CIE A Level Physics
CIE A Level Physics Physical Quantities and Units Exam Questions
Free CIE A Level Physics exam questions for Physical Quantities and Units, with a mark scheme for every pack. Covers SI base units, prefixes, homogeneity of equations, estimates, errors and uncertainties, and scalars and vectors.
Physical Quantities and Units exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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A-Level Physics Flashcards for 2027 Onwards
Every answer is written in mark scheme wording, so you learn exactly what examiners give marks for. Then test yourself with the questions on this page.
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About CIE A Level Physics Physical Quantities and Units
Physical quantities and units is Topic 1 of the Cambridge International AS & A Level Physics (9702) syllabus and sets up the language used in every other topic. You learn the SI base quantities and units, how to derive units for other quantities, how to check equations for homogeneity, and how to handle scalars and vectors. Uncertainties are also part of this topic. Examiners regularly ask you to express a unit in base units, to make sensible estimates of everyday quantities, and to add or resolve vectors using scale drawings or trigonometry.
What you need to know
- SI base units The base quantities are mass (kg), length (m), time (s), current (A), temperature (K) and amount of substance (mol). All other units are derived from these, for example the newton is kg m s^-2.
- Prefixes Know the prefixes from pico (10^-12) to tera (10^12), including nano (10^-9), micro (10^-6), milli (10^-3), kilo (10^3), mega (10^6) and giga (10^9).
- Homogeneity An equation is homogeneous if each term has the same base units. Homogeneity is necessary for an equation to be correct but does not prove it is correct, since numerical constants have no units.
- Estimates Make reasonable estimates of physical quantities, such as the mass of an adult (about 70 kg) or the power of a kettle (about 2 kW).
- Errors and uncertainties Distinguish systematic and random errors and precision and accuracy. Combine absolute uncertainties for sums and differences and percentage uncertainties for products, quotients and powers.
- Scalars and vectors Scalars have magnitude only (mass, speed, energy); vectors have magnitude and direction (displacement, velocity, force). Add coplanar vectors with a vector triangle and resolve a vector into perpendicular components F cos θ and F sin θ.
Common mistakes in Physical Quantities and Units
- Thinking a homogeneous equation must be correct. Homogeneity cannot detect wrong numerical factors such as a missing one half.
- Writing derived units such as N or J when the question asks for SI base units. Break every unit down to kg, m, s, A, K or mol.
- Adding vector magnitudes directly. Two forces of 6 N and 8 N at right angles give 10 N, not 14 N.
- Using sin and cos the wrong way round when resolving. The component adjacent to the angle uses cos θ.
- Treating weight and mass as having the same unit. Mass is in kg and weight is a force in N.
Try three Physical Quantities and Units exam-style questions
1. Show that the equation s = ut + ½at^2 is homogeneous. (2 marks)
Show the mark scheme answer
Units of s: m; units of ut: (m s^-1)(s) = m; units of at^2: (m s^-2)(s^2) = m; all terms have units of m so the equation is homogeneous.
2. Express the joule in SI base units. (2 marks)
Show the mark scheme answer
Work = force × distance; J = N m = (kg m s^-2)(m) = kg m^2 s^-2.
3. A cube has mass 50.0 ± 0.5 g and side 2.00 ± 0.02 cm. Calculate its density with its absolute uncertainty. (4 marks)
Show the mark scheme answer
Volume = 2.00^3 = 8.00 cm^3; density = 50.0/8.00 = 6.25 g cm^-3 (6250 kg m^-3); percentage uncertainty = 1% + 3 × 1% = 4%; density = (6.3 ± 0.3) × 10^3 kg m^-3.
Physical Quantities and Units FAQs
What does Cambridge AS & A Level Physics physical quantities and units cover?
It covers SI base units and prefixes, homogeneity of equations, estimating quantities, errors and uncertainties, and adding and resolving scalars and vectors.
How do I revise physical quantities and units?
Practise converting derived units into base units, learn the prefixes, and do plenty of vector addition and uncertainty questions until the rules are automatic.
Which units should I be able to express in base units?
At least the newton, joule, watt, pascal, coulomb, volt and ohm, since these appear often in homogeneity questions.
