A question on Pendulum experiment

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The program "Acceleration Spectrum" in "Tools" presents FFT of all accelerations components together. For studies of a pendulum it is interesting to separate them. So I give slightly modified version "Acceleration Spectrum 2" where ax (red), ay (blue) and az (yellow) are separated and presented in the same graph. The results for illustration are obtained with my, fortunately conserved, pendulum with smartphone put at the bob position with x-axis in the direction of motion and y-axis in the direction of the pendulum rod. One can easily see the primary frequency 0.5 Hz of the pendulum for ax and a double frequency for ay. The effect is many time discussed in the literature and recently becomes the topic of a special paper: Sophie Reinhold and Michael Ziese 2020 Eur. J. Phys. in press https://doi.org/10.1088/1361-6404/abc2db.