4-42 할리데이 11판 솔루션 일반물리학

$$\begin{cases} A:\text{Walk Case}\\ B:\text{Escalator Case}\\ C:\text{Walk and Escalator} \end{cases}$$ $$\begin{cases} t_A&=80\ut{s}\\ t_B&=45\ut{s} \end{cases}$$ $$\ab{a}$$ $$\begin{cases} v_A&=\cfrac{S}{t_A}\\ v_B&=\cfrac{S}{t_B}\\ \end{cases}$$ $$\begin{aligned} t_C &= \frac{S}{v_A+v_B}\\ &= \frac{S}{\frac{S}{t_A}+\frac{S}{t_B}}\\ &= \frac{1}{\frac{1}{t_A}+\frac{1}{t_B}}\\ &= \frac{1}{\frac{1}{80}+\frac{1}{45}}\ut{s}\\ &=\frac{144}{5}\ut{s}\\ &=28.8\ut{s}\\ &\approx 29\ut{s} \end{aligned}$$ $$\ab{b}$$ $$\text{Independent}$$