Question

A boat having a speed of $5km/h$ in still water, crosses a river of width $1km$ along the shortest possible path in $15$ minutes. The speed of the river in $km/h$ :

• A. $1$
  
• B. $3$
  
• C. $4$
  
• D. None of these

Answer 1

Answer: (B)

$3$

As the boat crosses the river of width 1 km along the shortest path in 15 min i.e.

(1/4) hr, so the crossing velocity is $v_c=\frac{1km}{\left(1/4\right)hr}=4km/hr$

Here, $v_c$ will be perpendicular to velocity of river i.e $v_r$.

Thus, obviously $v_c^2+v_r^2=v_{boat}^2$

or $4^2+v_r^2=5^2$ or $v_r=3km/hr$