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Density determines upthrust; force balance determines motion Small length errors can matter greatly in density Stress and pressure share units, but describe different loading Measuring Young modulus accurately 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 Measurements and uncertainty2.2.1 Dynamics3.2.1 Density and pressure3.2.4 Mechanical properties of matter3.4.2
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Estimate mass from size and material

OCR A AS Physics · Two ideas, one explanation

An estimate of volume and a plausible material density can give a useful estimate of mass. This also provides a check on measurements and unit conversions.

Estimated mass = density × estimated volume

Follow the reasoning

  1. Approximate the shape with a suitable geometric model.
  2. Estimate its dimensions and convert to consistent units.
  3. Use a sensible density and judge the result’s order of magnitude.
Specification outcomes (3)
  • 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.
  • 3.2.4(a) Define density; ρ = m / V.
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H156 v2.0 · May 2026

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