A boat, which has a speed of 5km/h in still water, crosses a river of width 1km along the shortest possible path in 15 minutes. What is the magnitude of velocity of the river water?
A
5km/h
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B
4km/h
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C
3km/h
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D
6km/h
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Solution
The correct option is C3km/h The speed of the boat in still water =vb=5km/h
Width of the river =s=1km
Time taken =t=15 minutes=14h
Speed of the boat w.r.t the river =vbr=st=114=4km/h
From the definition of relative velocity, we have →vbr=→vb–→vr
Hence |→vbr|2=|→vb|2+|→vr|2⇒vr=√(52−42)=3km/h
Hence the correct option is (c).