IB DP Physics · Exam special · SL & HL · new syllabus (first exams May 2025) · ~9 min read
IB Physics Paper 1A: Multiple-Choice Strategies and Traps
How to attack IB Physics Paper 1A multiple choice: timing with Paper 1B, the shortcuts examiners recommend, worked examples and a bank of the misconceptions flagged in the 2024–25 subject reports.
Paper 1 format & timingEliminationProportion & ratioUnits & dimensionsLimiting casesGraph meaningCause vs descriptionMisconception bank
RSby Riyaz Sir
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mentorriyaz.in
No tag = SL and HLHL ONLY HL students onlyDATA BOOKLET given in the examM1A1 how marks are awardedhighlight = comes up again and againHANDY worth memorisingTest yourself at the end
In 30 seconds
- Paper 1A is the multiple-choice half of Paper 1: 40 questions at HL and 25 at SL, sat in the same session as Paper 1B (the data-based questions). One clock covers both parts.
- There is no negative marking. A blank answer is a guaranteed zero, so never leave a question empty. The May 2025 reports saw more blanks towards the end of Paper 1A.
- About half the items test application (AO3), not recall. Most are designed to be answered without a calculator, even though one is now allowed.
By the end you can
- Plan your time between Paper 1A and Paper 1B.
- Use elimination, proportion, units and limiting cases to answer quickly.
- Recognise the misconceptions that examiners say catch most students.
1Format and timing
| Level | Paper 1A | Paper 1B | Shared time | Weighting of Paper 1 |
|---|---|---|---|---|
| HL | 40 MCQ (40 marks) | Data questions (20 marks) | 2 hours | 36% |
| SL | 25 MCQ (25 marks) | Data questions (20 marks) | 1 hour 30 minutes | 36% |
- A sensible split: about 1 minute per MCQ, so roughly 40 min at HL or 25 min at SL, then the rest for Paper 1B, which needs more reading and writing.
- Two passes. First pass: answer everything you can in under a minute and mark the rest. Second pass: return to the marked ones. Last two minutes: fill every blank.
- Shade carefully. Check that the question number on the answer sheet matches the question you're on, especially after skipping one.
Exam tipDon't let one tricky MCQ eat five minutes. It is worth the same 1 mark as the easiest question in the paper.
2Six techniques examiners recommend
1 · Eliminate
- Cross out options that are impossible: wrong units, the wrong direction, energy from nowhere, a speed above c. Two options often fail at a glance; choosing from two is a 50% guess even if you get stuck.
- Read all four options before choosing. The question asks for the best response.
2 · Use proportion, not full calculation
- Write how the answer depends on the variable that changes, then use the factor: new = old × factor.
Worked example
A satellite moves from an orbit of radius R to one of radius 2R (measured from the planet's centre). What happens to the gravitational field strength and the gravitational potential?
- g ∝ 1/r², so g becomes ¼ of its value.
- V ∝ −1/r, so V becomes half as negative (V/2).
Worked example
The wind speed at a turbine rises from 6 m s⁻¹ to 9 m s⁻¹. By what factor does the power available increase?
- P ∝ v³
- (9/6)³ = 1.5³ = 3.4 (3.375). Cube the ratio; don't cube the difference.
3 · Check units and dimensions
- Reduce every option to SI base units (kg, m, s, A, K, mol). Only one option usually matches. Examiners note that many students stop at derived units such as N or Ω.
Worked example
What are the SI base units of resistivity?
- ρ = RA/L, so units are Ω m.
- Ω = V A⁻¹ and V = W A⁻¹ = kg m² s⁻³ A⁻¹, so Ω = kg m² s⁻³ A⁻².
- ρ: kg m³ s⁻³ A⁻².
4 · Try a limiting case
- Put θ = 0° or 90°, a mass of zero, or an infinite resistance into each option. The correct expression must give a sensible result.
- Example: the component of weight down a slope must be 0 when θ = 0°, so mg sin θ survives and mg cos θ does not.
5 · Ask what the gradient and area mean
- Divide the y-unit by the x-unit for the gradient (m s⁻¹ ÷ s = acceleration); multiply them for the area (N × m = work).
- Instantaneous values come from a tangent; average values from the change between two points.
6 · Cause or description?
- Some items list four statements that are all true and ask which one explains the effect. Pick the cause, not the consequence.
- Example: a gas is compressed quickly and its temperature rises. "The molecules collide with the walls more often" is true but doesn't explain the rise. The cause is that the moving piston does work on the gas, increasing the molecules' kinetic energy.
3Worked mini-examples
Worked example
A diffraction grating has 500 lines per mm. Light of wavelength 600 nm falls on it normally. How many bright maxima are seen in total?
- d = 1/500 mm = 2.0 × 10⁻⁶ m
- n ≤ d/λ = 2.0 × 10⁻⁶ ÷ 6.0 × 10⁻⁷ = 3.3, so n_max = 3
- maxima: 3 on each side + the central one = 7 (always odd).
Worked example
A ray meets a glass surface at 40° to the surface. What is the angle of incidence?
- Angles are measured from the normal.
- i = 90° − 40° = 50°.
Worked example
A star has a parallax angle of 0.25 arcseconds. How far away is it?
- d (pc) = 1 / p (arcsec)
- d = 1/0.25 = 4 pc.
Worked example
A wooden block and an iron block of the same volume are both held completely under water. Which has the greater buoyant force?
- Buoyant force = weight of fluid displaced = ρfluid V g
- same V, same fluid: the forces are equal. The object's density doesn't matter.
4Misconception bank (from the subject reports)
Mechanics and thermal
| Trap students fall into | What is actually true |
|---|---|
| Buoyant force depends on the object's density | It depends on the fluid's density and the volume displaced (under 20% correct on one item) |
| A car turning at constant speed has no resultant force | It needs a centripetal (resultant) force towards the centre |
| Average and instantaneous acceleration are the same | Instantaneous = gradient of the tangent at that time |
| Power = KE ÷ time, using the final KE | Power = change in KE ÷ time |
| Adding mass to a mass–spring system changes the elastic PE at a given extension | Mass changes the frequency and the KE, not the EPE for a given extension |
| Compressing a gas heats it because the molecules collide more | Work done by the piston increases molecular KE |
| Coal is a secondary energy source | Coal, oil, gas, wind and sunlight are primary; electricity is secondary |
| Albedo = reflected ÷ absorbed | Albedo = reflected ÷ incident (incident = absorbed + reflected) |
Waves
| Trap students fall into | What is actually true |
|---|---|
| Particles in a wave move at the wave speed | Particles oscillate about fixed points; the wave speed is different (about 60% chose wave speed) |
| Reflection at a free end is inverted | It is not inverted; reflection at a fixed end is |
| Angles of refraction are measured from the surface | From the normal |
| A grating can give an even number of maxima | Always odd: the central maximum plus equal numbers on each side |
| Path-difference conditions work for any sources | The data-booklet conditions assume sources in phase; swap them for antiphase sources |
Fields and nuclear
| Trap students fall into | What is actually true |
|---|---|
| Doubling a planet's diameter doubles r in g = GM/r² | Use the radius; check whether the question gives diameter |
| Gravitational potential ∝ 1/r² | Potential ∝ 1/r (field strength ∝ 1/r²) |
| An induced current always opposes the magnet's motion in the same way | Lenz's law: the current opposes the change in flux; increasing and decreasing flux give opposite currents |
| A Compton-scattered photon gains energy | It loses energy: its wavelength increases |
| β⁺ decay happens in neutron-rich nuclei | β⁻ in neutron-rich nuclei; β⁺ in proton-rich nuclei |
Number traps
- Area conversions: 1 mm² = 10⁻⁶ m², not 10⁻³ m². 1 cm³ = 10⁻⁶ m³.
- Revolutions: use the average angular velocity when the rotation speeds up uniformly.
- SHM at t = 0: check whether the object starts at equilibrium (maximum speed) or at an extreme (zero speed) before choosing a sine or cosine graph.
5Last-week checklist
- Do at least two full Paper 1A sets under time, together with a Paper 1B, so you feel the shared clock.
- Learn the data booklet layout: in the exam you should find any equation in seconds.
- Re-read every misconception table above; for each one, say the correct physics out loud.
Linking questionRevise the physics behind these traps in A.2 Forces and momentum, C.3 Wave phenomena and D.1 Gravitational fields. Then try Paper 1B data skills and Paper 2 answers.
Test yourself — cover the answers first
Quick check
- Is there negative marking in Paper 1A?
- The distance to a planet's centre triples. What happens to g?
- What are the SI base units of the newton?
- A ray makes 25° with a surface. What is the angle of incidence?
- A grating gives n_max = 4. How many maxima are seen?
- Does the buoyant force on a submerged object depend on its density?
Show answers
- no; answer every question
- it becomes 1/9 of its value
- kg m s⁻²
- 65°
- 9
- no; on the fluid's density and the volume displaced
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