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Standard XII

Physics

Speed of Efflux

A man can swi...

Question

A man can swim at a speed of 5 km/h w.r.t. water. He wants to cross a 1.5 km wide river flowing at 3 km/h. He keeps himself always at an angle of 60 with the flow direction while swimming. The time taken by him to cross the river will be

0.25 hr.

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0.35 hr.

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0.45 hr.

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0.55 hr.

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Solution

The correct option is B 0.35 hr.
The man has to travel $1.5 k m$ perpendicular to the flow to cross the river.And the velocity of the man is at $60$ degree with flow, so component of man's velocity perpendicular to flow is $v S i n 60 = 5 \times \frac{\sqrt{3}}{2}$
time taken will be $\frac{d i s t a n c e}{s p e e d} = \frac{1.5}{5 \times \frac{\sqrt{3}}{2}} = 0.35 h r .$
so best option is B.

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