MATSEC AM Physics common mistakes
MATSEC AM Physics · Understand the traps and how to avoid them.
In projectile and circular-motion problems, treating the horizontal and vertical components as if they share the same time and acceleration, e.g. applying g to the horizontal velocity or using the launch speed in the centripetal formula without resolving it.
Students memorise the kinematics equations as a single set and do not build the habit of resolving vectors into perpendicular axes before writing equations; the SUVAT symbols are applied wholesale without checking which acceleration belongs to which direction.
What to do instead
Before substituting numbers, draw a free-body/velocity diagram, define axes, list the acceleration on each axis separately (horizontal a = 0 for projectiles, vertical a = g), and write one equation per axis. Practise by deriving the trajectory equation and the horizontal-range formula from scratch rather than recalling them.
Mixing up mass and weight, and using g in momentum or energy calculations where it does not belong, or omitting g where weight is required in moments and inclined-plane problems.
Everyday language uses 'weight' for mass, and the syllabus formulas W = mg, p = mv and E = mgh look superficially similar, so students substitute the wrong quantity under exam pressure.
What to do instead
Write the unit beside every quantity as you compute it (kg vs N) and treat the unit as a check. For moments and inclined planes, always resolve forces first and label the perpendicular component mg cos θ and the parallel component mg sin θ explicitly.
In circuit problems, applying the potential-divider formula to circuits containing more than one resistor in parallel, or ignoring internal resistance when asked for terminal potential difference.
The divider rule is learned as a formula V_out = V_in R2/(R1+R2) without the condition that the two resistors are in series and carry the same current; internal resistance is treated as an optional extra rather than part of the loop.
What to do instead
Redraw the circuit, reduce parallel groups to a single equivalent resistance first, then apply the divider only to genuine series pairs. For terminal p.d., always write ε = I(R + r) and V = ε − Ir and check the two answers agree.
Drawing field lines or flux patterns that cross each other, or that do not meet a conductor surface at right angles, and confusing the direction of conventional current with electron flow in Fleming's rules.
Field-line conventions are learned as pictures rather than as consequences of Gauss's law and of the fact that a conductor surface is an equipotential; the left-hand/right-hand rule pair is memorised without anchoring which hand goes with which physical situation.
What to do instead
State the rule aloud each time: left hand for motors (force on current), right hand for generators (induced current). For fields, sketch equipotentials as dashed lines perpendicular to field lines and verify no two field lines cross, since that would imply two directions of force at one point.
Using the wrong sign convention in the thin-lens or mirror equation and then reporting a virtual image as real, or forgetting that the image distance is negative for a virtual image.
Textbooks differ in convention (real-is-positive vs Cartesian), and students mix the two within one calculation; the sign carries physical meaning that is easy to lose when only plugging into 1/f = 1/u + 1/v.
What to do instead
Commit to one convention for the whole paper (real-is-positive), write it at the top of the answer, and finish every optics calculation with a one-line interpretation: 'v is negative, so the image is virtual, upright and on the same side as the object.'
Confusing the conditions for stationary waves with travelling waves, e.g. claiming nodes move, quoting the wrong node-to-node spacing, or using the wrong harmonic relationship for a pipe open at one end.
The superposition derivation is skipped in revision, so students recall the shape of the standing-wave diagram but not the underlying boundary conditions (node at a closed end, antinode at an open end).
What to do instead
Derive the standing-wave condition from first principles each time: node-to-node spacing is λ/2, node-to-antinode is λ/4. For pipes, write the boundary conditions explicitly, then obtain f_n = nv/2L for a string or open-open pipe and f_n = (2n−1)v/4L for a closed-open pipe.
Applying the first law of thermodynamics with the wrong sign for work done by or on the gas, or using Q = mcΔT when a phase change is involved.
The sign convention for W (work done by the gas vs on the gas) is stated differently in different textbooks, and specific latent heat is often forgotten once specific heat capacity is learned.
What to do instead
Write the convention at the start of the question (ΔU = Q − W, W = work done by the gas) and stick to it. For any heating problem, first ask whether the temperature is changing or the state is changing; if both, split the calculation into sensible-heat and latent-heat stages and add them.
In the photoelectric effect, using the intensity of light to argue that emitted electrons gain more kinetic energy, or treating threshold frequency and work function as unrelated quantities.
Classical wave intuition (brighter light = more energetic electrons) conflicts with the photon model, and the equation hf = φ + K_max is memorised without the meaning of each term.
What to do instead
Anchor the photon picture: one photon, one electron, energy hf; the work function φ is the minimum escape energy. Rehearse the three predictions (no emission below threshold regardless of intensity, K_max depends linearly on f not on intensity, saturation current depends on intensity) and explain each in one sentence.
Reporting a final answer with more significant figures than the data justify, or omitting units and the absolute uncertainty when a measurement is quoted.
Calculators display many digits and students equate precision with correctness; uncertainty propagation is often treated as a separate topic rather than part of every measurement answer.
What to do instead
Adopt a fixed reporting routine: round the final answer to the same number of significant figures as the least precise input, write the unit, and state the uncertainty to one significant figure with the value rounded to match (e.g. 3.42 ± 0.05 m). Practise this on every past-paper numerical question, not only on 'error analysis' questions.
Describing an experiment's method without identifying the controlled variables, the measurement instrument resolution, or a realistic source of systematic error, and drawing a 'best-fit' line that is not actually a line of best fit.
Practical skills are often revised by reading mark schemes rather than by performing or planning experiments, so students list apparatus but cannot explain why a method reduces a particular error or how to plot and interpret a graph.
What to do instead
For every required practical, write a one-page plan in the format: independent variable, dependent variable, controlled variables, instrument and resolution, procedure, main systematic error and how to eliminate it. When plotting, use a clear scale, plot all points, draw a single straight line with points balanced above and below, and use the gradient and intercept to extract the physical quantity.