Cameroon Baccalauréat Série F1 common mistakes
Cameroon Baccalauréat Série F1 · Understand the traps and how to avoid them.
In statics problems, the candidate writes the equilibrium equations but omits one reaction component (for example forgets the horizontal reaction at a built-in support, or treats a simple support as if it could carry moment), then solves the resulting system without checking that the number of unknowns equals the number of independent equations.
Candidates memorize the three scalar equations ΣFx=0, ΣFy=0, ΣM=0 and apply them mechanically, without first drawing a complete free-body diagram and classifying each support by the reactions it can actually provide. The isostatic/hyperstatic check is skipped because it feels like extra work.
What to do instead
Before writing any equation, draw the isolated body, replace each support by its standard reaction set (roller: one normal force; pin: two components; built-in: two forces plus a moment), count unknowns and compare with the number of equilibrium equations. State explicitly 'isostatic, 3 unknowns / 3 equations' or 'hyperstatic, needs compatibility'. Train on a set of five support types until the reaction sets are automatic.
When computing bending moments and shear forces in a beam, the candidate changes sign conventions partway through the calculation, or draws the shear-force and bending-moment diagrams without linking them (does not check that the shear diagram is the derivative of the moment diagram and that the moment is zero at a free end or hinge).
The candidate works section by section and re-derives the sign of each internal force from scratch, sometimes using the left-hand cut and sometimes the right-hand cut. The derivative relationship between V and M is known as a formula but never used as a self-check.
What to do instead
Fix one convention in writing at the top of the answer sheet (for example sagging moment positive, upward shear on the left face positive) and refer to it at every cut. After drawing the diagrams, verify three invariants: V equals the slope of M, M is extremal where V=0, and M vanishes at free ends and internal hinges. If an invariant fails, the error is located immediately rather than at the end.
In stress and strain problems, the candidate applies σ = F/S to a tensile member without checking whether the section is net or gross, and computes strain energy or elongation using the wrong length or the wrong modulus, mixing E (Young) with G (shear) or with the allowable stress.
The formula σ=F/S is learned as a single reflex and the candidate does not pause to identify which area is loaded (net section after a hole, welded throat area, bearing area). Symbols E, G, σe, σr are recalled as a blur because they were memorized as a list rather than as physical quantities with units.
What to do instead
Write the data table first: force, geometry, material, and the property needed, with units. Then select the formula by matching units (E in MPa, G in MPa, σ in MPa) and by the physical situation (tension, shear, bearing, welded joint). For a holed plate, always compute the net area (b − d)·e. Check the result against the order of magnitude of the material's yield strength.
On a technical drawing, the candidate dimensions from the drawing itself with a ruler or copies a dimension that is already given as a reference, instead of deriving the missing dimension from the dimension chain; tolerances are then written as symmetric ± values when the fit is specified as H7/g6.
The candidate confuses 'reading a drawing' with 'measuring a drawing'. The ISO fit notation is recognized visually but not decoded: the candidate does not know that H7 is a hole tolerance and g6 a shaft tolerance, and does not look up the numerical values in the tolerance tables.
What to do instead
Practice decoding fits systematically: letter = fundamental deviation (position), number = IT grade (width). For H7/g6, write the hole limits and shaft limits from the IT table before computing clearance max and min. For dimension chains, close the chain algebraically (cote-condition) rather than measuring. Never scale a drawing; if a dimension is missing, state the assumption explicitly.
When analysing a gear train, the candidate computes the overall ratio by adding the ratios of the stages, or forgets the direction of rotation and the effect of an idler wheel, and then uses the wrong torque in the output-shaft calculation.
The ratio i = Z2/Z1 is known for a single pair, but the product rule for a train and the rule that idler wheels change direction without changing the ratio are not consolidated. Torque and speed are confused because P = Cω is applied without checking which shaft the power refers to.
What to do instead
Draw the kinematic chain shaft by shaft, writing n and C at each shaft. Multiply the ratios of the meshing pairs (i_total = Π Z_driven/Z_driver); mark idlers as direction-only. Then propagate power with P = C·ω at each shaft, assuming P constant (minus efficiency) through the train. Verify that speed decreases when torque increases.
In bearing selection, the candidate compares the applied load directly with the basic dynamic load rating C without converting to the equivalent dynamic load P = X·Fr + Y·Fa, and ignores the speed and the required life L10h in the rating equation.
The catalogue page shows C prominently and the candidate treats it as an allowable load. The influence of axial load, rotation factor and life exponent (3 for ball bearings, 10/3 for roller bearings) is not internalized because the formula is memorized without the meaning of L10.
What to do instead
Always compute P first: identify Fr and Fa, take X and Y from the bearing table for the ratio Fa/Fr compared with e, then apply L10 = (C/P)^p and convert to hours with L10h = 10^6/(60n)·L10. Check that the selected bearing satisfies the required hours, and state the exponent used.
In machining calculations, the candidate uses cutting speed Vc in m/min directly with a diameter in millimetres in the formula n = 1000·Vc/(π·D), or omits the 1000 factor, producing a spindle speed off by a factor of a thousand; feed per tooth and feed per revolution are also interchanged.
Unit consistency is treated as a formality. The candidate copies the formula from memory without tracking mm versus m, and fz (mm/tooth) versus f (mm/rev) versus Vf (mm/min) are memorized as three similar symbols without their physical meaning.
What to do instead
Write the unit of every symbol next to it before substituting: Vc in m/min, D in mm, so the 1000 converts mm to m. Derive Vf = fz·Z·n and check that the result is in mm/min and plausible (typically tens to hundreds of mm/min). Sanity-check the spindle speed against the machine's available range.
When interpreting a heat-treatment specification, the candidate confuses hardening (trempe) with case hardening (cémentation) or with annealing (recuit), and predicts the wrong microstructure or the wrong property (surface hardness versus core toughness) for a given steel and treatment.
The French and English terminology overlap partially and the candidate learns the treatment names as a vocabulary list without attaching the temperature range, the cooling medium and the resulting microstructure to each one. The Fe-Fe3C diagram is known in shape but not used to justify the treatment.
What to do instead
Build a table with columns: treatment, temperature range, cooling, microstructure obtained, effect on hardness and toughness, typical application. Place each treatment on the Fe-Fe3C diagram (austenitizing range, eutectoid at 0.77 % C, Ms/Mf lines). For a gear, justify surface hardness plus tough core by case hardening or nitriding, not by a through-hardening alone.
In thermodynamics, the candidate applies the first law to a cycle or an open system without fixing the sign convention for work and heat, and uses the ideal-gas relation pV = nRT with the wrong units (pressure in bar, volume in litres, temperature in °C).
Sign conventions differ between physics and engineering textbooks and the candidate switches between them mid-problem. The gas constant R = 8.314 J/(mol·K) is used with mass instead of moles, and temperature is not converted to kelvin.
What to do instead
At the top of the answer, declare the convention used (for example W > 0 when work is done by the system, Q > 0 when heat is received). Convert all data to SI before substituting: p in Pa, V in m³, T in K. For an open system use the steady-flow energy equation with enthalpy, and check that a cycle gives ΣQ = ΣW.
In electrical and automation questions, the candidate selects a contactor or a motor protection device by rated current only, ignoring the starting current, the duty cycle and the difference between a circuit-breaker, a thermal relay and a fuse; in control logic, the candidate reads a ladder diagram without identifying the self-holding (auto-maintien) contact.
The devices are learned as names on a list rather than as functions in a power and control circuit. The candidate reads the schematic left to right as a sequence of symbols without tracing the energized path, so the self-holding branch and the normally-closed stop contact are missed.
What to do instead
Separate the power circuit (protection, contactor poles, motor) from the control circuit (stop NC in series, start NO in parallel with the contactor auxiliary contact). Trace the current path for 'start pressed' and for 'start released' to confirm the self-holding. Size protection on the full-load current and check the starting current against the protection curve.