Add materials on how to check if your model behaves as expected

[moorepants]

When I develop models I check various intermediate steps with known (simple) input/output pairs. Once the simulation can run you can then make it do very simple simulations to see it behaves as expected. It would be nice to have a chapter that shows how to do this for a medium complexity problem.

[Peter230655]

To me, looking at the total energy of the system has helped me many times. It is simple to do, and of course only a necessary condition for the equations to be right.

[moorepants]

The energy chapter gives a example of this with the jumper (although that wasn't my goal there).