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Vishrant Vasavada Grade:
        A river is flowing from west to east at speed of 5 m/minute . A man on south bank of river, capable of swimming at 10 m/min in still water, wants to swim across river in shortest time. He should swim in direction a)due north. b)30 east of north. c) 30 north of west. d) 60 east of north. Justify you answer.
7 years ago

Answers : (2)

Ramesh V
70 Points
										

 


 


-------------> V river = 5m/min     


V man = 10 m/minsuppose width of river is x mts


Case 1


if he travels due north


then his resultant velocty ( due to velocity of downstream of water) acts 30 north of east and the distane to be covered to reach the other bank increses to = x/cos 30


i.e., 2x /31/2 and resultant velocity is 11.2 m/min


so time taken is x / 9.7 mins


Case 2


if he travels due 30 north of east


then his resultant velocty ( due to velocity of downstream of water) actsdue north and the distane to be covered to reach the other bank will be x mts and resultant velocity is 8.7 m/min


so time taken is x / 8.7 mins


So case 1 takes least time


he should travel due north(a)


 


 

7 years ago
AskIITianExpert Srijan.iitd
8 Points
										

he should swim due north!!!


his velocity in moving water is equal to his own velocity plus the velocity of the river.


now to cross the river in smallest possible time he should maximise the velocity in the north direction.


suppose he move at an angle of @ from the bank then the component due north is 10sin@m/min. because river velocity is zero in the north direction.


now this velocity due noth will be maximum when @ tends to 90 degrees.


hence he should move due north that is 90 degrees to the bank.

7 years ago
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