A small glass marble falls through a column of liquid having a...

According to Stokes' law, the viscous drag force acting on a spherical body is given by:

$F_v = 6\pi\eta r v$

Substituting the given values:
$6\pi \times 10^{-4} = 6\pi \times 0.2 \times r \times 0.5$

Cancelling $6\pi$ from both sides:
$10^{-4} = 0.1 \times r$

$r = \dfrac{10^{-4}}{0.1} = 10^{-3} \text{ m} = 1 \text{ mm}$

The diameter of the marble is:
$d = 2r = 2 \times 1 \text{ mm} = 2 \text{ mm}$

Stopping at the radius leads to the incorrect answer of $1 \text{ mm}$.

Answer: $2 \text{ mm}$