Question

a man can row a boat with 4 kmph in still water if he is crossing river where the current is 8 kmph. width of river is 8km . the time he will cross the river in shortest path is k/√3 hour , where k =​

Answer 1



Given :    

Velocity of man wrt water, VMW=4 km/hr    

              

Velocity of river, VR=2 km/hr           

      

Distance covered in upstream or downward motion       d=2 km  each

For upstream motion of the boat :

Velocity of the man ,       VM=VMW−VW=4−2=2  km/h

∴ Time taken to go upstream       t1=22=1  hr

For downward motion of the boat  or  (return journey) :

Velocity of the man        VM=VMW+VW=4+2=6 km/h

∴ Time taken to go downstream       t

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Answer 2

Given :    

Velocity of man wrt water, Vm=4 km/hr    

Velocity of river, Vr=2 km/hr          

Distance covered in upstream or downward motion   d=2 km  each

To find :

For upstream motion of the boat :

Velocity of the man ,       Vm1=Vm−Vw=4−2=2  km/h

Therefore,

Time taken to go upstream       t1 = distance travelled/ velocity = 2/2=1  hr

For downward motion of the boat  :

Velocity of the man        Vm1=Vm+Vw=4+2=6 kmph

Therefore,

Time taken to go downstream t2=  distance travelled/ velocity

=2/6 = 1/3 hr = 20mins.

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