7-2 할리데이 11판 솔루션 일반물리학

$$ \begin{cases} \Delta x &= +5.0\ut{cm}\\ W_0&=30\ut{J} \end{cases} $$ $$W=\vec F \cdot \vec d,$$ $$ \begin{aligned} W(\phi)&=F d \cos \phi\\ W(0)&=F d\cos 0=30\ut{J}\\ \end{aligned} $$ $$ \therefore F d=30\ut{J} $$ $$ W(\phi)=30\cos \phi $$ $$\ab{a}$$ $$\phi=57\degree$$ $$ \begin{aligned} W(57\degree)&=30\cos 57\degree\ut{J}\\ &\approx 16.339171050450812\ut{J}\\ &\approx 16\ut{J}\\ \end{aligned} $$ $$\ab{b}$$ $$\phi=165\degree$$ $$ \begin{aligned} W(165\degree)&=30\cos 165\degree\ut{J}\\ &=-\frac{15(1+\sqrt3)}{\sqrt2}\ut{J}\\ &\approx -28.977774788672043\ut{J}\\ &\approx -29\ut{J}\\ \end{aligned} $$