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55 ideas · 69 connections
Springs and Newton’s second law Forces explain changes in motion Changing drag produces changing acceleration Density determines upthrust; force balance determines motion Small length errors can matter greatly in density Sample stiffness and Young modulus describe different things Stress and pressure share units, but describe different loading Measuring Young modulus accurately Estimate mass from size and material Motion3.1 Forces in action3.2 Physical quantities2.1.1 Measurements and uncertainty2.2.1 Dynamics3.2.1 Motion with non-uniform acceleration3.2.2 Density and pressure3.2.4 Springs3.4.1 Mechanical properties of matter3.4.2
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Density determines upthrust; force balance determines motion

OCR A AS Physics · Two ideas, one explanation

Upthrust is the weight of displaced fluid. Compare it with the object’s weight and other forces to decide whether the object floats, sinks or accelerates.

Upthrust = ρfluid Vdisplaced g

Follow the reasoning

  1. Find the volume of fluid actually displaced.
  2. Calculate upthrust and weight separately.
  3. Use the resultant to explain the motion or equilibrium.
Specification outcomes (5)
  • 3.2.4(a) Define density; ρ = m / V.
  • 3.2.4(c) Use p = hρg to calculate pressure in fluids; understand upthrust and Archimedes’ principle.
  • 3.2.1(a) Apply Newton’s second law, F = ma.
  • 3.2.1(c) Understand weight of an object, W = mg.
  • 3.2.1(d) Define and identify tension, normal contact force, upthrust and friction.
Open the official OCR AS specification ↗

H156 v2.0 · May 2026

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