IB DP Physics (2025) A2.3 Resultant Force & Newton's First Law

IB DP Physics lesson pack on resultant force, inertia and Newton’s first law for Topic A.2 Forces and momentum, SL and HL, first assessment 2025: how forces are added, and why a body already moving needs no force to keep going. Editable worksheet, 45-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

• The resultant force, the single force with the same effect as all the forces on a body together, found along one line from signed numbers.
• Two forces at right angles taken tip to tail, so Pythagoras gives the resultant, and a force at any angle resolved into F cos θ and F sin θ.
• Inertia, a body’s resistance to any change in its motion, measured by its mass and never drawn as a force.
• Newton’s first law: a body’s velocity changes only when a resultant force acts on it, so steady motion needs no cause.
• Translational equilibrium, the resultant force zero, in components and as a closed force triangle drawn to scale.

IB skills (Tool 3: Mathematics, with Inquiry 2: Collecting and processing data):

• Draw and interpret free-body diagrams (practised).
• Add and subtract vectors in the same plane (practised).
• Resolve vectors into two perpendicular components (practised).
• Construct and use scale diagrams (practised).
• Linearize graphs, only where appropriate (practised).
• Draw lines of best fit (practised).
• Interpret features of graphs: gradient and intercepts (practised).
• Identify and justify the removal of outliers in data (practised).

What’s included:

• Student Worksheet - 20 pages
• Teacher Key - 26 pages
• Worked Solutions - 16 pages
• Assessment - 13 pages
• Lesson deck - 45 slides

Eight questions and four multiple-choice items: 62 marks across 29 parts, and 11 figures, with six frames the students draw force arrows, a scale construction or a best-fit line onto.

Equilibrium is about the forces, never about the speed: a crate pushed steadily at 8.0 m s⁻¹ is in it, and so is the same crate at rest. Newton’s second law is not in this hour: F = ma belongs to the lesson that follows. 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:

• Free-Body Diagrams and Force Pairs, the lesson before this one
• What Is a Force? Contact and Field Forces, where these forces are named
• Resolving Vectors, the skill pack behind the component work
• Measuring the Acceleration of Free Fall, free, with the same graph work