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A boat B is moving westwards with a speed of $20kmh^{-1}$ and a boat C 200 km south of B is moving norhtwards with a speed of $20kmh^{-1}$. The time after which the distance between then becomes shortest is
Explanation
$\vec{V}_{CB} = 20\hat{j} + 20\hat{i} = 20\sqrt{2} km$ Time taken to reach shortest distance between bande = $200 \cos 45^{\circ}/20\sqrt{2} = (100\sqrt{2})/(20\sqrt{2}) = 5h$.