Friction is a phenomenon that we encounter in everyday life all the time. It is impossible to determine whether friction is harmful or beneficial. Taking even a step on slippery ice seems to be a difficult task; walking on a rough asph alt surface is a pleasure. Car parts without lubrication wear much faster.
The study of friction, knowledge of its basic properties allows a person to use it.
The force of friction in physics
The force arising from the movement or attempt of movement of one body on the surface of another, directed against the direction of movement, applied to moving bodies, is called the force of friction. The modulus of the friction force, the formula of which depends on many parameters, varies depending on the type of resistance.
The following types of friction are distinguished:
• rest;
• slip;
• rolling.
Any attempt to move a heavy object (cabinet, stone) from its place leads to the tension of a person's strength. At the same time, it is not always possible to set the object in motion. The friction of rest interferes with this.
Resting state
Calculation formula for static friction forcedoes not allow us to determine it accurately enough. By virtue of Newton's third law, the magnitude of the static resistance force depends on the applied force.
As the force increases, so does the friction force.
0 < Frest trouble < Fmax
Rest friction prevents nails driven into wood from falling out; buttons sewn with thread are firmly held in place. Interestingly, it is the resistance of rest that allows a person to walk. Moreover, it is directed in the direction of human movement, which contradicts the general state of affairs.
Sliding phenomenon
When the external force that moves the body increases to the value of the greatest static friction force, it starts to move. The force of sliding friction is considered in the process of sliding one body over the surface of another. Its value depends on the properties of the interacting surfaces and the force of the vertical action on the surface.
Calculation formula for the force of sliding friction: F=ΜP, where Μ is the coefficient of proportionality (sliding friction), P is the force of vertical (normal) pressure.
One of the driving forces is the sliding friction force, the formula of which is written using the reaction force of the support. Due to the fulfillment of Newton's third law, the forces of normal pressure and the reaction of the support are the same in magnitude and opposite in direction: Р=N.
Before you find the friction force, the formula of which takes a different form (F=M N), determine the reaction force.
The sliding resistance coefficient is introduced experimentally for two rubbing surfaces, depends on the quality of their processing and material.
Table. The value of the coefficient of resistance for various surfaces
| pp | Interacting surfaces | Value of sliding friction coefficient |
| 1 | Steel+ice | 0, 027 |
| 2 | Oak+oak | 0, 54 |
| 3 | Leather+cast iron | 0, 28 |
| 4 | Bronze+iron | 0, 19 |
| 5 | Bronze+cast iron | 0, 16 |
| 6 | Steel+steel | 0, 15 |
The largest force of static friction, the formula of which was written above, can be determined in the same way as the force of sliding friction.
This becomes important when solving problems to determine the strength of the driving resistance. For example, a book, which is moved by a hand pressed from above, slides under the action of the rest resistance force that arises between the hand and the book. The amount of resistance depends on the value of the vertical pressure force on the book.
Rolling phenomenon
The transition of our ancestors from drags to chariots is considered revolutionary. The invention of the wheel is the greatest invention of mankind. The rolling friction that occurs when a wheel moves over a surface is significantly inferior in magnitude to sliding resistance.
The emergence of rolling friction forces is associated with the forces of normal wheel pressure on the surface, has a nature that distinguishes it from sliding. Due to slight deformation of the wheel, pressure forces of different magnitude arise in the center of the formed platform and along its edges. This difference in forces determines the occurrence of rolling resistance.
The calculation formula for the rolling friction force is usually taken similarly to the sliding process. The difference is seen only in the values of the drag coefficient.
The nature of resistance
When the roughness of the rubbing surfaces changes, the value of the friction force also changes. At high magnification, two surfaces in contact look like bumps with sharp peaks. When superimposed, it is the protruding parts of the body that are in contact with each other. The total area of contact is insignificant. When moving or attempting to move bodies, the "peaks" create resistance. The magnitude of the friction force does not depend on the area of the contact surfaces.
It seems that two perfectly smooth surfaces should experience absolutely no resistance. In practice, the friction force in this case is maximum. This discrepancy is explained by the nature of the origin of forces. These are electromagnetic forces acting between the atoms of interacting bodies.
Mechanical processes that are not accompanied by friction in nature are impossible, because the ability to "turn off"there is no electrical interaction between charged bodies. The independence of the resistance forces from the mutual position of the bodies allows us to call them non-potential.
It is interesting that the friction force, the formula of which changes depending on the speed of the interacting bodies, is proportional to the square of the corresponding speed. This force includes the force of viscous resistance in a fluid.
Motion in liquid and gas
The movement of a solid body in a liquid or gas, liquid near a solid surface is accompanied by viscous resistance. Its occurrence is associated with the interaction of fluid layers entrained by a solid body in the process of movement. Different layer speeds are a source of viscous friction. The peculiarity of this phenomenon is the absence of fluid static friction. Regardless of the magnitude of the external influence, the body begins to move while in the liquid.
Depending on the speed of movement, the resistance force is determined by the speed of movement, the shape of the moving body and the viscosity of the fluid. Movement in water and oil of the same body is accompanied by resistance of different magnitude.
For low speeds: F=kv, where k is the proportionality factor depending on the linear dimensions of the body and the properties of the medium, v is the speed of the body.
The temperature of the fluid also affects the friction in it. In frosty weather, the car is warmed up so that the oil warms up (its viscosity decreases) and helps to reduce the destruction of the engine parts in contact.
Move speed up
Significant increase in the speed of the body can cause the appearance of turbulent flows, while the resistance increases dramatically. The values are: the square of the speed of movement, the density of the medium and the surface area of the body. The friction force formula takes on a different form:
F=kv2, where k is the proportionality factor depending on the shape of the body and the properties of the medium, v is the speed of the body.
If the body is streamlined, turbulence can be reduced. The body shape of dolphins and whales is a perfect example of the laws of nature that affect the speed of animals.
Energy Approach
To do the work of moving the body is hindered by the resistance of the environment. When using the law of conservation of energy, they say that the change in mechanical energy is equal to the work of friction forces.
The work of force is calculated by the formula: A=Fscosα, where F is the force under which the body moves a distance s, α is the angle between the directions of force and displacement.
Obviously, the resistance force is opposite to the movement of the body, whence cosα=-1. The work of the friction force, the formula of which is Atr=- Fs, the value is negative. In this case, mechanical energy is converted into internal energy (deformation, heating).
