... and inertial forces which act during the oscillation. These factors give rise to the property known as the natural frequency. An example of free vibrations would be how a tuning fork will continue to vibrate following a knock. Forced vibrations occur from externally varying forces and are independent of the balance between elastic and inertial forces which act during the oscillation. But when a forced frequency equals the natural one, resonance occurs. The most common example of a system which undergoes forced vibrations is one which includes an internal combustion engine. Usually the frequency of the vibratory oscillation caused by the engine shall pass through natural frequency. The less time the frequency of oscillation spends close to the natural frequency the better. This is why diesel engine vibrate more on start-up than petrol ones since the are slower to speed up. Self-excited vibrations are sustained by the vibratory motion itself, hence; ...

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