Friction is a phenomenon that we encounter in everyday life constantly. Determining whether friction is harmful or beneficial is not possible. Taking even a step on slippery ice is a tough task; walking on a rough asphalt surface is a pleasure. Parts of cars without lubrication wear out 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 motion, applied to the moving bodies, is called the friction force. The friction force modulus, 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 budge a heavy object (cabinet, stone) leads to stress of human forces. At the same time, it is not always possible to bring an object into motion. The friction of rest prevents this .
Dormancy
The calculated formula of the rest friction force does not allow to determine it quite accurately. By virtue of the action of Newton’s third law, the value of the rest resistance force depends on the applied force.
With increasing effort, the friction force also increases.
0 <F tr . Rest <F max
Friction of rest does not allow nails driven into a tree to fall out; buttons sewn with threads hold firmly in place. Interestingly, it is precisely the resistance of rest that allows a person to walk. Moreover, it is directed along the course of human movement, which contradicts the general state of things.
Slip phenomenon
With an increase in the external force moving the body to the value of the greatest rest friction force, it comes into motion. The sliding friction force 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 strength of the vertical action on the surface.
The calculated formula for the sliding friction force is F = μ, where μ is the proportionality coefficient (sliding friction), and P is the vertical (normal) pressure force.
One of the forces controlling the movement 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: P = N.
Before finding the force of friction, the formula of which takes a different form (F = μ N), determine the reaction force.
The slip resistance coefficient is introduced experimentally for two rubbing surfaces, depending on the quality of their processing and material.
Table. Resistance coefficient value for various surfaces
| No pp | Interacting surfaces | The value of the coefficient of friction |
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 greatest rest friction force, the formula of which was written above, can be determined in the same way as the sliding friction force.
This becomes important in solving problems of determining the strength of the driving resistance. For example, a book, which is moved by a hand pressed from above, slides under the influence 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.
The phenomenon of rolling
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 the wheel moves on the surface is significantly inferior in magnitude to the sliding resistance.
The occurrence
of rolling friction forces is associated with the forces of normal pressure of the wheel on the surface, has a nature that distinguishes it from sliding. Due to slight deformation of the wheel, different pressure forces arise in the center of the formed area and along its edges. This difference in forces determines the occurrence of rolling resistance.
The calculated formula for the rolling friction force is usually taken similarly to the slip process. The difference is visible only in the values of the resistance coefficient.
Nature of resistance
When the roughness of the rubbing surfaces changes, the value of the friction force also changes. At high magnification, two contacting surfaces look like bumps with sharp peaks. When superimposed precisely protruding parts of the body are in contact with each other. The total area of contact is insignificant. When moving or attempting to move bodies, “peaks” create resistance. The magnitude of the friction force does not depend on the area of contact surfaces.
It seems that two perfectly smooth surfaces should not have any resistance at all. In practice, the friction force in this case is maximum. This discrepancy is explained by the nature of the emergence 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 there is no way to “turn off” the electrical interaction of charged bodies. The independence of the resistance forces from the relative position of the bodies allows us to call them non-potential.
Interestingly, the friction force, the formula of which varies depending on the speed of motion of the interacting bodies, is proportional to the square of the corresponding speed. Such a force is the force of viscous resistance in a liquid.
Movement in liquid and gas
The movement of a solid 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 carried away by a solid during movement. Different velocity of the layers is a source of viscous friction. A feature of this phenomenon is the absence of liquid friction of rest. Regardless of the magnitude of external influences, the body begins to move, being in a 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 liquid. Movement in the water and oil of the same body is accompanied by resistance of different magnitude.
For low speeds: F = kv, where k is the coefficient of proportionality, 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 is heated (its viscosity decreases) and helps to reduce the destruction of the contacting engine parts.
Speed increase
A significant increase in the speed of the body can cause the appearance of turbulent flows, while the resistance increases sharply. 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 a different form:
F = kv 2 , where k is the coefficient of proportionality, depending on the shape of the body and the properties of the medium, v is the speed of the body.
By giving the body a streamlined shape, turbulence can be reduced. The body shape of dolphins and whales is a great example of the laws of nature that affect the speed of animals.
Energy approach
The work of moving the body is impeded by environmental resistance. When using the law of conservation of energy, it is said that a 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 movement.
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 has the form A mp = - Fs, is a negative value. In this case, mechanical energy is converted into internal energy (deformation, heating).