2. Motion of a solid body in a fluid. Equivalence criterion

Gravimetric concentration is possible thanks to the differences in particle motion in a fluid as a function of the simultaneous action of gravity (or centrifugal force) and one or more other forces, generally resistance to penetration in a more or less fluid medium (gas, water, homogeneous liquid or mixture composed of a fluid and solid particles held in suspension). The motion of a spherical particle in a fluid of low concentration is governed by the free sedimentation equation: $m=\frac{\text{d}v}{\text{d}t}=\frac{\text{π}{D}^3}{6}\gamma \left({\rho }_s-{\rho }_f\right)-\Phi \left(Re\right)\frac{\text{π}{D}^2}{4}{\rho }_f\frac{v^2}{2}$