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Question

Physics Question on Motion in a plane

The width of river is 1 km. The velocity of boat is 5 km/hr. The boat covered the width of river in shortest time 15 min. Then the velocity of river stream is

A

3 km / hr

B

4 km / hr

C

29 \sqrt 29 km / hr

D

41 \sqrt 41 km / hr

Answer

3 km / hr

Explanation

Solution

vResultant=1km1/4hr=4km/hrv_{ Resultant } = \frac{1 \, km}{ 1 / 4 \, hr } = 4 \, km /hr
vRiver=5242=3\therefore v_{ River} = \sqrt{ 5^2 - 4^2 } = 3 km /hr.
So, the correct answer is (A): 3 km / hr.