Friction is a phenomenon that we encounter ineveryday life constantly. Determine whether friction is harmful or useful, impossible. To make even a step on slippery ice is hard work, on a rough surface of asphalt the walk gives pleasure. Parts of vehicles without grease wear out much faster.
The force of friction in physics
The force that occurs when moving or tryingthe motion of one body along the surface of the other, directed against the direction of motion, applied to moving bodies, is called the force of friction. The frictional force module, 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 from the place (cabinet, stone) leads to the tension of man's forces. At the same time, the subject is not always brought into motion. The friction of rest disturbs this.
State of rest
The calculated formula for the frictional force of rest does not allow us to determine it accurately enough. Due to the third law of Newton, the magnitude of the resting resistance force depends on the applied effort.
0
Friction of rest does not allow nails driven into a treeto fall out; buttons sewn with thread, firmly held in place. It is interesting that it is the resistance of rest that allows one to walk. And it is directed in the course of the movement of a person, which contradicts the general state of things.
Slip phenomenon
With an increase in the external force driving the body, up tothe value of the greatest frictional force of rest it comes into motion. The sliding friction force is considered in the process of sliding one body over the surface of the other. Its value depends on the properties of the interacting surfaces and the force of the vertical action on the surface.
The design formula for the sliding friction force is: F = μP, where μ is the proportionality coefficient (sliding friction), P is the vertical (normal) pressure force.
Before determining the frictional force, the formula of which acquires a different form (F = μ N), determine the reaction force.
The coefficient of resistance in sliding is introduced experimentally for two friction surfaces, depends on the quality of their processing and material.
Table. The value of the coefficient of resistance for different surfaces
| No. пп | Interacting surfaces | The value of the coefficient of sliding 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 friction force of rest, the formula of which was written above, can be defined in the same way as the sliding friction force.
This becomes important when solving problemsdetermination of the drag force. For example, a book that is driven by a hand, pressed from above, slides under the action of a resistance to rest, arising between the hand and the book. The magnitude of the resistance depends on the value of the force of vertical pressure on the book.
Rolling phenomenon
The transition of our ancestors from dragons to chariotsis considered revolutionary. The invention of the wheel is the greatest invention of mankind. Rolling friction, which occurs when the wheel moves along the surface, is much inferior in magnitude to the resistance to sliding.
The calculating formula for the rolling friction force is usually taken in the same way as the sliding process. The difference is seen exclusively in the values of the coefficient of resistance.
Nature of resistance
When the roughness of friction surfaces changesthe value of the frictional force also changes. With a large magnification, the two contiguous surfaces look like irregularities with sharp peaks. When superimposed, the protruding parts of the body touch each other. The total area of contact is insignificant. When moving or trying to move bodies "peaks" create resistance. The magnitude of the frictional force does not depend on the area of the contact surfaces.
It seems that two perfectly smoothSurfaces must not resist absolutely. In practice, the frictional force in this case is maximal. This discrepancy is explained by the nature of the origin of forces. These are electromagnetic forces acting between atoms of interacting bodies.
Mechanical processes not accompanied byfriction in nature, are impossible, because there is no possibility of "disconnecting" the electric interaction of charged bodies. The independence of the resistance forces from the mutual position of the bodies makes it possible to call them nonpotential.
It is interesting that the force of friction, the formula of whichvaries depending on the speed of the interacting bodies, is proportional to the square of the corresponding velocity. To such force the force of viscous resistance in a liquid concerns.
Movement in a liquid and gas
Moving a solid in a liquid or gas,liquid near a solid surface is accompanied by a viscous resistance. Its origin is associated with the interaction of layers of fluid dragged by a solid body in the process of motion. The different velocity of the layers is a source of viscous friction. The peculiarity of this phenomenon is the absence of liquid friction of rest. Regardless of the magnitude of the external impact, the body comes into motion while in the liquid.
For small velocities: F = kv, where k is a coefficient of proportionality, depending on the linear dimensions of the body and the properties of the medium, v is the velocity of the body.
The temperature of the liquid also affects the friction in it.In frosty weather, the car is warmed up so that the oil heats up (its viscosity decreases) and helps to reduce the destruction of the contacting parts of the engine.
Increase driving speed
A significant increase in the speed of the body cancause the appearance of turbulent flows, while the resistance increases sharply. Value have: the square of the speed of motion, the density of the medium and the surface area of the body. The formula for the frictional force takes on a different form:
F = kv2, where k is a coefficient of proportionality, depending on the shape of the body and the properties of the medium, v is the velocity of the body.
If the body is streamlined, turbulence can be reduced. The shape of the body of dolphins and whales is a perfect example of the laws of nature that affect the speed of animals.
Energy Approach
The work of moving the body is impeded by the resistance of the environment. When using the law of conservation of energy, it is said that the change in mechanical energy is equal to the work of frictional forces.
Obviously, the resistance force is opposite to the displacement of the body, whence cosα = -1. The work of the friction force, whose formula has the form Atr = - Fs, the value is negative. At the same time, mechanical energy is converted into internal energy (deformation, heating).
