Mechanical Comprehension and Work-Sample Judgment
What this section asks
This domain combines Mechanical Comprehension with Work-Sample Judgment. Mechanical items test levers, moments, pulleys, gears, pressure, and basic force/weight relationships. Work-sample items present a realistic recruit task-damaged kit, a safety hazard, a conflicting instruction-and ask for the best next action. The judgment rubric usually rewards safety, procedure, accurate reporting, and teamwork over speed or personal convenience.
Core rules
- Moment = force × perpendicular distance from the pivot. Balance: effort × effort arm = load × load arm.
- A movable pulley shares load between two rope sections, giving a mechanical advantage of 2; a fixed pulley changes direction only.
- Meshed gears turn in opposite directions; larger gears turn more slowly and with greater torque.
- Mass is not weight: weight = mass × gravity (about 9.8 N/kg). Use newtons for force.
- In work-sample questions, identify the hazard and the rule first, then choose the option that controls risk and reports through the chain of command.
Worked example
A lever has a load 0.5 m from the fulcrum and an effort applied 1.5 m from the fulcrum. If the effort is 200 N, what load can be balanced? Moments balance: 200 N × 1.5 m = load × 0.5 m. Load = 300 ÷ 0.5 = 600 N. The longer effort arm gives a mechanical advantage of 3. A common wrong answer is 100 N, from dividing by the effort arm instead of the load arm.
Traps
Using slant length instead of perpendicular distance; mixing mass (kg) and weight (N); assuming a fixed pulley halves the effort; reversing gear direction; and in judgment items choosing the most dramatic action rather than the safest, procedure-compliant one. Avoid "tell no one" options when a defect affects safety.