Three forces act on the particle: the downward force of gravity, an upward force of buoyancy, and a drag force that acts opposite to the direction of motion of the particle. The equation relating these forces to the particle acceleration is:

,

where is the radius of the sphere; and are the densities of the fluid and the particle, respectively; is the gravitational constant; is the particle velocity; and is the drag coefficient that varies with the Reynolds number, , as follows [1]:

,

where is the viscosity in and is the particle diameter; CGS units are used throughout. The equation for is valid over the entire range of the available experimental data; use beyond is not reliable. For the equation follows the creeping flow result . You can calculate the terminal velocity, the Reynolds number, and the drag coefficient over a wide range of the variables , , , , and . The artificial values of gravity included in the calculation can be achieved particularly in space, but also on Earth.