How to explain the resonance phenomenon from an energy point of view?

Since the resonant frequency is close to the intrinsic frequency of the system, then at resonance the direction of the driving force during most of the period coincides with the direction of the body’s velocity. As a result, the driving force performs positive work for most of the period. If the natural frequency of the oscillatory system and the frequency of the driving force differ significantly, then the driving force performs positive work only during part of the period, in another part of it it does negative work. It is for this reason that the energy of the oscillatory system, and therefore. and the amplitude of the steady-state forced oscillations will be less, the more the cyclic frequency of the driving force will differ from the resonant frequency.