A boat which has a speed 5km/h in still water crosses a river of width 100m along the shortest possible path in 1.5min. The velocity of river water in km/h is
A
1
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B
3
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C
4
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D
4.1
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Solution
The correct option is B3 For shortest path velocity diagram is as shown Given velocity of boat in still water vbw=5km/h let vb be velocity of boat, vw be velocity of water Now time taken for boat to cross the river is t=1.5min=wvb=100vb vb=1001.5mmin
vb=1001.5×11000×60km/h vb=4km/h
Using pythagoras theorem ⇒vw=√v2bw−v2b=√52−42 ⇒vw=3km/h