PHY.1:4.2 - Recover the interactions that can change the requested result

Follow the physical route by which the proposed input affects the output. Identify the participants, interactions and constraints on that route. Include an exchange across the chosen boundary when it contributes to the result. In a deformation problem, distinguish the force applied at an end from weight distributed through the body. In a transport problem, distinguish material carried by motion from material spread by diffusion.

Use the applicable theory to express those contributions. B.5.TU supplies the passage from theory to the encountered case; A.3.3.TR helps when several interactions determine the same evolving state. State the physical premises still needed rather than filling an unknown interaction with a convenient equation.

Estimate the relative size or time scale of competing effects. Derive that comparison from their laws: which quantity multiplies each term, and what spatial or temporal variation makes the term large? In a regime where one effect is negligible for the output, retain the reason for that simplification and the conditions under which it can fail.

Bring the preparation into the same account. Geometry, contacts, constraints and initial conditions can alter the solution even when the material equations are unchanged. For a field, a boundary condition applies over a surface or interval; matching its value at one point may leave the intended problem different elsewhere.