Bifilar ballistic pendulum and accelerometer

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Thank you for comments and suggestions, Jens.

I will look for Ackley's pendulum. Yes, such a pendulum really behave as a simple gravity pendulum. I found a nice demonstration of this, but Ackley was not mentioned.

I thought about the autocorrelation, but with the rathe small noise it shoud not be better FFT or even simple direct measuring the signal period. (??) Fit gives much higher precision.