The coefficient of friction when sliding and rolling

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In terrestrial conditions, any moving body (or coming in motion) contact with the substance of the environment or with other bodies.This gives rise to forces that resist their motion.These forces are called frictional forces, they translate the motion of the mechanical energy into internal energy, which is accompanied by heating the body and the environment.

friction is internal and external.Internal (otherwise called viscosity) is the occurrence of tangential forces between the moving layers of liquid or gas, stop this movement.

In contrast, sliding friction occurs at the contact of solids in the form of force tangential to the surface and impeding their mutual displacement.It, in turn, is divided into static (static friction) and kinematic.Static friction appears when you try to move one body relative to another fixed.Kinematic exists between moving bodies in contact with each other.External friction can be divided into sliding and rolling friction.

What is the physical meaning of friction?It is helpful or harmful?At first glance, the friction only prevents us wear parts mechanisms, car tires, shoe soles are erased, and so on. D. And the creation of perpetual motion is impossible for that reason alone.But look closely closely.Friction disappear - we can neither walk nor flipping book or pull away the car or stop moving.A huge number of physical phenomena in the world based on friction.The two main achievements of mankind, determined the development of civilization - production of fire and the invention of the wheel - would be impossible without it.

Based on this phenomenon any irregularities bodies: one notch in contact always cling to the roughness of the other.For perfectly smooth (for example, carefully polished) surfaces, tightly adjacent to each other, the laws of molecular friction, based on the mutual attraction of molecules.

Learning science friction tribology.In 1781, French physicist C. Coulomb formulated the basic laws of dry friction.Empirically, scientists have found that the friction force F, the sliding occurs, is directly proportional to the force acting on a body N normal pressure.This relationship is as follows:

N: F = k ∙ N;

where the value of k - coefficient of friction (the coefficient of proportionality).Its value has been calculated as follows: The body is placed on the inclined plane and by changing the angle of inclination achieved its uniform motion.This friction force F is equal to the driving force P:

F = P ∙ sin a;

The force N (forces of normal pressure) is equal to P ∙ cos a;consequently k = tg a.The coefficient of friction is here the slope surface along which slides a body uniformly, ie. E. At a constant speed.

In practice, its value can be calculated only approximately.Body surface, usually in varying degrees contaminated are oxides, rust and other impurities.The friction coefficient is determined for pairwise combinations of various materials by experiments, brought in special reference tables.

When rolling friction arises because moving the wheel lightly pressed into the surface of the road, t. E. Have to overcome a small hillock.The harder the road, the less the bump and less friction force.Its value is calculated in this case by the formula: F = k ∙ N / r, in which r - the radius of the wheel.Consequently, the coefficient of rolling friction has a length dimension.Typically, it is expressed in centimeters as opposed to the sliding friction coefficient, is a dimensionless quantity.

As mentioned above, the internal friction coefficient exists not only to solids but also to liquids.The hydraulic system is often required to calculate the specific energy loss of the hydraulic system, resulting in tubing.They come in two types: actual losses that occur in a straight pipe with a uniform flow, and local loss which reason - flow deformation due to changes in channel shape (contraction, expansion, rotation).Hydraulic losses are calculated using the same value, which is called the "coefficient of hydraulic friction."