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IB Physics - Theme A.2

IB Physics forces and momentum

Forces questions become manageable when the system and interaction model are explicit. Build from a free-body diagram to Newton's laws, then connect force over time to impulse and momentum rather than treating each equation as a separate trick.

Framework
IB Diploma Programme Physics
Level
IB Diploma Physics SL and HL
Guide format
Concept and practice pathway

Start with the model

One system, one diagram, one consistent sign convention.

First name the object or group of objects you are analysing. Draw only external forces acting on that system, resolve components along useful axes, and decide whether the acceleration is known, zero, or part of the unknown. This prevents common errors such as adding motion arrows as forces or pairing action-reaction forces on one diagram.

Momentum provides a second view of interaction. A force-time graph records impulse, while an isolated-system model compares total momentum before and after an event. Keep these models distinct: Newton's second law describes the net interaction on a chosen system, while momentum conservation requires a justified boundary and negligible external impulse.

Study pathway

Four decisions that organize the topic.

Follow the sequence or open the step that matches your current gap. Destination pages keep their existing access rules.

  1. 01

    Define the system

    State what is inside the boundary and which external interactions can change its motion.

    Open this step
  2. 02

    Draw the force model

    Use free-body diagrams, components, and a declared positive direction before writing a sum of forces.

    Open this step
  3. 03

    Apply Newton's laws

    Connect resultant force to acceleration and distinguish interaction pairs from balanced forces.

    Open this step
  4. 04

    Move to impulse and momentum

    Read area from force-time graphs and test whether a system boundary supports conservation.

    Open this step

Use it deliberately

How to attack a forces question

Make the modelling choices visible before substitution. Most lost marks originate before the calculator is used.

  1. 01

    Circle the system and list external interactions in words before drawing arrows.

  2. 02

    Choose axes that simplify the geometry, then resolve every angled force into those axes once.

  3. 03

    Write the symbolic balance or momentum statement before inserting values and signs.

  4. 04

    Check whether the resulting direction, limiting case, and unit agree with the physical story.

Continue on GioPhysics

A mapped route, not a search-results wall.

Curriculum context on this page is public. Lessons, presentations, files, and practice follow the access message shown when you open them.

MapForces and momentum directorySee every mapped A.2 concept and its teaching sequence.DrawFree-body diagrams and net forceBuild the system boundary and force representation.ReasonNewton's laws of motionConnect interaction laws, resultant force, and acceleration.GraphImpulse and force-time graphsTurn area under a force-time graph into momentum change.PractiseForces applicationsApply force models through graduated problem pathways.

Questions before you begin

Clear scope, honest expectations.

Should I start an IB forces problem with an equation?

Usually start by defining the system, drawing the external forces, and choosing directions. The correct equation follows from that model.

How are impulse and momentum connected to force?

Impulse is the time accumulation of resultant force and equals the change in momentum. On a force-time graph, impulse is the signed area under the graph.

Is this guide only for IB Physics HL?

No. The mapped A.2 forces and momentum pathway is labelled SL + HL in the GioPhysics IB curriculum map.