Question

A boat $B$ is moving upstream with velocity $3\, m / s$ with respect to ground. An observer standing on boat observes that a swimmer $S$ is crossing the river perpendicular to the direction of motion of boat. If river flow velocity is $4 \,m / s$ and swimmer crosses the river of width $100 m$ in $50 \sec$, then

• A. velocity of swimmer w.r.t ground is $\sqrt{13} \,m / s$
• B. drift of swimmer along river is zero
• C. drift of swimmer along river will be $50\, m$
• D. velocity of swimmer w.r.t ground is $2 \,m / s$

Answer 1

Answer: (A)

$\vec{v}_{S B}=v \hat{j}=\vec{v}_s+3 \hat{i}$
$\vec{v}_S=\hat{j}-3 \hat{i}$ and $v=\frac{100}{50}=2 m / s$
$\therefore \left|\vec{v}_S\right|=\sqrt{v^2+(3)^2}=\sqrt{2^2+9}=\sqrt{13} \,m / s$
Drift $=50 \times 3=150\, m$