# Formula Stokes Friction Frictional force    Dynamic 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* indicates how viscous a fluid or gas is in which the particle is located. The greater the viscosity, the more viscous is the fluid or gas.