Question

The speed of a boat is 5 km/h in still water. It crosses a river of width 1 km along the shortest possible path in 15 min. The velocity of the river water is

Answer 1

Shortest possible path,s=1km

Time taken, t=15min=0.25hr

Speed in the direction of shortest path,

t

s

=

v

2

−u

2

Where, v=5kmh

−1

velocity of boat, u velocity of river.

t

s

=

v

2

−u

2

0.25

1

=

5

2

−u

2

u=3kmh

−1

Hence, velocity of river is 3kmh

−1

Answer 2

Answer:

3kmh-1

Explanation:

Net velocity of boat in river =52−u2−−−−−−√

t=DistanceVelocity⇒14=152−u2−−−−−−√⇒u=3kmh−1