Hello,
Your problem when assigning frequencies to fft results is only related to the shift of frequencies. The frequencies which you have assigned are corresponded to the results of matlab fftshift (fft after being centered in zero). Therefore you have to apply fftshift to your fft results as below:
T = 0.0017; %Time increment(in second)
t = (0 : T : (16.15-T))'; % 9500 samples
fs = 1/T; %Sample rate in Hertz
N = length(t); % Number of samples
TotalTime = T * N;
df = fs/N; %Frequency increment
% df = 1/ TotalTime;
f = -fs/2: df: fs/2-df; % shifted frequencies corresponding to matlab fftshift
FFT_V_x = fft(V_x); %Fourier Transform
FFT_V_x_magnitude = abs(fftshift(FFT_V_x)); %Absolute Values
plot(f ,FFT_V_x_magnitude)
If you don't get my explanation, I again suggest you to read this link for further explanation:
And about your doubt on sampling frequency, since you haven't resample your data, your sampling frequency and therefore assigned frequecies will not change. If you later have problem that the assigned frequencies have higher number of samples than your Vx and Ax vectors, try extrapolating your velocity and acceleration, or simply add zero at the beginning and end to make them fit in size (this does not change the dominant frequency components).
Hope these helps,
-Mona
