Center of inertia frequency can be considered as a definition of "system frequency" under transient conditions. You can verify that under equilibrium conditions, i.e., when all machines settle to a common speed we, center of inertia frequency also equals we.

In this module, we will concentrate on center of inertia movement and not on relative motion between rotors. We shall assume that relative motion is stable ( machines stay in synchronism).

We illustrate the concept of "system frequency" as a quantity affected by the aggregate of forces acting on the system by the familiar mass - spring analogy.