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A swimmer wants to cross a river straight. He swim at $5 km/hr$ in still water. A river 1 km wide flows at the rate of $3 km/hr$. Which of the following figure shows the correct direction for the swimmer along which he should strike? ($V_{s} \rightarrow$ velocity of swimmer, $V_{r} \rightarrow$ velocity of river, V $\rightarrow$ resultant velocity)
Explanation
The swimmer will cross straight if the resultant velocity of river flow and swimmer acts perpendicular to the direction of river flow. It will be so if the swimmer moves making an angle $\alpha$ with the upstream. i.e. goes along OB.