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A small ball of mass $M$, radius $R$ and density $\rho$ moves with terminal velocity through a container filled with glycerine of density $\sigma$. The viscous force acting on the ball is ( $g=$ gravitational acceleration)
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The correct answer is:
$M g\left[1-\frac{\sigma}{\rho}\right]$
Considering force balance on the ball falling with terminal velocity:
$F_v+\sigma V g=\rho V g$
where $F_v$ is the viscous force, $V$ is the volume of the ball.

Taking the ratio of equation (1) and (2):
$F_v=M g\left(1-\frac{\sigma}{\rho}\right)$
$F_v+\sigma V g=\rho V g$
where $F_v$ is the viscous force, $V$ is the volume of the ball.

Taking the ratio of equation (1) and (2):
$F_v=M g\left(1-\frac{\sigma}{\rho}\right)$
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