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Q. A small spherical ball falling through a viscous medium of negligible density has terminal velocity $v$. Another ball of the same mass but of radius twice that of the earlier falling through the same viscous medium will have terminal velocity

Mechanical Properties of Fluids

Solution:

If $\rho$ is the density of the ball and $\rho^{\prime}$ that of the another ball, $m$ for the balls are the same, but $r^{\prime}=2 r$
$\therefore m g=6 \pi r \eta v$ (by Stoke's law)
or, $ 6 \pi r \eta v=6 \pi 2 m v^{\prime}$ So, $v^{\prime}=\frac{v}{2}$