7-1 할리데이 11판 솔루션 일반물리학

$$ \begin{cases} B:\text{Box}\\ H:\text{Hand}\\ \end{cases} $$ $$ \begin{cases} m&=30.0\ut{kg}\\ g&=9.80665\ut{m/s^2}\\ \end{cases} $$ $$\ab{a}$$ $$\Sigma \vec F = 0 \Harr \Delta \vec v=0,$$ $$ \begin{aligned} \Sigma F_y&=2T-mg=0\\ T&=\frac{mg}{2}\\ &=15g\\ &=147.09975\ut{N}\\ &\approx 147\ut{N}\\ \end{aligned} $$ $$\ab{b}$$ $$ \begin{aligned} \Delta y_H&=2\Delta y_B\\ &=7.00\ut{cm} \end{aligned} $$ $$\ab{c}$$ $$ \begin{aligned} F&=mg\\ &=294.1995\ut{N}\\ &\approx 294\ut{N}\\ \end{aligned} $$ $$\ab{d}$$ $$ \begin{aligned} W&=\vec F\cdot \vec d\\ &=mgh\\ &=\frac{21g}{20}\\ &=10.2969825\ut{J}\\ &\approx 10.3\ut{J}\\ \end{aligned} $$