7-39 할리데이 11판 솔루션 일반물리학

$$ \begin{cases} F&=120\ut{N}\\ S&=1.8\ut{m}\\ \Delta K_a &= 42\ut{J}\\ \end{cases} $$ $$\ab{a}$$ $$ \begin{aligned} \Delta K_a &= \Sigma W =W_F+W_{mg}\\ W_{mg}&=\Delta K - W_F\\ &=42-(-120\cdot1.8)\\ &=258\ut{J}\\ &\approx 2.6\times10^2\ut{J}\\ \end{aligned} $$ $$\ab{b}$$ $$ \begin{aligned} \Delta K_b&=W_{mg}\\ &=258\ut{J}\\ &\approx 2.6\times10^2\ut{J}\\ \end{aligned} $$