Physical Quantities and Units · revision map · A-Level 9702, Topic 1
TOPIC 1 · PHYSICAL QUANTITIES AND UNITS
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
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QUANTITIES & UNITS1
Physical Quantities
Every physical quantity is a number paired with a unit; both are needed.
quantity = number × unite.g. mass of a person ≈ 70 kg, not just 70
- The unit must match the quantity it labels.
- Estimate orders of magnitude for everyday values.
SI Base Units & Homogeneity
Seven SI base units build every other (derived) unit.
kg · m · s · A · K · mol · cd
derived: N = kg m s−² · J = kg m² s−²
- Homogeneity: every term in an equation must have the same base units.
- Prefixes run pico (p) to tera (T): 10−¹² to 10¹².
Errors & Uncertainties
Every reading carries a band of doubt.
- Systematic error shifts all readings the same way.
- Random error scatters them; averaging reduces it.
- Accuracy is closeness to the true value; precision is the spread.
product / quotient: %Δy = %Δa + %Δb
Scalars & Vectors
Some quantities need a direction as well as a size.
- Scalar: magnitude only (mass, time, energy).
- Vector: magnitude and direction (force, velocity).
- Add coplanar vectors tip to tail (parallelogram rule).
- Resolve a vector into two perpendicular components.