Tại vị trí li độ góc \(\alpha \):
\(\begin{array}{l}\left\{ \begin{array}{l}{{\rm{W}}_d} = \dfrac{1}{2}m{v^2}\\{{\rm{W}}_d} = mgl(\cos \alpha - \cos {\alpha _0})\end{array} \right.\\ \Rightarrow v = \sqrt {2gl(\cos \alpha - \cos {\alpha _0})} \end{array}\)
Áp dụng định luật II Niuton:
\(\overrightarrow T + \overrightarrow P = m\overrightarrow a \)
Chiếu theo phương hướng tâm:
\(\begin{array}{l}T - P\cos \alpha = m{a_{ht}} = m\dfrac{{{v^2}}}{l}\\ \Leftrightarrow T = P\cos \alpha + m\dfrac{{{v^2}}}{l}\\= mg\cos \alpha + 2mg(\cos \alpha - \cos {\alpha _0})\\= mg(3\cos \alpha - 2\cos {\alpha _0})\end{array}\)
\(\begin{array}{l} \Rightarrow \left\{ \begin{array}{l}{T_{\max }} = mg(3 - 2\cos {\alpha _0})(VTCB)\\{T_{\min }} = mg\cos {\alpha _0}(VTB)\end{array} \right.\\ \Rightarrow \dfrac{{{T_{\max }}}}{{{T_{\min }}}} = \dfrac{{3 - 2\cos {\alpha _0}}}{{\cos {\alpha _0}}} = 1,02\\ \Rightarrow \cos {\alpha _0} = 0,99 \Rightarrow {\alpha _0} = 0,115(rad)\end{array}\)
