Resonance

Resonance: The increase in amplitude of oscillation of an electric or mechanical system exposed to a periodic external force whose frequency is equal to or some multiple of the natural frequency of the system.
Natural Frequency: The frequency an object will vibrate with after an external disturbance. These frequencies are usually discrete and depend only on the intrinsic physical quantities of the object.

In other words, all things have a natural frequency and if we put energy into a thing at its natural frequency, the energy could continue to build up to very high levels.

Example #1: Pushing a swing

	If you push a person on a swing, you must put the energy in at just the natural frequency of the swing or some multiple of it. In this animation, the energy is put in at the natural frequency of the pendulum. Notice how the energy builds constructively until it does work on the monkey's face.

	If you put the energy in too slow, you miss the swing being there to accept the energy. If you put it in too fast, you miss the swing more often. In this case the energy is put in at higher than the natural frequency of the pendulum. Notice that sometimes the energy is constructive and other times it is destructive. As a result of the destructive energy, the energy level does not build up.

Example #2: Shattering a glass

If a singer could match the natural frequency of the wine glass they could put more energy into the wine glass than it could handle. As the energy builds, the glass begins to deform beyond what its bonds can handle. The trick is to sing with the right frequency and being able to sustain that note. It is not about singing loud or horribly. It is also not possible for one note to shatter all glasses as each piece of glass would have its own natural frequency.

Example #3: Using Resonance to shatter a Kidney stone.

By tuning ultra sound waves to the natural frequency of a kidney stone, we can rely on resonance to pulverize the stone.