Resonance
If a musical instrument or human voice of a certain pitch produces a musical note, it can cause a nearby violin or piano string of the same pitch to begin to vibrate. This phenomenon is an example of resonance.
Many objects have a "natural" frequency at which they will oscillate or vibrate. (In actuality friction or wind resistance will cause the oscillation to eventually stop.) If a varying force is applied to an object with a certain natural frequency, and if the force also varies with that frequency, the object will begin to oscillate strongly. In the example above, sound waves are the force on the violin string, created by a human voice. The voice causes a resonant response in the violin.
A system can behave wildly if subjected to a resonating force. Soldiers routinely break their marching step when crossing a bridge, because the collective force of their feet has been known to cause a resonant response by the bridge, causing it to collapse. On November 7, 1940, the 5,939 ft (1,961 m) Tacoma Narrows suspension bridge in Washington state dramatically collapsed in a 42 mile per hour (68 km/hr) windstorm that caused huge resonant, twisting responses by the bridge, which thereby earned the nickname "Galloping Gertie." The bridge was rebuilt ten years later with the same structure, but the stiffness of the structure was changed (this changed the natural frequency of the bridge) so a wind of usual speed cannot resonate with the bridge. Today, no suspension bridge will be built today without a wind tunnel test. Spacecraft, aircraft, and ships, all subject to tremendous forces, must also be designed so that vibrations caused by their engines or by their movement through air do not come near resonant frequencies.
Electrical circuits carrying alternating current can exhibit a similar behavior. Circuits like those controlling a television antenna can be built so that their natural frequency is near the frequency of the electromagnetic waves carrying the television signal. In this way the circuit responds by absorbing the maximum amount of energy from a signal at the same frequency.
Atoms and their nuclei can also exhibit a resonant-like response to radiation. Electromagnetic waves of just the right frequency will induce changes in the structure of the atoms or nuclei, as they absorb the photons in the waves and change their energy levels. In the case of nuclear magnetic resonance, atomic nuclei placed in a moderately large magnetic field will absorb and emit radio waves at certain well-defined frequencies, a property exploited in magnetic resonance imaging, or MRIs.
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