7-47 할리데이 11판 솔루션 일반물리학

$$ \begin{cases} \vec d&=9.0\i+c\j\ut{m}\\ \vec F&=5.0\i-8.0\j\ut{N\\} \end{cases} $$ $$W=\vec F \cdot \vec S,$$ $$ \begin{aligned} W&=(5.0\i-8.0\j)\cdot(9.0\i+c\j)\\ &=45-8c \end{aligned} $$ $$\ab{a}$$ $$W=0,$$ $$45-8c=0$$ $$c=\frac{45}{8}=5.625\approx 5.6$$ $$\ab{b}$$ $$W\gt 0,$$ $$45-8c\gt0$$ $$c\lt\frac{45}{8}=5.625\approx 5.6$$ $$\ab{c}$$ $$W\lt 0,$$ $$45-8c\lt0$$ $$c\gt\frac{45}{8}=5.625\approx 5.6$$