La Rioja PAU Tech & Engineering II (UR) common mistakes
La Rioja PAU Tech & Engineering II (UR) · Understand the traps and how to avoid them.
In strength-of-materials problems, students compute stress with the force in newtons but leave the section area in mm², reporting a stress that is 10⁶ times too small or too large, and then carry that wrong value into the safety-factor comparison.
The exam gives sections in mm and forces in kN, and candidates mix units without converting to a single coherent system (N and m, or N and mm) before dividing. The habit comes from memorising σ = F/A without tracking the units through the calculation.
What to do instead
Before substituting, write the working unit system at the top of the page (e.g. N and mm, so stress comes out in MPa). Convert every magnitude once, at entry, and label each intermediate result with its unit. Check plausibility: structural steel yields around 250 MPa, so an answer of 0.0003 MPa or 3×10⁸ MPa signals a conversion error.
When drawing shear-force and bending-moment diagrams, students omit reaction forces at one support or misplace the point of contraflexure, producing diagrams that do not close to zero at the free end and a maximum moment at the wrong section.
They jump straight to the diagram without first solving the full free-body diagram, and they treat distributed loads as point loads at the wrong position (midpoint of the whole span rather than of the loaded length).
What to do instead
Follow a fixed sequence: draw the FBD, compute reactions by ΣF and ΣM, then integrate load to shear and shear to moment segment by segment, checking that shear returns to zero and moment to the end condition. Locate maxima where shear crosses zero, and mark the contraflexure where moment changes sign.
In mechanism problems (four-bar, slider-crank, gear trains), students apply the velocity ratio or Grashof condition as a memorised formula without checking which link is fixed or which gears are internal, giving a wrong transmission ratio or a wrong conclusion about whether the mechanism can rotate fully.
The formulas are learned in one canonical configuration, and the exam presents a variant (fixed shortest link versus fixed longest link, or an epicyclic train) where the sign convention or the reference link changes the result.
What to do instead
Redraw the mechanism with the fixed link clearly marked, label each link length or tooth count, and derive the ratio from the basic relation (ω₁r₁ = ω₂r₂, or the tabular method for epicyclic trains) rather than recalling a final formula. State the sign or direction explicitly, and verify with a limiting case.
In DC and AC circuit problems, students solve series-parallel networks by treating a parallel branch as series, or forget that in AC the impedance of a capacitor and inductor carry ±90° phase, so they add magnitudes arithmetically instead of as phasors.
The arithmetic habit from resistive circuits carries over, and the complex or phasor representation is applied mechanically without a picture of the phase relationships.
What to do instead
Redraw the circuit and mark nodes before simplifying; reduce parallel and series blocks step by step. For AC, write each impedance in rectangular or polar form (Z_R = R, Z_L = jωL, Z_C = 1/(jωC)) and add with complex arithmetic, then convert back to magnitude and phase. Sanity-check that the total impedance magnitude lies between the smallest and largest branch impedances.
When asked about a transformer, induction motor or DC machine, students state a qualitative claim (for example 'the transformer steps up power' or 'the motor runs at synchronous speed') that violates conservation of energy or slip, instead of using the equivalent relations.
The machines are studied descriptively, and the quantitative constraints (power balance, slip, turns ratio, back-emf) are not rehearsed as checkable rules.
What to do instead
Anchor each machine to its governing relations: transformer V₁/V₂ = N₁/N₂ with S₁ ≈ S₂ (power is conserved, not increased); induction motor slip s = (n_s − n)/n_s with rotor frequency sf₁; DC motor E = V − I_aR_a. In any answer, quote the relation, substitute, and check that efficiency stays below 100 % and slip lies between 0 and 1.
In energy and power questions, students compute an efficiency or a payback time using the wrong energy basis (for example mixing kWh with W, or comparing primary energy with delivered electricity) and report a payback period off by a factor of hours or a factor of the conversion efficiency.
Energy, power and time are used interchangeably in everyday language, and the distinction between primary energy and final energy is not reinforced by practice.
What to do instead
Write the dimensional identity first: E = P·t, with P in kW and t in h giving kWh. Convert every quantity to kWh or J before dividing. When comparing sources, state whether the figure is primary or final energy and apply the conversion efficiency explicitly. Check that the payback time has units of years or hours and is physically plausible.
In PLC and automation problems, students write a ladder or logic solution that ignores the scan cycle, using a self-holding circuit that drops out or a latch that never resets, or they confuse normally-open field contacts with normally-closed program contacts.
Ladder logic is treated as static Boolean algebra, and the physical state of a sensor (energised versus de-energised) is not separated from the program symbol used to represent it.
What to do instead
Trace the program rung by rung across one complete scan: read inputs, solve logic, write outputs. For every latch, identify the set condition and the reset condition explicitly. Draw the field device state next to each contact (energised/de-energised) and confirm the program symbol matches the intended action. Test with a truth table of input combinations before finalising.
In technical drawing, students dimension a part redundantly or ambiguously: the same dimension given twice, dimensions that do not sum to the overall size, or a tolerance stack that leaves the fit undefined, and they omit the scale or projection symbol.
The drawing is produced by copying shapes rather than by planning the dimension scheme, and the standards for dimensioning (no closed chains, reference dimensions in brackets, first- versus third-angle projection) are not applied systematically.
What to do instead
Before drawing, decide the datum faces and dimension from them, giving each feature one controlling dimension. Check that no dimension is repeated and that no chain closes on itself; if a dimension is for information only, bracket it as a reference. Add the projection symbol and scale, and verify the tolerance stack sums to the specified clearance.
In materials selection, students choose a material by name ('use aluminium because it is light') without matching the governing property index to the loading case, ignoring that stiffness, yield strength and toughness scale differently with density.
Material properties are memorised as a list rather than as a set of indices derived from the function, and the difference between strength-limited and stiffness-limited design is not distinguished.
What to do instead
Identify the function, objective and constraint first, then derive or select the appropriate index (for example E/ρ for a light stiff beam, σ_y/ρ for a light strong tie). Compare candidate materials on that index using a property chart or table, and state the trade-off (cost, corrosion, manufacturability) explicitly in the justification.
In the technology project section, students present a solution without a requirements list, budget breakdown or verification plan, so the design cannot be evaluated against criteria and the cost estimate omits labour, tolerances or contingency.
The project is treated as a description of a finished product rather than as a documented process, and the methodology (specification, generation, selection, detailed design, verification) is not rehearsed end to end.
What to do instead
Structure the answer to the project methodology: state the need and requirements with measurable criteria, list alternatives, justify the selection against those criteria, then give a cost breakdown by item (materials, components, labour, contingency) and a verification step for each requirement. Use a table so each requirement maps to evidence of compliance.