A river of width 120 m flows east at 3 m/s. A boat can move at 5 m/s relative to water. To cross the river in minimum possible time, the boat should be pointed:
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Question

A river of width 120 m flows east at 3 m/s. A boat can move at 5 m/s relative to water. To cross the river in minimum possible time, the boat should be pointed:

G1 Physics · Accepted Answer

Crossing time depends only on the velocity component perpendicular to the bank. This component is maximum when the entire 5 m/s boat velocity is directed perpendicular to the bank. Thus t m i n t_min t m ​ in = 120/5 = 24 s. The river causes drift, but does not increase this minimum crossing time.