{
  "schema_version": "1.0",
  "record_type": "mistake_journal_template",
  "item_id": "qs-fbd-boundary-001",
  "property": "QuietStride Learning",
  "subject": "Biomechanics",
  "topic": "Free-Body Diagrams: Get the Boundary Right",
  "prompt": "Which reasoning correctly solves the defined forearm model?",
  "options": {
    "A": "Remove the joint reaction because its elbow moment is zero.",
    "B": "Keep the joint reaction in force balance. Solve muscle force from moment balance.",
    "C": "Add a separate 18 N\u00b7m muscle moment alongside the muscle force.",
    "D": "Place both sides of every action\u2013reaction pair on the forearm diagram."
  },
  "correct_option": "B",
  "selected_option": null,
  "confidence_before": null,
  "learner_reasoning": null,
  "is_correct": null,
  "error_type": null,
  "conceptual_trap": "A zero moment arm removes a torque contribution. It does not remove the force.",
  "evidence_check": "The downward loads create 18 N\u00b7m. A 0.03 m muscle moment arm requires 600 N. Vertical balance then requires a 530 N downward reaction. These results follow from this simplified static model, not a real person\u2019s joint loading.",
  "corrected_rule": "Choose the system first; choose the moment reference second. Keep all external forces in force balance.",
  "source_url": "https://openstax.org/books/university-physics-volume-1/pages/12-2-examples-of-static-equilibrium",
  "transfer_question": "What changes if you take moments about the hand?",
  "transfer_answer": "The joint reaction now has a moment arm. The hand load does not. The physical forces remain unchanged.",
  "created_at": null,
  "review_due": null,
  "review_status": "unattempted"
}