FET common mistakes
FET · Understand the traps and how to avoid them.
Computing derivatives or integrals of trigonometric functions while the calculator is still in degree mode, or rounding intermediate values to two decimals before a sensitivity check in a numerical-methods item.
Candidates toggle to degrees for a statics or surveying item and forget to return to radians; numerical items invite premature rounding because the final answer choices look close together.
What to do instead
Before any calculus or series computation, set and verify radian mode, and carry at least six significant figures through intermediate steps. Estimate the magnitude of the result first so a degree/radian error (factor of about 57) is caught immediately.
Using a z-table and the population standard deviation when only a small-sample standard deviation is given, or treating events as independent because the problem does not explicitly say otherwise.
Normal-distribution drills dominate preparation, so the z procedure becomes automatic; independence is assumed because the multiplication rule is easier than constructing a joint or conditional setup.
What to do instead
Write the sample size and whether sigma is known before choosing z or t. For any 'both' or 'and' probability, state the independence assumption explicitly and check whether the wording implies sampling without replacement, which requires the hypergeometric or a conditional probability.
Mixing nominal and effective interest rates, or pairing an inflation-adjusted cash flow with a market MARR when discounting to present worth.
The factor tables are memorized as symbols, so the annual rate is inserted without checking whether the compounding period matches the payment period; ethics and economics items also contain distracting narrative.
What to do instead
Draw a cash-flow timeline, label every rate as nominal or effective and every cash flow as actual or constant dollars, and convert the rate to the payment period before selecting a factor. Use real MARR with constant-dollar flows and market MARR with actual-dollar flows, never a mixture.
Drawing a free-body diagram that omits a reaction or applies static equilibrium equations to a body that is clearly accelerating, and reversing the direction of friction on a block on the verge of motion.
Reactions at rollers, cables, and two-force members are mentally 'obvious' and get skipped; the impulse to sum forces to zero is strong even when the problem states an acceleration or an impending slip.
What to do instead
Isolate each body, mark every contact and its unknown, and label two-force members before writing any equation. Decide the state of motion first: if a is nonzero, use the equations of motion; if at impending slip, set friction opposite the impending relative motion at its limiting value.
Computing bending stress with a moment of inertia taken about the base or an arbitrary axis rather than the centroid, or applying the elastic flexure formula to a section loaded beyond yield.
The parallel-axis theorem is applied in the wrong direction under time pressure, and the formula sigma equals Mc over I is over-practiced relative to the yield check.
What to do instead
Locate the centroid first, compute I about that centroid, then transfer only if the axis genuinely shifts. Compare the resulting stress with the yield strength before trusting the elastic result; if it exceeds yield, the section is no longer elastic and the answer requires plastic or residual-stress reasoning.
Using gauge pressure on one side of the energy equation and absolute pressure on the other, or applying Bernoulli across a pump, turbine, or steep friction loss.
Bernoulli is the first equation recalled, and it is easy to forget that it assumes no shaft work and no losses; gauge and absolute pressures give identical differences only when both sides use the same reference.
What to do instead
Use the extended energy equation with head terms for pump, turbine, and loss whenever a machine or significant friction is present. Keep every pressure in the same reference frame and convert only at the end if the question asks for absolute or gauge.
Using Cp where Cv belongs, or applying the ideal-gas relation to a saturated mixture or a real substance near its critical point.
Constant-pressure problems are more common in coursework, and the ideal-gas equation is fast to write; the phase of the substance is often stated indirectly through quality or pressure.
What to do instead
Identify the substance and phase from the given state before selecting any relation. For a closed rigid tank use Cv, for a flow process use Cp; if quality or saturation is indicated, go to the property tables rather than the ideal-gas law.
Substituting peak voltage into a power calculation, or adding a real and a reactive power arithmetically instead of as a phasor sum.
Time-domain expressions and phasor quantities look interchangeable on the page, and the square root of three and RMS factors are easy to drop mid-calculation.
What to do instead
Convert every source to RMS phasor form before computing power, and write complex power as P plus jQ, summing magnitudes only after taking the vector sum. Check the power factor angle against the sign of Q to confirm whether the load is inductive or capacitive.
Ignoring instrument loading, or reading a calibration curve at the wrong span endpoint and reporting a steady-state error that actually belongs to a different system type.
Sensors are treated as ideal transducers, and steady-state error formulas are recalled by number of integrators without checking the input type (step, ramp, parabola).
What to do instead
State the instrument input impedance relative to the source and correct for loading. For control items, identify the system type and the input simultaneously, then use the matching error constant; verify the result with the final-value theorem when the closed-loop transfer function is given.
Applying a factor of safety to a load that is already factored, or choosing a safety factor without stating whether it is referenced to yield or to ultimate strength.
Design and safety items blend code-based load factors with strength reduction, and candidates apply every margin they remember because the problem 'feels' like it needs a safety factor.
What to do instead
Write the governing limit state explicitly, separate load factors from resistance or safety factors, and name the reference strength (yield for serviceability or onset of yielding, ultimate for rupture). Confirm that the selected standard, not habit, dictates which combination applies.