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

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

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

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

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

Rice. 3. Force of static friction

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

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

Let's act on the bar with a dynamometer.

Rice. 4. Measuring the friction force with a dynamometer

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

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

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

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

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

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

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

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

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

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

Rice. 8. Force of static friction in belt drives

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

Rice. 9. Force of sliding friction

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

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

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

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

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

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

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

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

Rice. 11. Surfaces of bodies at the microscopic level

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

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

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

Rice. 12. Occurrence of rolling friction force

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

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

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

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

The resistance force modulus depends on:

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

Conditions of the body surface (Fig. 15);

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

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

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

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

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

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

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

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

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

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

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

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

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

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

In the world around us, there are many physical phenomena: thunder and lightning, rain and hail, electric current, friction ... Our today's report is devoted 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 run a little, you can dashingly ride along the ice path. But try to do it on normal asphalt. However, it's not worth trying. Nothing will work. 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.

In the place where two bodies meet, there is always an interaction, which prevents the movement of one body on the surface of another. It is called friction. And the magnitude of this interaction is the force of friction.

Types of friction forces

Imagine that you need to move a heavy cabinet. Your strength is clearly not enough. Let's increase the "shifting" force. At the same time, the friction force also increases. rest. And it is directed in the direction opposite to the movement of the cabinet. Finally, the "shifting" force "wins" and the cabinet starts moving. Now the force of friction 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, had to watch how 2-3 people roll away a heavy car with a suddenly stalled engine. People pushing a car are not strong men, just a friction force 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, train wheels, 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 her "harmful nature", having accelerated on a bicycle or roller skates, one could enjoy riding indefinitely. For the same reason, a car with the engine turned off will continue to move by inertia for some time, and then stop.

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

• sliding friction;
• rolling friction;
• rest friction.

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

What is the nature of friction

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

At the points of contact, the molecules are 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 vibrates the atoms freed from attraction. Approximately the same as a spring released 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.

So there are two reasons for this phenomenon:

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

What does the force of friction depend on?

You've probably noticed how the sled slows down sharply if it runs into 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 move out together, the sled will stop faster. Therefore, the force of friction 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 - coefficient of friction, which reflects the dependence of the friction force on the material and the purity of its processing. Let's say if metal rolls on metal k=0.18, if you skate on ice k=0.02 (the 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 than the weight and depends on the angle of inclination. The steeper the hill, the easier it is to slide down and the longer you can drive.

And, having calculated the rest friction force of the cabinet using this formula, we will find out what force must be applied to move it from its place.

The work of the friction force

If a force acts on the body, under the action 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 prevents it. Therefore, the work she does will always be negative, i.e. with a minus sign no matter which direction the body is moving.

Friction is friend or foe

Friction forces accompany us everywhere, bringing tangible harm and ... great benefits. Imagine that friction has disappeared. An astounded observer would see how mountains collapse, trees uproot themselves from the ground, hurricane winds and sea waves endlessly dominate the earth. All bodies slide down somewhere, the transport falls apart into separate parts, since frictionless bolts do not fulfill their role, an invisible ugliness would untie all the laces and knots, furniture, not held by friction forces, slid into the lowest corner of the room.

Let's try to escape, escape from this chaos, but without friction we can't take a single step. After all, it is friction that helps us to 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 great benefits from it. For example, for dragging heavy loads, wheels were invented, replacing sliding friction with rolling, which is much less than sliding friction.

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

Have you noticed that all tires are rubber and black?

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

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

Is there friction in liquids?

When a body is stationary in water, there is no friction against the water. 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 in the middle of the river, because the current is much stronger there. Fish and marine animals are shaped so that the friction of their bodies on the water is minimal.

Designers give the same streamlining to submarines.

Our acquaintance with other natural phenomena will continue. Until we meet again, friends!

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In terrestrial conditions, friction always accompanies any movement of bodies. With all types of mechanical motion, some bodies come into contact either with other bodies or with the continuous liquid or gaseous medium surrounding them. Such contact always has a great influence on movement. There is a friction force directed opposite to the motion.

There are several types of friction:

Dry friction occurs when solid contacting bodies move relative to each other.

Viscous (otherwise liquid) friction occurs when solid bodies move in a liquid or gaseous medium, or when a liquid or gas flows past stationary solid bodies.

Friction occurs when a force is applied to a body that tries to move the body.

The causes of the friction force are: the roughness of the contacting surfaces and the mutual attraction of the molecules of the contacting bodies.

But what happens if you take two perfectly clean surfaces?

Tie a thread to the stem of a glass goblet and place it on a table covered with glass. If you pull the string, the glass will slide easily over the glass. Now dampen the glass with water. Moving the glass will become much more difficult. If you look closely at the glass, you can even notice scratches. The point is that the water removed grease and other substances that polluted the rubbing surfaces. A contact was formed between two perfectly clean surfaces, and it turned out that it was easier to make scratches (ie, tear out pieces of glass) than to tear off (move) a glass.

Ways to reduce friction force:

Grinding rubbing surfaces, applying lubricants and replacing sliding friction with rolling friction.

Friction forces are electromagnetic in nature.

What does the force of friction depend on?

From the kind of contact surfaces and from the magnitude of the load.
At one time, the great Italian artist and scientist Leonardo da Vinci, surprising those around him, conducted strange experiments: he dragged a rope along the floor, either in full length, or collecting it in rings. He studied: does the force of sliding friction depend on the area of ​​the bodies in contact?
As a result, Leonardo came to the conclusion that the force of sliding friction does not depend on the area of ​​the bodies in contact, which is also confirmed by modern scientists.

How to explain the occurrence of friction?

The contact surfaces of the bodies are never perfectly flat and have irregularities.

Moreover, the places of the protrusions on one surface do not coincide with the places of the protrusions on the other. But under compression, the pointed peaks are deformed and the contact area increases in proportion to the applied load. It is the resistance to shear in places of irregularities that is the cause of friction.

In addition, we must not forget that in the case of ideally smooth surfaces, resistance to movement will arise due to the forces of attraction between the molecules. This explains the effect on the friction force of the load - the pressing force and the properties of materials.

How to measure friction force?

This can be done with a dynamometer.
With a uniform movement of the body, the dynamometer shows a traction force equal to the friction force. For the convenience of measuring, sometimes, instead of pulling the book on the table, you can start moving the table itself, and hold the book in place by tying it to a spring. The force of friction will not change.

The unit of measurement of the friction force in SI (like any other force) is 1 Newton.

Which is more profitable: rolling or sliding?

Which is better, sliding or rolling? Of course, rolling is more profitable than sliding. Much less force is needed to keep rolling than to keep sliding at the same speed. Therefore, it is clear that in the summer they ride in a cart, and not in a sleigh.

But why do the wheels give way to skids in winter? The thing is that wheels are more profitable than skids only when they roll. And in order for the wheels to roll, there must be a solid, smooth road under them, and also non-slip.

EXPERIENCE. Comparison of sliding friction force and rolling friction force.

Place a round (not faceted) glass on the table and push it so that it slides with its bottom on the table. Moving, the glass will stop.
Now put the same glass on its side and push it with the same force. The glass, rolling, will move further. What's the matter?
The weight of the glass has not changed, its walls and bottom are made of the same glass, the table is the same.
The thing is that now the glass is rolling, not sliding, and its movement is slowed down by the rolling friction force, which is many times less than the sliding friction force. In many cases, it turns out to be 50 times more than rolling friction!

Friction always slows down motion; to overcome friction of all kinds, a huge amount of valuable fuel is consumed.
Friction causes wear on rubbing surfaces.

HISTORY OF THE STUDY OF FRICTION

The first study of the laws of friction belongs to the famous Italian scientist and artist Leonardo da Vinci (15th century):
the friction force arising from the contact of a body with the surface of another body is proportional to the pressing force, directed against the direction of movement and does not depend on the contact area of ​​the contacting surfaces.

He measured the friction force acting on wooden bars sliding along the board, and, placing the bars on different faces, determined the dependence of the friction force on the area of ​​\u200b\u200bsupport. But, unfortunately, the works of Leonardo da Vinci were not published.

However, only at the end of the 18th century, scientists G. Amonton and Sh.O. Coulomb introduced a new physical constant - the coefficient of friction (k).

After that, the formula for the friction force was derived:

Ftr = kN

Where N is the reaction force of the support, corresponding to the pressure force produced by the body on the surface.

If the body is on a horizontal surface, then N = Fstrand

Friction coefficient values ​​for various materials can be found in reference books.

It has been known for a long time that surfaces lubricated with grease or even simply wetted with water slide much more easily. In 1886, O. Reynolds created the first theory of lubrication.
And at the beginning of the 20th century, tribology appeared - the science that studies friction.

Sometimes friction is “harm”!

Friction slows down movement; to overcome friction of all kinds, a huge amount of valuable fuel is consumed.
Friction causes wear of rubbing surfaces: soles, car tires, machine parts are erased. They try to reduce harmful friction.

But sometimes friction is good!

Then they try to increase it, for example, when walking in ice.

What if there was no friction?

The Nobel Prize winner, Swiss physicist Charles Guillaume said: “Imagine that friction can be completely eliminated, then no body, whether it is the size of a stone block or small, like a grain of sand, will ever rest on one another, everything will slide and roll until will not be on the same level. If there were no friction, the Earth would be without unevenness, like a liquid.”

READ ALL ABOUT FRICTION

About friction for the curious..........

INTERESTING

An increase in the force of resistance to movement with an increase in speed leads to a steady uniform motion of a body when falling from a great height in a liquid or gas (for example, in the atmosphere). So a skydiver before opening the parachute can acquire a speed of only up to 50 m / s, and raindrops, depending on their size, reach speeds from 2 to 7 m / s.

The lowest coefficient of friction for a solid body (0.02) is Teflon known to you. Every modern person has pots and pans with non-stick Teflon coating in the kitchen.

If all the windows of a moving train are opened at the same time, then the air flow around it will deteriorate so much that the resistance to movement will increase by about a quarter.

Wetsuits that are specifically designed for spearfishing and freediving come with an ultra-smooth finish on the outside to reduce friction losses as you glide through the water.

QUESTION FOR EVERYONE!

The horse is pulling the cart. Where is the force of friction useful, and where is it harmful?
Ah, come on!

The friction force (Ftr.) is the force that occurs when the surfaces of two bodies come into contact and prevents their relative movement. It appears due to the electromagnetic forces arising from atoms and molecules at the point of contact of these two objects.

To stop a moving object, the force must act in the opposite direction with respect to the direction of motion. For example, if you push a book across a table, it will start moving. The force with which you acted on the book will move it. The book slides, then slows down and stops due to the influence of the friction force.

Features of friction forces

Friction, which was mentioned above, which manifests itself when objects move, is called external or dry. But it can also exist between parts or layers of one object (liquid or gaseous), this type is called internal.
The main feature is the dependence of friction on the speed of the relative motion of bodies.
There are other characteristic features:

• occurrence at contact of two moving bodies by surfaces;
• its action is parallel to the area of ​​contact;
• directed opposite to the body velocity vector;
• depends on the quality of surfaces (smooth or rough), interacting objects;
• the shape or size of an object moving in a gas or liquid affects the magnitude of the frictional force.

Types of friction

There are several types. Let's look at their differences. A book sliding on a table is affected by sliding friction.

sliding friction force

Where N is the reaction force of the support.

Pay attention to some situations:

If a person rides a bicycle, then the friction that occurs during the contact of the wheel with the road is rolling friction. This type of force is much smaller than the force of sliding friction.

Rolling friction force

Significantly smaller values ​​of this type of force are used by people using wheels, rollers and ball bearings in various moving parts of devices.

Charles Augustin Coulomb, in his work on the theory of friction, proposed to calculate the force of rolling friction as follows:

where λ is the coefficient of rolling friction, R is the radius of the roller or wheel, P is the weight of the body.
Imagine a situation in which a person is trying to move a sofa from place to place. A person acts on the sofa with some force, but cannot move it. This is because the sofa is not accelerating. That is, the result of the action of external forces on the sofa is zero. Therefore, the force of a person is compensated by a force equal in magnitude, but directed in the opposite direction. This is the static friction force.

F tr. n. acts in response to forces tending to cause the movement of a stationary object. If there is no external influence on a stationary object, then the magnitude of this force is zero. If an external influence appears (F), then the static friction force increases to a maximum, and then the body begins to move. The magnitude of the sliding friction force practically coincides with the maximum static friction force.

,
μ is the coefficient of friction.
Lubrication, most often in the form of a thin layer of liquid, reduces friction.
Liquids or gases are special media in which this type of force also manifests itself. In these media, friction only appears during the movement of the object. It is impossible to talk about the force of static friction in these media.

Friction force in liquids and gases

This type of force is called the resistance force of the medium. It slows down the movement of an object. The more streamlined shape of the object affects the magnitude of the drag force - it is significantly reduced. Therefore, in shipbuilding, streamlined hulls of ships or submarines are used.
The resistance force of the medium depends on:

• geometric dimensions and shape of the object;
• viscosity of a liquid or gaseous medium;
• state of the surface of the object;
• the speed of an object relative to the environment in which it is located.