2-3 할리데이 10판 솔루션 일반물리학

$$ \begin{cases} S &= 2.80\ut{km} \\ v_1 &= 6.00\ut{km/h} \\ v_2 &= -7.70\ut{km/h} \\ \Sigma t &= 35.0\ut{min} = \frac{35}{60}\ut{h} \end{cases} $$ $$ \begin{cases} t_1 &= \frac{S}{v_1} = \frac{2.80\ut{km}}{6.00\ut{km/h}} = \frac{28}{60}\ut{h} \\ t_2 &= \Sigma t - t_1 = \frac{35}{60}\ut{h} - \frac{28}{60}\ut{h} = \frac{7}{60}\ut{h} \end{cases} $$
(a) $$ \begin{aligned} \Ans &= \bar v = \frac{\Sigma x}{\Sigma t}= \frac{x_1+x_2}{\Sigma t} \\ &= \frac{S+v_2t_2}{\Sigma t} \\ &= \frac{2.80\ut{km}+(-7.70\ut{km/h})\cdot \frac{7}{60}\ut{h}}{\frac{35}{60}\ut{h}} \\ &= \frac{163}{50}\ut{km/h} \\ &= 3.26\ut{km/h} \end{aligned} $$
(b) $$ \begin{aligned} \Ans &= \frac{\Sigma\abs{x}}{\Sigma t}= \frac{\abs{x_1}+\abs{x_2}}{\Sigma t} \\ &= \frac{\abs{S}+\abs{v_2t_2}}{\Sigma t} \\ &= \frac{\abs{2.80\ut{km}}+\abs{(-7.70\ut{km/h})\cdot \frac{7}{60}\ut{h}}}{\frac{35}{60}\ut{h}} \\ &= \frac{317}{50}\ut{km/h} \\ &= 6.34\ut{km/h} \end{aligned} $$