Q 11-03-160JEE MainJEE Main 2019 (9 Apr, Shift 1)Easy
The stream of a river is flowing with a speed of $2\ \text{km h}^{-1}$. A swimmer can swim at a speed of $4\ \text{km h}^{-1}$. The direction of the swimmer with respect to the flow of the river, to cross the river straight, is
Answer: (C) $120^\circ$
To cross straight, the upstream component of the swimmer's velocity must cancel the river: $4\sin\alpha = 2$, where $\alpha$ is the angle from the straight-across direction.
$$\sin\alpha = \frac12 \Rightarrow \alpha = 30^\circ$$
Measured from the direction of flow, the swimmer heads at $90^\circ + 30^\circ = 120^\circ$.
Solution by Sreeraj P, M.Sc Physics