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ModelFixedRelations

EJS evaluates the statements on the Fixed Relations panel before the first evolution step, and after that between each evolution step.

The Evolution panel and the Fixed Relations panel complement each other. To show how that works out let us take the example of a simulation that works with position and velocity as variables for calculation of the dynamics, in which the kinetic energy is also calculated, for displaying that in the view. Then the correct place for calculating the kinetic energy is on the Fixed Relations panel.

More generally, Fixed Relations are expressed explicitly by expressions like: {$$ x_i = g_i(x_1,x_2,\dots,x_{i-1},x_{i+1},\dots,x_n) $$} where, as you can see, each variable appears only on one side of the equation.

Using again the example of a simulation that works with position and velocity as variables, with kinetic energy calculated on the Fixed Relations panel:

• If the Evolution evaluates ODEs then what needs to be in the Evolution panel are the equations in which the current position and velocity are the input for calculating the next position and velocity. The kinetic energy is not input for calculating the next position and velocity, so it doesn't need to be in the Evolution panel.
• If the user pauses the simulation and alters one of the dynamic variables, then the kinetic energy must immediately be recalculated, it would not look good if updating the kinetic energy would have to wait until the first evolution step has been calculated.

So while the statements in the Evolution panel and the Fixed Relations are similar (both deal with a change in one variable produced by changes in other variables), there are clear criteria to decide whether a particular calculation belongs in the Evolution panel or the Fixed Relations panel.