2-21 할리데이 10판 솔루션 일반물리학

$$ \begin{aligned} v_0 &= 130\ut{km/h} \\&= 130\ut{km/h}\cdot\frac{1000\ut{m}}{1\ut{km}}\cdot\frac{1\ut{h}}{3600\ut{s}} \\ &= \frac{325}{9}\ut{m/s} \end{aligned} $$ $$ \begin{cases} v_0 &= \frac{325}{9}\ut{m/s} \\ v &= 0 \\ S &= 210\ut{m} \\ a &= \Cons \end{cases} $$
(a) $$\Ans = \abs{a} =? $$ $$2aS = v^2-v_0^2, $$ $$ \begin{aligned} \Ans &= \abs{a} = \abs{\frac{v^2-v_0^2}{2S}} \\ &= \abs{\frac{(0)^2-\left(\frac{325}{9}\ut{m/s}\right)^2}{2(210\ut{m})}} \\ &= \frac{21125}{6804}\ut{m/s^2} \\ &\approx 3.104791299235744\ut{m/s^2} \\ &\approx 3.10\ut{m/s^2} \end{aligned} $$
(b) $$S=\frac{1}{2}(v+v_0)t, $$ $$ \begin{aligned} t&= \frac{2S}{v+v_0} \\ &= \frac{2\cdot210\ut{m}}{0+\frac{325}{9}\ut{m/s}} \\ &= \frac{756}{65}\ut{s} \\ &\approx 11.63076923076923\ut{s} \\ &\approx 11.6\ut{s} \end{aligned} $$