PHY.7:4.1 - Choose a principle for the physical question

State the consequence needed and the physical situation it concerns. Choose the retained participants, interactions and regime. PHY.5 helps decide which physical detail matters.

Recover an applicable variational principle from the physical theory, or propose one with a stated physical basis and conditional use. Identify what it assigns to a history and what property the realized history is required to have. A stationary action, minimum energy at equilibrium and a dissipative variational rule impose different conditions. Select the one appropriate to the question.

For a smooth classical configuration history q(t), a common form is

S[q] = integral from t0 to t1 of L(q,qdot,t) dt,

where L is the Lagrangian. A field description can instead integrate a density over space and time. The following steps show how to use such a principle; another physical principle can require a different functional or variation rule.

If the action or its physical grounds are missing, identify the interaction or assumption needed to construct them. A balance with a response law may settle the immediate question while the variational formulation remains open. The availability of a differentiation tool does not resolve that physical choice.