Question

The speed of boat is 5 Km/h in still water. It crosses a river of width 1 Km along shortest possible path in 15 min. The velocity of river water is

• A. 1 Km/h
• B. 3 Km/h
• C. 4 Km/h
• D. 5 Km/h

Answer 1

The correct option is B 3 Km/h

Shortest possible path,$s=1km$

Time taken, $t=15min=0.25hr$

Speed in the direction of shortest path, $\frac{s}{t}=\sqrt{v^2-u^2}$

Where, $v=5km{h}^{-1}$ velocity of boat, $u$ velocity of river.

$\frac{s}{t}=\sqrt{v^2-u^2}$

$\frac{1}{0.25}=\sqrt{5^2-u^2}$

$u=3km{h}^{-1}$

Hence, velocity of river is $3km{h}^{-1}$