Ncert Exercises & Solutions

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

NEETprep Answer:

$S p e e d o f t h e m a n, v_{m} = 4 k m / h$

Width of the river = 1 km

Time taken to cross the river = $\frac{width of the river}{speed of the river} = \frac{1}{4} h = \frac{1}{4} \times 60 = 15 \min$

$S p e e d o f t h e r i v e r, v_{r} = 3 k m / h$

$D i s \tan c e c o v e r e d w i t h f l o w o f t h e r i v e r = v_{r} \times t$ $= 3 \times \frac{1}{4} = \frac{3}{4} km$
$= \frac{3}{4} \times 1000 = 750 m$

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