PHY.7:4.6 - Return through the premise that changed

When an interaction changes, revise its action term or force contribution. When a support or boundary changes, revise the histories, velocity mapping and allowed variations before reusing the equations. When a coordinate description changes, transform the whole expression while keeping the physical history recoverable.

Equivalent expressions can describe the same motion. In the ordinary fixed-endpoint principle, adding dF(q,t)/dt changes the action only by endpoint values, so it preserves the interior equations. A use with different endpoint freedoms must also carry the changed boundary term. Distinguish this redescription from introducing another physical interaction.

If a computed consequence fails, locate whether the fault is the physical principle, admissible comparison, mathematical derivation or obtaining procedure. B.5.MPC.R provides the combined return. Seek additional physical evidence only where its possible result could alter the decision or action; a useful conditional derivation can already be used with its stated limits.