In’viscosity of liquids and gases is the property to resist the relative shift of particles, it manifests itself, when the fluid is in motion.In’viscosity is the result of friction between contacting layers of liquid, consequently, these layers move at different speeds.

To calculate the frictional force using Newton's law. This law describes the mechanical properties of continuous media and distributed to water, the air, alcohol, most Newtonian liquids and gases

When the motion of a viscous fluid between its neighboring layers , and also between the fluid and the walls of the channel the forces of internal friction and the resulting shear stresses., directed towards, opposite to movement. This leads to different speeds of the particles in the layers thread.

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Experience shows, that the liquid, which is  directly next to the wall of the channel, adhesion to it. Hence we conclude, the speed of the fluid layers in cross section , perpendicular to bed , increases from the walls of the channel to the middle layer. 

There are generally two types of fluid motion: laminar and turbulent. When Lunardi during the liquid moves layers, one layer glides over another, but layers are not mixed together, in turbulent motion the most characteristic feature is the presence of vortices.

The nature of the movement at first glance depends on the magnitude of fluid velocity, but in reality the important role played by other factors, in particular,  in’viscosity or internal friction. This property is inherent in almost all real fluids and is the result of the interaction of molecules with each other.

At laminar motion of layers of fluid with friction slide over each other.

The stronger the cohesive forces between particles of liquid, the greater the difference of velocities of two neighboring layers. In other words, the degree of speed change the speed of the fluid layers when moving perpendicular to its motion characterizes the strength of friction between layers.