IB DP Physics (2025) A2.6 Upthrust & Terminal Velocity

IB DP Physics lesson pack on upthrust, viscous drag and terminal velocity for Topic A.2 Forces and momentum, SL and HL, first assessment 2025. Stokes’ law, buoyancy, and the speed a body settles at. Editable worksheet, 49-slide lesson deck, teacher key, worked solutions and a questions-only assessment, supplied as self-contained HTML plus colour and black-and-white PDF.

What students learn

• Upthrust is the weight of the fluid a body displaces, and that is what decides whether the body sinks, floats or rises.
• Both traps in Fb = ρVg: the density is the fluid’s and never the body’s, and the volume is the fluid displaced, the whole body only when it is fully under.
• Viscous drag on a small sphere, Fd = 6πηrv, with the small-sphere and slow-flow conditions taught as part of Stokes’ law rather than a footnote to it.
• Terminal velocity as what those two buy together: drag grows with speed until the three forces cancel, and the speed against time curve is argued from the balance rather than plotted.

IB skills (Tool 3: Mathematics):

• Understand direct and inverse proportionality (practised).
• Select and manipulate equations (practised).
• Sketch graphs with labelled but unscaled axes (practised).
• Linearize graphs, only where appropriate (practised).
• Draw and interpret uncertainty bars (practised).

What’s included:

• Student Worksheet - 22 pages
• Teacher Key - 35 pages
• Worked Solutions - 20 pages
• Assessment - 12 pages
• Lesson deck - 49 slides

Seven structured questions and five multiple-choice items: 68 marks across 27 parts, and thirteen figures, six with teacher-only answer layers stripped from the student copy rather than hidden. Three questions, twenty-three marks, are worked on the board with the class; the rest go home as marked homework, including a hydrometer data question that straightens a curve, takes a mass from the gradient and carries one uncertainty bar.

It opens on two jars, one of water and one of warmed honey, and a bubble race the last question settles. Free-body diagrams and F = ma are assumed rather than taught again. Worksheet and deck open in any modern browser with nothing to install, and the worksheet is fully editable. SI units, IB notation and IB command terms throughout. Suitable for grades 11-12 (ages 16-19) as a lesson, cover lesson, homework or revision.

Related resources:

• Friction: Static and Dynamic, the lesson before this one
• Newton’s Second Law: F = ma, where the accelerations here come from
• Resultant Force, Inertia and Newton’s First Law, where zero resultant force is taught
• Uncertainties on a Graph, the skill pack behind the data question

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