ZdzisĹ‚aw Pluta, Tadeusz Hryniewicz, Adequate Theory of Oscillator: A Prelude to Verification of Classical Mechanics Part 3, ILCPA Volume 3, International Letters of Chemistry, Physics and Astronomy (Volume 3) https://www.scipress.com/ILCPA.3.11 Abstract: In the paper, the adequate theory of oscillator is presented, being a sort of prelude to verification of the classical theory of mechanics. The developed theory is based on a properly understood notion of energy, quantum value changes of its determined measures (potentials), as well as of such changes types of sites of full energetic states which presents the essence of the true principle of the energy conservation. In the first part of the paper the principle of energy conservation was considered. Then the energetic aspects of the oscillator motion, with an exemplary real system motion, were presented in the second Part. This part of the paper is devoted to the kinetics of a body in harmonic motion and verification of the adequate theory of oscillator. At the end, the determination of the gravity acceleration by means of mathematical pendulum is performed to confirm the presented findings. Keywords: Energy, Gravitation, Oscillator, Potential Field, Principle of Energy Conservation, Space-Time