# Formula: Stokes Friction Frictional force    Viscosity    Radius    Velocity

## Frictional force

Unit
The Stokes friction force decelerates the particle, i.e. it acts against its velocity. A requirement for the validity of this formula is that the size of the particle is greater than its mean free path. If this is not fulfilled, then the Cunningham correction should be used to get a more accurate result for friction force.

## Dynamic viscosity

Unit
Viscosity $$\eta$$ (pronounced: Eta) is a property of a fluid (liquid or gas) and describes how difficult it is to move a body through this fluid.

Liquid Viscosity $$\eta$$
Olive oil$$108 \cdot 10^{-3} \, \frac{\mathrm{kg}}{ \mathrm{m}\cdot \mathrm{s} }$$
Honey$$10\,000 \cdot 10^{-3} \, \frac{\mathrm{kg}}{ \mathrm{m}\cdot \mathrm{s} }$$
Glycerin$$1500 \cdot 10^{-3} \, \frac{\mathrm{kg}}{ \mathrm{m}\cdot \mathrm{s} }$$
Water$$1.008 \cdot 10^{-3} \, \frac{\mathrm{kg}}{ \mathrm{m}\cdot \mathrm{s} }$$
Tar$$100\,000 \cdot 10^{-3} \, \frac{\mathrm{kg}}{ \mathrm{m}\cdot \mathrm{s} }$$
Viscosity of some liquids.

Unit
Radius of the particle. In the formula, we assume that the particle is spherical.

## Velocity

Unit
Constant velocity of the particle directed opposite to the frictional force. The velocity can be the falling velocity, which occurs during falling due to friction.

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