A river flows at 2 km h⁻¹. A swimmer can swim at 4 km h⁻¹ relative to water. At what angle with respect to the direction of river flow should he swim to cross the river straight across?
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Question

A river flows at 2 km h⁻¹. A swimmer can swim at 4 km h⁻¹ relative to water. At what angle with respect to the direction of river flow should he swim to cross the river straight across?

G1 Physics · Accepted Answer

For zero downstream drift, the swimmer's component opposite the current must be 2 km h⁻¹. If θ is measured from the downstream flow direction: 4cosθ=-2, so cosθ=-1/2 and θ=120°. Hence option D.