Andalusia PAU Technology and Engineering II common mistakes
Andalusia PAU Technology and Engineering II · Understand the traps and how to avoid them.
When a problem gives a force system in 2D, the candidate sums forces algebraically without resolving components, or resolves only one of the forces and then adds magnitudes directly (e.g., adding a 30 N force at 40° to a 20 N horizontal force as 50 N).
School physics habits of 'adding forces' persist, and the candidate does not systematically draw the free-body diagram or set up ΣFx and ΣFy separately before combining. Under time pressure the vector nature of force is dropped.
What to do instead
Adopt a fixed routine for every statics item: draw the free-body diagram with all forces labelled, choose axes, write ΣFx=0 and ΣFy=0 as two separate equations, solve, then compute the resultant with Pythagoras and give its angle with atan(Fy/Fx), stating the quadrant explicitly. Practise five mixed problems where the answer requires both magnitude and direction.
In beam bending and torsion problems the candidate uses the wrong section property (area instead of second moment of area, or diameter instead of radius) or forgets to convert mm to m before substituting into σ = M·y/I or τ = T·r/J, producing answers off by factors of 10³ or 16.
Formulas are memorised as symbols without attaching units to each quantity, and the difference between I (m⁴), W (m³) and A (m²) is not firmly linked to what each formula physically needs.
What to do instead
Before substituting, write a unit table for every symbol in the formula (M in N·m, y in m, I in m⁴). Compute I explicitly for the given section (rectangle bh³/12, circle πd⁴/64) and check its order of magnitude. Do dimensional analysis on the final result: stress must come out in Pa = N/m².
For a mechanism, the candidate calculates the transmission ratio but inverts it (input/output instead of output/input) or ignores efficiency, so a gear train or screw jack answer is wrong in direction or magnitude even though the arithmetic is right.
The ratio is treated as a formula to plug into rather than a physical statement about speed and torque; the candidate does not check whether the output should spin faster or slower and with more or less torque.
What to do instead
State the ratio verbally first ('the driven wheel turns 3 times slower and with 3 times the torque'), then write i = ω_in/ω_out = z_out/z_in and confirm the numbers match the verbal statement. For power trains include η explicitly: P_out = η·P_in, and check η < 1 always.
In DC circuit problems with series-parallel combinations, the candidate applies the current divider where the voltage divider belongs, or assumes the same current flows through parallel branches, and does not verify with Kirchhoff's laws.
The two divider formulas look similar and are memorised without their conditions (same current for voltage division in series; same voltage for current division in parallel). The candidate stops at the first numeric answer without a consistency check.
What to do instead
After solving, verify with two checks: (1) KCL at each node - currents in equal currents out; (2) power balance - power supplied by sources equals ΣI²R. Rederive the dividers from Ohm's law once so the conditions are understood rather than recalled.
When asked about a three-phase or transformer machine, the candidate mixes line and phase quantities (uses 400 V where 230 V applies, or forgets √3) and confuses apparent, active and reactive power, giving a kVA figure labelled kW.
Star and delta relationships are memorised as isolated facts, and the power triangle is not drawn. The candidate also neglects the power factor when converting between P and S.
What to do instead
Always draw the star/delta diagram and label U_L, U_f, I_L, I_f before calculating. Write the power triangle with P = S·cosφ, Q = S·sinφ and check units (W, var, VA). For a balanced load use P = √3·U_L·I_L·cosφ and confirm the result against the per-phase calculation P = 3·U_f·I_f·cosφ.
In control problems the candidate identifies a system as open or closed loop by whether it 'has a sensor' rather than by whether the measured output feeds back to influence the input, and cannot distinguish the effects of P, I and D actions on steady-state error, overshoot and settling time.
The vocabulary of control is learned as definitions without a mental model of the loop; the candidate has not traced a signal around a block diagram to see where feedback actually closes.
What to do instead
For every example, trace the signal path and mark the comparison point: if the measured variable is subtracted from the reference and the error drives the actuator, it is closed loop. Build a one-page table of P/I/D effects (P: faster, reduces but does not eliminate offset; I: removes offset, can destabilise; D: damps, amplifies noise) and apply it to two or three concrete examples such as a thermostat and a level controller.
In pneumatic circuit questions the candidate designs or reads a circuit without distinguishing 3/2, 5/2 and 5/3 valves, or connects a double-acting cylinder so it cannot retract, and mislabels the numbering of ports and valve positions.
Symbols are memorised visually but the number of ports and positions is not linked to the actuator type; the candidate does not simulate the circuit mentally through a full cycle.
What to do instead
Learn the naming rule: first digit = number of ports, second = number of positions. For each circuit, trace the air path in both positions of every valve and confirm the cylinder extends and retracts. Practise drawing a double-acting cylinder controlled by a 5/2 valve with a one-way flow regulator on the retract line.
In manufacturing and metrology items the candidate confuses the machining operation with the machine tool (says 'a lathe mills the workpiece'), or misreads a vernier or micrometer by taking the wrong scale division or forgetting the resolution, and does not account for tool wear or temperature when discussing tolerances.
Process names are learned from lists rather than from the relative motion between tool and workpiece; instrument reading is a psychomotor skill that is under-practised, and the candidate reads the main scale only.
What to do instead
Anchor each process to the motion: turning rotates the workpiece against a single-point tool; milling rotates the cutter against a translating workpiece; drilling feeds axially. For instruments, practise ten readings of a 0.05 mm vernier and a 0.01 mm micrometer, always writing main scale + (coincident division × resolution) and stating the resolution in the answer.
In energy and project questions the candidate calculates efficiency as output/input in the wrong units (mixing kWh and J, or kW and W) or treats a renewable source as constant, and in project methodology omits the budget, schedule or environmental-impact section from a project outline.
Energy units are handled loosely because everyday language uses them interchangeably; project methodology is treated as a list of headings to name rather than a structure to populate with content.
What to do instead
Convert everything to SI before dividing: 1 kWh = 3.6 MJ, 1 kW = 1000 W. Write efficiency as η = useful energy out / energy in and check η ≤ 1. For project outlines, use a fixed skeleton - objectives, specifications, budget, schedule (Gantt), environmental impact, risk - and write at least one concrete line for each heading.
In technical drawing items the candidate produces or interprets views that violate first-angle projection (placing the left view on the right), omits hidden lines, or dimensions with redundant or missing dimensions (over-dimensioning a chain, or leaving a feature undimensioned because it was dimensioned on another view).
The projection convention is memorised as a rule without the physical model of the object being folded out of the box; dimensioning is done by adding numbers where they fit rather than by a systematic plan.
What to do instead
For every drawing, place the object in the first-angle 'glass box', unfold it, and check that the view on the right of the front view is the left-hand view. Dimension by planning first: identify each feature, assign exactly one dimension per feature, avoid closed chains, and mark the datum. Compare your drawing against the standard symbols for centre lines, hidden lines and dimension arrows.