Common Exam Mistakes and Revision Planning

Two students with the same physics knowledge can walk out of the same exam with very different marks. The difference is rarely more content — it is avoiding the handful of recurring mistakes that quietly bleed marks, and revising in ways that actually build recall rather than just feeling productive. This lesson gathers the errors examiners see again and again across every topic, and pairs them with a revision approach grounded in how memory really works.

By the end of this lesson you should recognise the commonest exam mistakes in physics and how to avoid each, and be able to build a revision plan around active recall, spaced practice and past papers.

---

The Commonest Exam Mistakes

Mistake	Why it costs marks	The fix
Missing or wrong units	An answer with no unit rarely gets full marks; a wrong unit can lose the accuracy mark	Always write the unit; check it matches the quantity
Not converting units	Using g, cm or minutes instead of kg, m, s wrecks the arithmetic	Convert to base SI units before substituting
No working shown	Method marks are lost; a wrong answer scores zero	Show equation → substitution → answer, every time
Rearranging slips	Dividing when you should multiply loses the answer	Rearrange the letters first, substitute last
Wrong significant figures	Over- or under-rounding loses the accuracy mark	Match the data; round only at the end
Describe vs explain confusion	Describing when asked to explain caps the marks	Read the command word; explain needs "because"/"so"
Confusing similar terms	Mixing up mass/weight, speed/velocity, current/voltage	Learn precise definitions; watch the vocabulary
Ignoring embedded instructions	Missing "use the graph" or "to 2 s.f."	Underline every instruction, not just the command word

Every one of these is a technique error, not a knowledge gap — which is exactly why they are so frustrating and so fixable. A student who eliminates them can gain a grade without learning any new physics.

Exam Tip: Keep a personal "error log" as you do practice questions — jot down every mark you lose and why. After a few papers, patterns emerge (maybe you keep forgetting units, or mixing up mass and weight), and you can target that specific weakness instead of vaguely "revising more".

---

Confusable Terms to Keep Straight

Physics has pairs of terms that sound related but mean different things — and questions exploit the confusion:

These are NOT the same	The difference
Mass vs weight	Mass (kg) is the amount of matter; weight (N) is the force of gravity on it, $W = mg$W=mg
Speed vs velocity	Speed is scalar (size only); velocity is a vector (size and direction)
Distance vs displacement	Distance is total path; displacement is straight-line from start to end
Current vs voltage	Current (A) is rate of flow of charge; voltage/p.d. (V) is energy per charge
Series vs parallel	Series: one loop, same current; parallel: branches, same p.d.
Renewable vs non-renewable	Renewable replenishes on a human timescale; non-renewable runs out
Reflection vs refraction	Reflection bounces off; refraction bends on entering a new medium

Getting these right is often the whole mark. Writing "weight" when you mean "mass", or "speed" when the question needs the direction too, is a classic self-inflicted loss.

Exam Tip: For every confusable pair, learn a one-line discriminator — "mass is in kilograms, weight is a force in newtons". Rehearsing the distinction, not just each term alone, means you pick the right word under exam pressure when the two are easily muddled.

These confusions are so common partly because everyday language blurs distinctions that physics keeps sharp. In ordinary speech we use "weight" when we mean mass (a bag of sugar "weighs" a kilogram), we say something is "going fast" without any sense of direction, and we talk loosely about "power" and "energy" as if interchangeable. The exam demands the physicist's precision instead: weight is a force measured in newtons and changes with gravitational field strength, whereas mass is the amount of matter in kilograms and does not; velocity carries a direction that speed does not; power is a rate (energy per second) while energy is a quantity. Because the everyday habit pulls you toward the imprecise word, you have to consciously override it — which is exactly why rehearsing the discriminators pays off. When you meet one of these terms in a question, pause for a heartbeat and check you are using the physics meaning, not the loose everyday one.

Exam Tip: Beware that everyday language actively misleads you on physics terms — we casually say "weight" for mass and "fast" without direction. Consciously switch into the precise physics meaning of each term in the exam, because the habit of ordinary speech is pulling you toward the answer that loses the mark.

---

Calculation Mistakes in Detail

Because ~40% of marks are maths and application, calculation slips are the most costly category:

• Forgetting to square in $E_k = \dfrac{1}{2}mv^2$Ek​=21​mv2 — square the speed before multiplying.
• Powers of ten mis-keyed on the calculator — use the EXP button, sanity-check the magnitude.
• Rearranging the wrong way — for $V = IR$V=IR, resistance is $R = \dfrac{V}{I}$R=IV​, not $R = VI$R=VI.
• Rounding too early — carry the full value through, round only the final answer.
• Dropping the unit — a bare number loses the final mark.

A quick sanity check catches many of these: does the answer's size make sense? A car with a kinetic energy of 3 J, or an efficiency of 140%, is a signal you have slipped somewhere.

Exam Tip: After every calculation, pause for a two-second sanity check: is the magnitude sensible and is the unit right? Spotting that an efficiency came out above 100% or a person's mass came out as 7 kg lets you catch and fix an error before you move on.

---

A Worked Example of Catching a Mistake

To see the sanity check earn its keep, follow a student making — and catching — a typical error. A 60 kg cyclist has a kinetic energy of 3000 J. Calculate their speed. The student writes the equation $E_k = \dfrac{1}{2}mv^2$Ek​=21​mv2 and rearranges to $v = \sqrt{\dfrac{2E_k}{m}}$v=m2Ek​​​. So far so good. Substituting: $v = \sqrt{\dfrac{2 \times 3000}{60}} = \sqrt{\dfrac{6000}{60}} = \sqrt{100} = 10\text{ m/s}$v=602×3000​​=606000​​=100​=10 m/s. A sanity check confirms this is sensible — 10 m/s is a brisk but plausible cycling speed — and the unit (m/s) is right for a speed. The answer stands.

Now watch a student who forgets to take the square root. They compute $\dfrac{2 \times 3000}{60} = 100$602×3000​=100 and write $v = 100\text{ m/s}$v=100 m/s. Here the sanity check saves them: 100 m/s is 360 km/h, absurd for a cyclist, which flags that something is wrong. Re-checking the working, they spot the missing square root and correct the answer to 10 m/s. Without the habit of asking "is this magnitude physically sensible?", that error sails through and costs the answer mark.