NSSCO Physics (6118) common mistakes
NSSCO Physics (6118) · Understand the traps and how to avoid them.
Converts an area or volume by applying the linear prefix factor only once, for example writing 3.0 cm² = 3.0 × 10⁻² m² or 2.0 cm³ = 2.0 × 10⁻² m³.
The candidate memorises 'centi = 10⁻²' but does not raise that factor to the power of the dimension; area and volume conversions are especially common in density, pressure and upthrust calculations.
What to do instead
Write the unit identity first, such as 1 cm = 10⁻² m, then square or cube both sides: 1 cm² = 10⁻⁴ m² and 1 cm³ = 10⁻⁶ m³. Convert before substituting into ρ = m/V or P = F/A, then check that the units cancel to the required SI unit.
Reads a measuring cylinder, thermometer or ammeter to more decimal places than the scale allows, or ignores a zero error, then carries those extra digits into a calculation.
Candidates often treat a digital display or a finely marked scale as exact and believe more decimal places mean a better answer; they also forget to subtract a zero reading before using the instrument reading.
What to do instead
Record the reading to the smallest scale division plus one estimated digit, and state the uncertainty, for example 24.0 ± 0.5 cm³. If the instrument has a zero error, subtract it before calculating. Keep the final answer to the same number of significant figures as the least precise measurement.
Treats the gradient of a distance–time graph as acceleration or the area under it as distance, and confuses the gradient and area of a velocity–time graph.
The two graph types look similar, and candidates memorise 'gradient' and 'area' without linking them to the physical quantity represented by each axis.
What to do instead
Before calculating, write down the axis units: for a distance–time graph the gradient is m/s, so it is speed or velocity; the area is not distance. For a velocity–time graph the gradient is m/s², so it is acceleration; the area is displacement. Use the units of gradient and area to decide which operation is valid.
On an inclined plane, writes N = mg or swaps sin and cos when resolving the weight, so the normal force, friction and acceleration are all wrong.
Candidates memorise 'mg sinθ down the slope' without drawing the angle, and they assume the normal force always equals the weight as it does on a horizontal surface.
What to do instead
Draw a free-body diagram and mark the angle θ between the slope and the horizontal. Resolve weight into mg sinθ along the slope and mg cosθ perpendicular to it. If no other force acts perpendicular to the slope, then N = mg cosθ, so friction is μmg cosθ. Check that the component along the slope is smaller than mg.
Drops the sign when a ball rebounds from a wall or two trolleys move toward each other, then adds the speeds as if all momentum were in one direction.
Momentum is treated as a scalar because the word 'momentum' is often practised with one moving object; candidates forget that velocity, and therefore momentum, has direction.
What to do instead
Choose and state a positive direction before writing anything. Give velocities on the opposite side a negative sign, then use total momentum before = total momentum after with those signs. Check that a rebound gives a change in momentum larger than mv, not zero.
Calculates work as W = Fs even when the force is at an angle to the displacement, or when the force is perpendicular to the motion, instead of using the component of force along the displacement.
The formula W = Fs is learned first with a pushing force in the same direction as motion, and candidates do not link work to the component F cosθ.
What to do instead
Identify the angle θ between the force and the displacement. Use W = Fs cosθ; if θ = 90°, the work done by that force is zero. For power, use P = W/t, and for constant speed in the direction of the driving force use P = Fv. Check that only the component along the motion transfers energy.
Uses Q = mcΔT during melting or boiling, or uses Q = mL while the temperature is changing, because both formulas contain Q and m.
Candidates match the symbols in the formula to the data without considering the physical state; they also miss that temperature stays constant during a phase change.
What to do instead
Sketch a heating or cooling curve and label the sloped sections and the flat sections. Use Q = mcΔT only when the temperature changes within one state, and Q = mL only at a constant-temperature phase change. For a full process, add the separate energy terms in order.
Substitutes wavelength in centimetres directly into v = fλ, or swaps period and frequency by writing T = f instead of T = 1/f, giving a speed off by a factor of 100 or a period/frequency answer that is inverted.
Prefix conversions and reciprocals are done mentally under time pressure, and the wave equation is often practised only with metres and hertz.
What to do instead
Convert wavelength to metres before using v = fλ, and write the unit of every quantity beside it. Remember f = 1/T and T = 1/f, then check that frequency in hertz means oscillations per second. A quick check: a longer period must mean a smaller frequency.
Applies series and parallel circuit rules backwards: adds currents in a series circuit, adds voltages across parallel branches, or adds resistances directly for resistors in parallel.
The rules are memorised as lists without a mental model of charge flow and potential difference; candidates also confuse 'parallel' with 'more resistance'.
What to do instead
Draw the circuit and trace one path for series or branching points for parallel. In series: same current, voltages add, R_total = R₁ + R₂ + ... . In parallel: same voltage, currents add, and 1/R_total = 1/R₁ + 1/R₂ + ... . Check that parallel resistance is smaller than the smallest branch resistance.
Balances a nuclear equation by mass number only and ignores charge or atomic number, so beta decay or alpha decay products have the wrong Z; or calculates remaining activity after several half-lives as N₀/2 × (t/T).
The conservation of nucleon number is visible in the superscripts, while proton number conservation is less obvious; half-life is also treated as a simple division rather than repeated halving.
What to do instead
For every nuclear equation, write two conservation checks: total mass number A and total atomic number Z must be equal on both sides. For decay, alpha emission reduces A by 4 and Z by 2; beta emission leaves A unchanged and increases Z by 1. For half-life, count the number of half-lives n = t/T and use N = N₀(1/2)^n, not N₀/2 × n.