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Terminal velocity
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Two small spherical metal balls, having equal masses, are made from materials of densities $\rho_1$ and $\rho_2$ ($\rho_1=8\rho_2$) and have radii of 1 mm and 2 mm, respectively. They are made to fall vertically (from rest) in a viscous medium whose coefficient of viscosity equals $\eta$ and whose density is $0.1\rho_2$. The ratio of their terminal velocities would be:$v_T=\frac{2r^2g(\rho-\sigma)}{9\eta}$. $\frac{v_1}{v_2}=\left(\frac{1}{2}\right)^2\frac{8\rho_2-0.1\rho_2}{\rho_2-0.1\rho_2}=\frac{1}{4}\times\frac{7.9}{0.9}=\frac{79}{36}$.