In Fig. 4( c), the frequency of the impressed force
has increased beyond resonance. Now the inertia force
exceeds the spring force, and the difference must be
supplied by the impressed force. This is the condition
that exists in the springless (sometimes called "brute
force") system, where the spring force along the line
of action is negligible, and the impressed force equals
the inertia force, leading it by 180 deg. at zero damping.
From the foregoing illustrations, it will be seen that
very little power will be required, in the natural-frequency
condition represented by Fig. 4(b), to apply
the impressed force needed to maintain the steadystate
oscillation, if the frequency ratio remains near
unity and damping is light and constant. In practice,
the frequency ratio is set at a value slightly below unity,
to provide some marginal stability against the effects
of increasing loading.