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Forces explain changes in motion Measuring motion without hiding uncertainty 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 Why wire diameter matters in resistivity A Planck-constant experiment needs uncertainty analysis Instrument choice sets the measurement limits Estimate mass from size and material Motion3.1 Forces in action3.2 Implementing1.1.2 Physical quantities2.1.1 Measurements and uncertainty2.2.1 Kinematics3.1.1 Dynamics3.2.1 Density and pressure3.2.4 Mechanical properties of matter3.4.2 Resistivity4.2.4 Photons4.5.1
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Small length errors can matter greatly in density

OCR A AS Physics · Two ideas, one explanation

Density depends on mass and volume. When volume is calculated from a diameter or side length cubed, its fractional uncertainty is amplified.

ρ = m/V For V ∝ d³: percentage uncertainty in V ≈ 3 × that in d

Follow the reasoning

  1. Write volume in terms of the measured dimensions.
  2. Apply the appropriate powers when combining uncertainties.
  3. Combine the mass and volume contributions for density.
Specification outcomes (2)
  • 3.2.4(a) Define density; ρ = m / V.
  • 2.2.1(c) Calculate absolute and percentage uncertainties when data are combined by addition, subtraction, multiplication, division or raising to powers.
Open the official OCR AS specification ↗

H156 v2.0 · May 2026

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