Forced Vibration. If a system is subjected to an external force (often, a repeating type
of force), the resulting vibration is known as forced vibration. The oscillation that arises in
machines such as diesel engines is an example of forced vibration.
If the frequency of the external force coincides with one of the natural frequencies of
the system, a condition known as resonance occurs, and the system undergoes dangerously
large oscillations. Failures of such structures as buildings, bridges, turbines, and airplane
wings have been associated with the occurrence of resonance.