Ohio State University Sum of Sinusoidal Components and Fourier Series Problems

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Ohio State University

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I don't really see how you can take a fourier series expansion of this function, any help is appreciated

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m (10 points) You want to push a small child on a swing, keeping the oscillation amplitude small (for safety, and to make it a simple, damped harmonic oscillator). You notice that the amplitude decays very slowly, decreasing by lle after very many oscillations, so you decide you will give the child a small, short push when the swing pauses at its maximum amplitude. But, you will only do this once for every 5 cycles. You characterize the swing-oscillator with the standard parameters m, 0,=Vk/m and B =b/2m, and you characterize your push with the net FO v . you make vo while ensuring that the swing never reaches a velocity greater than Vmax · Throughout this problem, use approximations whenever possible, keeping only the lowest order terms of B/0, a) Determine the Fourier series amplitudes of the components that make up your push. Please be clear about the frequency of each term, and show that many of the lowest frequency amplitudes are all the same. b) Determine which three components create the largest responses. c) What is the ratio of " where Vmax is the maximum velocity of the steady state V Max solution? This is a ratio of velocities, so it should be unitless.
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Explanation & Answer

Please view explanation and answer below.

Because you push every five cycles, the frequency of your pushing force is ωpush = ω0/5, and its period is Tpush = 2π/ωpush = 10π/ω0.
We can write a Fourier series as a sum of sinusoidal components with frequencies ωn = nω0/5, where n = 0, 1, 2, …
Since your push is short, we will also approximate it as a delta function, F(t) ≈ v0mδ(t), so that the velocity kick ∆v = ∆p/m = ∫F(t)dt/m =
v0, as expected. The fourier expansion therefore takes the form F(t) = v0mδ(t) = A0 + ∑n≥1 Ancos(ωnt).
To find the Fourier series amplitudes A0 and An, we can integrate both sides multiplied by a cosine factor over a single period of
pushing:
∫0→Tpush v0mδ(t)cos(ωℓt)dt = v0m = ∫0→Tpush {A0 + ∑n≥1 Ancos(ωnt)}cos(ωℓt)dt, where ωℓ has an arbitrary index ℓ = 0, 1, 2, …
First, cons...


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