  In his famous book entitled \textit{Theory of Oscillations}, Nicolas Minorsky
wrote: "\textit{each time the system absorbs energy the curvature of its
trajectory decreases} and \textit{vice versa}". According to the \textit{Flow
Curvature Method}, the location of the points where the \textit{curvature of
trajectory curve}, integral of such planar \textit{singularly dynamical
systems}, vanishes directly provides a first order approximation in
$\varepsilon$ of its \textit{slow invariant manifold} equation. By using this
method, we prove that, in the $\varepsilon$-vicinity of the \textit{slow
invariant manifold} of generalized Li\'{e}nard systems, the \textit{curvature
of trajectory curve} increases while the \textit{energy} of such systems
decreases. Hence, we prove Minorsky's statement for the generalized Li\'{e}nard
systems. Then, we establish a relationship between \textit{curvature} and
\textit{energy} for such systems. These results are then exemplified with the
classical Van der Pol and generalized Li\'{e}nard \textit{singularly perturbed
systems}.
