For nonconservative systems the energy of the system does not remain constant along its trajectories
in the Poincaré phase plane. Consequently, the level curves of the energy function are
not integral curves for the system. However, if we know that the energy is decreasing along a
trajectory, then it follows that the trajectory is moving toward a state that has a lower total
energy. This state may correspond to a critical point or a limit cycle (defined in Section 12.6).
We will demonstrate this behavior in the following examples.
A nonconservative system with nonlinear damping has the form