So far, we have only investigated oscillating systems that are set into motion abruptly. When an oscillating system is driven by a continually varying force, the amplitude of the motion varies in response to the driving frequency. If the driving frequency happens to match the natural frequency of the system, the largest possible amplitude of vibration occurs. This is a phenomenon known as "resonance". Perhaps the most famous unintentional resonance in history is that of the Tacoma Narrows Bridge and its unfortunate collapse.
