If particles $A$and $B$ are moving with velocities ${\mathrm{v}}_A$ and ${\mathrm{v}}_B$, respectively. The relative velocity of $A$ with respect to $B$is defined as ${\mathrm{v}}_{AB}={\mathrm{v}}_A-{\mathrm{v}}_B$. The driver of a car travelling Southward at $30 {\mathrm{kmh}}^{-1}$ observes that wind appears to be coming from the West. The driver of another car travelling Southward at $50 {\mathrm{kmh}}^{-1}$ observes that wind appears to be coming from the South. West.