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Question

A river is flowing from West to East at a speed of 5 m/min. A man on the south bank of the river, capable of swimming at speed 10 m/min in still water, wants to swim across the river in the shortest time. What angle with the bank of the river should he swim at? (in degree) .

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Solution

Velocity of river, Vr=5 m/min
Velocity of man w.r.t river, Vm=10 m/min

Let the width of the river be d and assume the man heads in a direction making angle θ with the bank of the river.


On breaking component of Vm along and perpendicular to river flow


We can see that component Vmsinθ will be responsible for crossing the river.

Time taken to cross river,
t=width of riverperpendicular component of velocity

t=dVmsinθ

For t to be minimum, sinθ should be maximum, and maximum value of sinθ is 1 at θ=π2=90.

Man should head at angle of 90 with the bank of river i.e. in North direction.

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