The drag force on an obstacle is commonly written as

(1)

where is the drag coefficient, is the density of the medium, v is the flow speed, and A is the cross-sectional area. This law is valid when , so the resistive force is dominated by inertia.

For neutral atoms,

(2)

where T is the temperature,

(3)

is the binary diffusion coefficient, D is the diffusion coefficient, and n is the number density.

Drag, Drag Coefficient, Spin-Down Time