Question

A man can swim with a speed of $4\,km\,h^{-1}$ in still water. He crosses a river $1\,km$ wide that flows steadily at $3\,km\,h^{-1}$. If he makes his strokes normal to the river current, how far down the river does he go when he reaches the other bank?

• A. $500\,m$
• B. $600\,m$
• C. $750\,m$
• D. $850\,m$

Answer 1

Answer: (C)

$750\,m$

Answer 2

Time to cross the river, $t=\frac{\text{Width of river}}{\text{Speed of man}}$ $=\frac{1\,km}{4\,km\,h^{-1}}$ $=\frac{1}{4}h$ Distance moved along the river in time $t$, $s=v_{r}\times t=3\,km\,h^{-1}\times\frac{1}{4}h=\frac{3}{4}km$ $=750\,m$