A spherical ball of radius 1mm and density 10 . 5g cc ^ - 1 is...

Viscous force on the ball when it attains constant velocity i.e., terminal velocity is,

${F}_{v} = \left(mg - {F}_{B}\right)\left(\text{As}, a = 0\right)$

Here, ${F}_{B}$ is buoyant force.

$⇒ {F}_{v} = V{ρ}_{b}g - V{ρ}_{g}g = Vg\left({ρ}_{b} - {ρ}_{g}\right) = \dfrac{4}{3}\pi{\left({10}^{- 3}\right)}^{3} \times 9 . 8\left[\left(10 . 5 - 1 . 5\right) \times {10}^{3}\right] \approx 3696 \times {10}^{- 7}N$

Thus, $x = 7$.