Question

A copper ball of radius '$r$' travels with a uniform speed '$v$' in a viscous fluid. If the ball is changed with another ball of radius '$2r$', then new uniform speed will be :-

• A. $v$
• B. $2v$
• C. $4v$
• D. $8v$

Answer 1

The correct option is C $4v$

The terminal velocity of ball moving through a liquid of density $\rho$ is given by

$v=\frac{2r^{2}g}{9\eta }(d-\rho )$

Hence $v\propto r^2$

Hence for $r^{\prime }=2r$

$v^{\prime }=4v$