Question

A man can swim with a speed of $4.0km/hr$ in still water. How long does he take to cross a river $1.0km$ wide, if the river flows steadily at $3.0km/hr$ and he makes his strokes normal to the river current? How far down the river does he go, when he reaches the other bank?

Answer 1

Since he makes his strokes normal to the river current, the movement of the river will have no effect on the time of crossing the bank.

The time will be, $T=\frac{Distance}{Speed}=\frac{1}{4}hr=0.25hr=15min$.

In the same time, he will go down by

$Distance=Speed$ x $time=3km/hr$ x $0.25hr=0.75km=750m$