11-43 할리데이 11판 솔루션 일반물리학

$$\begin{cases} \omega&=1.5\ut{rev/s}\\ I_i&=8.0\ut{kg\cdot m^2}\\ I_f&=2.0\ut{kg\cdot m^2}\\ \end{cases}$$ $$\ab{a}$$ $$\Delta \Sigma \vec L=0,$$ $$\begin{aligned} 0&=L_f-L_i\\ &=I_f\omega_f-I_i\omega_i\\ \end{aligned}$$ $$\begin{aligned} \omega_f&={I_i\over I_f}\omega_i\\ &=6\ut{rev/s}\\ \end{aligned}$$ $$\ab{b}$$ $$\put \begin{cases} \RE : \text{Rotational Kinetic Energy}\\ \end{cases}$$ $$\begin{aligned} {E_f\over E_i}&={\RE_f\over \RE_i}\\ &={{1\over2}I_f{\omega_f}^2\over {1\over2}I_i{\omega_i}^2}\\ &={I_f\over I_i}\cdot\br{{\omega_f}\over {\omega_i}}^2\\ &={I_f\over I_i}\cdot\br{{{1}\over {\omega_i}}\cdot{I_i\over I_f}\omega_i}^2\\ &={I_i\over I_f}\\ &=4.0\\ \end{aligned}$$ $$\ab{c}$$ $$\begin{aligned} \text{Moving Stone by Man} \end{aligned}$$