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Two boats A and B move in a river, both having the same speed relative to the flowing river water. Boat A appears to move perpendicular to the river bank when viewed by an observer who is drifting along with the river current. Boat B actually moves perpendicular to the river bank as seen by a stationary observer on the ground. Which of the following statements are INCORRECT?

1. As seen from the ground, boat B moves faster than boat A
2. As seen from the ground, boat A moves faster than boat B
3. As seen by the drifting observer (moving with current), boat B moves faster than boat A
4. As seen by the drifting observer (moving with current), boat A moves faster than boat B

Correct answer: As seen from the ground, boat A moves faster than boat B

Solution

Boat A moves at speed u as seen from ground (since the drifting observer sees it go straight across, its ground velocity has no downstream component). Boat B's ground speed = sqrt(u² + v²) > u. So from the ground, B is faster than A (option A is correct, option B is incorrect).

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