It's defined as the frequency that an object vibrates at when there is no driving force. What's a driving force? IS the natural frequency the frequency at which the atoms inside vibrate? People usually use the example of hitting a baseball bat or striking a tuning. Isn't the hitting a driving force? And when you hit a tuning fork harder, the frequency of the vibration doesn't change. The amplitude changes. Can someone explain that? Does anyone have like a good animation of wave propagating through air and changing amplitude?
If you hit a tuning fork with a hammer, you provide a driving force. Once your hammer is no longer in contact with the tuning fork, there is no longer any driving force, and the fork vibrates at it's natural frequency.