Question

Two balls $A$ and $B$ are flowing through water with same velocity as shown in figure. If radius of ball $A$ is twice the radius of ball $B$, then find the ratio of drag force acting on $A$ and $B$. Assume that Stoke's law is valid.

• A. $4:1$
• B. $3:2$
• C. $2:1$
• D. $2:3$

Answer 1

The correct option is C $2:1$

We know,
Drag force, $F_D=6\pi \mu rv$
$\therefore$ Drag force on ball $A$,
$F_A=6\pi \mu r_{A}v$
Similarly,
Drag force on ball $B$,
$F_B=6\pi \mu r_{B}v$

$\therefore \frac{F_A}{F_B}=\frac{r_A}{r_B}$
Given,
$r_A=2r_B$
$\Rightarrow \frac{F_A}{F_B}=\frac{2}{1}$