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i like this site. it is very helpful :) http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reson.html
everything can vibrate with its own frequency. Imagine that u re pushing ur friend on a swing, when u push him, he will oscillate to and fro in a certain time interval, period(p). Note that the time interval is always the same regardless of his mass and the time interval is still the same even u applied a greater or lesser force to push him. The natural resonance occur when no external force applied to the oscillation system. Imagine that if u only push him once and let him oscillate till the end, u will find that the period and frequency is the same, just the amplitude decreasing cuz of air resistance, that frequency iis its natural frequency. But if there is external force appiled like when : u push ur friend when he swing back to u and u keep doing it again and again, the frequency of the oscillation will be different than u let him oscillate on his own, so this is not its natural frequency because u keep applying external force. Another example of natural resonance, imagine u have an iron rod, u hung it with a thread. And u hit it with different force, u will notice the frequency is the same, just different in the amplitude of the sound regarding to the force u used to hit it.
, this might help. http://ocw.mit.edu/courses/physics/8-03-physics-iii-vibrations-and-waves-fall-2004/video-lectures/