Phy 515 class notes for Spring, 2000

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Phy 515, Week 13

17 Apr How can you use Fourier series to solve a differential equation? For an example, look at the forced harmonic oscillator. If the forcing function is periodic, you can write it using a series of functions that satisfy periodic boundary conditions.

Fext( t ) = S cn eniwt

Here w = 2p/T and T is the period of the applied force. If you try to solve the equation

m d2x / dt2 = Fext( t )

the first step will be to solve the inhomogeneous equation for one of these terms in the Fourier series, then to add the results. The result is a series representation of the solution for x( t ). This solution has the notable property that if the natural frequency of the oscillator coincides with any one of the harmonics of the forcing function, then you get resonance.


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