Physics
Core concepts
Physics underpins every system you will maintain. You need: matter (states, density, specific gravity), mechanics (mass vs weight, force, moments, centre of gravity, stress and strain, levers), motion (velocity, acceleration, Newton's laws), energy (kinetic, potential, work, power), heat (temperature scales, expansion, heat transfer by conduction, convection and radiation), pressure (Pascal's law, hydrostatic pressure, gauge vs absolute), sound, and basic light behaviour.
Traps to watch
- Mass vs weight: mass in kg is constant; weight is a force in newtons and varies with gravity.
- Gauge vs absolute pressure: absolute = gauge + atmospheric. Questions often quote one and expect the other.
- Moment arms: the perpendicular distance from the pivot, not the length of the lever along its axis.
- Specific gravity: a ratio with no units; density of water is 1000 kg/m³.
- Temperature scales: convert with K = °C + 273 (or 273.15); a temperature difference converts 1:1 between °C and K.
Worked example
A force of 200 N acts at 0.5 m from a pivot. Moment = 200 × 0.5 = 100 N·m. To balance it with a 50 N force, the required arm is 100 ÷ 50 = 2.0 m. Note the balance condition: sum of clockwise moments = sum of anticlockwise moments.
Rule of thumb
For any mechanics question, sketch the free-body diagram and mark distances from the pivot. Most errors are geometry errors, not arithmetic.