Resonance A driving force in tune with the natural frequency (sometimes known as the resonant frequency) of an oscillator can buildup larger amplitudes than the oscillator could alone. This buildup is known as resonance or sympathetic vibration. Imagine swinging on a swing. If someone pushes you at the right time, the amplitude of your swing increases. The pushing has to be at the correct frequency, however. If someone pushes you at random intervals, the chances are it will not cause you to swing very much higher, but if the pushing is in tune with your natural frequency, you can go very high indeed. This principle applies to more than swings. Put two tuning forks of equal frequencies side by side, but not touching. Strike one tuning fork so that you can hear its tone, and then suddenly silence it. You can still hear a faint tone. This is because the second tuning fork has started vibrating sympathetically. Resonance is something engineers have to be quite careful to remember. In 1940, the Tacoma Narrows bridge, now better known as "Galloping Gurdy" collapsed when a steady wind provided the energy for the bridge to begin oscillating in simple harmonic motion. The amplitude of the oscillation continued to increase, and the bridge began twisting as well, until eventually it collapsed. These days engineers try to avoid this sort of resonance by adjusting their designs. It is also for this reason that army companies "break step" when traveling over bridges. Bridges have been known to collapse because the frequency of the marching of many soldiers all in step caused sympathetic vibration in the bridges. To Top