Question

A boat man could row his boat with a speed $10\,m/sec$. He wants to take his boat from $P$ to a point $Q$ just opposite on the other bank of the river flowing at a speed $4\,m/sec$. He should row his boat:

• A. At right angle to the stream
• B. At an angle of sin$^{-1} (2/5)$ with $PQ$ up the stream
• C. At an angle sin$^-{1} (2/5)$ with $PQ$ down the stream
• D. At an angle cos$^{-1} (2/5)$ with $PQ$ down the stream

Answer 1

Answer: (B)

He should row his boat at an angle $\theta$ such that
$\sin \theta=\frac{v}{u}=\frac{4}{10}=\frac{2}{5}$
$\therefore \theta=\sin ^{-1}\left(\frac{2}{5}\right)$