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Consider the differential equation describing the motion of a simple pendulum:
where denotes the length of the pendulum, the gravitational acceleration and the angle between the pendulum and vertically downwards. In this system there is a conserved quantity H (the Hamiltonian), which is given by
With this defined, one can plot a curve of constant H in the phase space of system. The phase space is a graph with along the horizontal axis and on the vertical axis – see the thumbnail to the right. The type of resulting curve depends upon the value of H.
If then no curve exist ( must be imaginary).
If then the curve will be a simple closed curve (see curve) which is nearly circular for small H and becomes "eye" shaped when H approaches the upper bound. These curves correspond to the pendulum swinging periodically from side to side.
If then the curve is open, and this corresponds to the pendulum forever swinging through complete circles.
In this system the separatix is the curve that corresponds to . It separates (hence the name) the phase space into two distinct areas. The region inside the separatrix has all those phase space curves which correspond to oscillating motion back and forth of the pendulum (whose equation of motion is given above), whereas the region outside the separatrix has all the phase space curves which correspond to the motion with the pendulum continuously turning through vertical planar circles.
Logan, J. David, Applied Mathematics, 3rd Ed., 2006, John Wiley and Sons, Hoboken, NJ, pg. 65