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
Concept. Shortest time: head straight across. Shortest path: head upstream so the resultant is straight across.
Step 1. Shortest time. $t_1 = \dfrac{120}{5} = 24$ s.
Step 2. Shortest path. Crossing speed $\sqrt{25 - 9} = 4$ m/s: $t_2 = 30$ s.
Step 3. Difference. $6$ s.
Answer. $6$ s (Option B).
Solution by Sreeraj P, M.Sc Physics