2-60 할리데이 11판 솔루션 일반물리학

$$ \begin{cases} v_A&=15\ut{km/h}\\ v_B&=30\ut{km/h}\\ \end{cases} $$ $$ \begin{aligned} \bar v&=\frac{S_A+S_B}{t_A+t_B}\\ &=\frac{2S}{\cfrac{S}{v_A}+\cfrac{S}{v_B}}\\ &=\frac{2}{\cfrac{1}{v_A}+\cfrac{1}{v_B}}\\ &=\frac{2}{\cfrac{1}{15}+\cfrac{1}{30}}\ut{km/h}\\ &=20\ut{km/h} \end{aligned} $$