(iii) Critical case between oscillatory and non-oscillatory motion :

Damping corresponding to this case is called critical damping, c_c

                                                                                          (3.25)

Any damping can be expressed in terms of the critical damping by a non-dimensional number called the damping ratio

                                                                                                                              (3.26)

Response corresponding to the critical damping case is shown in Figure 3.4 for various initial conditions.