What this covers
Physical Quantities is part of the Measurement chapter. We teach it the way it is tested: the concept in plain English, then a worked example, then a question the student tries while the teacher checks the method.
How it is examined
It can appear in Paper 1 (objective) and Paper 2 (structured), and where an experiment applies, in Paper 3. We do not predict which questions appear; we prepare the technique for all of them.
A common mistake
How to study it
Learn the definition precisely, practise one or two SPM-style questions with full working, and link it to the rest of Measurement. If it keeps costing marks, a one-to-one lesson fixes exactly that.
Measurement · Formulas · Exam Papers
What you need to know
Every measurement in Physics is expressed as a physical quantity, and these fall into two groups: base quantities and derived quantities. There are seven SI base quantities, each with its own base unit, length (metre, m), mass (kilogram, kg), time (second, s), electric current (ampere, A), temperature (kelvin, K), amount of substance (mole, mol), and luminous intensity (candela, cd).
Derived quantities are formed by multiplying or dividing base quantities, so their units are derived too. Density, for example, is mass divided by volume, giving the derived unit kg m⁻³.
Quantities are also classed as scalar (magnitude only, such as mass or time) or vector (magnitude and direction, such as displacement or force). Prefixes convert awkward numbers into manageable ones: nano (10⁻⁹), micro (10⁻⁶), milli (10⁻³), kilo (10³), mega (10⁶), giga (10⁹).
A value such as 3.0 cm must be converted before it enters any formula: 3.0 cm = 3.0 × 10⁻² m = 0.030 m. Since SPM Physics has no formula sheet, every base unit, symbol and prefix must be memorised accurately, including the correct case, m for milli and M for mega.
Worked example
Question: The density of a small metal block is defined as ρ = m / V. A block has a mass of 54 g and a volume of 20 cm³. Show that the derived unit of density is kg m⁻³, then find the density in that unit.
First convert to SI base units: m = 54 g = 0.054 kg, and V = 20 cm³ = 20 × 10⁻⁶ m³ = 2.0 × 10⁻⁵ m³. Deriving the unit: unit of ρ = unit of m ÷ unit of V = kg ÷ m³ = kg m⁻³.
Substituting the values: ρ = m / V = 0.054 kg ÷ 2.0 × 10⁻⁵ m³ = 2700 kg m⁻³. This matches the known density of aluminium, so the working is consistent.
The key skill tested here is not the arithmetic alone but the conversion of every quantity into SI base units before the formula is applied, and the derivation of the unit from the defining equation rather than from memory alone.
How it is examined
Paper 1 objective items usually ask students to state the SI unit of a given quantity, identify whether a listed quantity is a base or derived quantity, or classify a quantity as scalar or vector. Paper 2 structured questions may ask candidates to define a physical quantity, convert a value using prefixes, or derive the unit of a derived quantity from its defining formula, for example deriving kg m⁻³ from ρ = m / V. Paper 3 practical questions require quantities to be recorded with the correct SI unit and an appropriate number of significant figures when tabulating experimental readings.
Common mistakes include writing lowercase m for mega instead of uppercase M, confusing mass in kg with weight in N, leaving units off derived quantities, and forgetting to convert centimetres or grams into metres and kilograms before substituting into a formula. Another frequent error is treating a vector quantity as though direction does not matter, which loses marks even when the magnitude is correct.
Careful unit tracking through every step of a calculation is essential for full marks.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)