2-53 할리데이 10판 솔루션 일반물리학

$$\begin{cases} K = \text{Key}\\ B = \text{Boat}\\ \end{cases}$$ $$\begin{cases} x_{K0} = 45\ut{m}\\ x_{K1} = 0\ut{m}\\ v_{K0} = 0\\ a_K = -g = -9.80665\ut{m/s^2}\\ \end{cases}$$ $$\begin{cases} x_{B0} = -12\ut{m}\\ x_{B1} = 0\ut{m}\\ v_B = \Cons\\ t=t_{K0\to1} = t_{B0\to1}\\ \end{cases}$$ $$v_B=?$$ $$\Delta x = v_0t+\frac{1}{2}at^2,$$ $$\Delta x_{K0\to1} = \frac{1}{2}(-g)t^2$$ $$t^2=\frac{2(-x_{K0})}{-g}$$ $$t=\sqrt{\frac{2x_{K0}}{g}}$$ $$\begin{aligned} \Ans&=v_B=\frac{\Delta x_B}{t}\\ &=\Delta x_B\sqrt{\frac{g}{2x_{K0}}}\\ &=(12\ut{m})\sqrt{\frac{9.80665\ut{m/s^2}}{2(45\ut{m})}}\\ &=\frac{1}{50}\sqrt{\frac{196133}{5}}\ut{m/s}\\ &\approx 3.961141249690549\ut{m/s}\\ &\approx 4.0\ut{m/s}\\ \end{aligned}$$