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How do I calculate the effective motion for several excited harmonic oscillators?

Physics Asked on April 2, 2021

Let’s say I have several harmonic oscillators, e.g. springs, where each has a displacement $x(t)$ and velocity $v(t)$ with total energy $E = frac{1}{2} m v^2 + frac{1}{2} k x^2 $.

Now these oscillators are subject to a force $F(t)$. This force could be anything, but it is the same for each oscillator. The relative phases of the oscillators are a free parameter.

I am only interested in the center-of-mass motion of all oscillators combined. My question is: Is there a way to calculate the center-of-mass motion without having to calculate each oscillator individually and then summing up the motion?

I feel like this is a very basic question, but I don’t know what to google.

One Answer

Assuming the spring constants and masses to be identical, you can see that the COM position is the average of the positions of each individual oscillator. Now sum the equations of motion for all the individual oscillators together and divide by their number to find an equation of motion for x. For generic spring constants and masses this won't work, as the COM motion can take the form of any finite Fourier series, which requires more than the 2 initial conditions of a 2nd order ODE to specify.

Answered by Oscar Emil Sommer on April 2, 2021

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