Q 11-03-029NEETJEE MainMedium
A river $120$ m wide flows at $3$ m/s. A boat can move at $5$ m/s in still water. How much longer does it take to cross along the shortest path than in the shortest possible time?
Answer: (B) $6$ s
Shortest time: head straight across. $t_1 = \dfrac{120}{5} = 24$ s (the boat drifts downstream).
Shortest path: head upstream so that the resultant is straight across. Crossing speed $= \sqrt{5^2 - 3^2} = 4$ m/s, so $t_2 = \dfrac{120}{4} = 30$ s.
Difference $= 30 - 24 = 6$ s.
Solution by Sreeraj P, M.Sc Physics