11-56 할리데이 11판 솔루션 일반물리학

$$\begin{cases} m&=5.4\ut{kg}\\ \vec r&=4.0t^2\i-(2.0t+6.0t^2)\j \end{cases}$$ $$\begin{aligned} \vec v &=\dyt{\vec r}\\ &=\dt\bra{4t^2\i-(2t+6t^2)\j}\\ &=8t\i+\br{-2-12t}\j\taag1 \end{aligned}$$ $$\begin{aligned} \vec L &=m\br{\vec r\times\vec v}\\ &=8mt^2\k \end{aligned}$$ $$\ab{a}$$ $$\vec \tau_\net=\dyt{\vec L},$$ $$\begin{aligned} \vec \tau_\net &=\dt\br{8mt^2\k}\\ &=16mt\k\\ &=86.4t\k\ut{N\cdot m}\\ &\approx 86t\k\ut{N\cdot m}\\ \end{aligned}$$ $$\ab{b}$$ $$\begin{aligned} \dyt{\vec L}\gt0,\\ ~\text{Increase} \end{aligned}$$