SEC G3 Combined Science Physics component K326/K327
CP4: Dynamics
Use resultant force and motion reasoning from a clear force model.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Dynamics begins with one chosen object: identify every external force on it, combine those forces, and then relate the resultant to its acceleration.
Balanced and unbalanced forces
Zero resultant force means no acceleration, so an object remains at rest or continues at constant velocity. A non-zero resultant changes speed, direction, or both. These are the relationships to apply; memorised statements of Newton's laws are not required.
An action-reaction pair belongs to two interacting objects. The forces have equal magnitude and opposite direction, but they do not cancel because they act on different bodies.
Free-body diagrams
Draw only forces acting on the selected object. Use labelled arrows and keep horizontal and vertical components distinct in two-dimensional cases.
Do not add motion arrows as forces. Velocity describes motion, whereas a force diagram records interactions.
Resultant force and friction
Resultant force equals mass times acceleration. With fixed mass, a larger resultant produces a larger acceleration; with fixed resultant, a larger mass produces a smaller acceleration.
Friction opposes relative movement, or the tendency for relative movement, between surfaces. It provides grip when needed, but it can also slow motion and transfer mechanical energy into heating.
Formulae and relationships
| Relationship | Use |
|---|---|
| Relate resultant force to acceleration. |
Worked examples
Example 1: A trolley has forward and backward forces. Find its acceleration.
- Choose forward as positive.
- Find the resultant: forward.
- Calculate forward.
Answer: forward.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Draw one object boundary and every external force before calculating the resultant.
- Use balanced forces for rest or constant velocity and unbalanced forces for acceleration.
- Keep action-reaction pairs on two different interacting bodies and explain friction separately.
Official outcome coverage
K326 CP4: 4 mapped outcomes, references CP4(a), CP4(b), CP4(c), CP4(d). Check the official K326 syllabus.
K327 CP4: 4 mapped outcomes, references CP4(a), CP4(b), CP4(c), CP4(d). Check the official K327 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Investigate balanced and unbalanced forces with a low-friction moving system. Change one of resultant force or mass, control the other, measure motion consistently, repeat trials, and decide whether the evidence supports the predicted acceleration change.
Exam traps and retrieval check
Avoid these traps
- Putting an action and reaction on the same free-body diagram.
- Assuming motion requires a forward resultant.
- Using one applied force instead of the resultant.
Check from memory
What does zero resultant force imply?
Zero acceleration.
Where do action-reaction forces act?
On two different interacting objects.
What is the unit of resultant force?
The newton, N.
Official Combined Science scope
This shared Combined Physics owner serves both K326 and K327. Balanced and unbalanced forces, action-reaction pairs, free-body diagrams, resultant force, acceleration, and friction.