GACE Physics (718) common mistakes
GACE Physics (718) · Understand the traps and how to avoid them.
Writing g = 10 m/s² or 9.8 m/s² into a calculation when the paper's data sheet specifies a value (use the stated exercise value), or dropping the unit of g entirely and then treating a weight as a mass in later steps.
Candidates memorise one value from textbooks or from a different year's paper and never check the front of the question paper; the unit is dropped because g is treated as a bare number in the formula W = mg.
What to do instead
Before starting a calculation, circle the constants printed on the paper (g, e, h, c, mass of electron) and write them at the top of your answer booklet. In every mechanics line, carry units through: write W = mg = 2.0 kg × 10 m/s² = 20 N, not 2.0 × 10 = 20.
Resolving forces on an inclined plane by using the angle with the horizontal in the component that should use the angle with the vertical, e.g. writing the component of weight along a slope of angle θ as mg cos θ instead of mg sin θ.
The formula is memorised as a pattern without drawing the triangle; candidates do not sketch the weight vector and the slope and identify which angle is actually between the weight and the normal.
What to do instead
Always draw the free-body diagram first and mark the angle θ on it. Then resolve along and perpendicular to the plane by geometry, not memory: the component of weight down the slope is mg sin θ, the normal reaction is mg cos θ. Check with the limits θ = 0 (no component down the slope) and θ = 90° (full weight down the slope).
Using the thin-lens or mirror equation with a real-is-positive sign convention, then reporting a virtual image as real (or a negative focal length as if it were positive), and confusing magnification m = v/u with m = −v/u.
Different textbooks and the candidate's own notes use different sign conventions; the substitution is done mechanically and the sign of the answer is never interpreted physically.
What to do instead
State your convention explicitly at the top of the optics answer (e.g. 'real-is-positive, distances measured from the lens'). After computing, interpret the sign in words: a negative v means a virtual image on the same side as the object. For a converging lens, check that an object inside the focal length gives a virtual, magnified, upright image - if your numbers say otherwise, the sign handling is wrong.
Treating e.m.f. and terminal potential difference as interchangeable, so that a circuit with internal resistance is solved as if the terminal p.d. equalled the e.m.f., and the lost volts are never accounted for.
Introductory problems often use ideal cells, and the phrase 'lost volts' is memorised without linking it to the current through the internal resistance.
What to do instead
Write the energy equation every time: ε = I(R + r) = V_terminal + Ir. Label V_terminal as the p.d. across the external resistor and Ir as the lost volts. Check the limiting cases: open circuit (I = 0, V_terminal = ε) and short circuit (R = 0, V_terminal = 0).
Confusing the direction of induced current or force using Fleming's left- and right-hand rules, and applying them with the field, current and motion in the wrong fingers.
The two rules look similar and are memorised as gestures without a physical anchor; candidates do not connect the left-hand rule to motors (force on a current) and the right-hand rule to generators (induced current).
What to do instead
Anchor each rule to its device: left hand = motor effect (current in a field experiences a force); right hand = generator effect (motion in a field induces a current). In the left-hand rule, first finger = field, second finger = current, thumb = force. Test the answer against Lenz's law: the induced current must oppose the change producing it.
Mixing Celsius and kelvin in the gas law or in specific-heat and latent-heat calculations, e.g. substituting 27 into pV = nRT, or using Δθ and ΔT inconsistently.
Temperature differences are numerically equal in °C and K, so candidates assume absolute temperatures are also interchangeable, and the ideal-gas equation is memorised without the 'absolute' condition.
What to do instead
Convert to kelvin as the first line of any gas calculation: T = θ + 273 (use the value on the data sheet). Note that Δθ = ΔT so differences need no conversion, but pV = nRT and Charles's/Gay-Lussac's laws require absolute temperature. Sanity-check: a gas at 0 °C must not give zero volume or zero pressure.
In the photoelectric effect, treating the work function as a threshold frequency or mixing up the stopping potential with the photon energy, and omitting that maximum kinetic energy equals eV_s.
The equation hf = φ + K_max is memorised as symbols without the physical meaning of each term, and the electron-volt is not linked to the stopping potential measurement.
What to do instead
Write the energy balance in words first: photon energy = work function + maximum kinetic energy of the emitted electron. Then K_max = eV_s, where V_s is the stopping potential that just stops the fastest electrons. Check units: φ and K_max both in joules or both in eV, never mixed. Use the threshold condition f = f_0 when K_max = 0 to find φ = hf_0.
Reading a vernier caliper or micrometer screw gauge to the wrong precision - e.g. recording a micrometer reading as 2.34 cm instead of 2.34 mm, or ignoring the zero error correction and reporting the raw scale reading.
The main-scale and thimble/vernier scales are read separately and the least count is not applied; zero error is treated as a footnote rather than a step in the measurement.
What to do instead
Write the least count on the answer line before reading (use the actual instrument marking or supplied specification; 0.01 mm or 0.05 mm are examples, not universal resolutions). Record the raw reading, then subtract the zero error with its sign, then convert to SI. Quote the final value with the correct number of decimal places and the unit, e.g. (2.34 ± 0.01) mm.
Computing a gradient from a graph by dividing the difference in y-values by the difference in x-values taken from the origin rather than from two widely separated points on the line of best fit, and using data points instead of the fitted line.
The formula 'rise over run' is applied to the first and last tabulated points, which may not lie on the best-fit line; the candidate does not realise the gradient must be read from the drawn line.
What to do instead
Choose two points on the line of best fit at least half the graph width apart, mark their coordinates on the graph, and show the subtraction explicitly. Never use raw table values unless they lie on the line. State the gradient with its unit, since the unit is the ratio of the axis units.
Equating maximum oscillator speed with maximum acceleration.
Velocity and acceleration are different derivatives, so a position that maximizes one need not maximize the other.
What to do instead
Use a = -omega squared x and v squared = omega squared times (A squared - x squared). Acceleration vanishes at equilibrium while speed is greatest there.
Assuming every periodic oscillation has amplitude-independent period.
The ideal linear spring result is remembered without the assumptions that produced its differential equation.
What to do instead
Identify the restoring relation and approximation first. Use amplitude independence only for the specified linear SHM model; a large-angle pendulum is a counterexample.