Question

A boat is sent across the river with a velocity of $8\text{km/h}$ in a direction perpendicular to flow of river. If resultant velocity of boat is $10\text{km/h}$, then velocity of river flow is

• A. $18\text{km/h}$
• B. $2\text{km/h}$
• C. $6\text{km/h}$
• D. $4\text{km/h}$

Answer 1

The correct option is C $6\text{km/h}$

Velocity of boat w.r.t. river, $v_b=8\text{km/h}$

Let velocity river flow $=\overrightarrow{u}$ and resultant velocity of boat $=\overrightarrow{v_r}$
From fig, $v_r^2=v_b^2+u^2$
$\therefore u=\sqrt{v_r^2-v_b^2}=\sqrt{100-64}=6{\text{km h}}^{-1}$