Question
For an oscillating pendulum, establish the relation, $\frac{\text{d}^2\theta}{\text{dr}^2}+\omega^2=\theta=0,$ Where $\omega^2=\frac{\text{g}}{\text{l}}$ and $\theta$ is small angular displacement.

Answer

Restoring force is provided by the portion $\text{mg}\sin\theta$ of gravitational force. Since, it acts perpendicular to length l, the restoring torque $=-\text{mg}\sin\theta\text{l}$

Also, $\tau=\text{l}\alpha=\text{m}\text{l}^2\alpha$

$\therefore\text{ml}^2\alpha=-\text{mg}\sin\theta.\text{l}$

$\alpha=-\frac{\text{g}\sin\theta}{\text{l}}$

For small angles ofoscillation, $\sin\theta\cong\theta.$

$\therefore\alpha=-\frac{\text{g}}{\text{l}}.\theta$

$\frac{\text{d}^2\theta}{\text{dt}^2}=-\frac{\text{g}}{\text{l}}.\theta$

i.e. $\frac{\text{d}^2\theta}{\text{dt}^2}+\omega^2\theta=0.$

giving $\omega=\sqrt{\frac{\text{g}}{\text{l}}}$

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

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