--- name: math-thumb-applied-mathematics description: "Apply Chapter 18 (Applied Mathematics: Scaling, Regimes, and Model Sanity) of Mathematical Rules of Thumb to solve, check, or teach problems. Use it to reject inconsistent models and use dimensionless ratios to choose a plausible first approximation." --- # Applied Mathematics: Scaling, Regimes, and Model Sanity Use this chapter to help the reader make a checked mathematical decision. All 14 numbered rules are available in [the chapter source](references/chapter.md). [The workbook](references/notebook.md) contains a lab, exercises, solutions, and the full rule checklist. [The local rule index](references/rules.json) supplies discovery metadata. ## Start from the reader's task Infer solve, learn, or audit mode from the request. In solve mode, use their supplied numbers and target; in learn mode, use the workbook or their chosen rule; in audit mode, inspect their actual calculation before replacing it. Gather only missing information that changes the choice: Output and accuracy target; variables with dimensions; geometry; characteristic length, speed and time; property values; relevant regimes. If the question falls outside this chapter, say which mathematical operation is missing and suggest a relevant chapter. If the whole-book skill is available, it can carry the task onward, but this chapter works independently. ## Select and apply a rule 1. Define the output, domain, and decision accuracy. List variables with units. 2. Check dimensional homogeneity, signs, conservation, symmetries, and easy limits before solving. 3. Choose characteristic scales that make typical variables order one; allow different scales in different regions. 4. Form independent dimensionless groups and identify plausible dominant balances. 5. Compute relevant regime ratios with explicitly defined \(U,L,T\), properties, and geometry. 6. Select the simplest model consistent with those ratios, then substitute the approximation back into every discarded term. 7. Compare with data or a higher-fidelity model near thresholds; escalate when multiple ratios are order one or uncertainty crosses regimes. Read the selected complete profile, including its equation, “How to read it,” and “How to use it.” The compact graph assumptions are search cues, not a substitute for the profile. Preserve the numbered citation and role. **Independent**, **Workflow**, and **Specialized** describe the relationship to a calculation; exactness, approximation, bound, diagnostic, and heuristic describe a different dimension. Use verified inputs, show the substitution and units, and interpret the result in the reader's decision. Verify by an appropriate bound, alternative computation, limiting case, residual with conditioning, or sensitivity check. If a required condition fails, reject that use and give the specific missing information or alternative method; do not calculate a plausible-looking answer from an invalid formula. **Essential boundary:** Dimensionally consistent does not mean physically correct. State characteristic scales explicitly and re-check discarded terms after choosing a regime. For a sufficient independent result, stop with the decision it supports. For a workflow or specialized rule, name the downstream calculation still needed. A numerical demonstration is evidence for that instance, not a universal proof. ## Teach and check understanding Use the [workbook](references/notebook.md) for guided practice. Start with 18.1.1, 18.2.4, 18.3.5 when the reader wants a starting exercise. Ask for an attempt, offer a relevant hint, and reveal the answer when requested or when teaching requires it. Do not force a quiz when the reader asked for a worked solution. Check whether the reader can explain the controlling quantity, apply the rule to a changed input, identify an invalid use, and distinguish a final answer from a preparatory step. Track only demonstrated work. Give a short prerequisite explanation when needed; avoid requiring completion of earlier chapters. ## Return a usable result Include the chosen rule numbers, assumptions that matter, calculation, verification, and next action. For ongoing work, offer this compact record: question; inputs and units; rules; claim type; book role; assumption status; result and error; check; decision; unresolved next step. Write a progress file only when asked or within an already authorized notebook-editing task. The source chapter is a fixed book snapshot. Preserve its mathematical qualifications and historical evidence gaps. Use outside material only when the reader's task needs it, verify material facts appropriately, and identify that material separately from the book. ## Illustrated exploration Open [the browser reader](assets/reader.html) or [the saved illustrated notebook](assets/notebook.ipynb). [Equation cards](references/equations.json) record the book rule, formula, fixed inputs, supported choices, assumptions and executed default results. - **C18-D01: Screen a lumped thermal model**: rule 18.3.5. - **C18-D02: Compare transport time scales**: rule 18.3.2. - **C18-D03: Construct a dimensionless flow comparison**: rule 18.3.1. - **C18-D04: Feel the square law of diffusion time**: rule 18.3.4. Use a saved illustration only when its conditions fit. Browser controls select finite precomputed choices; they do not calculate arbitrary reader inputs. For different inputs, make a checked calculation using the selected rule. Explain what changes, and never claim the notebook ran or the browser was viewed unless it did. Offer prediction questions for learning; answer direct requests without a mandatory quiz.