Physics — Relative Velocity
Combining velocities and river-crossing.
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\mathbf{v}_{AB} = \mathbf{v}_A - \mathbf{v}_B
\mathbf{v}_{resultant} = \mathbf{v}_{boat} + \mathbf{v}_{water}
v_{rel} = \sqrt{v_1^2 + v_2^2}\ \text{(perpendicular)}
\text{closing speed of approaching bodies}