Physics
What you must understand
Physics underpins every system you will maintain. Know matter and its states, mass versus weight, density, force, pressure (including atmospheric, gauge and absolute), work, energy, power, and the gas laws. Understand heat transfer (conduction, convection, radiation), thermal expansion, and the relationship between temperature, pressure and volume. Study mechanics: velocity, acceleration, Newton's laws, momentum, friction, and moments (torque = force × perpendicular distance). Understand simple machines and mechanical advantage, plus basic fluid dynamics including Bernoulli's principle and the venturi effect, which explain carburettor and pitot-static behaviour.
Traps in questions
- Mass versus weight: mass in kg is constant; weight is a force in newtons and varies with gravity.
- Gauge versus absolute pressure: absolute = gauge + atmospheric. Questions often quote one and expect the other.
- Torque arm: the distance must be perpendicular to the force line of action.
- Gas law confusion: Boyle (constant T), Charles (constant P), combined gas law - check which variable is held constant.
- Thermal expansion direction: different materials expand at different rates; that is why dissimilar-metal joints and bimetal strips behave as they do.
Worked example
A force of 200 N acts at 0.5 m from a pivot, perpendicular to the arm. Moment = 200 × 0.5 = 100 N·m. If the force were applied at 30° to the arm, the perpendicular component is 200 × sin 30° = 100 N, giving 50 N·m.
Rule of thumb
Always sketch the free-body diagram and mark the direction of every force and moment before substituting numbers.