Physical Science has two papers. Paper 1 (Physics) is 150 marks. Paper 2 (Chemistry) is 150 marks. That is 300 marks total.
And students are throwing away 40 to 60 of those marks on the same avoidable mistakes every single year. Not hard questions. Not trick questions. Basic errors that the DBE examiner reports call out year after year after year.
Mr Sawaya has been teaching Physical Science for 30 years. He knows exactly what goes wrong. This post lays out the 10 biggest mark killers and tells you how to fix each one before exam day.
In This Post You Will Learn
✓ The 10 mistakes that cost the most marks across Paper 1 and Paper 2
✓ Which paper and topic each mistake affects most
✓ The exact fix for each one
✓ How to check your own work before handing in
✓ A quick reference checklist for both papers
Mistake 1: Not Converting Units
This is the number one mark killer across both papers.
The formulas work in SI units. If you plug in cm instead of m, or cm³ instead of dm³, your answer is wrong by a factor of 100 or 1000.
| Given Unit | Must Convert To | How |
|-----------|----------------|------------------------|
| cm | m | Divide by 100 |
| mm | m | Divide by 1000 |
| cm³ | dm³ | Divide by 1000 |
| cm³ | m³ | Divide by 1 000 000 |
| g | kg | Divide by 1000 |
| kJ | J | Multiply by 1000 |
| km/h | m/s | Divide by 3.6 |The fix: Before you substitute into any formula, check every unit. Write the conversion as a separate line of working. The examiner can see you did it and you avoid errors.
We covered unit issues in detail in Stoichiometry Grade 12 - Mole Calculations Made Easy.
Mistake 2: Not Drawing Free Body Diagrams
Newton's Laws questions carry 25+ marks in Paper 1. The first step for EVERY Newton's Laws problem is drawing a free body diagram.
Students skip this step and try to do the calculation in their heads. They forget a force. They get the direction wrong. They lose 4 to 6 marks on a single question.
The fix: Draw the free body diagram FIRST. Label every force with its name and direction. Only then write F_net = ma and substitute.
Forces you must never forget:
| Force | Direction | When to Include |
|--------------------|--------------------|-----------------------------|
| Weight (mg) | Downward | Always |
| Normal force (N) | Perpendicular to surface | Always (if on a surface) |
| Friction (f) | Opposite to motion | If surface is rough |
| Applied force (F) | As stated | If pushing/pulling |
| Tension (T) | Along the string | If connected by string/rope |Mistake 3: Mixing Up Positive and Negative Directions
In mechanics, you must choose a positive direction and stick with it for the entire question.
Students choose "right is positive" then forget and treat left-acting forces as positive too. The signs get scrambled and the whole answer falls apart.
The fix: Write your sign convention at the top of every mechanics question.
Example: "Let right be positive" or "Let the direction of motion be positive."
Then apply it consistently. Every force in the chosen direction is positive. Every force against it is negative.
Mistake 4: Wrong Mole Ratio in Stoichiometry
Chemistry calculations in Paper 2 depend on balanced equations and mole ratios. Students use 1:1 ratios by default instead of reading the actual coefficients.
If the equation says 2HCl + Ca(OH)₂ → CaCl₂ + 2H₂O, the ratio of HCl to Ca(OH)₂ is 2:1, not 1:1.
The fix: Write out the mole ratio from the balanced equation before calculating. Circle the coefficients. Use them.
We broke this down step by step in Stoichiometry Grade 12 - Mole Calculations Made Easy.
Mistake 5: Giving Incomplete Definitions
Theory questions are free marks. But students write vague answers and get 0 out of 2.
| Term | Wrong Answer | Correct Answer (CAPS) |
|--------------------|--------------------------|-----------------------------------------------------------|
| Net force | "The total force" | "The resultant force acting on an object" |
| Photoelectric effect| "Light makes electrons" | "Electrons are ejected from a metal surface when light of suitable frequency shines on it" |
| Doppler Effect | "Sound changes" | "The apparent change in frequency of a wave due to relative motion between source and observer" |The fix: Memorise definitions from the CAPS document word for word. The examiner marks against a specific memo. If your wording is too vague, you get 0 even if you "kind of" know the concept.
We covered key definitions in Photoelectric Effect Grade 12 - How to Answer Theory Questions and Doppler Effect and Waves Grade 12 - Everything Explained.
Mistake 6: Using the Wrong Energy Equation
Work, energy and power has multiple formulas. Students use conservation of energy when friction is present, or use the work-energy theorem incorrectly.
| Situation | Correct Approach |
|---------------------------------|------------------------------------|
| No friction, no applied force | Ek_i + Ep_i = Ek_f + Ep_f |
| Friction present | Ek_i + Ep_i + W_nc = Ek_f + Ep_f |
| Any situation (always works) | W_net = change in Ek |The fix: Read the question for the word "rough" (friction) or "smooth" (no friction). That tells you which equation to use. If in doubt, the work-energy theorem works in every situation.
We covered this in full in Work, Energy and Power Grade 12 - Conservation Explained Simply.
Mistake 7: Getting Doppler Signs Wrong
The Doppler formula has plus and minus signs that depend on direction. Students memorise it wrong or forget the rule and guess.
TOP (listener): + towards, - away
BOTTOM (source): - towards, + awayThe fix: After calculating, do a logic check. Source coming towards you? You should hear a HIGHER frequency. If your answer is lower, your signs are wrong.
Full breakdown in Doppler Effect and Waves Grade 12 - Everything Explained.
Mistake 8: Not Stating Direction in Mechanics Answers
In physics, many quantities have direction. Force, velocity, acceleration, momentum. If you give a magnitude without a direction, you lose the mark.
Wrong: "The acceleration is 2.5 m/s²."
Right: "The acceleration is 2.5 m/s² to the right" or "The acceleration is 2.5 m/s² up the incline."
The fix: For every vector quantity, include direction in your final answer. Write it as part of the sentence. Do not assume the examiner knows which direction you mean.
Mistake 9: Confusing Kc Changes With Equilibrium Shifts
In chemical equilibrium, students say "adding more reactant increases Kc." It does not.
Kc only changes with temperature. Adding reactant shifts the equilibrium forward but Kc stays the same.
| Change | Does Equilibrium Shift? | Does Kc Change? |
|---------------------|------------------------|-----------------|
| Add reactant | Yes (forward) | No |
| Remove product | Yes (forward) | No |
| Increase temperature | Yes | Yes |
| Add catalyst | No | No |The fix: Memorise this rule. Temperature changes Kc. Nothing else does. The exam asks this directly almost every year.
Full guide in Chemical Equilibrium Grade 12 - Le Chatelier's Principle Made Simple.
Mistake 10: Poor Time Management
Paper 1 and Paper 2 are both 150 marks in 3 hours. Same as maths.
Students spend 30 minutes on a 6-mark question and then rush through 20 marks of straightforward questions at the end.
The fix: 1 minute per mark, roughly. A 6-mark question gets 6 to 7 minutes. If you have been on it for 15 minutes, move on.
| Paper 1 Suggested Time |
|--------------------------------------------------|
| Q1-2: Mechanics (Newton's Laws) 35 min |
| Q3-4: Momentum, Work-Energy 30 min |
| Q5-6: Waves, Doppler 20 min |
| Q7-8: Electricity 40 min |
| Q9-10: EM Induction, Photoelectric 25 min |
| Checking: 20 min || Paper 2 Suggested Time |
|--------------------------------------------------|
| Q1-3: Matter, Intermolecular forces 25 min |
| Q4-5: Equilibrium, Acids/Bases 35 min |
| Q6-7: Electrochemistry 25 min |
| Q8-9: Organic Chemistry 30 min |
| Q10: Stoichiometry/Calculations 20 min |
| Checking: 25 min |For full live lessons on every topic, see our Grade 12 Physical Science tuition page.
Your Pre-Submission Checklist
Run through this before you hand in each paper:
✓ Did I convert all units to SI before substituting?
✓ Did I draw free body diagrams for every mechanics question?
✓ Did I state a positive direction for mechanics problems?
✓ Did I include direction in all vector answers?
✓ Did I check the mole ratio from the balanced equation?
✓ Did I use the correct energy equation (check for "rough" or "smooth")?
✓ Did I do a logic check on my Doppler answer (higher or lower)?
✓ Did I write definitions in full using CAPS wording?
✓ Did I attempt every question (no blanks)?
✓ Did I check my calculator entries for obvious errors?
How These Mistakes Appear in the NSC Exam
These mistakes span both papers.
Paper 1 is 150 marks covering mechanics, waves, electricity, electromagnetic induction, and the photoelectric effect.
Paper 2 is 150 marks covering chemical bonding, equilibrium, acids and bases, electrochemistry, organic chemistry, and stoichiometry.
The DBE examiner report for 2023 specifically highlighted unit conversion errors in electricity calculations, missing directions in mechanics answers, incorrect use of conservation of energy when friction was present, and vague definitions in theory questions.
These are the same issues raised in the 2022, 2021, and 2020 reports. The pattern has not changed. The students who fix these errors jump 15 to 30 marks. The ones who do not keep losing the same marks year after year.
You now know all 10. Fix them before exam day.
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