= 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.
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