A.3.3.TR:4.2 - Write each contribution as a relation

For a discrete action, write its enabling condition and its relation between the values before and after the action. A prime can mark an after-value: x is the counter before a write, and x’ is its value after the write. State what remains unchanged when the action affects only part of the state.

An action may leave several after-values possible. Keep the relation broad enough to represent them. An expression x’ = f(x,u) is appropriate when the supplied state and input u determine that after-value; a relation R(x,x’,u) can retain several possibilities.

For continuous evolution, recover the subject laws relating quantities, rates and interactions. These may be written as differential equations, algebraic constraints and any event conditions needed by the model. Include the initial and boundary conditions that the selected law requires. With constrained bodies, identify the forces exchanged through their connection and combine the equations, as :5.2 demonstrates.

The form of an equation does not establish its physical applicability. Recover its domain meaning and conditions from the subject account. When that account is missing, name the missing relation and return the conditional result already obtainable.