Resonance

Resonance has wide ranging practical applications. Any machine or structure is likely to be subjected to periodic forces, either as a result of its own operation (e.g., the motor in any vehicle imposes an oscillation or vibration on every part of the vehicle) or through the action of some external agent (e.g., wind exerts a periodic force on buildings and structures through vortex shedding). If you keep your eyes and ears open you will notice countless examples of forced oscillations.

Forced oscillations can prevent machines operating efficiently, as when an unevenly loaded spin drier cannot achieve its normal working speed because much of its energy is being diverted into a violent wobbling. More seriously, forced oscillations can result in fatigue failure of metal components at stresses well below the tensile strength of the metal, simply as a result of repeated flexing (like breaking a piece of wire by bending it to and fro). If resonance occurs, forced oscillations can be violent and may have catastrophic results (as in the Tacoma Narrows bridge collapse). An understanding of forced oscillations is clearly essential to engineering.

Forced oscillations are not always destructive; sometimes engineers and scientists can make positive use of them. Nor is the phenomenon confined to mechanical oscillations. Microwave ovens heat food as a result of a forced oscillation of the molecules within the food, particularly water molecules, which are polar (they are permanently charged positive at one end and negative at the other, see figure D21). Infra-red absorption spectroscopy, which is an important technique for chemists, involves the forced oscillation of atoms or groups of atoms within a molecule. The conversion of radio waves to electric currents in an aerial is an example of a forced oscillation, and the operation of a tuning circuit in a radio relies on resonance.