Mainly, a door alarm system is used to measure gravitational acceleration. The magnetic switch component of the door alarm is turned on or off as the magnetic pendulum bob approaches or recedes from it. The audio signals is recorded by a smartphone as a time series, permitting the measurement of the period of oscillations and thus the determination of the gravitational acceleration. A satisfactory match between the experimental and the locally accepted value is attained. The setup is also an apparent audio-based demonstration of the periodicity of the motion of the simple pendulum.
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