A boat goes across a river in shortest possible time with a velocity 12km/h . The magnitude of its resultant speed in flowing water is 13km/h. The magnitude of velocity of water flow in the river is
A
5km/h
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B
7km/h
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C
9km/h
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D
1km/h
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Solution
The correct option is A5km/h Let →Vbr be the velocity of boat w.r.t river and →Vr be the velocity of water flow in river.
According to question, →Vbr=12km/h (−→Vbr is perpendicular to the flow of the river for shortest time)
and
Velocity of boat w.r.t ground, →Vb=13km/h
We know that →Vbr=→Vb−→Vr⇒→Vb=→Vbr+→VrVb=√V2br+V2r Vb=√122+V2r 13=√144+V2r V2r=25⇒Vr=5km/h