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The speed of a swimmer in still water is $20 m/s$. The speed of river water is $10 m/s$ and is flowing due east. If he is standing on the south bank and wishes to cross the river along the shortest path, the angle at which he should make his strokes w.r.t. north is given by:
Explanation
Velocity of swimmer w.r.t. river $V_{SR} = 20 m/s$ Velocity of river w.r.t. ground $V_{RG} = 10 m/s$ $\vec{V}_{SG} = \vec{V}_{SR} + \vec{V}_{RG}$ $\sin \theta = \left| \frac{{\vec{V}}_{RG}}{{\vec{V}}_{SR}} \right| \Rightarrow \sin \theta = 10/20$ $\Rightarrow \sin \theta = 1/2 \therefore \theta = 30^{\circ}$ west i.e., to cross the river along the shortest path, swimmer should make his strokes $30^{\circ}$ west.