The velocity of a small ball of mass 0 . 3g and density 8g cc ^ - 1...

Let ${F}_{V}$ be the viscous force and ${F}_{B}$ be the buoyant force acting on the ball.

Then, when body moves with constant velocity,

${F}_{V} + {F}_{B} = Mg$

Or, ${F}_{V} = Mg - {F}_{B}$

{F}_{V} = {ρ}_{B}Vg - {ρ}_{L}Vg = \left({ρ}_{B} -\right)\left({ρ}_{L}\right)Vg, where, $V$ is the volume of small ball.

$= \left(8 - 1 . 3\right) × {10}^{3} × \dfrac{0 . 3 × {10}^{- 3}}{8 × {10}^{3}} × 10$

$= \dfrac{6 . 7 × 0 . 3}{8} × {10}^{- 2}$

$= \dfrac{67 × 3}{8} × {10}^{- 4} = 25 . 125 × {10}^{- 4} = 25 × {10}^{- 4}N$

Thus, the value of $x = 25$.