Solving Rotation Condition for Equation of Motion

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SUMMARY

The discussion focuses on determining the minimum angular velocity required to maintain continuous rotation in a pendulum described by the equation of motion \(\ddot{\theta} + k^2\sin\theta = 0\). By substituting \(\omega = \dot{\theta}\) and applying the chain rule, the equation transforms into \(\omega \frac{d\omega}{d\theta} + k^2 \sin \theta = 0\). The solution to this differential equation yields \(\frac{1}{2} \omega^2 = k^2 \cos \theta + C\). To ensure that the pendulum can reach the position \(\theta = \pi\), the constant \(C\) must satisfy specific energy conditions.

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If I know the equation of motion of the following form

[tex]\ddot{\theta} + k^2\sin\theta = 0[/tex]

(for pendulum for example). What's the condition (minimum angular velocity) to keep it rotate instead of just oscillation?
 
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Writing [tex]\omega = \dot \theta[/tex], that equation becomes (using the chain rule)

[tex]\omega \frac{d\omega}{d\theta} + k^2 \sin \theta = 0[/tex]

and solving that differential equation gives

[tex]\frac12 \omega^2 = k^2 \cos \theta + C.[/tex]

Once you have that, what condition on C do you need for [tex]\theta = \pi[/tex] to be possible? (Note that each side of this equation corresponds nicely to energy.)
 

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