→Natural Frequency : "When a system capable of oscillating is given some initial displacement from its equilibrium position and left free, it begins to oscillate. Thus the oscillations performed by the system in the absence of any type of resistive forces are known as natural (free) oscillations."
→If such a system is driven by an energy source at a frequency that is near the natural frequency, the amplitude of the oscillations is found to be large. This phenomenon is known as resonance.
→Example : A familiar example of this phenomenon is a child on a swing. The swing has a natural frequency for swinging back and forth like a pendulum. If the child pulls on the rope at regular intervals and the frequency of the pulls is almost the same as frequency of swinging, the amplitude of the swinging will be large.