Q. 12

A river is flowin A. due north

B. 300 east of north

C. 300 north of west

D. 600 east of north.

Answer :

Let us consider the width of the river be . Let the man swims at some angle with velocity . This is shown below in the figure



Where, =distance travelled by man and is constant


By Pythagoras theorem in above triangle formed, we have



Differentiating above equation with respect to time, we get




Negative sign is due to decrease in the distance, covered by man, as time goes. Man can also go along through the river with velocity .



Therefore, the equation (1) becomes




Therefore, time taken by the Man to pass the river with velocity along the line making angle is



Therefore, the man takes shortest time when is maximum. is maximum when . Thus, we say that the man should swim along that is towards north so that he crosses the river in shortest possible time.

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