Viscous drag

Viscous drag is resistance to relative motion arising from viscosity. It transfers momentum between a moving object and its surrounding fluid, with mechanical energy dissipated as heat.

For a rigid sphere of radius $a$ moving at speed $U$ through an unbounded Newtonian fluid with dynamic viscosity $\mu$, Stokes law gives drag magnitude $F=6\pi \mu aU$ in the creeping-flow limit. The force opposes relative motion. This linear law requires a small Reynolds number; a quadratic drag law appropriate to other flow regimes cannot be substituted without checking the assumptions.