Mechanical Reasoning and Applied Physics
What the domain tests
This domain tests basic mechanics and physics applied to fire-service equipment and building systems: levers, moments, pulleys, gears, pressure, hydraulics, friction, work, power, energy, density, thermal expansion and simple electricity. You are expected to choose the correct relationship and apply it to a practical situation, often with unit conversion.
Core rules to apply
- Moment: moment = force × perpendicular distance from the pivot. For balance, clockwise moments equal anticlockwise moments.
- Pressure: pressure = force ÷ area. In a hydraulic system, pressure is transmitted equally, so F1/A1 = F2/A2.
- Flow and power: power = pressure × flow rate when units are consistent. Work = force × distance; power = work ÷ time.
- Pulleys and gears: a movable pulley can halve the effort but the rope must be pulled twice as far. Gear ratio relates teeth and turns; direction reverses between meshed external gears.
- Friction: opposes relative motion and converts mechanical energy to heat. It reduces mechanical advantage in real systems.
- Thermal expansion: most solids and liquids expand when heated; enclosed liquids can create large pressure rises.
Worked example
A hydraulic press has an input piston area of 0.01 m² and an output piston area of 0.05 m². A force of 200 N is applied to the input piston. Find the output force. Reasoning: pressure = 200 ÷ 0.01 = 20,000 Pa. The same pressure acts on the output piston, so output force = pressure × area = 20,000 × 0.05 = 1,000 N. The force is multiplied by 5 because the area is 5 times larger. The output piston moves only one-fifth as far, so energy is not multiplied.
Traps that cost marks
Use perpendicular distance for moments, not the length of a sloping lever. Do not mix Pa with kPa or N with kN. Distinguish gauge pressure from absolute pressure. Do not assume mechanical advantage is free: force multiplication trades against distance or speed. In pulley questions, count the rope sections supporting the load. In gear questions, check whether the question asks for speed, torque or direction.
Exam technique
Sketch the system and mark forces and distances. Write the formula with units. Check whether the answer is physically plausible: a small input piston should not produce a smaller output force when the output area is larger.