Newton's law of friction

The Newtonian law of friction (by Isaac Newton ) is the friction force F _r at a plate which is pulled at a uniform speed through a fluid:

${\displaystyle F_{\mathrm {r} }=A\cdot \eta \cdot {\frac {\mathrm {d} v}{\mathrm {d} y}}}$

The following applies:

A is the area with which the body rests on the fluid .

${\displaystyle \eta }$is the dynamic viscosity of the fluid.

${\displaystyle {\frac {\mathrm {d} v}{\mathrm {d} y}}}$ are the vertical velocity gradients in the fluid.

If the friction of the fluid can be described in this way, then it is a Newtonian fluid .

Explanation

The lower platen is at rest ( ) while the upper platen is moved at a constant speed . Thus, the upper liquid layer has speed and the lower layer has no speed. The resulting speed gradient does not have to be linear (as shown in the picture), which is why a differential speed gradient is defined . It is assumed that the individual liquid layers do not mix and that there is a laminar flow . ${\displaystyle v=0}$ ${\displaystyle v=v_{0}}$ ${\displaystyle v_{0}}$ ${\displaystyle {\frac {\mathrm {d} v}{\mathrm {d} y}}}$