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...