\frac{4}{3} \cdot π R^{3} ρ g = k π R^{2} v^{2} \Rightarrow v = \sqrt{\frac{4 ρ R g}{3 k}}

${\frac{4}{3}\cdot\pi R^3\rho g=k\pi R^2v^2 \quad\Rightarrow\quad v= \sqrt{\frac{4\rho Rg}{3k}}}$

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