friction with the water does not occur. But as soon as it starts moving, friction arises, i.e. Water resists the movement of any bodies in it.

This means that the shore, creating friction, “slows down” the water. And, since the friction of water on the shore reduces its speed, you should not swim into the middle of the river, because the current there is much stronger. Fish and sea animals are shaped in such a way that the friction of their bodies with the water is minimal.

Designers give the same streamlining to submarines.

Our acquaintance with other natural phenomena will continue. See you again, friends!

If this message was useful to you, I would be glad to see you

« Physics - 10th grade"

Remember what friction is.
What factors is it due to?
Why does the speed of movement of the block on the table change after a push?

Another type of force dealt with in mechanics is frictional forces. These forces act along the surfaces of bodies when they are in direct contact.

Friction forces in all cases prevent the relative motion of contacting bodies. Under certain conditions, friction forces make this movement impossible. However, they not only slow down the movement of bodies. In a number of practically important cases, the movement of a body could not occur without the action of friction forces.

Friction that occurs during relative movement of contacting surfaces of solid bodies is called dry friction.

There are three types of dry friction: static friction, sliding friction and rolling friction.

Rest friction.

Try moving a thick book lying on the table with your finger. You apply some force to it, directed along the surface of the table, and the book remains at rest. Consequently, a force arises between the book and the surface of the table, directed opposite to the force with which you act on the book, and exactly equal to it in magnitude. This is the friction force tr. You push the book with more force, but it still stays in place. This means that the friction force tr increases by the same amount.

The frictional force acting between two bodies stationary relative to each other is called force static friction.

If a body is acted upon by a force parallel to the surface on which it is located, and the body remains motionless, this means that it is acted upon by a static friction force tr, equal in magnitude and directed in the opposite direction to the force (Fig. 3.22). Consequently, the force of static friction is determined by the force acting on it:

If the force acting on a body at rest even slightly exceeds the maximum force of static friction, then the body will begin to slide.

The greatest value of the friction force, at which sliding does not yet occur, is called maximum static friction force.

To determine the maximum static friction force, there is a very simple, but not very accurate quantitative law. Let there be a block on the table with a dynamometer attached to it. Let's conduct the first experiment. Let's pull the dynamometer ring and determine the maximum static friction force. The block is acted upon by the force of gravity m, the normal reaction force of the support 1, the tension force 1, the dynamometer springs and the maximum static friction force tr1 (Fig. 3.23).

Let's place another similar block on the block. The force of pressure of the bars on the table will increase by 2 times. According to Newton's third law, the normal reaction force of support 2 will also increase by 2 times. If we measure the maximum static friction force again, we will see that it has increased as many times as the force 2 has increased, i.e. 2 times.

Continuing to increase the number of bars and measuring each time the maximum force of static friction, we will be convinced that

>the maximum value of the modulus of the static friction force is proportional to the modulus of the normal reaction force of the support.

If we denote the module of the maximum static friction force by F tr. max, then we can write:

F tr. max = μN (3.11)

where μ is a proportionality coefficient called the friction coefficient. The friction coefficient characterizes both rubbing surfaces and depends not only on the material of these surfaces, but also on the quality of their processing. The friction coefficient is determined experimentally.

This dependence was first established by the French physicist C. Coulomb.

If you place the block on the smaller face, then F tr. max will not change.

The maximum static friction force does not depend on the area of ​​contact between the bodies.

The static friction force varies from zero to a maximum value equal to μN. What can cause a change in the friction force?

The point here is this. When a certain force is applied to a body, it shifts slightly (imperceptibly to the eye), and this displacement continues until the microscopic roughness of the surfaces are positioned relative to each other in such a way that, hooking on one another, they will lead to the appearance of a force that balances the force. As the force increases, the body will again move slightly so that the smallest surface irregularities will cling to each other differently, and the friction force will increase.

And only at > F tr. max, no matter the relative position of the surface roughnesses, the friction force is not able to balance the force , and sliding will begin.

The dependence of the sliding friction force modulus on the acting force modulus is shown in Figure 3.24.

When walking and running, the soles of the feet are subject to static friction unless the feet slip. The same force acts on the drive wheels of the car. The driven wheels are also acted upon by a static friction force, but this time braking the movement, and this force is significantly less than the force acting on the drive wheels (otherwise the car would not be able to move).

For a long time, it was doubted that a steam locomotive could run on smooth rails. They thought that the friction braking the driven wheels would be equal to the friction force acting on the driving wheels. It was even proposed to make the drive wheels geared and lay special geared rails for them.

Sliding friction.

When sliding, the friction force depends not only on the state of the rubbing surfaces, but also on the relative speed of the bodies, and this dependence on speed is quite complex. Experience shows that often (though not always) at the very beginning of sliding, when the relative speed is still low, the friction force becomes somewhat less than the maximum static friction force. Only then, as the speed increases, does it grow and begin to exceed F tr. max.

You've probably noticed that a heavy object, such as a box, is difficult to move, but then moving it becomes easier. This is precisely explained by the decrease in friction force when sliding occurs at low speed (see Fig. 3.24).

At not too high relative speeds of movement, the sliding friction force differs little from the maximum static friction force. Therefore, it can be approximately considered constant and equal to the maximum static friction force:

F tr ≈ F tr. max = μN.

The force of sliding friction can be reduced many times by using a lubricant - most often a thin layer of liquid (usually some type of mineral oil) - between the rubbing surfaces.

Not a single modern machine, such as a car or tractor engine, can operate without lubrication. A special lubrication system is provided for in the design of all machines.

The friction between layers of liquid adjacent to solid surfaces is much less than between dry surfaces.

Rolling friction.

The rolling friction force is significantly less than the sliding friction force, so it is much easier to roll a heavy object than to move it.

The friction force depends on the relative speed of the bodies. This is its main difference from the forces of gravity and elasticity, which depend only on distances.

Resistance forces during the movement of solid bodies in liquids and gases.

When a solid body moves in a liquid or gas, it is acted upon by the drag force of the medium. This force is directed against the speed of the body relative to the medium and slows down the movement.

The main feature of the drag force is that it appears only in the presence of relative motion of the body and the environment.
The force of static friction in liquids and gases is completely absent.

This leads to the fact that with the effort of your hands you can move a heavy body, for example, a floating boat, while moving, say, a train with your hands is simply impossible.

The modulus of the resistance force F c depends on the size, shape and state of the surface of the body, the properties of the medium (liquid or gas) in which the body moves, and, finally, on the relative speed of movement of the body and the medium.

The approximate nature of the dependence of the modulus of the resistance force on the modulus of the relative velocity of the body is shown in Figure 3.25. At a relative speed equal to zero, the drag force does not act on the body (F c = 0). As the relative speed increases, the drag force grows slowly at first, and then faster and faster. At low speeds of movement, the resistance force can be considered directly proportional to the speed of movement of the body relative to the medium:

F c = k 1 υ, (3.12)

where k 1 is the resistance coefficient, depending on the shape, size, state of the surface of the body and the properties of the medium - its viscosity. It is not possible to calculate the coefficient k 1 theoretically for bodies of any complex shape; it is determined experimentally.

At high speeds of relative motion, the drag force is proportional to the square of the speed:

F c = k 2 υ 2 , υ, (3.13)

where k 2 is the resistance coefficient different from k 1 .

Which of the formulas - (3 12) or (3.13) - can be used in a particular case is determined experimentally. For example, for a passenger car, it is advisable to use the first formula at approximately 60-80 km/h; at higher speeds, the second formula should be used.

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 differ from elastic and gravitational forces by 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

Provided that there is relative motion of two bodies in contact with each other, the friction forces arising in such a 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 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 contacting parts of mechanisms and additional fuel consumption for vehicles. Various lubricants (air or liquid cushions) serve as a means of counteracting this. Another effective method is to replace sliding with 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.