8-72 할리데이 11판 솔루션 일반물리학

$$ \begin{cases} m&= 78\ut{kg}\\ v&=64\ut{m/s}\\ g&=9.80665\ut{m/s^2} \end{cases} $$ $$ \put \begin{cases} \KE : \text{Kinetic Energy}\\ \GE : \text{Gravitational Potential Energy}\\ \ME : \text{Mechanical Energy}\\ \end{cases} $$ $$\ab{a}$$ $$ \begin{aligned} \dyt{\GE}&=\dyt{mgy}\\ &=mg\dyt{y}\\ &=mgv\\ &=4992g\\ &=48954.7968\ut{W}\\ &\approx 4.9\times10^4\ut{W}\\ &\approx 49\ut{kW}\\ \end{aligned} $$ $$\ab{b}$$ $$ \begin{aligned} \dyt{\ME}&=\dt(\KE+\GE)\\ &=\dyt{\KE}+\dyt{\GE}\\ &=\dyt{\GE}~~\(\because \dyt{v}=0\)\\ &=48954.7968\ut{W}\\ &\approx 4.9\times10^4\ut{W}\\ &\approx 49\ut{kW}\\ \end{aligned} $$