IB DP Physics (2025) A2.2 Free-Body Diagrams & Force Pairs

IB DP Physics lesson pack on free-body diagrams and force pairs for Topic A.2 Forces and momentum, SL and HL, first assessment 2025: Newton’s third law, and every force acting on one chosen body. Editable worksheet, 43-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

• A force is one side of an interaction between two bodies, named as the force of one body on another: the push of the thumb on the clay, the pull of the Earth on the lorry.
• Newton’s third law, and the five tests a force pair must pass: equal in size, opposite in direction, same line, same type, and acting on two different bodies.
• Why only one force of a pair can appear on a given free-body diagram, and how to write the partner of any force, naming both bodies.
• A free-body diagram of one chosen body: each arrow started where its force acts, its length showing the size, annotated Fg, FN, FT, Ff or Fapp.
• The three things that never go on it: the forces the body exerts on other bodies, the bodies touching it, and 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 - 16 pages
• Teacher Key - 21 pages
• Worked Solutions - 12 pages
• Assessment - 9 pages
• Lesson deck - 43 slides

Nine questions and 10 figures: three multiple-choice items and 44 marks of written work across 18 parts, with five blank frames the students draw the force arrows onto.

The only apparatus is a lump of modelling clay per pair. Forces are drawn one at a time here and never added together: this hour stops at a completed free-body diagram, and resultants, translational equilibrium and Newton’s first and second laws come in the lessons that follow. 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:

• What Is a Force? Contact and Field Forces, the lesson before this one
• Foundations of Motion: Distance and Displacement, a free lesson pack to try first
• Unit Conversion, the free skill pack behind the gram to kilogram step
• Resolving Vectors, the skill pack for the component work later in the topic