Interesting facts about the force of friction in physics. Friction Types of friction forces What does the friction force depend on? Friction in mechanisms and machines Interesting facts
Introduction.
We encounter friction at every step. But, despite the great role that friction plays in our lives, a sufficiently complete picture of the occurrence of friction has not yet been created. This is not even due to the fact that friction is of a complex nature, but rather to the fact that friction experiments are very sensitive to surface treatment and therefore difficult to reproduce.
When talking about friction, there are three somewhat different physical phenomena: resistance when a body moves in a liquid or gas; it is called liquid friction; the resistance that occurs when a body slides over some surface is sliding friction, or dry friction; resistance arising from the rolling of the body - rolling friction .
The history of the emergence of the force of friction
The first formulation of the force of friction is attributed to Leonardo da Vinci. He argued that the friction force arising from the contact of a body with the surface of another body is proportional to the load (pressing force), directed against the direction of motion and does not depend on the area of contact.
Leonardo's model was rediscovered 180 years later by G. Amonton and received its final formulation in the works of Coulomb (1781). Amonton and Coulomb introduced the concept of the friction coefficient as the ratio of the friction force to the load, giving it the value of a physical constant that completely determines friction force for any pair of contacting materials. So far, this formula
where P is the pressing force, and Ftr is the friction force, is the only formula that appears in physics textbooks, and the values of the friction coefficient ftr for various materials (steel on steel, steel on bronze, cast iron on leather, etc.) are included in standard engineering handbooks and serve as the basis for traditional technical calculations.
However, already in the 19th century it became clear that the Amonton-Coulomb law does not give a correct description of the friction force, and the coefficients of friction are by no means universal characteristics. First of all, it was noted that the coefficients of friction depend not only on what materials are in contact, but also on how smoothly the contact surfaces are processed. It also turned out that the force of static friction differs from the force of friction during motion. To recall what is usually understood by static friction, let us present the scheme of the simplest experiment (Fig. 1).
We will try to move the body from its place by pulling the cable with a spring dynamometer. With a small movement of the end of the cable, the body remains in place: the force developed by the dynamometer spring is not enough. It is usually said that a friction force develops on the contacting surfaces, balancing the applied force. We gradually increase the displacement and with it the elastic force applied to the body. At some point, it turns out to be enough to move the body from its place. The reading of the dynamometer recorded at this moment is usually called the force of static friction, which characterizes the limiting possibilities of the motionless (static) adhesion of bodies. If we continue to slowly pull the cable, then the body will move along the surface. It turns out that the dynamometer readings recorded during the movement will not be the same as at the moment of starting off. Usually, the friction force during slow movement is less than the force of breakaway, static friction. Coulomb studied precisely the force of friction during slow mutual movement of contacting bodies and found that this force does not depend on the magnitude of the speed, but only on the direction of movement (always directed against the movement.
The end of the 19th century was marked by remarkable achievements in the study of viscosity, that is, friction in liquids. Probably, it has been known since prehistoric times that surfaces lubricated with grease or even simply wetted with water slide much easier. Lubrication of rubbing surfaces has been used since the inception of technology, but only O. Reynolds in 1886 gave the first theory of lubrication.
In the presence of a sufficiently thick lubricant layer, which ensures the absence of direct contact between rubbing surfaces, the friction force is determined only by the properties of the lubricating layer. The static starting force is zero, and with increasing speed, the resistance to movement increases. If there is not enough lubrication, then all three mechanisms operate: the force of static resistance to starting off, the Coulomb force and the force of viscous resistance.
So, by the end of the 19th century, the picture of the dependence of the friction force on the speed, presented by the graph (Fig. 2, a), became clear. But already on the threshold of the 20th century, doubts arose about the correctness of this picture at very low speeds. In 1902, Striebeck published data indicating that in the absence of lubrication, the drag force does not immediately drop from the starting force to the Coulomb force, but a gradual drop in force occurs with increasing speed - an effect opposite to hydrodynamic viscosity. This fact has been repeatedly verified in the future and is now commonly referred to as the Stribeck effect. The picture of the dependence of the friction force on the speed (Fig. 2, b.).
The rapidly developing technology of the 20th century required more and more attention to the study of friction. In the 30s, research in the field of friction became so intense that it was necessary to single it out as a special science - tribology, lying at the intersection of mechanics, physics of surface phenomena and chemistry (the creation of new lubricants is the business of chemists). In the US alone, more than 1,000 researchers are currently working in this area, and more than 700 articles are published annually in world science.
The modern picture of friction.
In order to understand at least the basics of tribology, one should first of all turn to the topography of the surfaces of the parts of real mechanisms that are in contact with each other. These surfaces are never perfectly flat, they have micro-roughnesses. The places of protrusions on one surface do not at all coincide with the places of protrusions on the other. As one of the pioneers of tribology, F. Bowden, figuratively put it, "the imposition of two solid bodies one on top of the other is similar to the imposition of the Swiss Alps on the inverted Austrian Alps - the contact area turns out to be very small." However, under compression, the pointed "mountain peaks" are plastically deformed, and the true contact area increases in proportion to the applied load. It is the resistance to the relative shear of these contact zones that is the main source of motion friction. The shear resistance itself in ideal contact is determined by intermolecular interaction, which depends on the nature of the contacting materials.
Thus, the influence of two main factors is explained: the load (pressure force) and the properties of the materials. However, there are two complicating circumstances. First, metal surfaces in air are quickly covered with a thin film of oxides, and in fact, contact is not between purely metal surfaces, but between oxide films that have a lower shear resistance. The penetration of any liquid or paste-like lubricant generally changes the contact pattern. Secondly, with a relative shear, not only sliding along the contact pads is carried out, but also elastic deformation of the protrusions, peaks. Let us select schematically only two peaks (practically the slope of their slopes is about 10?-20?, but for clarity they are drawn in Fig. 3 steeper). When trying to move in a horizontal direction, one peak begins to bend the other, that is, it first tries to smooth the road, and then slides along it. The width of the peaks is small (on the order of hundredths of a millimeter), and within such microdisplacements leading role it is the elastic resistance that plays, that is, the force must obey Hooke's law, be proportional to the displacement. In other words, with microdisplacements, the contact surfaces appear to be connected, as it were, by numerous springs. But after the upper peak in the course of movement crosses the lower one (and both of them are flattened), the spring breaks until it meets a new obstacle. Thus, after the application of a longitudinal force tending to move two bodies, the following four main regimes may arise: regimes
I elastic microdisplacements, mode
II slip on the contact areas of the soft surface layer (oxide films), mode
III, when, at a higher speed, the squeezed out liquid lubricant creates a lifting force that violates most direct contacts and thereby reducing the friction force,
IV, when direct contacts disappear altogether, one body "floats" over the other in the lubricating layer and the viscous resistance increases with increasing speed.
Not going - just going
Because it's icy
But it falls great!
Why isn't anyone happy?
Such a naive nursery rhyme at first glance - and how much it contains when you look at it from a physical point of view! After all, it is precisely in it that the system of a contradictory attitude to the notorious force of friction is contained. This constant battle, where two concepts compete with each other - the harm and the benefits of the force of friction, will never have a winner. After all, what is convenient and beneficial for one person is often quite the opposite for another - bad, as in this poem.
Remember the story of Nikolai Nosov about the ice slide that the guys built in the yard? And when they all left for dinner, the one who did not participate in the construction came out. He tried to climb on it, but only hurt himself, but he could not climb. And the kid guessed to sprinkle sand on the ice - it became very convenient to climb to the very top even on ice! So, having strengthened with the help of sand between the slippery ice and the sole, the boy realized that the use of friction allows him to overcome obstacles.
But after lunch, the kids came out with ice cubes to ride their slide to their heart's content. But it wasn’t there: the sled does not go on the sand! For them, this situation turned the other way, showing the harm of friction.
We observe similar cases in winter, when the boys roll out ice paths and rush along them with a run, covering the distance in a matter of minutes! And then elderly people hobble, slip on the snow-covered rolls and fall, breaking their arms and legs. Here again are illustrative examples, where in the same case both the harm and the benefit of the force of friction coexist.
It is to reduce the friction force that skiers lubricate their skis with special ointments in order to increase speed when moving. Skating rinks, where skaters or figure skaters are engaged, are periodically watered and cleaned - also to reduce friction. And footpaths, on the contrary, are sprinkled with sand or ash so that no one falls on them. Some innovators even came up with the idea of sticking pieces of sandpaper to the soles of winter boots and boots just to increase the friction force.
The same thing happens with the wheels of cars. It's no secret that with the onset of winter, drivers "shoe" their iron horses in special "winter tires". Otherwise, without a useful friction force, the car skids when cornering, it drives, and often the driver does not manage well. And how the accident ends, everyone knows for himself.
Something we are all about winter, but about ice, but about falling. But are there other moments in everyday life where you can clearly see how the harm and benefit of the force of friction compete with each other? Of course have! They are everywhere. Even in our room.
Here, for example, is a huge and heavy wardrobe. It stands to itself, as if rooted to the spot, and does not move. And if the force of friction suddenly disappeared, what could happen then? And this hulk would have gone from the lightest push around the room! And it is still unknown if we could manage to dodge it. Good friction force, useful!
But my mother decided to rearrange the furniture. And you need to move this notorious closet to another wall. One, two, take it! Three - four, tensed up! Only everything turns out to be useless: the heavier the object, the stronger the friction force holds on to it. Terrible, nasty power!
Again they compete with each other - the harm and benefit of the force of friction. No competition needed! You just need to know physical laws well and be able to extract practical benefits from this knowledge. Not needed in this Means, it should be reduced: to make the contact surfaces smoother, slippery. For this, someone advises smearing the floor with soap or oil, someone puts a wet rag under the legs of a heavy object. And now - one - two - and you're done! Moved quite easily a sort of colossus from its place.
The force of friction constantly accompanies us throughout our lives, just as Somewhere it creates inconvenience for us, but somewhere we cannot do without it. But be that as it may, it exists, and our task is to learn how to use physical laws so that our life becomes more convenient and comfortable.
The science
European scientists have provided a modern explanation for the origin of sliding friction between solid objects. Despite the fact that friction is one of the fundamental phenomena of modern applied physics, This phenomenon has not ceased to be studied for many centuries.. Up to the present day, it was believed that mechanical wear resistance and the presence (or absence) of liquid lubrication are among the main factors affecting friction, but the fundamental causes of sliding friction remained unknown.
Dr. Lacey Makkonen, Senior Researcher at the Technical Research Center in Finland, presented his own explanation of the origin of sliding friction between solid objects. His theory fully confirms the fact that the amount of friction also depends on the so-called surface energy of the materials in question. At the same time, friction has a significant effect on many of the phenomena that we encounter every time (such as, for example, the absorption of energy).
The new thermodynamic model created by Makkonen is the first of its kind to quantify the coefficient of friction of materials, taking into account the surface energy of materials. The model actually shows that friction occurs when materials come into contact at the nanoscale level, being a consequence of the formation of new bonds at the atomic level. This theory complements the explanation of the origin of the friction force and the presence of frictional heating during dry friction. It can also be used to more accurately calculate friction coefficients for combinations of different materials.
The constructed model also makes it possible to more accurately control friction processes by selecting a specific surface of materials or by using lubricating layers, taking into account the presence of surface energy between them. It is noteworthy that this theory confirms the opinions of many physicists that there are noticeable inaccuracies in the well-known tables with the coefficients of friction presented in them for various materials (especially for homogeneous ones).
Do you know that back in 1500, the brilliant Leonardo da Vinci was very interested in what the friction force depends on and what it is? The strange experiments that he conducted caused considerable surprise among his students, and what else could be expected from people who see how a talented scientist drags a rope across the floor, either unwound to its full length, or tightly twisted. These and other similar experiments allowed him a little later (in 1519) to conclude: the friction force that appears when one body contacts the surface of another directly depends on the load (pressing force), does not depend on the interaction area and is directed in the opposite direction from the movement side.
Formula discovery
180 years have passed, and the Leonardo model was rediscovered by G. Amonton, and in 1781 S. O. Coulomb gave her the final formulation in his works. The merit of these two scientists is that they introduced such a physical constant as the coefficient of friction, thereby making it possible to derive a formula by which it is possible to calculate what the friction force is equal to for a particular pair of interacting materials. Until now, this expression
F t = k t x P, where
P is the pressing force (load), and k t is the coefficient of friction, from year to year it migrates to various textbooks and textbooks in physics, and the coefficients themselves have long been calculated and are contained in standard engineering reference books. It would seem that finally, with this phenomenon, complete clarity has come, but it was not there.
New nuances
In the 19th century, scientists became convinced that the formulation proposed by Amonton and Coulomb is not universal and absolutely correct, and the friction force depends not only on the coefficients and the applied load. In addition, there is a third factor - the quality of the surface treatment. Depending on whether it is smooth or rough, the friction force will take different meaning. In principle, this is quite logical: moving a sliding object is much easier compared to moving an object with an uneven surface. And at the end of the 19th century, new achievements in the study of viscosity appeared, and it became clear how the friction force acts in liquids. And although the lubrication of rubbing surfaces was used from the very beginning of the emergence of technology, only in 1886, thanks to O. Reynolds, a coherent theory devoted to lubrication appeared.
So, if it is enough, and there is no direct contact between two objects, the friction force depends only on its hydrodynamics. And if there is not enough lubricant, then all three mechanisms are activated: the Coulomb force, the force of viscous resistance, and the force that prevents starting from a place. Do you think this theory put an end to the study of this phenomenon? That's right, no. At the turn of the 20th century, it turned out that at low speeds, in the absence of lubrication, the Stribeck effect occurs. Its essence is that when there is no lubrication, the resistance force does not immediately decrease from the starting force to the level of the Coulomb force, but gradually decreases as the speed increases. In the twentieth century, further research in this area brought so much new information that it needed to be systematized somehow. As a result, a whole science appeared - tribology, which studies how the friction force works in nature. In the United States alone, the number of scientists working in this field has exceeded one thousand people, and in the world over 700 articles are published annually on this topic. It is curious what else interesting scientists will be able to discover? Wait and see!
Municipal budgetary educational institution
"Pervomaiskaya secondary school"
Pervomaisky
Research work
"The force of friction and its useful properties"
Completed by: Platon Alexey,
student 9 - "D" class
Supervisor:
,
Physics teacher
Pervomaisky
Tambov region
2012
1. Introduction 3
2. Research public opinion. 4
3. What is friction (a little theory). 5
3.1. Friction of rest. 5
3.2. Sliding friction. 6
3.3. Rolling friction. 6
3.4. History reference. 8
3.5. Friction coefficient. 9
3.6. The role of friction forces. eleven
4. Results of experiments. 12
5. Design work and conclusions. 13
6. Conclusion. fifteen
7. List of used literature. 16
1. Introduction
Problem:To understand whether we need friction force and to find out its useful properties.
How does the car accelerate, and what force slows it down when braking? Why does the car "skid" on a slippery road? What causes rapid wear of parts? Why can't a car come to a sudden stop when accelerating to high speeds? How are plants held in the soil? Why is a live fish difficult to hold in your hand? How to explain the high percentage of injuries and traffic accidents during sleet in winter?
The laws of friction provide answers to these and many other questions related to the motion of bodies.
From the above questions it follows that friction is both a harmful and beneficial phenomenon.
In the 18th century, a French physicist discovered the law according to which the force of friction between solids 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 coefficient of friction. The coefficient of friction lies in the range from 0.5 to 0.15. Although since then many hypotheses have been put forward to explain this law, there is still no complete theory of the friction force. Friction is determined by the properties of the surface of solids, and they are very complex and have not yet been fully explored.
Basic goals this project : 1) To study the nature of friction forces; to investigate the factors on which friction depends; consider the types of friction.
2) Find out how a person received knowledge about this phenomenon, what is its nature.
3) Show what role the phenomenon of friction or its absence plays in our life; answer the question: “What do we know about this phenomenon?”
4) Create demonstration experiments; explain the results of the observed phenomena.
Tasks: To 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 regularities and dependences of the friction force; think over and create demonstration experiments proving the dependence of the friction force on the force of normal pressure, on the properties of the contacting surfaces, on the speed of the relative motion of bodies.
To achieve these goals, this project worked in the following areas:
1) Research of public opinion;
2) Study of the theory of friction;
3) Experiment;
4) Design.
The urgency of the problem. The phenomenon of friction is very common in our life. All movements of bodies in contact with respect to each other always occur with friction. The force of friction always affects to a greater or lesser extent the nature of the movement.
Hypothesis. The friction force is useful, depends on the kind of rubbing surfaces, and the pressure force.
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, on the speed of movement in nature. It is also necessary to take this into account in technology and in everyday life.
Scientific interest lies in the fact that in the process of studying this issue, some information was obtained on the practical application of the phenomenon of friction.
2. Research of public opinion.
Goals: show what role the phenomenon of friction or its absence plays in our life; answer the question: “What do we know about this phenomenon?”
Proverbs and sayings were studied, in which the friction force of rest, rolling, sliding is manifested, human experience was studied in the application of friction, ways to combat friction.
Proverbs and sayings:
There will be no snow, there will be no trace.
A quiet cart will be on the mountain.
Difficult to swim against the water.
You love to ride, love to carry sleds.
Patience and work will grind everything.
From that, the cart sang that it had not eaten tar for a long time.
And scribbles, and rolls, and strokes, and rolls. And all with 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 faster, the coin will begin to heat up, reminding us that heat is released during friction - a fact known to man of the Stone Age, because it was in this way that people first learned to make fire.
Friction enables us to walk, sit, work without fear that books and notebooks will fall off the table, that the table will slide until it hits a corner, and the pen slips out of our fingers.
Friction contributes to stability. The carpenters level the floor so that the tables and chairs stay where they are.
However, a little friction on ice can be successfully used technically. Evidence of this is the so-called ice roads, which were arranged for the removal of timber from the felling site to railway or to fusion points. On such a road, which has smooth ice rails, two horses pull a sled 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 most ancient Sumerian cities - Uruk - the remains of massive wooden wheels, which are 4.5 thousand years old, were found. The wheels are studded with copper nails for the obvious purpose of protecting the wagon train from wear and tear.
And in our era, the fight against wear of technical devices is the most important engineering problem, the successful solution of which would save tens of millions of tons of steel, non-ferrous metals, and drastically reduce the production of many machines and spare parts for them.
Already in antiquity, engineers had at their disposal such essential means to reduce friction in the mechanisms themselves, as a replaceable metal plain bearing lubricated with grease or olive oil, and even a rolling bearing.
The world's first bearings are belt loops that support the axles of antediluvian Sumerian carts.
Bearings with replaceable metal inserts were well known in Ancient Greece where they were used in well gates and mills.
Of course, friction plays a positive role in our life, but it is also dangerous for us, especially in winter, during the period of ice.
3. What is friction (a little theory)
Goals:to study the nature of friction forces; to investigate the factors on which friction depends; consider the types of friction.
Friction force
If we try to move the closet, we will immediately see that it is not so easy to do it. His movement will be hindered by the interaction of the 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 do they have in common?
3.1. Friction of rest
In order to find out the essence of this phenomenon, you can conduct a simple experiment. Let's put the block on an inclined board. If the angle of inclination of the board is not too large, the bar may remain in place. What will keep it from sliding down? Friction of rest.
Let's press our hand to the notebook lying on the table and move it. The notebook will move relative to the table, but rest in relation to our palm. How did we make this notebook move? With the help of rubbing the rest of the notebook against the hand. The static friction moves loads on a moving conveyor belt, prevents shoelaces from untying, keeps nails driven into a board, etc.
The static friction force 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 than. For example, for a wooden block 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 static friction force, we will move the body from its place, and it will begin to move. The static friction will then be replaced by sliding friction.
3.2. Sliding friction
What causes the sled that rolls down the mountain to stop gradually? 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 motion of the body. Causes of the friction force:
1) Roughness of surfaces of contacting bodies. Even those surfaces that look smooth, in fact, always have microscopic irregularities (protrusions, depressions). When one body slides over the surface of another, these irregularities catch on to each other and thereby interfere with movement;
2) intermolecular attraction acting at the points of contact of rubbing bodies. There is attraction between the molecules of a substance at very small distances. Molecular attraction is manifested in those cases when the surfaces of the contacting bodies are well polished. So, for example, with the relative sliding of two metals with very clean and even surfaces, processed in vacuum using a special technology, the friction force is much stronger than the friction force between the wood blocks with each other, and further sliding becomes impossible.
3.3. rolling friction
If the body does not slide on the surface of another body, but, like a wheel or a cylinder, rolls, then the friction that occurs at the point of their contact is called rolling friction. The rolling wheel is somewhat pressed into the roadbed, and therefore there is always a small tubercle in front of it, which must be overcome. It is precisely the fact that the rolling wheel constantly has to run into the tubercle that appears in front, and the rolling friction is due. At the same time, the harder the road, the less rolling friction. With the same loads, the rolling friction force is much less than the sliding friction force (this was noticed in antiquity). So, the legs of heavy objects, such as beds, pianos, etc., are provided with rollers. In engineering, to reduce friction in machines, rolling bearings, otherwise called ball and roller bearings, are widely used.
These types of friction are referred to as dry friction. We know why the book doesn't fall through the table. But what prevents her 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. It is felt to the touch, and under the microscope it is clear that the surface solid body most of all resembles a mountainous country. Countless protrusions cling to each other, deform a little and prevent the book from slipping. 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 start to slide. Obviously, at the same time, the “chipping off” of the tubercles begins, the gap molecular bonds unable to withstand the increased load. The friction force is still acting, but it will already be the sliding friction force. It is not difficult to detect the "cleavage" of the tubercles. The result of this "chipping" is the wear of rubbing parts.
It would seem that the more carefully the surfaces are polished, the less the friction force should be. To a certain extent this is so. Grinding reduces, for example, the frictional force between two steel bars. But not limitless! The friction force suddenly begins to increase with a further increase in the smoothness of the surface. This is unexpected, yet 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 the molecules of neighboring surfaces. After all, these are very short-range forces. When a certain perfection of grinding is achieved, the surfaces will approach so much that the cohesive forces of the molecules will 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 torn, just as the bonds within the tubercles themselves are destroyed on rough surfaces. The breaking of molecular bonds is the main difference between friction forces and elastic forces. When elastic forces arise, such discontinuities do not occur. Because of this, the friction forces depend on the speed.
Often popular books and science fiction stories paint a picture of a frictionless world. So you can very clearly show both the benefits and harms of friction. But we must not forget that friction is based on the electric forces of interaction of molecules. The destruction of friction would actually mean the destruction of electrical forces and, consequently, the inevitable complete disintegration of matter.
But knowledge about the nature of friction did not come to us by itself. This was preceded by a big research work experimental scientists for several centuries. Not all knowledge took root easily and simply, many required multiple experimental verifications and proofs. The brightest minds of recent centuries have studied the dependence of the friction force modulus on many factors: on the area of contact between surfaces, on the type of material, on the load, on surface irregularities 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 the scientists who participated in these studies set up experiments in which work was done to overcome the force of friction.
3.4. History reference
It was 1500 . The great Italian artist, sculptor and scientist Leonardo da Vinci conducted strange experiments, which surprised his students.
He dragged along the floor, now a tightly twisted rope, then the same rope in its entire length. He was interested in the answer to the question: does the force of sliding friction depend on the size of the area of bodies in contact in motion? The mechanics of that time were deeply convinced that what more area touch, the greater the friction force. They reasoned something like this: the more such points, the greater the force. 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 a stunning conclusion: the force of sliding friction does not depend on the area of the bodies in contact. 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 the area.
2. Does not depend on the material.
3. It 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 in this way. For the first three - the same, for the fourth - it depends. On the fifth - does not depend. It turned out, and Amonton confirmed such an unexpected conclusion by Leonardo da Vinci about the independence of the friction force from the area of the bodies in contact. But at the same time, he did not agree with him that the force of friction does not depend on the speed of sliding; he believed that the sliding friction force depends on the speed, but he did not agree with the fact that the friction force depends on the surface roughness.
During the eighteenth and nineteenth centuries, there were up to thirty studies on the subject. Their authors agreed on only one thing - the friction force is proportional to the force of normal pressure acting on the bodies in contact. There was no agreement on other issues. The experimental fact continued to bewilder even the most prominent scientists: the friction force does not depend on the area of the rubbing bodies.
1748 . Active member Russian Academy Leonhard Euler published his answers to five questions about friction. For the first three - 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 deeper study of the laws of friction. The outstanding French physicist Coulomb took up the solution of the problem of friction and devoted two years to this. He set up experiments at a shipyard in one of the ports of France. There he found those practical production conditions in which the friction force played a very important role. Coulomb 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 normal pressure force, depends on the material of the contacting bodies, depends on the sliding speed and on the degree of smoothness of the rubbing surfaces. In the future, scientists became interested in the question of the effect of lubrication, and types of friction were identified: liquid, clean, dry and boundary.
Right answers
The force of friction does not depend on the area of the bodies in contact, but depends on the material of the bodies: the greater the force of normal pressure, the greater the force of friction. Precise measurements show that the modulus of the sliding friction force depends on the modulus of the relative velocity.
The friction force depends on the quality of the processing of rubbing surfaces and the increase in the friction force as a result. If the surfaces of the bodies in contact are carefully polished, then the number of points of contact with the same force of normal pressure increases, and, consequently, the friction force also increases. Friction is associated with overcoming molecular bonds between contacting bodies.
3.5 Friction coefficient
The force of friction depends on the force that presses the 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 an experiment with a tribometer, the force of normal pressure is the weight of the bar. 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 bar. Let us now double the force of normal pressure by placing weights on the bar. Putting additional weights on the cup, we again make the bar move evenly.
The force of friction will then double. On the basis of such experiments, it was found that, with the material and condition of the rubbing surfaces unchanged, 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 rubbing surfaces is called the coefficient of friction. The coefficient of friction is measured by an abstract number showing what part of the force of normal pressure is the force of friction
μ depends on a number of reasons. Experience shows that the friction between bodies of the same substance, generally speaking, is 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 heterogeneous ones.
Affects friction and the quality of processing of rubbing surfaces.
When the quality of processing of these surfaces is different, then the dimensions of the roughness on the rubbing surfaces are also not the same, the stronger the adhesion of these roughnesses, i.e., the greater the friction μ. Therefore, the same material and quality of processing of both friction surfaces corresponds to the largest value font-size: 14.0pt; line-height: 115%"> interaction forces. If in the previous formula under F tr meant the force of sliding friction, then μ will denote the coefficient of sliding friction, 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, we put 2 identical bars on the skids of the tribometer and measure the friction force between the skids and the "double" bar. Then we put them on the runners separately, interlocking with each other, and again measure the friction force. It turns out that, despite the increase in the area of 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. Such, at first glance, a strange result of the experiment is explained very simply. By increasing the area of rubbing surfaces, we thereby increased the number of irregularities engaging with each other on the surface of the bodies, but at the same time we reduced the force with which these irregularities are pressed against each other, since we distributed the weight of the bars over a large 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, 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 (lubrication). Lubrication is widely used in engineering to reduce the forces of harmful friction.
3.6. The role of friction forces
In technology and Everyday life frictional forces play a huge role. In some cases, friction forces are beneficial, in others they are harmful. The force of friction holds driven nails, screws, nuts; holds threads in matter, tied knots, etc. In the absence of friction, it would be impossible to sew clothes, assemble a loom, put together a box.
Friction increases the strength of structures; without friction, neither the laying of the walls of a building, nor the fixing of telegraph poles, nor the fastening of parts of machines and structures with bolts, nails, screws can be carried out. Without friction, plants could not be held in the soil. The presence of static friction allows a person to move on the surface of the Earth. Walking, a person pushes the Earth back from himself, and the Earth pushes the person forward with the same force. The force that propels 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 away with a force greater than the ultimate static friction force, the foot slides backwards, making walking difficult. Remember how hard it is to walk on slippery ice. To make it easier to walk, it is necessary to increase the static friction. For this purpose, the slippery surface is sprinkled with sand. This also applies to the movement of an electric locomotive, a car. The wheels connected to the engine are called drive wheels.
When the driving wheel, with the force generated by the engine, pushes the rail back, then a force equal to the static friction and applied to the wheel axle moves the electric locomotive or car forward. So the friction between the driving wheel and the rail or the ground is useful. If it is small, then the wheel is slipping, and the electric locomotive or car is standing still. Friction, for example, between the moving parts of a running machine is harmful. To increase friction, sprinkle the rails with sand. It is very difficult to walk and move in cars in icy conditions, since the static friction is very small. In these cases, sand is sprinkled on the sidewalks and chains are put on the wheels of cars to increase the rest friction.
The force of friction is also used to keep 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 against the wheel rim in one way or another. Air brakes are the most common, in which the brake pad is pressed against the wheel using compressed air.
Let us consider in more detail the movement of a horse pulling a sled. The horse puts his legs and tenses his muscles in such a way that, in the absence of resting friction, the legs would slide backwards. In this case, forces of static friction directed forward arise. On the sled, which the horse pulls forward through the traces with force , sliding friction force acting backwards from the ground. In order for the horse and sled 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 sled. 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 strength of the horse's muscles. Therefore, even when the horse's legs do not slip, still he sometimes cannot move the heavy sledge. When moving (when sliding began), the friction force decreases somewhat; therefore, it is often enough just to help the horse move the sled from its place, so that later it can carry it.
4. Experimental results
Target:find out the dependence of the sliding friction force on the following factors:
From the load;
From the area of contact of rubbing surfaces;
From rubbing materials (with dry surfaces).
Equipment: laboratory dynamometer with spring force 40 N/m; round demonstration dynamometer (limit - 12N); wooden bars - 2 pieces; a set of cargoes; wooden board; a piece of metal sheet; flat cast iron bar; ice; rubber.
Experimental results
1. Dependence of the sliding friction force on the 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. The 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 tissue |
|
0.9N | 1, 4N |
In the study of the friction force from the materials of rubbing surfaces, we use one bar with a mass of 120 g and different contact surfaces. We use the formula:
We calculated sliding friction coefficients for the following materials:
No. p / p | Rubbing materials (dry surfaces) | Coefficient of friction (when moving) |
Wood by wood (average) | 0,28 |
|
Wood on wood (along the fibers) | 0,07 |
|
wood for metal | 0,39 |
|
wood for cast iron | 0,47 |
|
tree on ice | 0,033 |
5. Design work and conclusions
Goals:create demonstration experiments; explain the results of the 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 devices and tools, we took: a wooden ruler, knives, sandpaper, a grinding wheel.
Experience #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 load on its feet is buried in the sand, and a metal ball is placed on its surface. When the box is shaken, the figurine sticks out of the sand, and the ball sinks into it. When the sand is shaken, 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".
An experience№ 2 Point of knives in workshops. Surface treatment of parts with sandpaper. The phenomena are based on the splitting of notches between contacting surfaces.
Experience #3With repeated unbending and bending of the wire, the bending point heats up. This is due to friction between the individual layers of metal.
Also, when rubbing a coin on a horizontal surface, the coin heats up.
Many phenomena can be explained by the results of these experiments.
For example, the case in the workshops. While working at the machine, I had smoke between the rubbing surfaces of the moving parts of the machine. This is due to 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 a person has long been using knowledge about the phenomenon of friction, obtained empirically. Beginning with XV - XVI centuries, knowledge about this phenomenon becomes scientific: experiments are carried out to determine the dependence of the friction force on many factors, regularities are clarified.
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 mass; from the kind of contacting surfaces; on the speed of the relative motion of bodies; on the size of uneven or rough surfaces. But it does not depend on the area of \u200b\u200bcontact.
Now we can explain all the regularities observed in practice by the structure of matter, by the force of interaction between molecules.
We conducted a series of experiments, did about the same experiments as the scientists, and got about the same results. It turned out that experimentally we confirmed all the statements made by us.
We have created a series of experiments to help understand and explain some of the "difficult" observations.
But, perhaps most importantly, we realized how great it is to acquire knowledge ourselves, and then share it with others.
List of used literature.
1. Elementary textbook of physics: Study guide. At 3 pm / Ed. . T.1 Mechanics. Molecular physics. M.: Nauka, 1985.
2., Leprosy of mechanics and technology: Book. for students. – M.: Enlightenment, 1993.
3. Bytko, parts 1 and 2. Mechanics. Molecular physics and heat. M.: graduate 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
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