Stokes' Law states that the viscous drag force ($F_d$) acting on a small spherical body of radius $r$ moving with velocity $v$ through a fluid of viscosity $\eta$ is: $F_d=6\pi \eta rv$

• The force is directly proportional to the radius ($r$) of the sphere.
• The force is directly proportional to the velocity ($v$) of the sphere. This law assumes laminar flow (low Reynolds number) and that the fluid is infinite in extent.