IB DP Physics lesson pack on free-body diagrams, force pairs and Newton’s third law, Topic A.2 Forces and momentum, SL and HL, first assessment 2025. Students draw every force acting on one chosen body, and nothing else. Editable worksheet, 44-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
- Every force is a push or a pull of one body on another, and its name gives both bodies: the push of the thumb on the clay.
- Newton’s third law: the two forces of a pair are equal in size, opposite in direction, on the same line and of the same type, and act on two different bodies.
- How to write the partner of any force, and why only one force of a pair goes on a free-body diagram.
- A free-body diagram of one chosen body: each arrow starts where its force acts, its length shows the size, and it carries its symbol (Fg, FN, FT, Ff or Fapp).
- What never goes on it: forces the body exerts on other bodies, the bodies it touches, or an arrow for its motion.
IB skills (Tool 3: Mathematics):
- Draw and interpret free-body diagrams, with forces at the point of application or the centre of mass (taught).
- Draw and label vectors: magnitude, point of application, direction (practised).
- Apply SI prefixes and units (practised).
What’s included:
- Student Worksheet - 15 pages
- Teacher Key - 22 pages
- Worked Solutions - 12 pages
- Assessment - 9 pages
- Lesson deck - 44 slides
Six written questions and three multiple-choice items: 44 marks across 20 parts, and 13 figures. Your class works four questions, 25 marks, in the lesson, and the rest go home. Students draw the force arrows onto five blank frames.
The only apparatus is a lump of modelling clay per pair. Forces are drawn one at a time and never added together. The lesson stops at a completed free-body diagram. Resultants, translational equilibrium and Newton’s first and second laws come in later lessons. 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.
Part of the A.2 Forces and Momentum Complete Unit Bundle, 30% off buying the packs separately.
Related resources:
- What Is a Force? Contact and Field Forces, the lesson before this one
- Resultant Force, Inertia and Newton’s First Law, the lesson that comes next
- Friction: Static and Dynamic, where those diagrams gain a friction arrow
- Foundations of Motion: Distance and Displacement, a free lesson pack to try first
Updated 3 Nov 2026: please reprint.
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