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Units keep motion calculations consistent A physical quantity needs both a number and a unit Estimate mass from size and material Quantities and units2.1 Physical quantities2.1.1 SI units2.1.2 Kinematics3.1.1 Density and pressure3.2.4
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A physical quantity needs both a number and a unit

OCR A AS Physics · Two ideas, one explanation

A numerical value is meaningful only alongside its unit. Estimating the expected scale before calculating can reveal a missed prefix or an implausible result.

Physical quantity = numerical value × unit

Follow the reasoning

  1. State the quantity being measured or calculated.
  2. Choose or convert to a consistent unit.
  3. Compare the result with an estimate of a realistic scale.
Specification outcomes (5)
  • 2.1.1(a) Physical quantities have a numerical value and a unit.
  • 2.1.1(b) Make estimates of physical quantities listed in the specification.
  • 2.1.2(a) Système Internationale (S.I.) base quantities and their units – mass (kg), length (m), time (s), current (A), temperature (K), amount of substance (mol).
  • 2.1.2(b) Derived units of S.I. base units, such as momentum (kg m s⁻¹) and density (kg m⁻³).
  • 2.1.2(e) Use prefixes and their symbols to indicate decimal submultiples or multiples of units (pico, nano, micro, milli, centi, deci, kilo, mega, giga, tera).
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H156 v2.0 · May 2026

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