Решение

$T=2\pi\sqrt{\frac{\beta}{\alpha}}$

$E_k=\frac{m\upsilon^2}{2}\Rightarrow \beta =\frac{m}{2}$

$E_p=m'gx=m\frac{x}{l}gx=\frac{mg}{l}x^2\Rightarrow \alpha =\frac{mg}{l}$

$T=2\pi\sqrt{\frac{l}{2g}}$