7-30 할리데이 11판 솔루션 일반물리학

$$ \begin{cases} \vec S&=-1.80\ut{m}\\ F_1&=6.00\ut{N}\\ F_2&=9.00\ut{N}\\ F_3&=14.00\ut{N}\\ \theta &=60.0\degree\\ \end{cases} $$ $$\ab{a}$$ $$\Sigma \vec F = 0 \Harr \Delta \vec v=0,$$ $$\Sigma F_y=0$$ $$ \begin{aligned} \Sigma F_x &= -F_1 +F_2\cos\theta\\ &=\frac{3}{2}\ut{N}\\ \end{aligned} $$ $$ \begin{aligned} W&=\Sigma F_x \cdot \vec S\\ &= \frac{3}{2}\cdot (-1.8)\\ &= -\frac{27}{10}\ut{J}\\ &= -2.7\ut{J}\\ &= -2.70\ut{J} \end{aligned} $$ $$\ab{b}$$ $$\text{decrease}$$