Skip to content

Salam Academy

Sign in Create account

LearnCambridge97022025–2027Paper 2 · AS Level Structured QuestionsPhysical quantities and units

Paper 2 · Module

Physical quantities and units

3 subtopics≈ 95 min in total

Start the module: SI units, prefixes and estimates Open graphing calculator

SI units, prefixes and estimates
Every quantity has a magnitude and a unit. All SI units are built from six base units (kg, m, s, A, K, mol) used in this course. Writing a derived unit in base units lets you check whether an equation is homogeneous. Prefixes and powers of ten keep numbers manageable, and good physicists can estimate any everyday quantity to the right order of magnitude.
≈ 30 min
Errors and uncertainties
Random errors scatter readings either side of the true value and are reduced by repeating and averaging. Systematic errors shift every reading the same way and aren't reduced by repeating. Precise readings agree with each other; accurate ones are close to the true value. When quantities are added or subtracted, add absolute uncertainties; when multiplied or divided, add percentage uncertainties; for a power, multiply the percentage by the power.
≈ 35 min
Scalars and vectors
Scalars have magnitude only; vectors have magnitude and direction. Vectors add tip-to-tail; for two at right angles use Pythagoras and trigonometry, and for other angles use a scale drawing or the cosine rule. Any vector can be resolved into two perpendicular components: $F\cos\theta$ along the reference direction and $F\sin\theta$ perpendicular to it.
≈ 30 min