7-18 할리데이 11판 솔루션 일반물리학

$$ \begin{cases} \theta &= 50\degree\\ F_r&=70\ut{N}\\ d&=0.60\ut{m}\\ \Delta K_1 &=80\ut{J} \end{cases} $$ $$ \begin{cases} \Delta K_1 = \Sigma W_1 = W_{mg}+W_F\\ \Delta K_2 = \Sigma W_2 = W_{mg} \end{cases} $$ $$ \begin{aligned} \Ans &= \Delta K_2-\Delta K_1\\ &=W_{mg}-(W_{mg}+W_F)\\ &=-W_F\\ &=-\vec F \cdot \vec d\\ &=-(-42\ut{J})\\ &=42\ut{J} \end{aligned} $$