CG PET common mistakes
CG PET · Understand the traps and how to avoid them.
Applying a physics formula with values in grams, centimetres, or minutes without converting to SI units first, so the numeric answer is off by powers of ten.
CG PET physics items often state data in mixed units (g, cm, minute, eV) and candidates substitute numbers directly into memorised formulas, trusting the final answer to 'look right' rather than tracking the unit of each quantity.
What to do instead
Before substituting, write a one-line unit ledger converting every given quantity to SI (kg, m, s, joule) and carry units through the calculation so the final unit is checked, not just the number.
Treating the direction of friction or normal reaction as fixed by convention rather than by the actual motion or contact geometry, leading to wrong sign in force or work calculations.
Candidates memorise 'friction opposes motion' and 'N = mg' as rules and apply them to inclined planes, accelerating lifts, and blocks on moving surfaces where the normal force and friction direction differ.
What to do instead
Draw a free-body diagram for each body, mark the contact surfaces, and derive N from the perpendicular force balance and friction direction from relative slipping before writing any equation.
Assuming a circuit current or voltage is unchanged after adding or removing a component, instead of re-solving the network with Kirchhoff's laws or equivalent resistance.
Textbook problems often feature simple series-parallel circuits, so candidates pattern-match to a remembered result and skip the fact that inserting an ammeter, voltmeter, or extra resistor redistributes current and terminal voltage.
What to do instead
Redraw the circuit with ideal-meter assumptions stated (ammeter zero resistance, voltmeter infinite), compute equivalent resistance from the source outward, then find branch currents and check that they sum correctly at each node.
Balancing a chemical equation by adjusting subscripts inside formulas rather than coefficients in front of them, producing a species that does not exist.
Under time pressure candidates focus on making atom counts equal and change the smallest visible number, forgetting that subscripts define the compound's identity while coefficients alone may be varied.
What to do instead
Balance by inspection or half-reaction method using only coefficients, then verify each element's atom count on both sides and confirm charges balance for ionic equations before moving on.
Mixing up the mole concept quantities: using molar mass where molarity is needed, or forgetting that molarity depends on solution volume, not solvent volume.
Mole, molar mass, molarity, molality, and normality share symbols and similar-sounding definitions, and candidates substitute whichever formula they recall first, especially in dilution and titration items.
What to do instead
Write the definition of each quantity beside the problem (moles = mass/molar mass; molarity = moles solute / litres of solution), state which is being asked, and check units cancel to the requested one.
Predicting reaction direction or spontaneity from Le Chatelier's principle alone, ignoring whether the change affects equilibrium constant or only the position of equilibrium.
Candidates learn 'shift right/left' rules and misapply them to temperature changes, where K itself changes, and to catalysts, which alter rate but not position, so answers about K, yield, and rate get conflated.
What to do instead
Classify each change as affecting K (temperature), position only (concentration, pressure via volume), or neither (catalyst), and check whether the question asks about K, equilibrium position, or rate before answering.
Dropping a solution branch or sign when solving equations with squares, absolute values, or logarithms, giving one root where two exist or an extraneous root that fails the original equation.
Candidates square both sides or cancel a variable factor to simplify quickly, which loses information, and then report the first root found without substituting back into the original equation.
What to do instead
Note where each step is not reversible (squaring, multiplying by an expression that may be zero, taking logs), list all candidate roots, and verify each in the original equation or stated domain before finalising.
Applying a standard derivative or integral rule without checking the chain rule factor, e.g. differentiating sin(3x) as cos(3x) or integrating e^(2x) as e^(2x).
Formula sheets list basic rules with a single variable, and candidates reproduce them from memory while ignoring the inner function, a habit reinforced by practising only simple examples.
What to do instead
For every composite function, explicitly name the inner function and multiply by its derivative (or divide the integral by it), then differentiate the result back to confirm it returns the integrand.
Using the wrong trigonometric identity or forgetting the general solution when solving trigonometric equations, reporting only the principal value.
Candidates memorise one form of identities and the principal-value answer, overlooking periodicity and the second solution in the given range, so answers miss valid angles.
What to do instead
Reduce to a single trigonometric function, write the general solution with the period term, then enumerate all angles inside the specified interval and check each by substitution.
Misreading the question's final demand, e.g. computing a probability when the question asks for odds, or giving a magnitude when a direction is required, and selecting the matching distractor.
Exam distractors are deliberately built from intermediate results, and candidates stop as soon as their number appears among the options instead of confirming it answers the exact quantity requested.
What to do instead
Underline the requested quantity and its required form before computing, and after obtaining a result restate the question in one sentence and confirm the answer matches that sentence, not just an option.