5-2 할리데이 11판 솔루션 일반물리학

$$\begin{cases} m&=7.80\ut{kg}\\ v_x(0)&=4\ut{m/s}\\ v_x(2)&=-1\ut{m/s}\\ g&=9.80665\ut{m/s^2} \end{cases}$$ $$\begin{aligned} a_x&=\frac{v_x(2)-v_x(0)}{2-0}\\ &=\frac{-5}{2}\ut{m/s^2}\\ &=-g\sin\theta\\ \end{aligned}$$ $$\therefore \sin\theta=\frac{5}{2g}$$ $$\begin{aligned} F_N&=mg\cos\theta\\ &=7.8 g \sqrt{1-\frac{25}{4 g^2}}\\ &\approx 73.96456027109808\ut{N}\\ &\approx 74.0\ut{N}\\ \end{aligned}$$