Friction in the life of plants and animals. "The force of friction in our lives

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

What is friction force?

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

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

Types of friction forces

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

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

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

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

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

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

What is the nature of friction

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

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

This means that there are two reasons causing this phenomenon:

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

What does friction force depend on?

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

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

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

Ftr = kN Where:

Ftr - friction force.

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

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

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

Work of friction force

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

Is friction friend or foe?

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

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

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

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

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

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

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

Is there friction in liquids?

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

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

Designers give the same streamlining to submarines.

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

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>>Physics: The role of friction forces

Another type of force dealt with in mechanics is frictional forces. These forces act along the surface of bodies when they are in direct contact.
Friction forces in all cases prevent the relative motion of contacting bodies. Under certain conditions, friction forces make this movement impossible. However, friction forces not only slow down the movement of bodies. In a number of practically important cases, the movement of a body could not occur without the action of friction forces.
The importance of friction forces can be seen using the example of a moving car ( Fig.4.13). The friction force acting from the Earth's surface on the driven wheels and the air resistance force are directed backward and can only slow down the movement. The only external force that can increase the speed of a car is friction acting on the drive wheels. If this force did not exist, the car would slip in place, despite the rotation of the drive wheels.

In the same way, the frictional force acting on the soles of our feet imparts to our body the acceleration necessary to start or stop moving.
The work of the engine that drives the drive wheels and the efforts of the leg muscles cause friction forces to appear.
By preventing slippage, the force of friction does a useful job, accelerating the car or our own body. But without effort from the engine or leg muscles, increasing speed due to friction is impossible.
Thus, on the one hand, it is necessary to take all measures to reduce the frictional forces that impede movement, lubricating the rubbing parts of the engine and giving the car a shape in which air resistance is minimal, and on the other hand, it is necessary to increase useful friction by sprinkling, for example, the road sand in icy conditions.
Friction forces depend on the state of the rubbing solid surfaces, and when a solid body moves in water or air, on the relative speed of movement, on the size and shape of this body.
Friction is a phenomenon that accompanies us everywhere and everywhere. In some cases it is useful, and we try in every possible way to increase it. In others it is harmful, and we lead with it

???
1. Look around you. Do you see the beneficial effect of friction forces?
2. Why are notches made on the jaws of a vice and pliers?
3. Why do car tires have a relief pattern (tread)?

G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics 10th grade

If you have corrections or suggestions for this lesson,

All movements of bodies in contact relative to each other always occur with friction: the wheel axle experiences friction in the bearing, and its rim experiences friction on the rail; the door opens with a creaking sound, indicating friction in the hinges; a ball rolling on a horizontal table stops under the influence of rolling friction forces. When we study the movement of a body and exclude friction from consideration, then, while simplifying the problem, we at the same time, to one degree or another, distort the actual state of affairs. In all the experiments that we cited to illustrate the laws of motion, we assumed that there was no friction. In reality, friction forces always influence to a greater or lesser extent the nature of movement.

The role of friction is not always limited to inhibiting the movements of bodies. In many cases, movement, such as walking, becomes possible only due to the action of friction forces, in particular static friction. When walking, we place our feet on the ground in such a way that they would have to slide backwards if the static friction force did not exist (indeed, when we try to walk on smooth ice, our feet slide backwards). Since the static friction force acts in the direction opposite to the one in which sliding should occur, a static friction force appears directed forward. It imparts forward acceleration to the human body.

The situation is approximately the same in all self-propelled vehicles (bicycle, car, electric locomotive). The carriage engine causes the drive wheels to rotate. If there were no static friction force, then the carriage would remain in place and the wheels would begin to slip, so that the points of the wheel that were currently touching the ground or rails would slip backwards. The resulting static friction force acting on the wheels from the ground is directed forward and imparts acceleration to the vehicle or, by balancing other forces acting on the vehicle, maintains its uniform motion. If this friction force is insufficient (for example, on ice), then the crew does not move and the wheels slip. On the contrary, if a moving carriage whose wheels are rotating slows down the rotation of the wheels without slowing down the speed of the carriage itself, then in the absence of frictional forces the wheels would begin to slide forward along the ground; This means that in reality there is a frictional force directed backwards. The action of the brakes is based on this.

If a train is attached to an electric locomotive, then as soon as the electric locomotive moves forward, the coupling will stretch and an elastic force of the coupling will arise, which will act on the train: this is the traction force. If you increase the force acting on the wheels from the engine, the static friction force will also increase, and therefore the traction force. The greatest traction force is equal to the greatest static friction force of the drive wheels. With a further increase in forces from the engine, the wheels will begin to slip and traction may even decrease.

An equally important role is played by static friction forces in non-self-propelled vehicles. Let's take a closer look at the movement of a horse pulling a sleigh (Fig. 72). The horse places its legs and tenses its muscles in such a way that, in the absence of resting friction forces, the legs would slide backwards. In this case, static friction forces arise, directed forward. On a sled, which the horse pulls forward through the tracks with force, a sliding friction force directed backwards acts from the ground. In order for the horse and sleigh to gain acceleration, it is necessary that the friction force of the horse's hooves on the road surface be greater than the friction force acting on the sleigh. However, no matter how great the coefficient of friction of the horseshoes on the ground, the force of static friction cannot be greater than the force that should have caused the hooves to slide (§ 64), that is, the force of the horse’s muscles. Therefore, even when the horse’s legs do not slide, he still sometimes cannot move the heavy sleigh. When moving (when sliding begins), the friction force decreases slightly; therefore, it is often enough just to help the horse move the sleigh so that he can then carry it.

66.1. Explain the role of friction forces in transmitting motion from one pulley to another through a drive belt.

It is worth mentioning the role of friction in nature. An example is the rough legs of insects to improve traction with the surface, or, conversely, the smooth bodies of fish, covered with mucus to reduce friction with the water.

In nature, animals and plants have long learned to adapt and use the force of friction to their advantage. A person must do the same in order to ensure a comfortable existence on planet Earth.

More examples of friction force in nature:

· we can walk on earth

· squirrels jumping on tree branches

· sloth hanging on a branch

· the bird can sit on a branch

water wears away stone

formation of planets and comets

· it rains and water flows into the depression, although the stone lies and does not roll into the depression (the friction force of water is less than that of the stone)

· huge boulders lie on the edges of the rocks and do not fall down - they are held by the force of friction

Friction force is the force that occurs when one body moves over the surface of another. It is always directed opposite to the direction of movement. The friction force is directly proportional to the force of normal pressure on the rubbing surfaces and depends on the properties of these surfaces. The laws of friction are related to the electromagnetic interaction that exists between bodies.

Distinguish between friction external And internal.

External friction occurs when there is relative movement of two solid bodies in contact (sliding friction or static friction).

Internal friction observed during relative movement of parts of the same solid body (for example, liquid or gas).

Distinguish dry and liquid (or viscous) friction.

Dry friction occurs between the surfaces of solids in the absence of lubrication.

Liquid(viscous) is the friction between a solid and a liquid or gaseous medium or its layers.

Dry friction, in turn, is divided into friction slip and friction rolling.

Let's consider the laws of dry friction (Fig. 4.5).

Rice. 4.5

Rice. 4.6

Let us act on a body lying on a stationary plane with an external force, gradually increasing its modulus. At first, the block will remain motionless, which means that the external force is balanced by some force directed tangentially to the rubbing surface, opposite to the force. In this case, there is the static friction force.

It has been established that the maximum static friction force does not depend on the area of ​​contact between the bodies and is approximately proportional to the modulus normal pressure forces N:

μ 0 – static friction coefficient, depending on the nature and condition of the rubbing surfaces.

When the module of the external force, and therefore the module of the static friction force, exceeds the value F 0, the body will begin to slide along the support - static friction F friction will be replaced by sliding friction F sk (Fig. 4.6):

where μ is the sliding friction coefficient.

Rolling friction occurs between a spherical body and the surface on which it rolls. The rolling friction force obeys the same laws as the sliding friction force, but the friction coefficient is μ; there is much less here.

Let's take a closer look at the sliding friction force on an inclined plane (Fig. 4.7).

A body located on an inclined plane with dry friction is subject to three forces: gravity, normal support reaction force and dry friction force. The force is the resultant of the forces and ; it is directed downwards along the inclined plane. From Fig. 4.7 it is clear that

F = mg sin α, N = mg cos α.

Rice. 4.7

If – the body remains motionless on an inclined plane. The maximum angle of inclination α is determined from the condition ( F tr) max = F or μ mg cosα = mg sinα, therefore, tan α max = μ, where μ is the dry friction coefficient.

F tr = μ N = mg cosα,
F = mg sinα.

When α > α max the body will roll with acceleration

a = g(sinα - μcosα),
F ck = ma = F-F tr.

If extra power F external force directed along the inclined plane is applied to the body, then the critical angle α max and the acceleration of the body will depend on the magnitude and direction of this external force.

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

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

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

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

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

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

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

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

11 question. Rotation of a rigid body about a fixed axis. Moment of inertia. Steiner's theorem. Basic equation of rotational motion. Kinetic energy of a rotating body. Calculation of the moment of inertia about a fixed axis. Rotational motion of a rigid body relative to a fixed axis. Basic equation for the dynamics of rotational motion. Kinetic energy of a body during rotational motion.

Have you tried driving a car in icy conditions? The pleasure is not pleasant. The same, however, as being a pedestrian at the same time of year. When the road is covered with a crust of ice, we say: poor grip. What does this mean?

This means that there is very little friction between the wheels and the road. And if this is useful in the case of moving loads by dragging, for example, on a sled, then it is very harmful in a situation where it is necessary to sharply brake or change the direction of movement. The role of friction in human life is enormous, this cannot be denied. And our task comes down to use friction force as efficiently as possible in everyday life and in technology to make life easier.

The role of friction in everyday life

The role of friction in everyday life comes down to the fact that we can walk and drive, that objects do not slip out of our hands, that shelves and pictures hang on the walls and do not fall, we even wear clothes thanks to friction, which holds the fibers in the threads , and threads in the structure of fabrics. But friction can also play a negative role. It is because of this that the moving parts of various mechanisms heat up and wear out. In such cases, they try to reduce it. There are several ways to reduce friction. One of them is the introduction of lubricant between the rubbing surfaces. Lubrication reduces the contact of bodies, and it is not the bodies that rub, but the layers of liquid. And friction in liquid is much less than dry friction.

Friction force in technology

Another way to reduce friction is to use ball and roller bearings. The inner ring of the bearing is placed on the shaft of a mechanism, and the outer ring is fixed in the body of the machine or machine tool. And when the shaft begins to rotate, it does not slide, but rolls on balls or rollers between the bearing rings. And we know that the rolling friction force is much less than sliding friction. Therefore, rotating parts wear out much more slowly. An air cushion, a reduction in the area of ​​contacting bodies, and grinding are also used. For example, to reduce the friction between ice and skates, skates are sharpened, making the contact surface smaller, and the ice is ground, making it as smooth as possible. They also reduce friction when cutting anything at home and at work, sharpening the knives as sharply as possible. The role of friction in technology is not always negative, as it might seem. After all, for example, when we replace the force of sliding friction with rolling friction in order to reduce the interaction of rubbing surfaces, we should remember that if there were no friction at all, then the wheels or balls in the bearings would simply spin without setting the body in motion.

The role of friction in nature

It is worth mentioning the role of friction in nature. An example is the rough legs of insects to improve traction with the surface, or, conversely, the smooth bodies of fish, covered with mucus to reduce friction with the water. In nature, animals and plants have long learned to adapt and use the force of friction to their advantage. A person must do the same in order to ensure a comfortable existence on planet Earth.