11-59 할리데이 11판 솔루션 일반물리학

$$\begin{cases} {I_f\over I_i}&=85\%\\ \end{cases}$$ $$\Delta \Sigma \vec L=0,$$ $$\begin{aligned} 1 &={L_f\over L_i}\\ &={I_f\omega_f\over I_i\omega_i}\\ \end{aligned}$$ $${\omega_f\over \omega_i}={I_i\over I_f}\taag1$$ $$\put \begin{cases} \RE : \text{Rotational Kinetic Energy}\\ \end{cases}$$ $$\begin{aligned} Ans &={\RE_f\over\RE_i}\\ &={{1\over2}I_f{\omega_f}^2\over{1\over2}I_i{\omega_i}^2}\\ &={I_f\over I_i}\cdot\br{\omega_f\over \omega_i}^2\\ &={I_f\over I_i}\cdot\br{I_i\over I_f}^2\\ &={I_i\over I_f}\\ &={100\over85}\\ &={20\over17}\\ &\approx 1.1764705882352942\\ &\approx 1.2\\ \end{aligned}$$