The equation expressing the force of viscous resistance on a smooth, rigid sphere moving in a viscous fluid under standard temperature and pressure, namely

F=3πηDV

where F is the force of viscous resistance, π = 3. 1416, η is the fluid viscosity, D is the diameter of the sphere, and V is the velocity of fall (or movement). Applying Stokes' law to gravity sedimentation as used in particle-size analysis of soil by pipette or hydrometer methods, the resulting sedimentation equation is

V=2gr2(d1-d2)/9η

where g is the acceleration of gravity, r is the "equivalent" radius of a particle, d1 is the soil-particle density, and d2 is the fluid density. Stokes' law applied to centrifugation yields still another equation for V.