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Q.

A boat moves relative to water with a velocity which is ‘n’ times the river flow
a) If n < 1 boat can not cross the river
b) If n = 1 boat can not cross the river without drifting
c) If n > 1 boat can cross the river along shortest path
d) Boat can cross the river what ever is the value of n excluding zero value

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a

only a is correct

b

a, b are correct

c

c, d are correct

d

b, c & d are correct

answer is D.

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Detailed Solution

Vb=nVw

To follow the shortest path, the direction of boat has to be at some angle with water flow , the resultant velocity of boat is,

Vbg=Vb2-Vw2=n2Vw2-Vw2=Vwn2-1

for a real value of Vbg , n>1

if n > 1 , Vbg > 0 , so, boat can cross the river along shortest path.

Let θ is the angle between direction of boat and shortest path.

V=Vbcosθ=nVwcosθ ;

 V=Vw-Vbsinθ=Vw1-nsinθ

To cross the river, θ>0.

if n = 1 , V0 , so boat can not cross the river without drifting.

if n0, V0, so, Boat can cross the river whatever is the value of n excluding zero value

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A boat moves relative to water with a velocity which is ‘n’ times the river flowa) If n < 1 boat can not cross the riverb) If n = 1 boat can not cross the river without driftingc) If n > 1 boat can cross the river along shortest pathd) Boat can cross the river what ever is the value of n excluding zero value