Research work “The force of friction and its useful properties. Start in science

Friction force.

Lesson-experiment. 7th grade. A basic level of.

Teacher: Lesnova E.Yu.

Target: to familiarize students with the phenomenon of friction. Experimentally establish what this force depends on. Continue the formation of skills to use instruments, analyze and compare the results of experiments.

Equipment: a dynamometer, a board - smooth on one side, rough on the other, a wooden block with hooks, a set of weights, a cuvette with water, a trolley on wheels.

The class is divided into 4 groups. Each group is given a task card. You have 2 minutes to complete each task. If the group does not cope with the task, the teacher offers hints. The conclusions of the experiment are recorded in a notebook.

Lesson Plan

The study of new material, the systematization of the studied.

Reflection.

homework

Teacher's message

Filling in the table

Conduct experiments and explain results.

Recording conclusions in a notebook.

Answers on questions. Recording homework.

Tasks for groups.

Exercise 1.

Find out what and how the modulus of sliding friction depends on.

Task 2.

Compare, with the same masses of bodies, the modules of sliding and rolling friction forces.

Task 3.

Compare, with the same masses of bodies, the modules of dry and liquid sliding friction.

Hint #1 (To task 1)

Find out how the modulus of the friction force depends on the type of surfaces and the pressure force.

Hint #2 (To task 2)

1. Using a horizontally placed dynamometer, evenly move a wooden block with two weights first along smooth surface boards, then rough. Compare the dynamometer readings. Make a conclusion.

2. Using a horizontally placed dynamometer, move the wooden block evenly over the rough surface of the board - first with one load, then with two, three. Compare the dynamometer readings. Make a conclusion.

Hint #1 (To task 2)

Measure the sliding friction modulus and the rolling friction modulus.

Hint #2 (To task 2)

1. Using a horizontal dynamometer, first measure the rolling friction force while moving the wheeled cart evenly with six weights inside.

2. Remove the wheels and measure the sliding friction while moving the cart without the wheels (with the same weights). Compare the dynamometer readings. Make a conclusion.

Hint #1 (To task 3)

Find out how the modulus of the friction force depends on the movement of a wooden block on a solid and liquid surface.

Hint #2 (To task 3)

1. Using a horizontal dynamometer, first measure the friction force by moving the block evenly across a hard surface.

2. Using a horizontal dynamometer, first measure the friction force by moving the bar evenly across the surface of the liquid in the vessel. Compare the dynamometer readings. Make a conclusion.

During the classes.

1 .Motivation. Any discovery is accompanied by experience, the talent of a researcher, and even chance. Today in the lesson we will also try to make small, but independent discoveries. We work in groups. The rules are written on the board.

2 . Learning new material. teacher pushes a wooden block wooden board.

What happened to the speed of the bar? Why does the speed of the bar change? What force caused the body to stop? This is the force of friction and we will study it in the lesson.

Let's continue filling in the table, using paragraph No. 24. I take 8 minutes to work.

direction

Measurement method

Graphic image

Reasons for the emergence of strength

The filling of the table is checked-3min.

The teacher explains what is different kinds friction: friction force of sliding, rolling, dry friction on the surface, liquid friction.

Work in groups on assignments.

After discussion, the results of the experiments are discussed and recorded in a notebook.

3. Reflection.

And now everyone will express their attitude to the lesson, starting their statement with the words:

1. The most important conclusions about the force of friction are

2. Do you know that today at the lesson I learned….

3. Most of all I remember today ....

4. the most interesting was...

If a person, by his industriousness, reaches the truth in something, then this is his discovery.

D / C: read the notes in the notebook, give examples of useful and harmful friction.

Exercise 1.

Task 2.

Task 3.

Task 4.

Exercise 1.

Find out what and how the modulus of sliding friction depends on.

Task 2.

Compare, with the same masses of bodies, the modules of sliding and rolling friction forces.

Task 3.

Compare, with the same masses of bodies, the modules of dry and liquid sliding friction.

Task 4.

Compare the modulus of sliding friction force from the area of ​​the contacting surfaces.

Hint #1 (To task 1)

Hint #2 (To task 1)

Hint #1 (To task 2)

Hint #2 (To task 2)

Hint #1 (To task 3)

Hint #2 (To task 3)

Hint #1 (To task 4)

Measure the sliding friction force modulus for different areas of the contact surfaces.

Hint #2 (To task 4)

1.Using a horizontal dynamometer, first measure the friction force by moving the block evenly across the surface of the board so that it is in contact with the board with a larger area.

2. Using a horizontal dynamometer, first measure the friction force by moving the block evenly across the surface of the board so that it is in contact with the board with a smaller area.

HOW TO WORK IN A GROUP

of their strength.

Speak on behalf of the group honorable.

HOW TO WORK IN A GROUP

Be conscientious towards your comrades, work to the fullest of their strength.

Listen to each member of the group carefully without interrupting.

Speak briefly, clearly so that everyone can speak

Support each other despite intellectual differences.

When rejecting an idea, be polite and don't forget to offer an alternative.

If no one can start talking, start clockwise from the captain (coordinator)

Speak on behalf of the group honorable. This is not done by kamikaze, but by its plenipotentiary representative trained by the entire group.

HOW TO WORK IN A GROUP

Be conscientious towards your comrades, work to the fullest of their strength.

Listen to each member of the group carefully without interrupting.

Speak briefly, clearly so that everyone can speak

Support each other despite intellectual differences.

When rejecting an idea, be polite and don't forget to offer an alternative.

If no one can start talking, start clockwise from the captain (coordinator)

Speak on behalf of the group honorable. This is not done by kamikaze, but by its plenipotentiary representative trained by the entire group.

HOW TO WORK IN A GROUP

Be conscientious towards your comrades, work to the fullest of their strength.

Listen to each member of the group carefully without interrupting.

Speak briefly, clearly so that everyone can speak

Support each other despite intellectual differences.

When rejecting an idea, be polite and don't forget to offer an alternative.

If no one can start talking, start clockwise from the captain (coordinator)

Speak on behalf of the group honorable. This is not done by kamikaze, but by its plenipotentiary representative trained by the entire group.

HOW TO WORK IN A GROUP

Be conscientious towards your comrades, work to the fullest of their strength.

Listen to each member of the group carefully without interrupting.

Speak briefly, clearly so that everyone can speak

Support each other despite intellectual differences.

When rejecting an idea, be polite and don't forget to offer an alternative.

If no one can start talking, start clockwise from the captain (coordinator)

Speak on behalf of the group honorable. This is not done by kamikaze, but by its plenipotentiary representative trained by the entire group.

HOW TO WORK IN A GROUP

Be conscientious towards your comrades, work to the fullest of their strength.

Listen to each member of the group carefully without interrupting.

Speak briefly, clearly so that everyone can speak

Support each other despite intellectual differences.

When rejecting an idea, be polite and don't forget to offer an alternative.

If no one can start talking, start clockwise from the captain (coordinator)

Speak on behalf of the group honorable. This is not done by kamikaze, but by its plenipotentiary representative trained by the entire group.

HOW TO WORK IN A GROUP

Be conscientious towards your comrades, work to the fullest of their strength.

Listen to each member of the group carefully without interrupting.

Speak briefly, clearly so that everyone can speak

Support each other despite intellectual differences.

When rejecting an idea, be polite and don't forget to offer an alternative.

If no one can start talking, start clockwise from the captain (coordinator)

Speak on behalf of the group honorable. This is not done by kamikaze, but by its plenipotentiary representative trained by the entire group.

Exercise 1.

Find out what and how the modulus of sliding friction depends on.

Task 2.

Compare, with the same masses of bodies, the modules of sliding and rolling friction forces.

Task 3.

Compare, with the same masses of bodies, the modules of dry and liquid sliding friction.

Task 4.

Compare the modulus of sliding friction force from the area of ​​the contacting surfaces.

Exercise 1.

Find out what and how the modulus of sliding friction depends on.

Task 2.

Compare, with the same masses of bodies, the modules of sliding and rolling friction forces.

Task 3.

Compare, with the same masses of bodies, the modules of dry and liquid sliding friction.

Task 4.

Compare the modulus of sliding friction force from the area of ​​the contacting surfaces.

Exercise 1.

Find out what and how the modulus of sliding friction depends on.

Task 2.

Compare, with the same masses of bodies, the modules of sliding and rolling friction forces.

Task 3.

Compare, with the same masses of bodies, the modules of dry and liquid sliding friction.

Exercise 1.

Find out what and how the modulus of sliding friction depends on.

Task 2.

Compare, with the same masses of bodies, the modules of sliding and rolling friction forces.

Task 3.

Compare, with the same masses of bodies, the modules of dry and liquid sliding friction.

Hint #1 (To task 1)

Find out how the modulus of the friction force depends on the type of surfaces and the pressure force.

Hint #2 (To task 1)

1. Using a horizontally placed dynamometer, evenly move a wooden block with three weights, first on a smooth surface of the board, then on a rough one. Compare the dynamometer readings. Make a conclusion.

2. Using a horizontally placed dynamometer, move the wooden block evenly over the rough surface of the board - first with one load, then with two, three. Compare the dynamometer readings. Make a conclusion.

Hint #1 (To task 2)

Measure the sliding friction modulus and the rolling friction modulus.

Hint #2 (To task 2)

1. Using a horizontal dynamometer, first measure the rolling friction force while moving the wheeled cart evenly with six weights inside.

2. Remove the wheels and measure the sliding friction while moving the cart without the wheels (with the same weights). Compare the dynamometer readings. Make a conclusion.

Hint #1 (To task 3)

Find out how the modulus of the friction force depends on the movement of a wooden block on a solid and liquid surface.

Hint #2 (To task 3)

1. Using a horizontal dynamometer, first measure the friction force by moving the block evenly across a hard surface.

2. Using a horizontal dynamometer, first measure the friction force by moving the bar evenly over the surface of the liquid in the cuvette. Compare the dynamometer readings. Make a conclusion.

Hint #1 (To task 1)

Find out how the modulus of the friction force depends on the type of surfaces and the pressure force.

Hint #2 (To task 1)

1. Using a horizontally placed dynamometer, evenly move a wooden block with three weights, first on a smooth surface of the board, then on a rough one. Compare the dynamometer readings. Make a conclusion.

2. Using a horizontally placed dynamometer, move the wooden block evenly over the rough surface of the board - first with one load, then with two, three. Compare the dynamometer readings. Make a conclusion.

Hint #1 (To task 2)

Measure the sliding friction modulus and the rolling friction modulus.

Hint #2 (To task 2)

1. Using a horizontal dynamometer, first measure the rolling friction force while moving the wheeled cart evenly with six weights inside.

2. Remove the wheels and measure the sliding friction while moving the cart without the wheels (with the same weights). Compare the dynamometer readings. Make a conclusion.

Hint #1 (To task 3)

Find out how the modulus of the friction force depends on the movement of a wooden block on a solid and liquid surface.

Hint #2 (To task 3)

1. Using a horizontal dynamometer, first measure the friction force by moving the block evenly across a hard surface.

2. Using a horizontal dynamometer, first measure the friction force by moving the bar evenly over the surface of the liquid in the cuvette. Compare the dynamometer readings. Make a conclusion.

direction

Measurement method

Graphic image

Reasons for the emergence of strength

The text of the work is placed without images and formulas.
Full version work is available in the "Files of work" tab in PDF format

Introduction

Winter is a favorite time for many kids in the Kama region! After all, you can slide down a hill with a 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 going so far without ice.

The purpose of this project: unraveling the mystery of the force of friction.

Tasks:

trace the historical experience of mankind in the use and application of this phenomenon;

find out the nature of the friction force;

conduct experiments confirming the regularities and dependences of the friction force;

to understand where a student of the 2nd grade can meet with the force of friction;

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

1) Research of public opinion;

2) The study of theory;

3) Experiment;

4) Design.

Hypothesis: the force of friction is necessary in people's lives.

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

1 . What is friction (a little theory)

Goals: study the nature of friction forces.

Friction force

Why so snow slide is it better to go on ice? How does the car accelerate, and what force slows it down when braking? How are plants held in the soil? Why live fish hard to hold? How to explain the danger of ice in winter period? It turns out that all these questions are about the same thing!

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.

Any body, moving along the surface, catches on its irregularities and experiences resistance. This resistance is called 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.

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. What determines the amount of friction force? Everyday experience shows that the more strongly the surfaces of bodies are pressed against each other, the more difficult it is to cause their mutual sliding and to maintain it. We will try to prove this experimentally.

1.1 Role of friction forces

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

Firstly, we would not be able to walk, the wheels of cars would spin in place to no avail, clothespins would not be able to hold anything ...

Secondly, the causes that generate friction would disappear. During the sliding of one object over another, it is as if the microscopic tubercles are meshing with each other. But if these tubercles did not exist, this would not mean that it would be easier to move an object or drag it. There would be a so-called STICKING effect, which is easy to detect when trying to move a stack of books in a glossy cover 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 around. But then how would these particles stick together? That is, inside the various bodies the desire to “live in a company” would disappear, and the substance would fall apart to the smallest detail, like a LEGO house.

Here are some unexpected conclusions that can be reached if we assume the absence of friction. As with everything that hinders us, we must fight it, but it will not be possible to completely get rid of it, and it is not necessary!

In technology and Everyday life frictional forces play an enormous 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. Strength, driving man forward, equal to the static friction force between the sole of the foot and the Earth.

How stronger man pushes the earth back more strength friction applied to the leg, and the faster the person moves.

It is very difficult to walk and drive in icy conditions because there is very little friction. 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 by the brakes. The most common are air brakes that are powered by compressed air.

2. Design work and conclusions

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

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

Experience #1

Let's go back to the story of my downhill ride.

Once, my dad and I were skating down an ice slide. At first I moved out without ice. And I managed to get only to the end of the ice slope. Then I decided to get out on a plastic ice rink, and my distance almost doubled!

Now, I understand that the force of friction the first time I rolled was greater, it made my body slow down faster. But even in this experiment, the hardness of the bodies matters. My winter suit is much softer than a plastic ice cap. This means that the suit interacts more with the slide and produces a greater friction force. Rigid ice is less "engaged" with the slide, and friction is less!

Experience #2

On a piece of cardboard one toothpick wide, and two toothpicks long, with plasticine, attach a toothpick across the cardboard in the middle. Then fold the edges of the cardboard. Draw a spider on colored paper. We draw a spider so that its body is larger than a rectangle. Glue a piece of cardboard to the back of the spider. Cut the thread to the length of your hand. We will thread the needle and stretch it through the cardboard. Pull 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 tight, it touches the toothpick and FRICTION occurs between them. Friction keeps the spider from sliding down.

Experience No. 3

This experiment shows what the friction force 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.

Experience No. 4

With 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 against a surface, the coin heats up.

Experience No. 5

This simple experiment shows the application of the force of friction.

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 the notches between the contacting surfaces.

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

I really want to give examples of fairy tales. In the fairy tale "Gingerbread Man" - the force of friction helps the main character to get out of difficult situations(“Kolobok lay down, lay down, 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 rolled into the canopy ...”). In the fairy tale "Ryaba the Hen" - the lack of friction force led to trouble ("The mouse ran, wagged its tail, the testicle rolled, fell and broke). In the fairy tale "Turnip" - the friction of turnips on the surface of the earth made the whole family rally. The Snow Queen with her magic, she easily overcame the force of friction (“The sleigh drove around the square twice. Kai quickly tied his sledge to it and rolled it”).

It is interesting to look at famous works differently!

3. Public opinion survey

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.

All these proverbs indicate that people have noticed the existence of friction forces for a long time. The people reflect in proverbs and sayings the efforts that must be made to overcome the forces of friction.

Take a coin and rub it on a rough surface. We will feel 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 is not only a brake on movement. This is also main reason wear 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, with the obvious purpose of protecting the convoy from rapid wear.

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 important means for reducing friction in the mechanisms themselves as a replaceable metal bearing lubricated with grease or olive oil.

Of course, friction plays a positive role in our life. No body, whether it be the size of a stone block or a grain of sand, will ever rest on one another, everything will slide and roll. If there were no friction, the Earth would be without irregularities, like liquids.

I learned so many interesting and new things about the secrets of the force 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. Skiing is everyone's favorite winter activity. Speed, the whistle of a fresh wind, a storm of overflowing 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 baby under 3 years old, you should not go to a busy hill with which children 7-10 years old and older ride.

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

3. If a child is already riding a “busy” slide of different ages, an adult must be sure to follow him. 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 hills near the carriageway of motorways be used as slides.

Rules of conduct on a busy mountain:

Climbing a snow or ice slide should only be done at a climbing point equipped with steps; it is forbidden to climb a hill where others slide down towards you.

Do not move out until the previous descender has moved aside.

Do not linger below when you have moved out, but quickly crawl away or roll to the side.

Do not cross the icy path.

To avoid injury, you can not ride while standing on your feet and squatting.

Try not to slide backwards or head forward (on your stomach), but always look ahead, both when descending and when ascending.

If a passer-by walks past the hill, wait until he passes, and only then make the descent.

If it is impossible to get away from a collision (a tree, a person, etc.) on the way, then you should try to fall on your side on the snow or roll away from the ice surface.

Avoid skiing hills with uneven ice.

In case of injury, immediately provide first aid to the victim, report this to the emergency call service 01.

At the first sign of frostbite, or if you feel unwell, stop riding immediately.

Various means for skiing now available great amount, so you can find the right one for you to enjoy riding from any hill: from steep icy to gently sloping, covered with fresh snow.

Convenient means of transportation on the ice slide:

Ledyanka plastic. The simplest and cheapest device for skiing in winter. They are intended for single skiing on icy and rolled snow slopes. The ice rinks are designed for children from 3 years old, because. It's hard for kids to manage. The plate-shaped ice cube becomes uncontrollable if you sit in it with your feet.

ice trough it is very unstable, at the slightest unevenness it strives to fall on its side - thus, having flown up on a springboard, you can land upside down. Ledyanki are not designed for springboards or any other obstacles, because. any sharp jump on the hill is fraught with backfire for the coccyx and spine rider!

Ordinary"Soviet" sled great for any snowy slopes. You can steer and brake with your feet. Falling sideways to avoid a dangerous collision is also quite easy and safe.

Snow scooter. For family skiing, you should not choose a snow scooter - it is designed for one or two kids aged 5 to 10 years. More than once, cases were noticed when snow scooters clung to an obstacle (tree root, snow mound) with the front skid and turned over. It is difficult to get off the snow scooter high speed, and the speed is vehicle develops considerable on any slope and accelerates quickly. The brakes are located at the front, which increases the risk of rolling over your head when trying to brake hard. If an adult is traveling with high mountain together with the child, putting the baby on a snow scooter in front, it will be very difficult for them to steer, brake and evacuate in case of danger.

Cheesecakes. Recently, inflatable sleds are increasingly found on our slides. The most common inflatable circles are “sled-cheesecakes”. The cheesecake is light and rides well even in fresh snow on a completely unrolled hill. It is best to ride cheesecakes from gentle snowy slopes without obstacles in the form of trees or other people. As soon as the movement speed increases, the cheesecake becomes quite dangerous. Cheesecakes accelerate with lightning speed, and the speed is higher than that of a sled or a snow scooter on a similar slope, and it is impossible to jump off the cheesecake at speed. You can’t ride on cheesecakes from slides with springboards - when landing, the cheesecake is very springy. Even if you do not fly off, you can get severe back injuries and cervical spine. A good version of the “cheesecake” is a small inflatable ice rink (about 50 cm in diameter) - it’s easy to fall on your side (get off).

Carefully consider the choice of slides and means for skiing!

The hill is a place of increased danger, and not just another entertainment on winter walk along with building snowmen and feeding birds! When riding children with adults, it is important not to forget that speed depends on mass. That is, the steeper and "icier" the hill or the larger the mass ("dad is big and strong, it's not scary with him"), the more deadly the force of the collision. That is why even in cars children are required to be carried fastened in car seats, and not in the arms of adults and not fastened together with an adult with one belt. The force of friction is not a magical force, it will not allow you to stop instantly!

Conclusion

We found out that a person has long been using knowledge about the phenomenon of friction, obtained empirically.

Now we know exactly when the friction force occurs.

We have created a series of experiments to help understand and explain some of the "difficult" phenomena of nature.

We have identified literary works that talk about the force of friction.

Most importantly, we realized how great it is to get knowledge ourselves, and then share it with others.

List of used literature

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

2. Ivanov A.S., Prokaza A.T. World of mechanics and technology: Book for students. - M.: Enlightenment, 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 know the world: Physics / comp. A.A. Leonovich, ed. O.G. Hinn. - M .: LLC "Firm" AST Publishing House ". 2010.-480s.

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

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

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

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

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

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

ROLLING AND SLIDING

Place the book on an angle and put a pencil on it. Will it slide or won't it slide?
It depends how you put it. If you put it along the slope, the pencil will not slip even with a large slope. What if across?
Oh, how it rolled! Especially if it is round, not hexagonal.

You can say: think, I also have a scientific experience! What is interesting in it?
And the interesting thing about this experiment is that when the pencil rolls, the friction is much less than when it crawls. Rolling is easier than dragging. Or, as physicists say, rolling friction is less than sliding friction.

That is why people invented wheels. In ancient times, wheels were not known, and even in summer, goods were carried on sledges. On the wall of an ancient temple in Egypt, a picture is carved: a huge stone statue is being driven across the ground on a sleigh.

Rollers, and then wheels, appeared already several thousand years ago, sliding friction was replaced by more beneficial rolling friction.

Modern technology has made the following important step: bearings appeared, which are sliding, ball and roller.

To move a thick book on the table with one finger, you need to make some effort.

And if you put two round pencils under the book, which will be in this case roller bearings, the book will move easily with a slight push with the little finger.

Since rolling friction is much less than sliding friction, in technology they try to replace sliding bearings with ball or roller bearings. Even in an ordinary adult bicycle, there are ball bearings in the wheel hubs, in the steering column, on the crank axle, on the pedal axles.
Cars, motorcycles, tractors, railroad cars - all these machines roll on ball and roller bearings.

REST FRICTION

Place a hexagonal pencil on top of the book parallel to the spine. Slowly lift the top edge of the book until the pencil begins to slide down. Slightly reduce the slope of the book and secure it in this position by placing something under it.

Now the pencil, if you put it on the book again, will not move out. It is held in place by the force of friction - the force of static friction. But if this force is slightly weakened - and for this it is enough to click on the book with your finger - and the pencil will crawl down until it falls on the table. The same experiment can be done, for example, with a pencil case, a matchbox, an eraser, etc.

The friction force of motion (under other identical conditions) is usually less than the friction force of rest. In this case, she was unable to hold the pencil on the inclined plane.
By the way, think about why it is easier to pull a nail out of the board if you rotate it around its axis?

ACROBAT WHEEL GOES

Before we finish talking about friction, let's make one more fun toy.
Cut out an acrobat figure from thick paper. Put it on a pen inserted on a sharply sharpened round pencil. A day is now a pencil with an acrobat obliquely into the ring of scissors. Holding the scissors horizontally, move them gently in a circle.

Ah, how our acrobat went wheel!
He's involved in two movements at once. First, the end of the pen with the acrobat on the nib describes large circles. And secondly, the handle does not slide along the ring of the scissors, but rolls over it. And the handle, together with the acrobat, rotates around its axis. From the combination of these two movements, such wonderful wheels are obtained. A living acrobat will hardly be able to repeat them!

Where is the friction, you ask?
Yes, in the ring of scissors. If it were not there, the handle would immediately fall down, it would not be able to hold even in an inclined position. And one more thing: if there was no friction between the ring and the handle, the handle would not run around the ring and the acrobat would not tumble so beautifully.

BRAKE IN THE EGG

Experience 1

hang up a raw egg on thin string. To prevent the lace from slipping off the vertical egg, use adhesive tape, sticking small pieces of it on the places where the lace is located.

Hang a hard-boiled egg nearby. Twist each string with the egg in one direction for the same number of turns. When the laces are twisted, release the eggs at the same time. You will see that a boiled egg behaves differently than a raw one: it spins much faster.

In a raw egg, its protein and yolk try to remain stationary (this is their inertia) and, by their friction against the shell, slow down its rotation.

In a boiled egg, the protein and yolk are no longer liquid substances and represent, together with the shell, as if one whole, so there is no braking and the egg rotates faster.

This experiment can be done without hanging the eggs: just twist them with your fingers on a large plate.

Experience 2

It is even more interesting to do such an experiment.
Take two identical saucepans with two ears (you can also use toys). Connect the ears with a rope or thin wire, and tie another rope to the middle so that the pan is in balance. Hang both pots on these ropes and pour water into one of them, and the same amount of cereal in the other. Now twist the ropes on the same number revs and release. The result will be similar to the experiment with eggs.

When the pans have spun, try to quickly stop them, and then release them again. It turns out that the pot of water continues to rotate. Well, how can you explain this phenomenon?

Sources: F. Rabiz "Experiments without instruments"; "Funny Physics" L. Galpershtein

Let's do some experiments.

Experience 1. Place a wooden block on wooden table. We attach a dynamometer to the bar and begin to apply force to the dynamometer. The dynamometer pointer will show that a force will act on the bar, which increases with the growth of our efforts. The bar, despite the increase in strength, remains at rest for some time. Rest is possible when the action of forces on the body is compensated. Therefore, it can be assumed that between the bar and the table there is some kind of force opposite to the force acting from the dynamometer. This force was called the static friction force (Fig. 4.35).

It is marked with a letter. Experience shows that with an increase in force, the static friction force will also increase.

Let's continue our experiments.

Experience 2. We will increase the force acting from the dynamometer. At some point in time, the bar will still move from its place, and will continue to move uniformly and rectilinearly under the action of some constant force. The uniformity of the movement of the bar means that a force acts on our bar, preventing its movement. It is equal in absolute value to the force and is directed opposite to it. (fig.4.36).

This force began to be called the force of sliding friction and is denoted by the letter.

Experience 3. Let's repeat this experiment by placing a wooden block on a glass table. We will find that the results of the experiment will not change. Will only change numerical value forces , and . And this means that friction forces arise on any contact surfaces. This type of friction is called dry friction.

French physicists Charles Augustin Coulomb and Guillaume Amonton studied the forces of dry friction. Experimentally, they established the following laws of dry friction:

1. The maximum static friction force is equal to the sliding friction force

2. The force of sliding friction is directly proportional to the force normal pressure, i.e.

where m - coefficient of proportionality, which is determined by the type of material, contacting surfaces, the quality of their processing, etc. This proportionality factor is called the coefficient of sliding friction.

3. The force of dry friction does not depend on the area of ​​the contacting surfaces.

Formula (1) is called the Coulomb-Amonton law.

Experience 4. Let us place a bar and a wooden cylinder of the same mass on a horizontal table and set them in motion in the same direction with the same speed (Fig. 4.37).

Experiment will show that the cylinder will move a much greater distance than the bar. This means that the friction force acting on the cylinder is much less than the sliding friction force of the bar. It is necessary to pay attention to the fact that during the movement the bar comes into contact with the table surface with only one of its surfaces, and the cylinder rolls along it. The force of friction that occurs when a body rolls over a surface is called rolling friction. The magnitude of this force is found by the formula

In this formula k - coefficient of rolling friction.

It should be noted that the physical meaning of the coefficient of sliding friction and the coefficient of rolling friction is completely different. Therefore, they cannot be compared.

After conducting these experiments, we found out that there are three types of dry friction: static friction, sliding friction, rolling friction.

It turned out that in nature there is also liquid friction, which occurs between contacting layers of liquid and gas. The resistance force arising from the movement of solid bodies in liquids and gases is also the force of liquid friction. I. Newton studied liquid friction. Liquid friction is much less than dry friction. The laws of liquid friction established by Newton are quite complex and you will learn them with further study of physics.

Let's try to understand the cause of the dry friction forces. The surfaces of the bodies in contact have roughness, most often invisible to the naked eye (Fig. 4.38).

The surface roughness of one body engages with the surface roughness of the body in contact with it, and in this case deformation occurs, an elastic force appears that prevents the relative movement of the bodies in contact. This is the force of dry friction, which, like the force of elasticity, has an electromagnetic nature.

Let us now try to explain the laws of dry friction established by Coulomb and Amonton. So, if the body lies on a horizontal table, then the roughness of both surfaces is not deformed along this surface. Therefore, the friction force between them is zero. As soon as we act on the body with a dynamometer along the table, roughness deformations will occur and a friction force will appear, equal to the dynamometer readings and oppositely directed. If at the same time the body remains at rest, then this force will be the rest friction force. With an increase in the tension force, the static friction force will also increase, because. roughness deformation increases. But, sooner or later, there will be a breakdown between the engagement of roughness, and the body will begin to move. At the moment of the beginning of the movement, the static friction force reaches its maximum value and in the future it practically does not change. The force of friction acting in the process of movement is called the force of sliding friction. Therefore, the maximum static friction force is equal to the sliding friction force.

Something else is also obvious: if the body is pressed to the surface, everything with greater strength(Fig. 4.38), then the engagement between the roughness of the contacting surfaces will increase, which will lead to an increase in the friction force. This is easy to prove empirically: with an increase in the pressure force on the body, the dynamometer readings will increase. This proves the Coulomb-Amonton law.

Fig.4.39a and b

If you place a wooden block on the table with its different faces, having different areas, and each time to move it evenly and rectilinearly with the help of a dynamometer (Fig. 4.40), then you can find that the friction force remains unchanged, i.e. the force of friction does not depend on the area of ​​the contacting surfaces. This confirms the third law of dry friction.

It is also clear that if the contact surfaces are polished, then the friction force will decrease. This is due to the fact that the size of the roughness is reduced.

It turns out that if the surfaces are polished so that their roughness (bumps, cavities) becomes commensurate with the size of atoms, then the friction force will increase sharply. This is because as the distance between atoms decreases, the electromagnetic forces of their interaction increase.

It should be noted that in the case when a body moves along a horizontal surface under the action of a force directed along this surface, then gravity will act as a normal pressure force mg . In this case, the sliding friction force will be equal to:

Friction like any physical phenomenon can be both harmful and beneficial. In the case when friction is harmful, they try to reduce it. To do this, grease is used, replacing dry friction with liquid, using a magnetic or air cushion, using ball, roller bearings or wheels, replacing sliding friction with rolling friction.

When friction is useful, they try to increase it. Sidewalks and roads are sprinkled with sand in ice, spikes are used on shoes or tires, or materials in contact with large coefficient friction, e.g. rubber materials.

It is difficult to imagine what would happen on Earth if the forces of dry friction disappeared.

Questions for self-control:

1. What force is called the force of friction?

2. How does friction force arise?

3. What is the nature of the friction force?

4. What is the difference between the force of static friction and the force of sliding friction?

5. What kind of friction is called dry?

6. What are the results of the study of dry friction by Coulomb and Amonton?

7. When does the rolling friction force occur?

8. On what factors does the coefficient of sliding friction depend?

9. How will the friction force change if we increase a) the area of ​​contact between two bodies; b) heat the body; c) sand the mating surfaces?

10. Give examples of harmful and beneficial manifestations of friction forces.

11. What kind of friction is called liquid and how does it arise?

12. Why are rubbing parts lubricated, for example, with grease?

13. Write an abstract about the research on dry friction conducted by the French physicists Sh.O. Coulomb and G. Amonton.

Municipal budgetary educational institution

"Pervomaiskaya secondary school"

Pervomaisky

Research work

"The force of friction and its beneficial features»

Completed by: Platon Alexey,

student 9 - "D" class

Supervisor:

,

Physics teacher

Pervomaisky

Tambov region

2012

1. Introduction 3

2. Research of public opinion. four

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 black ice 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 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 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.

The main objectives of 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; to think over and create demonstration experiments proving the dependence of the friction force on the force of normal pressure, on the properties of 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 important means for reducing 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 with 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 adjacent 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 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 the larger the contact area, 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 . Member of the Russian Academy of Sciences 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 force of friction 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 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 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 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 in everyday life, friction 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 was supposed to cause 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 blocks- 2 pieces; a set of cargoes; wooden board; a piece sheet metal; 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 ways surface treatments).

	1 varnished	2 wooden	3 tissue
	0.9H		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. Moscow: Higher school, 1972.

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

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