Saturday, December 26, 2015

frequency - Natural and Resonance frequencies of a damped oscillator


The damped oscillator equation is


\begin{equation} m\ddot{x}+b\dot{x}+kx=0 \end{equation}


And its solution has natural frequency $\omega_0$



\begin{equation} \omega_0=\sqrt{\frac{k}{m}-(\frac{b}{2m})^2} \end{equation}


However, when one adds a driving force to the equation


\begin{equation} m\ddot{x}+b\dot{x}+kx=D\cos(\Omega t + \phi) \end{equation}


the resonance frequency $\Omega=\omega_R$ that maximizes amplitude is


\begin{equation} \omega_R=\sqrt{\frac{k}{m}-2(\frac{b}{2m})^2} \end{equation}


I'm wondering why the resonance frequency isn't the natural frequency. I've read this formulas in the wikipedia page of the harmonic oscillator.




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