Issue 87. Example was linear.

I provided  a linear example for what was supposed to be a
nonlinear example.

09-Nonlinear-Filtering.ipynb

@@ -310,11 +310,9 @@
     "\n",
     "The Kalman filter that we have developed uses linear equations, and so the filter can only handle linear problems. But the world is nonlinear, and so the classic filter that we have been studying to this point can have very limited utility. \n",
     "\n",
-    "There can be nonlinearity in the process model. Suppose we wanted to track the motion of a weight on a spring, such as an automobile's suspension. The equation for the motion with $m$ being the mass, $k$ the spring constant, and $c$ the damping force is \n",
+    "There can be nonlinearity in the process model. Suppose we want to track an object falling through the atmosphere. The acceleration of the object depends on the drag it encounters. Drag depends on air density, and the air density decreases with altitude. In one dimension this can be modelled with the nonlinear differential equation\n",
     "\n",
-    "$$m\\frac{d^2x}{dt^2} + c\\frac{dx}{dt} +kx = 0$$\n",
-    "\n",
-    "There is no linear solution for $x(t)$ for this second order differential equation, and therefore we cannot design a Kalman filter using the theory that we have learned.\n",
+    "$$\\ddot x = \\frac{0.0034ge^{-x/22000}\\dot x^2}{2\\beta} - g$$\n",
     "\n",
     "A second source of nonlinearity comes from the measurements. For example, radars measure the slant range to an object, and we are typically interested in the aircraft's position over the ground. We invoke Pythagoras and get the nonlinear equation:\n",
     "\n",