text: The force of viscosity on a small sphere moving through a viscous fluid is given by:
  where (in SI units):
  *  is the frictional force – known as Stokes' drag – acting on the interface between the fluid and the particle (newtons, kg m s);
  *  (some authors use the symbol ) is the dynamic viscosity (Pascal-seconds, kg m s);
  *  is the radius of the spherical object (meters);
  *  is the flow velocity relative to the object (meters per second).
  Stokes' law makes the following assumptions for the behavior of a particle in a fluid:
  *Laminar flow
  *No inertial effects (zero Reynolds number)
  *Spherical particles
  *Homogeneous (uniform in composition) material
  *Smooth surfaces
  *Particles do not interfere with each other.
  Depending on desired accuracy, the failure to meet these assumptions may or may not require the use of a more complicated model. To 10% error, for instance, velocities need be limited to those giving Re < 1.
  For molecules Stokes' law is used to define their Stokes radius and diameter.
  The CGS unit of kinematic viscosity was named "stokes" after his work.