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Question

# A boat moves relative to water with a velocity 'v' which is n times less than the river flow velocity u. At what angle to the north direction must the boat move to minimize drifting if the river flows from west to east?

A

θ=sin1n

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B

θ=sin11n

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C

θ=cos11n

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D

none of these

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Solution

## The correct option is B θ=sin−11n In this case, the velocity of boat is less than the river flow velocity. Hence boat cannot reach the point directly opposite to its starting point. i.e. drift can never be zero.) Suppose boat starts at an angle θ from the normal direction up stream as shown. Component of velocity of boat along the river, vx = u−v sinθ and velocity perpendicular to the river, vy = v cosθ Time taken to cross the river is t =dvy=dv cosθ Drift x = (vx)t = (u−v sinθ)dv cosθ=udvsecθ−d tanθ The drift x is minimum, when dxdθ= 0, or (udv)(secθ.tanθ)−d sec2θ=0or uvsinθ=1⇒sinθ=vu i.e., for minimum drift, the boat must move at an angle θ=sin−1(vu)=sin−11n from normal direction.

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