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Springs and Newton’s second law Forces explain changes in motion Changing drag produces changing acceleration Terminal motion still transfers energy Test terminal velocity with repeated measurements Density determines upthrust; force balance determines motion Motion3.1 Forces in action3.2 Planning1.1.1 Dynamics3.2.1 Motion with non-uniform acceleration3.2.2 Density and pressure3.2.4 Power3.3.3 Springs3.4.1
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Changing drag produces changing acceleration

OCR A AS Physics · Two ideas, one explanation

As a falling object speeds up, drag usually increases. The resultant force becomes smaller, so its acceleration falls until terminal velocity is reached.

Downward resultant = weight − drag − upthrust

Follow the reasoning

  1. Draw all forces, including upthrust when significant.
  2. Find the resultant at the speed being considered.
  3. Use Newton’s second law to infer the acceleration.
Specification outcomes (6)
  • 3.2.2(a) Define drag as the frictional force experienced by an object travelling through a fluid.
  • 3.2.2(c) Describe motion of objects falling in a uniform gravitational field in the presence of drag.
  • 3.2.2(d)(i) Define and describe terminal velocity.
  • 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(e) Draw and interpret free-body diagrams.
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H156 v2.0 · May 2026

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