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 moving at speed through an unbounded Newtonian fluid with dynamic viscosity , Stokes law gives drag magnitude 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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