We find the force of friction. The formula for the force of friction

Friction is a phenomenon that we face in everyday life all the time. Determine whether friction is harmful or useful, it is impossible. To make even a step on slippery ice is a hard task, on a rough surface of asphalt a walk is a pleasure. Parts of vehicles without grease 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 to move one body over the surface of the other, directed against the direction of motion, applied to moving bodies, is called the force of friction. The friction 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 human forces. In this case, the movement of the subject of the lead is not always obtained. The friction of rest disturbs this .

State of rest

The calculated formula for the friction force of rest does not allow to determine it sufficiently accurately. Due to the third law of Newton, the magnitude of the resting resistance force depends on the applied effort.

With increasing force, the friction force also increases.

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Friction of rest does not allow the nails driven into the wood to 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 man, which contradicts the general state of things.

Slip phenomenon

With the increase in the external force driving the body, up to the value of the greatest friction 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 frictional force of sliding: F = μР, where μ is the proportionality coefficient (sliding friction), Р is the force of vertical (normal) pressure.

One of the driving forces is the sliding friction force, the formula of which is recorded 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 equal in magnitude and opposite in direction: P = N.

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 rubbing surfaces, depends on the quality of their processing and material.

Table. The value of the coefficient of resistance for different surfaces

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 in solving the problems of determining the force of the moving resistance. 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 vertical pressure force on the book.

The phenomenon of rolling

The transition of our ancestors from dragons to chariots is 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 significantly inferior in magnitude to the resistance to sliding.

The appearance of frictional forces is associated with the forces of normal wheel pressure on the surface, has a nature that distinguishes it from sliding. Due to the slight deformation of the wheel, different pressure forces occur at the center of the formed area and along its edges. This difference in forces determines the occurrence of resistance during rolling.

The calculation formula for the rolling friction force is usually taken in the same way as the sliding process. The difference is seen only in the values of the coefficient of resistance.

Nature of resistance

When the roughness of rubbing surfaces changes, the 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 contact area 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 smooth surfaces should not experience any resistance at all. 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 that are not accompanied by friction 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 frictional force, whose formula varies depending on the speed of the interacting bodies, is proportional to the square of the corresponding velocity. This force is due to the viscous drag force in the liquid.

Movement in a liquid and gas

The displacement of 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.

Depending on the speed of movement, the resistance force is determined by the speed of motion, the shape of the moving body and the viscosity of the fluid. Movement in water and oil of the same body is accompanied by different in great resistance.

For small velocities: F = kv, where k is the proportionality coefficient, which depends 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 velocity of the body can cause 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 = kv ² , 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.

The work force is calculated by the formula: A = Fscosα, where F is the force by which the body moves by a distance s, α is the angle between the directions of force and displacement.

Obviously, the resistance force is opposite to the displacement of the body, whence cosα = -1. The work of the frictional force, the formula of which has the form A _m = - Fs, is negative. At the same time, mechanical energy is converted into internal energy (deformation, heating).