View source for Viscous drag

If an object moves slowly through a liquid, it will experience a viscous drag which is approximately proportional to the velocity. For a sphere falling through a liquid in a velocity regime where viscosity is the dominant effect and turbulence can be neglected, the viscous drag is

<math> F_{drag} = -6\pi a \eta v</math>

where a is the radius of the sphere, η is the viscosity of the fluid the sphere is in and v is the velocity of the sphere.