Objects that can vibrate tend to do so at their natural frequency. This natural frequency is known as the resonant frequency. A swing and a tuning fork are two examples of objects that will oscillate at a resonant frequency. The resonant frequency is dependent on the design of the object and the materials that it is made of.
Resonance is when the object is exposed to vibrations that are equal to the objects resonant frequency. In this situation, the external vibration is vibrating at the driving frequency. Resonance occurs when a weak vibration from one object causes a strong vibration in another object. Resonance can become a problem when the amplitude of the vibrations becomes too great – the object may be destroyed.
Resonance can be thought of using the swing analogy: A swing will vibrate back and forth at its natural or resonant frequency. If you provide a force and push the swing at the exact moment it reaches its highest point you will be providing a driving frequency and this will result in resonance – the swing will continue to move with greater amplitude.
Resonance can also be observed when a bow moves across the strings of a violin or when you blow across the mouthpiece of a bottle.
When resonance occurs a transfer of energy will take place. The energy from the object providing the driving frequency (forced vibration – pushing a swing) will transfer energy to the object oscillating at a natural frequency (free vibration – the swing). This energy transfer will occur at maximum efficiency whilst ever the driving frequency matches the natural frequency.