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` 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

-------------> V river = 5m/min

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

Case 1if 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 /3

^{1/2}and resultant velocity is 11.2 m/minso time taken is

x / 9.7mins

Case 2if 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 / 8.7minsSo case 1 takes least time

he should travel due north(a)

7 years ago

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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