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Undamped Free Response

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Free vibration occurs because there is an initial disturbance to the system.
The initial disturbance is referred to as an initial condition, which can
be either a displacement of the mass, an initial velocity of the mass,
or both. Frequency of the system is determined by the mass and spring constant
of the system, initial conditions do not affect the vibration frequency
of an undamped system, therefore it is called the natural frequency. Vibration
amplitude and the phase angle are also determined by initial conditions.
Theoretically, an undamped free vibration system will vibrate forever once
it is started.

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DANIEL J. INMAN, Engineering Vibration, Prentice Hall, Englewood Cliffs,
New Jersey,1994