2-33 할리데이 10판 솔루션 일반물리학

$$ \begin{cases} v_0 &= -20\ut{m/s} \\ S &= -60\ut{m} \\ g &=9.80665\ut{m/s^2} \end{cases}$$ $$t = ? $$ $$ S = v_0t+\frac{1}{2}at^2, $$ $$ \begin{aligned} \\ S &= v_0t+\frac{1}{2}(-g)t^2 \\ -60 &= -20t-\frac{1}{2}(9.80665)t^2 \\ t&=\frac{400 \left(-1000\pm \sqrt{3941995}\right)}{196133}\ut{s} \\ &=\frac{400 \left(-1000+ \sqrt{3941995}\right)}{196133}\ut{s} \ (\because t \ge 0) \\ &\approx 2.009750105836867\ut{s} \\ &\approx 2.0\ut{s} \end{aligned} $$