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).
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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):
| F tr = μ N, | (4.4.1) |
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 |
– 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 sk = 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.
Friction force (Ftr.) is a force that arises when the surfaces of two bodies come into contact and prevents their relative movement. It appears due to electromagnetic forces generated by atoms and molecules at the point of contact of these two objects.
To stop a moving object, the force must act in the opposite direction to the direction of movement. For example, if you push a book across a table, it will start moving. The force you apply to the book will move it. The book slides, then slows down and stops due to friction.
Features of friction forces
The friction mentioned above, which appears when objects move, is called external or dry. But it can also exist between parts or layers of one object (liquid or gaseous); this type is called internal.
The main feature is the dependence of friction on the speed of relative motion of bodies.
There are other characteristic features:
- occurrence when two moving bodies come into contact with surfaces;
- its action is parallel to the area of contact;
- directed opposite to the body velocity vector;
- depends on the quality of surfaces (smooth or rough) and interacting objects;
- The shape or size of an object moving in a gas or liquid affects the magnitude of the frictional force.
Types of friction
There are several types. Let's look at their differences. A book sliding on a table is subject to sliding friction.
Sliding friction force
Where N is the ground reaction force.
Please note some situations:
If a person rides a bicycle, then the friction that occurs during contact of the wheel with the road is rolling friction. This type of force is significantly less than the sliding friction force.
Rolling friction force
Significantly smaller values of this type of force are used by people using wheels, rollers and ball bearings in various moving parts of devices.
Charles Augustin Coulomb, in his work on the theory of friction, proposed calculating the rolling friction force as follows:
,
μ - friction coefficient.
Lubricant, most often in the form of a thin layer of liquid, reduces friction.
Liquids or gases are special media in which this type of force also manifests itself. In these environments, friction occurs only when the object is moving. It is impossible to talk about the force of static friction in these media.
Friction force in liquids and gases
This type of force is called the resistance force of the medium. It slows down the movement of an object. The more streamlined shape of the object affects the magnitude of the drag force - it decreases significantly. Therefore, in shipbuilding, streamlined hulls of ships or submarines are used.
The resistance force of the medium depends on:
- geometric dimensions and shape of the object;
- viscosity of a liquid or gaseous medium;
- state of the object's surface;
- the speed of an object relative to the environment in which it is located.
Bodies interact with each other in different ways. One type of interaction is friction. Before we understand the intricacies of dry and viscous friction, we will answer two questions. What is friction force and when does it occur?
What is friction force?
Friction force is a force that arises when bodies come into contact and impedes their relative motion.
Friction occurs due to the interaction between atoms and molecules of bodies when they come into contact with each other.
The nature of friction force is electromagnetic.
As with any other interaction, Newton's third law is true for friction. If a frictional force acts on one of two interacting bodies, then a force of the same magnitude acts on the other body in the opposite direction.
There are dry and viscous friction, static friction force, sliding friction force, rolling friction force.
Dry friction is friction that occurs between solid bodies in the absence of a liquid or gaseous layer between them. The friction force is directed tangentially to the contacting surfaces.
Let's imagine that a body, for example, a block lying on a table, is acted upon by some external force. This force tends to move the block from its place. While the bodies are at rest, the static friction force and, in fact, the external force act on the block. The static friction force is equal to the external force and balances it.
When the external force exceeds a certain limiting value F t r. m a x , the block moves from its place. It is also acted upon by a friction force, but this is no longer a static friction force, but a sliding friction force. The sliding friction force is directed in the direction opposite to the movement and depends on the speed of the body.
When solving physical problems, the sliding friction force is often taken equal to the maximum static friction force, and the dependence of the friction force on the relative speed of motion of bodies is neglected.
The figure above shows the real and idealized characteristics of dry friction. As we can see, in fact, the sliding friction force changes depending on the speed, but the changes are not so great that they cannot be neglected.
The friction force is proportional to the force of the normal reaction of the support.
F t r = F t r. m a x = μ N .
What is the coefficient of sliding friction?
μ is a proportionality coefficient, which is called the sliding friction coefficient. It depends on the materials of the contacting bodies and their properties. The sliding friction coefficient is a dimensionless quantity not exceeding unity.
Rolling friction forces arise when bodies roll. They are usually neglected when solving problems.
Viscous friction in liquids and gases
Viscous friction occurs when bodies move in liquids and gases. The viscous friction force is also directed in the direction opposite to the movement of the body, but its magnitude is much less than the sliding friction force. There is no static friction in viscous friction.
Calculating the viscous friction force is more complex than calculating the sliding friction force. At low speeds of movement of a body in a liquid, the force of viscous friction is proportional to the speed of the body, and at high speeds - to the square of the speed. In this case, the proportionality coefficients depend on the shape of the bodies; it is also necessary to take into account the properties of the medium itself in which the movement occurs.
For example, the viscous friction forces in water and oil will be different, since these liquids have different viscosities.
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« Physics - 10th grade"
Remember what friction is.
What factors is it due to?
Why does the speed of movement of the block on the table change after a push?
Another type of force dealt with in mechanics is frictional forces. These forces act along the surfaces 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, they 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.
Friction that occurs during relative movement of the contacting surfaces of solid bodies is called dry friction.
There are three types of dry friction: static friction, sliding friction and rolling friction.
Rest friction.
Try moving a thick book lying on the table with your finger. You apply some force to it, directed along the surface of the table, and the book remains at rest. Consequently, a force arises between the book and the surface of the table, directed opposite to the force with which you act on the book, and exactly equal to it in magnitude. This is the friction force tr. You push the book with more force, but it still stays in place. This means that the friction force tr increases by the same amount.
The frictional force acting between two bodies stationary relative to each other is called force static friction.
If a body is acted upon by a force parallel to the surface on which it is located, and the body remains motionless, this means that it is acted upon by a static friction force tr, equal in magnitude and directed in the opposite direction to the force (Fig. 3.22). Consequently, the force of static friction is determined by the force acting on it:
If the force acting on a body at rest even slightly exceeds the maximum force of static friction, then the body will begin to slide.
The greatest value of the friction force, at which sliding does not yet occur, is called maximum static friction force.
To determine the maximum static friction force, there is a very simple, but not very accurate quantitative law. Let there be a block on the table with a dynamometer attached to it. Let's conduct the first experiment. Let's pull the dynamometer ring and determine the maximum static friction force. The block is acted upon by the force of gravity m, the normal reaction force of the support 1, the tension force 1, the dynamometer springs and the maximum static friction force tr1 (Fig. 3.23).
Let's place another similar block on the block. The force of pressure of the bars on the table will increase by 2 times. According to Newton's third law, the normal reaction force of support 2 will also increase by 2 times. If we measure the maximum static friction force again, we will see that it has increased as many times as the force 2 has increased, i.e. 2 times.
Continuing to increase the number of bars and measuring each time the maximum force of static friction, we will be convinced that
>the maximum value of the modulus of the static friction force is proportional to the modulus of the normal reaction force of the support.
If we denote the module of the maximum static friction force by F tr. max, then we can write:
F tr. max = μN (3.11)
where μ is a proportionality coefficient called the friction coefficient. The friction coefficient characterizes both rubbing surfaces and depends not only on the material of these surfaces, but also on the quality of their processing. The friction coefficient is determined experimentally.
This dependence was first established by the French physicist C. Coulomb.
If you place the block on the smaller face, then F tr. max will not change.
The maximum static friction force does not depend on the area of contact between the bodies.
The static friction force varies from zero to a maximum value equal to μN. What can cause a change in the friction force?
The point here is this. When a certain force is applied to a body, it shifts slightly (imperceptibly to the eye), and this displacement continues until the microscopic roughness of the surfaces are positioned relative to each other in such a way that, hooking on one another, they will lead to the appearance of a force that balances the force. As the force increases, the body will again move slightly so that the smallest surface irregularities will cling to each other differently, and the friction force will increase.
And only at > F tr. max, no matter the relative position of the surface roughnesses, the friction force is not able to balance the force , and sliding will begin.
The dependence of the sliding friction force modulus on the acting force modulus is shown in Figure 3.24.
When walking and running, the soles of the feet are subject to static friction unless the feet slip. The same force acts on the drive wheels of the car. The driven wheels are also acted upon by a static friction force, but this time braking the movement, and this force is significantly less than the force acting on the drive wheels (otherwise the car would not be able to move).
For a long time, it was doubted that a steam locomotive could run on smooth rails. They thought that the friction braking the driven wheels would be equal to the friction force acting on the driving wheels. It was even proposed to make the drive wheels geared and lay special geared rails for them.
Sliding friction.
When sliding, the friction force depends not only on the state of the rubbing surfaces, but also on the relative speed of the bodies, and this dependence on speed is quite complex. Experience shows that often (though not always) at the very beginning of sliding, when the relative speed is still low, the friction force becomes somewhat less than the maximum static friction force. Only then, as the speed increases, does it grow and begin to exceed F tr. max.
You've probably noticed that a heavy object, such as a box, is difficult to move, but then moving it becomes easier. This is precisely explained by the decrease in friction force when sliding occurs at low speed (see Fig. 3.24).
At not too high relative speeds of movement, the sliding friction force differs little from the maximum static friction force. Therefore, it can be approximately considered constant and equal to the maximum static friction force:
F tr ≈ F tr. max = μN.
The force of sliding friction can be reduced many times by using a lubricant - most often a thin layer of liquid (usually some type of mineral oil) - between the rubbing surfaces.
Not a single modern machine, such as a car or tractor engine, can operate without lubrication. A special lubrication system is provided for in the design of all machines.
The friction between layers of liquid adjacent to solid surfaces is much less than between dry surfaces.
Rolling friction.
The rolling friction force is significantly less than the sliding friction force, so it is much easier to roll a heavy object than to move it.
The friction force depends on the relative speed of the bodies. This is its main difference from the forces of gravity and elasticity, which depend only on distances.
Resistance forces during the movement of solid bodies in liquids and gases.
When a solid body moves in a liquid or gas, it is acted upon by the drag force of the medium. This force is directed against the speed of the body relative to the medium and slows down the movement.
The main feature of the drag force is that it appears only in the presence of relative motion of the body and the environment.
The force of static friction in liquids and gases is completely absent.
This leads to the fact that with the effort of your hands you can move a heavy body, for example, a floating boat, while moving, say, a train with your hands is simply impossible.
The modulus of the resistance force F c depends on the size, shape and state of the surface of the body, the properties of the medium (liquid or gas) in which the body moves, and, finally, on the relative speed of movement of the body and the medium.
The approximate nature of the dependence of the modulus of the resistance force on the modulus of the relative velocity of the body is shown in Figure 3.25. At a relative speed equal to zero, the drag force does not act on the body (F c = 0). As the relative speed increases, the drag force grows slowly at first, and then faster and faster. At low speeds of movement, the resistance force can be considered directly proportional to the speed of movement of the body relative to the medium:
F c = k 1 υ, (3.12)
where k 1 is the resistance coefficient, depending on the shape, size, state of the surface of the body and the properties of the medium - its viscosity. It is not possible to calculate the coefficient k 1 theoretically for bodies of any complex shape; it is determined experimentally.
At high speeds of relative motion, the drag force is proportional to the square of the speed:
F c = k 2 υ 2 , υ, (3.13)
where k 2 is the resistance coefficient different from k 1 .
Which of the formulas - (3 12) or (3.13) - can be used in a particular case is determined experimentally. For example, for a passenger car, it is advisable to use the first formula at approximately 60-80 km/h; at higher speeds, the second formula should be used.
Thanks to this force, cars slow down at traffic lights, a boat stops in the water, and a wheel slips in a hole. As you already understand, in this article we will figure out how to solve problems on friction force.
The friction force is electromagnetic in nature. This means that this force is manifested as a result of the interaction of the particles that make up the substance.
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What you need to know about friction force to solve problems
Friction is one of the types of interaction between bodies that occurs when they come into contact.
The friction force is always directed in the direction opposite to the movement and tangential to the contacting surfaces. Dry friction occurs between solid bodies, and when bodies move in liquids or gases they speak of viscous friction.
We have already established the nature of this force. In addition, you need to know that there are different types of friction forces:
- static friction;
- sliding friction;
- rolling friction (when bodies roll over each other);
- resistance of the medium (for movement in a liquid).
Here is an example of the types of friction force: the block lies on the table and no one touches it. In this case, only gravity and the normal ground reaction force act. If we start pushing the block, but so hard as to move it, it will be acted upon by a static friction force, which, according to Newton’s third law, is equal to the external force applied to the block. The static friction force has a limiting value. If the external force is greater than this value, the block will begin to slide along the table. In this case, they talk about the sliding friction force. And here is the simplest formula for the friction force:
“Mu” is the coefficient of sliding friction. This is a dimensionless quantity that depends on the materials of the interacting bodies and the quality of their surfaces. The friction coefficient does not exceed unity.
When solving simple physical problems, the sliding friction force is often taken to be equal to the maximum static friction force.
Questions on the topic “Friction Force”
Question 1. What does friction force depend on?
Answer. Let's take a look at the formula above and the answer will come to you. The friction force depends on the properties of the contacting bodies, the force of the normal reaction of the support, and the speed of the relative movement of the bodies.
Question 2. Does the force of friction depend on the area of contacting surfaces?
Answer. No, area does not affect the force of friction.
Question 3. In what ways can you reduce or increase the force of friction?
Answer. You can reduce the coefficient of friction by making dry friction viscous. To increase the friction force, it is necessary to increase the pressure on them.
Question 4. The body is at rest on a plane. Does friction force act on it?
Answer. If no external forces act on the body, then the static friction force, according to Newton’s third law, is equal to zero.
Question 5. Which of these forces is the largest in magnitude: static friction force, rolling friction force or sliding friction force?
Answer. The sliding friction force is of greatest importance.
Question 6. What are some examples of the beneficial effects of friction?
Answer. Among the useful uses of friction force, one can highlight the operation of vehicle brakes and the production of fire by primitive people.
Friction problems with solutions
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Task No. 1. Finding the friction force
Condition
A block of mass 5 kilograms slides along a horizontal surface. The sliding friction force is 20 N. Find the friction force if the mass of the block is halved and the friction coefficient remains unchanged.
Solution
Let's apply the formulas:
Answer: 10 N.
Task No. 2. Finding the coefficient of friction
Condition
A body slides along a horizontal plane. Find the coefficient of friction if the friction force is 5 N and the pressure force of the body on the plane is 20 N.
Solution
The force of body pressure on the plane is equal to the force of the normal support reaction.
Answer: 0,25
Task No. 3. Finding the friction force and friction coefficient
Condition
A skier weighing 60 kg, having a speed of 10 m/s at the end of the descent, stops 40 s after the end of the descent. Determine the friction force and friction coefficient.
Solution
First, let's find the acceleration with which the skier is moving. Then, using Newton’s second law, we find the force that acts on it:
Answer: 15 N; 0.025.
Task No. 4. Finding the friction force
Condition
A block with a mass of 20 kg moves uniformly along a horizontal surface under the action of a constant force directed at an angle of 30° to the surface and equal to 75 N. What is the coefficient of friction between the block and the plane?
Solution
First, let's use Newton's second law, given that the acceleration is zero. Then we find the projections of force on the vertical and horizontal axes:
Answer: 0,4
Task No. 5. Finding the static friction force
Condition
A box with a mass of 10 kg stands on a horizontal floor. The coefficient of friction between the floor and the box is 0.25. A force of 16 N is applied to the box in the horizontal direction. Will it move? What is the force of friction between the box and the floor?
Solution
Let's calculate the maximum static friction force:
Since the applied force is by condition less than the maximum static friction force, the box will remain in place. The friction force between the floor and the box, according to Newton's third law, is equal to the applied force.
Answer: 16 N.
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