History of the emergence of friction force. The harm and benefits of friction Interesting facts about the force of friction

Issue 19

In a physics video lesson from the Academy of Entertaining Sciences, Professor Daniil Edisonovich will talk about the force of friction. It turns out that there are different types of friction force. And they act differently. In the phenomena familiar to us, we will learn to distinguish the action of these forces, which means we will be able to use them if necessary.

Types of friction force

The interaction that occurs at the point of contact of two bodies and prevents their movement relative to each other is called friction. And the force that characterizes this interaction is called the friction force. When we try to move a heavy cabinet while cleaning, we will immediately feel how something is interfering with it. And the work of the friction force will interfere with the movement. We encounter friction at every step, literally. After all, without friction we will not be able to take a step. It is friction forces that keep our feet on the surface of the earth. Everyone knows that walking on a slippery surface (for example, ice) is not an easy task. This suggests that friction can be a very useful thing. The friction force is distinguished according to the reasons for the occurrence of the friction force. The first reason is surface roughness. This is well understood using the example of floor boards or the surface of the Earth. And if the surface is smoother, for example, the surface of ice, then the roughnesses are almost invisible, but they are still there. These roughnesses and irregularities cling to each other and interfere with movement (sliding). With the first type of friction force, everything is clear. The second reason for the emergence of friction force is intermolecular attraction, which acts at the points of contact of rubbing bodies. The second reason appears mainly only in the case of very well polished bodies. And most often we are dealing with the first cause of friction forces. If necessary, lubricant is used to reduce friction. A layer of lubricant, most often liquid, separates the rubbing surfaces, and layers of liquid rub against each other, the friction force in which is several times less. What types of friction force exist? In total, there are three types of friction force: sliding friction, static friction and rolling friction. When we tried to move the cabinet from its place, it was held by the force of static friction. Residual friction holds nails driven into the wall, prevents shoelaces from unraveling spontaneously, and also holds our closet in place. The sliding friction force, like the static friction force, is directed in the direction opposite to the applied force. In the case when a body does not slide on a surface, but rolls, the friction that arises at the point of contact is called rolling friction. The rolling wheel is slightly pressed into the road, and a small bump forms in front of it, which has to be overcome. This is what causes rolling friction. The harder the road, the less rolling friction. This is why driving on the highway is much easier than driving on sand. In the vast majority of cases, rolling friction is significantly less than sliding friction. That is why wheels, bearings, and so on are widely used.

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 prevents it. Therefore, the work it does is will always be negative, i.e. with a minus sign, no matter which way 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 against 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

Municipal budgetary educational institution

"Pervomaiskaya Secondary School"

Pervomaisky village

Research work

“Friction force and its beneficial properties”

Completed by: Platon Alexey,

student of 9th – “D” grade

Supervisor:

physics teacher

Pervomaisky village

Tambov region

2012

1. Introduction 3

2. Public opinion research. 4

3. What is friction (a little theory). 5

3.1. Static friction. 5

3.2. Sliding friction. 6

3.3. Rolling friction. 6

3.4. Historical information. 8

3.5. Friction coefficient. 9

3.6. The role of friction forces. 11

4. Experimental results. 12

5. Design work and conclusions. 13

6. Conclusion. 15

7. List of used literature. 16

1. Introduction

Problem:Understand whether we need friction force and find out its beneficial properties.

How does a car accelerate, and what force slows it down when braking? Why does a car skid on a slippery road? What causes rapid wear of parts? Why can’t a car, having accelerated to high speeds, suddenly stop? How do plants stay in the soil? Why is it difficult to hold a live fish in your hand? How can we explain the high percentage of injuries and traffic accidents during icy conditions in winter?

The answers to these and many other questions related to the movement of bodies are provided by the laws of friction.

From the above questions it follows that friction is both a harmful and beneficial phenomenon.

In the 18th century, a French physicist discovered a law according to which the friction force between solid bodies does not depend on the area of contact, but is proportional to the reaction force of the support and depends on the properties of the contacting surfaces. The dependence of the friction force on the properties of the contacting surfaces is characterized by the friction coefficient. The friction coefficient ranges from 0.5 to 0.15. Although many hypotheses have been put forward since then to explain this law, a complete theory of friction force still does not exist. Friction is determined by the surface properties of solids, and they are very complex and have not yet been fully studied.

Main goals of this project : 1) Study the nature of friction forces; explore the factors on which friction depends; consider types of friction.

2) Find out how a person acquired knowledge about this phenomenon, what its nature is.

3) Show what role the phenomenon of friction or its absence plays in our lives; answer the question: “What do we know about this phenomenon?”

4) Create demonstration experiments; explain the results of observed phenomena.

Tasks: Trace the historical experience of mankind in the use and application of this phenomenon; find out the nature of the phenomenon of friction, the laws of friction; conduct experiments confirming the patterns and dependencies of the friction force; think over and create demonstration experiments that prove the dependence of the friction force on the force of normal pressure, on the properties of contacting surfaces, on the speed of relative motion of bodies.

To achieve our goals, we worked on this project in the following areas:

1) Public opinion research;

2) Study of the theory of friction;

3) Experiment;

4) Design.

Relevance of the problem. The phenomenon of friction occurs very often in our lives. All movements of bodies in contact relative to each other always occur with friction. The force of friction always influences, to a greater or lesser extent, the nature of movement.

Hypothesis. The useful frictional force depends on the type of rubbing surfaces and the force of pressure.

Practical significance consists in applying the dependence of the friction force on the reaction force of the support, on the properties of the contacting surfaces, and on the speed of movement in nature. It is also necessary to take this into account in technology and in everyday life.

Scientific interest is that in the process of studying this issue, some information was obtained about the practical application of the phenomenon of friction.

2. Public opinion research.

Goals: show what role the phenomenon of friction or its absence plays in our lives; answer the question: “What do we know about this phenomenon?”

We studied proverbs and sayings in which the force of static, rolling, and sliding friction is manifested; we studied human experience in the use of friction and ways to combat friction.

Proverbs and sayings:

There will be no snow, there will be no trace.

There will be a quiet cart on the mountain.

It's hard to swim against the water.

If you love to ride, you also love to carry sleds.

Patience and work will grind everything down.

That’s why the cart began to sing because it had not eaten tar for a long time.

And he scribbles, and plays, and strokes, and rolls. And all in language.

He lies that he sews with silk.

Take a coin and rub it on a rough surface. We will clearly feel the resistance - this is the force of friction. If you rub too quickly, the coin will begin to heat up, reminding us that friction generates heat - a fact known to Stone Age man, because this is how people first learned to make fire.

Friction gives us the opportunity to walk, sit, and work without fear that books and notebooks will fall off the table, that the table will slide until it hits a corner, and that the pen will slip out of our fingers.

Friction promotes stability. Carpenters level the floor so that the tables and chairs remain where they were placed.

However, small friction on ice can be successfully exploited technically. Evidence of this is the so-called ice roads, which were built to transport timber from the logging site to the railway or to rafting points. On such a road, which has smooth ice rails, two horses pull a sleigh loaded with 70 tons of logs.

Friction is not only a brake on movement. This is also the main reason for the wear and tear of technical devices, a problem that man also faced at the very dawn of civilization. During excavations of one of the oldest Sumerian cities - Uruk - the remains of massive wooden wheels were discovered, which are 4.5 thousand years old. The wheels are covered with copper nails for the obvious purpose of protecting the convoy from rapid wear and tear.

And in our era, the fight against wear and tear of technical devices is the most important engineering problem, the successful solution of which would save tens of millions of tons of steel and non-ferrous metals, and sharply reduce the production of many machines and spare parts for them.

Already in ancient times, engineers had at their disposal such important means for reducing friction in the mechanisms themselves as a replaceable metal plain bearing, lubricated with fat or olive oil, and even a rolling bearing.

The world's first bearings are considered to be belt loops that supported the axles of the antediluvian Sumerian carts.

Bearings with replaceable metal liners were well known in ancient Greece, where they were used in well gates and mills.

Of course, friction also plays a positive role in our lives, but it is also dangerous for us, especially in winter, when there is ice.

3. What is friction (a little theory)

Goals:study the nature of friction forces; explore the factors on which friction depends; consider types of friction.

Friction force

If we try to move the cabinet, we will immediately see that it is not so easy to do. His movement will be hampered by the interaction of his legs with the floor on which he stands. There are 3 types of friction: static friction, sliding friction, rolling friction. We want to find out how these species differ from each other and what they have in common?

3.1. Static friction

In order to find out the essence of this phenomenon, you can conduct a simple experiment. Place the block on an inclined board. If the angle of inclination of the board is not too large, the block may remain in place. What will keep it from sliding down? Static friction.

Let's press our hand to the notebook lying on the table and move it. The notebook will move relative to the table, but will rest relative to our palm. What did we use to make this notebook move? Using static friction between the notebook and your hand. Static friction moves loads on a moving conveyor belt, prevents shoelaces from untying, holds nails driven into a board, etc.

The force of static friction can be different. It grows along with the force that strives to move the body from its place. But for any two bodies in contact it has a certain maximum value, which cannot be greater. For example, for a block of wood resting on a wooden board, the maximum static friction force is approximately 0.6 of its weight. By applying a force to the body that exceeds the maximum force of static friction, we will move the body and it will begin to move. In this case, static friction will be replaced by sliding friction.

3.2. Sliding friction

What causes a sled to gradually stop as it rolls down the mountain? Due to sliding friction. Why does a puck sliding on ice slow down? Due to sliding friction, always directed in the direction opposite to the direction of movement of the body. Reasons for the occurrence of friction force:

1) Roughness of the surfaces of contacting bodies. Even those surfaces that look smooth, in fact, always have microscopic irregularities (protrusions, depressions). When one body slides along the surface of another, these irregularities catch on each other and thereby interfere with movement;

2) intermolecular attraction acting at the points of contact of rubbing bodies. Attraction occurs between molecules of a substance at very short distances. Molecular attraction manifests itself in cases where the surfaces of contacting bodies are well polished. So, for example, when two metals with very clean and smooth surfaces, processed in a vacuum using a special technology, slide relative, the friction force turns out to be much stronger than the friction force between blocks of wood with each other, and further sliding becomes impossible.

3.3. Rolling friction

If a body does not slide on the surface of another body, but, like a wheel or cylinder, rolls, then the friction that arises at the point of their contact is called rolling friction. The rolling wheel is somewhat pressed into the road surface, and therefore there is always a small bump in front of it that must be overcome. It is precisely the fact that the rolling wheel constantly has to run over the bump that appears in front that causes rolling friction. Moreover, the harder the road, the less rolling friction. At the same loads, the rolling friction force is significantly less than the sliding friction force (this was noticed in ancient times). Thus, the legs of heavy objects, for example, beds, pianos, etc., are equipped with rollers. In technology, rolling bearings, otherwise called ball and roller bearings, are widely used to reduce friction in machines.

These types of friction are referred to as dry friction. We know why the book doesn't fall through the table. But what stops it from slipping if the table is slightly tilted? Our answer is friction! We will try to explain the nature of the friction force.

At first glance, it is very simple to explain the origin of the friction force. After all, the surface of the table and the cover of the book are rough. This can be felt to the touch, and under a microscope it can be seen that the surface of a solid body most closely resembles a mountainous country. Countless protrusions cling to each other, become slightly deformed and prevent the book from sliding off. Thus, the static friction force is caused by the same molecular interaction forces as ordinary elasticity.

If we increase the tilt of the table, the book will begin to slide. Obviously, this begins to “chip off” the tubercles, breaking molecular bonds that are unable to withstand the increased load. The friction force still acts, but it will be the sliding friction force. It is not difficult to detect the “chipping” of the tubercles. The result of this “chipping” is wear of the rubbing parts.

It would seem that the more thoroughly the surfaces are polished, the less the friction force should be. To a certain extent this is true. Grinding reduces, for example, the frictional force between two steel bars. But not infinitely! The frictional force suddenly begins to increase as the surface smoothness further increases. This is unexpected, but still understandable.

As the surfaces are smoothed, they fit closer and closer to each other.

However, as long as the height of the irregularities exceeds several molecular radii, there are no interaction forces between molecules of neighboring surfaces. After all, these are very short-range forces. When a certain polishing perfection is achieved, the surfaces will come so close that the adhesive forces of the molecules come into play. They will begin to prevent the bars from moving relative to each other, which provides the static friction force. When smooth bars slide, the molecular bonds between their surfaces are broken, just as the bonds inside the tubercles themselves are broken on rough surfaces. The breaking of molecular bonds is the main difference between frictional forces and elastic forces. When elastic forces arise, such ruptures do not occur. Because of this, friction forces depend on speed.

Often popular books and science fiction stories paint a picture of a world without friction. This way you can very clearly show both the benefits and harms of friction. But we must not forget that friction is based on the electrical forces of interaction between molecules. The destruction of friction would actually mean the destruction of electrical forces and, therefore, the inevitable complete disintegration of matter.

But knowledge about the nature of friction did not come to us by itself. This was preceded by extensive research work by experimental scientists over several centuries. Not all knowledge took root easily and simply; many required repeated experimental testing and proof. The brightest minds of recent centuries have studied the dependence of the modulus of friction force on many factors: on the area of contact of surfaces, on the type of material, on load, on surface unevenness and roughness, on the relative speed of movement of bodies. The names of these scientists: Leonardo da Vinci, Amonton, Leonard Euler, Charles Coulomb - these are the most famous names, but there were also ordinary workers of science. All scientists participating in these studies conducted experiments in which work was done to overcome the force of friction.

3.4. Historical background

The year was 1500 . The great Italian artist, sculptor and scientist Leonardo da Vinci conducted strange experiments, surprising his students.

He dragged across the floor, either a tightly twisted rope, or the same rope at full length. He was interested in the answer to the question: does the force of sliding friction depend on the area of the bodies touching in motion? Mechanics of that time were deeply convinced that the larger the contact area, the greater the friction force. They reasoned something like this: the more such points, the greater the power. It is quite obvious that on a larger surface there will be more such points of contact, so the friction force should depend on the area of the rubbing bodies.

Leonardo da Vinci doubted and began to conduct experiments. And I got an amazing conclusion: the force of sliding friction does not depend on the area of the contacting bodies. Along the way, Leonardo da Vinci studied the dependence of the friction force on the material from which the bodies are made, on the magnitude of the load on these bodies, on the sliding speed and the degree of smoothness or roughness of their surface. He got the following results:

1. Does not depend on area.

2. Does not depend on the material.

3. Depends on the magnitude of the load (in proportion to it).

4. Does not depend on sliding speed.

5. Depends on surface roughness.

1699 . The French scientist Amonton, as a result of his experiments, answered the same five questions. For the first three - the same, for the fourth - it depends. On the fifth - it does not depend. It worked, and Amonton confirmed Leonardo da Vinci’s unexpected conclusion about the independence of the friction force from the area of contacting bodies. But at the same time, he did not agree with him that the friction force does not depend on the sliding speed; he believed that the force of sliding friction depends on speed, but he did not agree that the force of friction depends on the roughness of the surfaces.

During the eighteenth and nineteenth centuries, there were up to thirty studies on this topic. Their authors agreed on only one thing - the friction force is proportional to the force of normal pressure acting on the contacting bodies. But there was no agreement on other issues. The experimental fact continued to puzzle even the most prominent scientists: the force of friction does not depend on the area of the rubbing bodies.

1748 . Full member of the Russian Academy of Sciences Leonhard Euler published his answers to five questions about friction. The first three were the same as the previous ones, but in the fourth he agreed with Amonton, and in the fifth - with Leonardo da Vinci.

1779 . In connection with the introduction of machines and mechanisms into production, there is an urgent need for a more in-depth study of the laws of friction. The outstanding French physicist Coulomb took up solving the problem of friction and devoted two years to it. He conducted experiments at a shipyard in one of the ports of France. There he found those practical production conditions in which the frictional force played a very important role. The pendant answered all questions - yes. The total friction force, to some small extent, still depends on the size of the surface of the rubbing bodies, is directly proportional to the force of normal pressure, depends on the material of the contacting bodies, depends on the sliding speed and the degree of smoothness of the rubbing surfaces. Subsequently, scientists became interested in the question of the influence of lubrication, and types of friction were identified: liquid, pure, dry and boundary.

Correct Answers

The friction force does not depend on the area of the contacting bodies, but depends on the material of the bodies: the greater the normal pressure force, the greater the friction force. Accurate measurements show that the modulus of the sliding friction force depends on the modulus of the relative speed.

The friction force depends on the quality of processing of the rubbing surfaces and the resulting increase in friction force. If you carefully polish the surfaces of contacting bodies, then the number of points of contact with the same force of normal pressure increases, and therefore the friction force increases. Friction is associated with overcoming molecular bonds between contacting bodies.

3.5.Friction coefficient

The friction force depends on the force pressing a given body against the surface of another body, i.e., on the force of normal pressure N and on the quality of rubbing surfaces.

In the experiment with a tribometer, the normal pressure force is the weight of the block. Let us measure the force of normal pressure equal to the weight of the cup with weights at the moment of uniform sliding of the block. Let us now double the force of normal pressure by placing weights on the block. By placing additional weights on the cup, we again make the block move evenly.

The friction force will double. Based on similar experiments, it was established that, with unchanged material and condition of the rubbing surfaces, the force of their friction is directly proportional to the force of normal pressure, i.e.

The value characterizing the dependence of the friction force on the material and the quality of processing of the rubbing surfaces is called the friction coefficient. The friction coefficient is measured by an abstract number showing what part of the normal pressure force is the friction force

μ depends on a number of reasons. Experience shows that friction between bodies of the same substance is, generally speaking, greater than between bodies of different substances. Thus, the coefficient of friction of steel on steel is greater than the coefficient of friction of steel on copper. This is explained by the presence of molecular interaction forces, which are much greater for homogeneous molecules than for dissimilar ones.

Affects friction and the quality of processing of rubbing surfaces.

When the quality of processing of these surfaces is different, then the sizes of the roughnesses on the rubbing surfaces are also unequal, the stronger the adhesion of these roughnesses, i.e., the greater the μ of friction. Consequently, the same material and quality of processing of both rubbing surfaces corresponds to the highest value font-size:14.0pt;line-height:115%"> interaction forces. If in the previous formula under F tr meant the sliding friction force, then μ will denote the sliding friction coefficient, but if FTp replace with the largest value of the static friction force F max ., then μ will denote the coefficient of static friction

Now let's check whether the friction force depends on the area of contact of the rubbing surfaces. To do this, put 2 identical bars on the tribometer runners and measure the friction force between the runners and the “double” bar. Then we put them on the runners separately, interlocking with each other, and measure the friction force again. It turns out that, despite the increase in the area of the rubbing surfaces in the second case, the friction force remains the same. It follows that the friction force does not depend on the size of the rubbing surfaces. This, at first glance strange, result of the experiment is explained very simply. By increasing the area of the rubbing surfaces, we thereby increased the number of irregularities on the surface of the bodies that engage each other, but at the same time reduced the force with which these irregularities press against each other, since we distributed the weight of the bars over a larger area.

Experience has shown that the force of friction depends on the speed of movement. However, at low speeds this dependence can be neglected. While the speed of movement is low, the friction force increases with increasing speed. For high speeds of movement, an inverse relationship is observed: with increasing speed, the friction force decreases. It should be noted that all established relationships for the friction force are approximate.

The friction force varies significantly depending on the state of the rubbing surfaces. It decreases especially strongly in the presence of a liquid layer, such as oil, between the rubbing surfaces (lubricant). Lubricants are widely used in technology to reduce harmful friction forces.

3.6. The role of friction forces

In technology and in everyday life, friction forces play a huge role. In some cases, friction forces are beneficial, in others they are harmful. Frictional force holds nails, screws, and nuts driven in; holds threads in fabric, knots tied, etc. In the absence of friction, it would be impossible to sew clothes, assemble a machine, or put together a box.

Friction increases the strength of structures; Without friction, it is impossible to lay the walls of a building, or fasten telegraph poles, or fasten parts of machines and structures with bolts, nails, and screws. Without friction, plants would not be able to stay in the soil. The presence of static friction allows a person to move on the surface of the Earth. While walking, a person pushes the Earth back, and the Earth pushes the person forward with the same force. The force that moves a person forward is equal to the static friction force between the sole of the foot and the Earth.

The more a person pushes the Earth back, the greater the static friction force applied to the leg, and the faster the person moves.

When a person pushes the Earth with a force greater than the maximum static friction force, the leg slides backward, making walking difficult. Let's remember how difficult it is to walk on slippery ice. To make walking easier, you need to increase static friction. For this purpose, the slippery surface is sprinkled with sand. The same applies to the movement of an electric locomotive or car. The wheels connected to the engine are called drive wheels.

When the drive wheel, with the force generated by the engine, pushes the rail back, a force equal to static friction and applied to the wheel axis moves the electric locomotive or car forward. So, friction between the drive wheel and the rail or the Earth is beneficial. If it is small, then the wheel slips, and the electric locomotive or car stands still. Friction, for example, between moving parts of a working machine is harmful. To increase friction, sand is sprinkled on the rails. In icy conditions it is very difficult to walk and drive cars, since static friction is very low. In these cases, sidewalks are sprinkled with sand and chains are put on car wheels to increase static friction.

Friction is also used to hold bodies at rest or to stop them if they are moving. The rotation of the wheels is stopped with the help of brake pads, which are pressed in one way or another against the wheel rim. The most common are air brakes, in which the brake pad is pressed against the wheel using compressed air.

Let's take a closer look at the movement of a horse pulling a sleigh. The horse places its legs and tenses its muscles in such a way that, in the absence of resting friction forces, the legs would slide backwards. In this case, static friction forces directed forward arise. On a sleigh, which the horse pulls forward through the lines with force , The sliding friction force acts from the ground and is directed backwards. In order for the horse and sleigh to gain acceleration, it is necessary that the friction force of the horse's hooves on the road surface be greater than the friction force acting on the sleigh. However, no matter how great the coefficient of friction of the horseshoes on the ground, the static friction force cannot be greater than the force that should have caused the hooves to slide, that is, the force of the horse’s muscles. Therefore, even when the horse’s legs do not slide, he still sometimes cannot move the heavy sleigh. When moving (when sliding begins), the friction force decreases slightly; therefore, it is often enough just to help the horse move the sleigh so that he can then carry it.

4. Experimental results

Target:find out the dependence of sliding friction force on the following factors:

From the load;

From the area of contact of rubbing surfaces;

From rubbing materials (on dry surfaces).

Equipment: laboratory dynamometer with a spring stiffness of 40 N/m; round demonstration dynamometer (limit - 12N); wooden blocks - 2 pieces; set of loads; wooden plank; a piece of metal sheet; flat cast iron bar; ice; rubber.

Experimental results

1. Dependence of sliding friction force on load.

m, (g)			1120
FTP(H)

2. Dependence of the friction force on the contact area of the rubbing surfaces.

S(cm2)
FTP(H)	0,35	0,35	0,37

3. Dependence of the friction force on the size of the irregularities of the rubbing surfaces: wood on wood (various methods of surface treatment).

	1 varnished	2 wooden	3 fabric
	0, 9Н		1, 4Н

When studying the friction force from materials of rubbing surfaces, we use one block weighing 120 g and different contact surfaces. We use the formula:

We calculated sliding friction coefficients for the following materials:

No.	Friction materials (on dry surfaces)	Friction coefficient (while moving)
	Wood by wood (average)	0,28
	Wood on wood (along the grain)	0,07
	Wood for metal	0,39
	Wood on cast iron	0,47
	Tree on ice	0,033

5. Design work and conclusions

Goals:create demonstration experiments; explain the results of observed phenomena.

Friction experiments

After studying the literature, we selected several experiments that we decided to carry out ourselves. We thought through the experiments and tried to explain the results of our experiments. As instruments and tools we took: a wooden ruler, knives, sandpaper, a sharpening wheel.

Experience No. 1

A cylindrical box with a diameter of 20 cm and a height of 7 cm is filled with sand. A light figurine with a weight on its legs is buried in the sand, and a metal ball is placed on its surface. When the box is shaken, the figure sticks out of the sand, and the ball sinks in it. When shaking the sand, the friction forces between the grains of sand are weakened, it becomes mobile and acquires the properties of a liquid. Therefore, heavy bodies “sink” in the sand, and light ones “float”.

Experience№ 2 Knife point in workshops. Processing the surfaces of parts using sandpaper. The phenomena are based on the splitting of notches between contacting surfaces.

Experience No. 3When the wire is repeatedly extended and bent, the bend area heats up. This occurs due to friction between individual layers of metal.

Also, when a coin is rubbed against a horizontal surface, the coin heats up.

The results of these experiments can explain many phenomena.

For example, the case in workshops. While working at the machine, I encountered smoke between the rubbing surfaces of the moving parts of the machine. This is explained by the phenomenon of friction between contacting surfaces. To prevent this phenomenon, it was necessary to lubricate the rubbing surfaces and thereby reduce the friction force.

6. Conclusion

We found out that people have long used knowledge about the phenomenon of friction, obtained experimentally. Starting from XV - XVI centuries, knowledge about this phenomenon becomes scientific: experiments are carried out to determine the dependence of the friction force on many factors, and patterns are revealed.

Now we know exactly what the friction force depends on and what does not affect it. More specifically, the friction force depends on: the load or body weight; on the type of contacting surfaces; on the speed of relative motion of bodies; on the size of irregularities or surface roughness. But it does not depend on the contact area.

Now we can explain all the patterns observed in practice by the structure of matter, the strength of interaction between molecules.

We conducted a series of experiments, performed approximately the same experiments as scientists, and obtained approximately the same results. It turned out that experimentally we confirmed all the statements we made.

We created a series of experiments to help understand and explain some “difficult” observations.

But, probably, the most important thing is that we realized how great it is to gain knowledge ourselves, and then share it with others.

List of used literature.

1. Elementary physics textbook: Study guide. In 3-xt. /Ed. . T.1 Mechanics. Molecular physics. M.: Nauka, 1985.

2., Leprosy of mechanics and technology: Book. for students. – M.: Education, 1993.

3. By the way, parts 1 and 2. Mechanics. Molecular physics and heat. M.: Higher School, 1972.

4. Encyclopedia for children. Volume 16. 1 Biography of physics. Journey into the depths of matter. Mechanical picture of the world/Chapter. Ed. . – M.: Avanta+, 2000

· http://demo. home. nov. ru/favorite. htm

· http://gannalv. *****/tr/

· http://ru. wikipedia. org/wiki/%D0%A2%D1%80%D0%B5%D0%BD%D0%B8%D0%B5

· http://class-fizika. *****/7_tren. htm

· http://www. *****/component/option, com_frontpage/Itemid,1/

Kungurova E.V. (Perm, primary school teacher, MAOU “Gymnasium No. 1”)

1. Elementary physics textbook: Study guide. In 3-xt. /Under the editorship of G.S. Landsberg. T.1 Mechanics. Molecular physics. M.: Nauka, 1985.

2. Ivanov A.S., Leprosa A.T. The world of mechanics and technology: A book for students. – M.: Education, 1993.

3. Encyclopedia for children. Volume 16. Physics Part 1 Biography of physics. Journey into the depths of matter. Mechanical picture of the world/Chapter. Ed. V.A.Volodin. – M.:Avanta+, 2010

4. Children's encyclopedia. I explore the world: Physics/comp. A.A. Leonovich, ed. O.G. Hinn. – M.: LLC “Firm Publishing House AST”. 2010.-480 p.

5. http://demo.home.nov.ru/favorite.htm

6. http://gannalv.narod.ru/tr/

7. http://ru.wikipedia.org/wiki/%D0%A2%D1%80%D0%B5%D0%BD%D0%B8%D0%B5

8. http://class-fizika.narod.ru/7_tren.htm

9. http://www.physel.ru/component/option,com_frontpage/Itemid,1/

10. http://62.mchs.gov.ru/document/1968180.

This article is an abstract presentation of the main work. The full text of the scientific work, applications, illustrations and other additional materials are available on the website of the III International Competition of Scientific Research and Creative Works of Students “Start in Science” at the link: https://www.school-science.ru/0317/11/28780

Winter is the favorite time of many kids in the Kama region! After all, you can slide down a hill with the breeze, drive quietly through a fabulous winter forest and have fun skating with friends. I love winter fun too!

Problem: to understand what prevented me from traveling so far without an ice pack.

Target of this project: revealing the secrets of friction force.

Tasks:

trace the historical experience of mankind in the use and application of this phenomenon;
find out the nature of friction force;
conduct experiments confirming the patterns and dependencies of the friction force;
understand where a 2nd grade student can encounter friction force;
develop recommendations for classmates “Smart winter holidays”.

To achieve our goals, we worked on this project in the following areas:

1) public opinion research;

2) study of theory;

3) experiment;

4) design.

Hypothesis: frictional force is necessary in people's lives.

The scientific interest lies in the fact that in the process of studying this issue, some information was obtained about the practical application of the phenomenon of friction.

1. What is friction (a little theory)

Objectives: to study the nature of friction forces.

Friction force

Why is it better to ride down a snowy hill on ice? How does a car accelerate, and what force slows it down when braking? How do plants stay in the soil? Why is it difficult to hold a live fish in your hand? How can we explain the danger of ice in winter? It turns out that all these questions are about the same thing!

The answers to these and many other questions related to the movement of bodies are provided by the laws of friction. From the above questions it follows that friction is both a harmful and beneficial phenomenon.

Any body moving along a surface catches on its unevenness and experiences resistance. This resistance is called friction force. Friction is determined by the surface properties of solids, and they are very complex and have not yet been fully studied.

If we try to move the cabinet, we will immediately see that it is not so easy to do. His movement will be hampered by the interaction of his legs with the floor on which he stands. What determines the magnitude of the friction force? Everyday experience shows: the more you press the surfaces of bodies against each other, the more difficult it is to cause their mutual sliding and maintain it. We will try to prove this experimentally.

1.1.The role of friction forces

Let's imagine that one day something strange happened on Earth! Let's turn to a thought experiment, imagine that in the world some wizard managed to turn off friction. What would this lead to?

Firstly, we wouldn’t be able to walk, car wheels would spin pointlessly in place, clothespins wouldn’t be able to hold anything...

Secondly, the causes of friction would disappear. When one object slides in another way, it seems as if the microscopic tubercles are catching on each other. But if these tubercles were not there, this would not mean that moving an object or dragging it would become easier. A so-called sticking effect would arise, which is easy to detect when trying to move a stack of glossy-covered books along the surface of a polished table.

This means that if there were no friction, there would not be these tiny attempts of each particle of matter to keep its neighbors near it. But then how would these particles stay together? That is, inside various bodies the desire to “live in company” would disappear, and the substance would fall apart to the smallest detail, like a Lego house.

These are the unexpected conclusions that can be reached if we assume the absence of friction. Like everything that bothers us, we need to fight it, but we won’t be able to completely get rid of it, and we don’t need to!

The more a person pushes the Earth back, the greater the frictional force applied to the leg, and the faster the person moves.

In icy conditions it is very difficult to walk and drive cars, since there is very little friction. In these cases, sidewalks are sprinkled with sand and chains are put on car wheels to increase static friction.

Friction is also used to hold bodies at rest or to stop them if they are moving. The rotation of the wheels is stopped using the brakes. The most common are air brakes, which operate using compressed air.

2. Design work and conclusions

Goals: create a demonstration experiment; explain the results of observed phenomena.

After studying the literature, my dad and I did several experiments. We thought through the experiments and tried to explain their results.

Let's get back to the story of my ride on the slide.

One day my dad and I were sliding down an ice slide. At first I drove down without ice. And I only managed to get to the end of the icy slope. Then I decided to slide down on a plastic ice skate, and my distance almost doubled!

Now, I understand that the friction force was greater the first time I rolled down, it made my body slow down faster. But in this experiment, the hardness of bodies also matters. My winter suit is much softer than a plastic ice cap. This means that the suit interacts more with the slide and produces greater frictional force. A hard ice cube “adheses” less to the slide, and there is less friction!

On a piece of cardboard one toothpick wide and two toothpicks long, use plasticine to attach a toothpick across the cardboard in the middle. Then we fold the edges of the cardboard. Let's draw a spider on colored paper. Let's draw the spider so that its body is larger than a rectangle. Glue the cardboard to the back of the spider. Cut the thread as long as your arm. Thread the needle and pull it through the cardboard. Stretch the thread with the spider and hold it vertically. Then loosen the thread a little. How will the spider behave?

When the thread is pulled tightly, it touches the toothpick and friction occurs between them. Friction prevents the spider from sliding down.

This experiment shows what the force of friction depends on.

Let's take a sheet of paper. Let's put it between the pages of a thick book lying on the table. Let's try to pull out the sheet. Let's do the experiment again. Now let's put the sheet almost at the very end of the book. Let's try to pull it out again. Experience shows that it is easier to pull a sheet from the top of a book than from the bottom. This means that the stronger the surfaces of bodies are pressed against each other, the greater their interaction, that is, the greater the friction force.

When the wire is repeatedly extended and bent, the bend area heats up. This occurs due to friction between individual layers of metal. Also, when a coin is rubbed against a surface, the coin heats up.

This simple experiment shows the application of friction force.

Sharpening knives in workshops. When a knife becomes dull, it can be sharpened with a special device. The phenomenon is based on the smoothing of notches between contacting surfaces.

The results of these experiments can explain many phenomena in nature and human life. Now that I knew the secret of the force of friction, I realized that it is described in many fairy tales! This was another discovery for me!

I really want to give examples of fairy tales. In the fairy tale “Kolobok”, the force of friction helps the main character get out of difficult situations (“Kolobok lay there, lay there, took it and rolled - from the window to the bench, from the bench to the floor, along the floor to the door, jumped over the threshold - and into the canopy and rolled..."). In the fairy tale “The Ryaba Hen,” a lack of frictional force led to trouble (“The mouse ran, wagged its tail, the egg rolled, fell and broke”). In the fairy tale “Turnip,” the friction of the turnip on the surface of the earth forced the whole family to unite. The Snow Queen, with her magic, easily overcame the force of friction (“The sleigh drove around the square twice. Kai quickly tied his sleigh to it and rolled”).

It's interesting to look at famous works differently!

3. Public opinion research

Objectives: to show what role the phenomenon of friction or its absence plays in our lives; answer the question: “What do we know about this phenomenon?”

We studied proverbs and sayings in which the force of static, rolling, and sliding friction is manifested; we studied human experience in the use of friction and ways to combat friction.

Proverbs and sayings:

There will be no snow, there will be no trace.
If you drive more quietly, you will continue.
There will be a quiet cart on the mountain.
It's hard to swim against the water.
If you love to ride, you also love to carry sleds.
Patience and work will grind everything down.
That’s why the cart began to sing because it had not eaten tar for a long time.
And he scribbles, and plays, and strokes, and rolls. And all in language.
He lies that he sews with silk.

All the above proverbs indicate that people noticed the existence of friction force a long time ago. People reflect in proverbs and sayings the efforts that need to be made to overcome friction forces.

Take a coin and rub it on a rough surface. We will feel resistance - this is the force of friction. If you rub too quickly, the coin will begin to heat up, reminding us that friction generates heat - a fact known to Stone Age man, because this is how people first learned to make fire.

Already in ancient times, engineers had at their disposal the most important means for reducing friction in the mechanisms themselves, such as a replaceable metal bearing lubricated with fat or olive oil.

Of course, friction also plays a positive role in our lives. No body, be it the size of a boulder or a grain of sand, can ever rest on one another; everything will slide and roll. If there were no friction, the Earth would be uneven, like a liquid.

I learned so many interesting and new things about the secrets of friction. You need to fight it wisely in order to develop unprecedented speed. I decided to tell my classmates about how to ride the slides correctly and safely.

Winter is a time of fun and fun games. Slide riding is everyone's favorite winter activity. Speed, the whistle of a fresh wind, a storm of overwhelming emotions - in order for your vacation to be not only pleasant, but also safe, you should think about choosing both slides and sleds.

1. With a child under 3 years old, you should not go on a busy slide, where children 7-10 years old and older ride.

2. If the slide causes you concern, first let an adult ride it; without a child, try the descent.

3. If a child is already riding on a multi-age “live” slide, he must be supervised by an adult. It is best if one of the adults watches the descent from above, and someone from below helps the children quickly clear the way.

4. Under no circumstances should railway embankments and slides be used as slides near roadways.

Bibliographic link

Makarova E. THE AMAZING FORCE OF FRICTION // Start in science. – 2017. – No. 4-3. – P. 519-523;
URL: http://science-start.ru/ru/article/view?id=813 (access date: 01/19/2020).

Under 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. A friction force arises, directed opposite to the movement.

There are several types of friction:

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

Viscous (aka liquid) friction occurs when solids move in a liquid or gaseous medium, or when a liquid or gas flows past stationary solids.

Static friction occurs when a force is applied to a body that attempts to move that body.

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

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 thread, the glass will easily slide along the glass. Now wet 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 contaminated the rubbing surfaces. Contact was formed between two perfectly clean surfaces, and it turned out that making scratches (i.e. tearing out pieces of glass) was easier than tearing off (moving) a glass.

Ways to reduce friction:

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

Friction forces are electromagnetic in nature.

What does friction force depend on?

Depending on the type of contacting surfaces and 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, sometimes the full length, sometimes collecting it in rings. He studied: does the force of sliding friction depend on the area of contacting bodies?
As a result, Leonardo came to the conclusion that the force of sliding friction does not depend on the area of contacting bodies, which is confirmed by modern scientists.

How to explain the occurrence of friction?

The contacting surfaces of 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 during compression, the pointed peaks are deformed and the contact area increases in proportion to the applied load. It is the shear resistance in places of irregularities that causes 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 molecules. This explains the influence on the frictional force of the load - the pressing force and the properties of the materials.

How to measure friction force?

This can be done using a dynamometer.
When the body moves uniformly, the dynamometer shows the traction force equal to the friction force. For ease of measurement, sometimes instead of pulling the book across the table, you can start moving the table itself and hold the book in place by tying it to a spring. This will not change the friction force.

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

What is more profitable: rolling or sliding?

What's better - sliding or rolling? Of course, rolling is more profitable than sliding. To maintain rolling, you need to apply much less force than to maintain sliding at the same speed. Therefore, it is understandable that in the summer they travel in a cart and not on a sleigh.

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

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

Place a round (not faceted) glass on the table and push it so that its bottom slides along the table. Having moved, the glass will stop.
Now put the same glass on its side and push it with the same force. The glass, having rolled, 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 greater than rolling friction!

Friction always slows down movement; A huge amount of valuable fuel is consumed to overcome friction of all types.
Friction causes wear of the rubbing surfaces.

HISTORY OF FRICTION STUDY

The first study of the laws of friction belongs to the famous Italian scientist and artist Leonardo da Vinci (15th century):
the friction force that arises when a body comes into contact 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 blocks sliding along a board, and by placing the bars on different faces, he determined the dependence of the friction force on the area of support. But, unfortunately, Leonardo da Vinci's works 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 friction coefficient (k).

After this, a formula for the friction force was derived:

Ftr = kN

Where N is the ground reaction force corresponding to the pressure force exerted 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 since ancient times that surfaces lubricated with grease or even simply moistened with water glide 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; A huge amount of valuable fuel is consumed to overcome friction of all types.
Friction causes wear and tear on rubbing surfaces: soles, car tires, and machine parts wear out. They try to reduce harmful friction.

But sometimes there is benefit in friction!

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

What if there was no friction?

Nobel Prize winner, Swiss physicist Charles Guillaume said: “Let us imagine that friction can be completely eliminated, then no body, be it 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 irregularities, like a liquid.”

READ ALL ABOUT FRICTION

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

INTERESTING

An increase in the force of resistance to movement with increasing speed leads to steady uniform motion of a body when falling from a great height in a liquid or gas (for example, in the atmosphere). Thus, before the parachute opens, a parachutist 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.

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

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

Wetsuits, which are specially designed for spearfishing and freediving, are produced with an ultra-smooth coating on the outside to reduce friction losses when sliding in the water.

QUESTION FOR EVERYONE!