First slide

Motion in a plane

Question

A swimmer wishes to cross a 500 m wide river flowing at 5 km/h. His speed relative to water is 3 km/h. The shortest possible time to cross the river is

Moderate

A

10 min
B

20 min
C

5 min
D

40 min

Solution

When the velocity of swimmer relative to water is perpendicular to the direction of flow of river, the swimmer can cross the river in minimum possible time.
$∴ t_{\min} = \frac{(500 / 1000)}{3} h = \frac{60}{6} \min = 10 \min$

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