Question

A boat $B$ is moving upstream with velocity $3m/s$ 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 $4m/s$ and swimmer crosses the river of width $100m$ 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 $50m$
• D. velocity of swimmer w.r.t ground is $2m/s$

Answer 1

Answer: (A)

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