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JEE Main 2026 Jan 21 Shift 2, Physics Q14: Motion In Two And Three Dimension

JEE Main2026Jan 21, Shift 2Physics
Q.

A river of width 200 m is flowing from west to east with a speed of 18 km/h. A boat, moving with speed of 36 km/h in still water, is made to travel one-round trip (bank to bank of the river). Minimum time taken by the boat for this journey and also the displacement along the river bank are ______ and _____ respectively.

  1. A

    20 s and 100 m

  2. B

    40 s and 0 m

  3. C

    40 s and 200 m

  4. D

    40 s and 100 m

Solution

Convert speeds: river km/h m/s; boat km/h m/s.

Minimum-time crossing requires the boat's velocity to point straight across the river (perpendicular to flow). Crossing speed across river m/s. Time across s. Round trip (across and back) s.

During those 40 s, the river drags the boat along the bank by m. So net displacement along the bank is 200 m.

Concept behind this question

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