O-Level and SEC G3 Physics K323
P3: Dynamics
Draw a force model first, then apply resultant force, mass, acceleration, momentum, and impulse relationships.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Dynamics links forces to changes in motion. Strong answers begin with an object boundary and a free-body diagram, distinguish mass from weight, and apply Newton-law relationships to the forces acting on the selected object.
Forces and free-body diagrams
Contact forces include friction, air resistance, tension and normal force. Non-contact forces include gravitational, electrostatic and magnetic forces. Draw only forces acting on the chosen body, with arrows beginning on that body and labels identifying each interaction.
Balanced forces give zero resultant force, so velocity remains constant. An unbalanced resultant produces acceleration. For a static point mass acted on by three forces, a closed vector triangle represents equilibrium when drawn to scale.
Mass, weight, gravitational field and inertia
Mass measures the amount of matter and also the resistance to change in motion called inertia. Weight is the gravitational force on that mass. Gravitational field strength is force per unit mass, so weight changes with field strength but mass does not.
An action-reaction pair acts on two different interacting bodies. The two forces are equal in magnitude, opposite in direction and of the same interaction type. They do not cancel on one free-body diagram because they act on different objects.
Friction, falling and terminal velocity
Friction opposes relative motion or attempted motion between surfaces. It can be useful for walking and braking, but it can also dissipate mechanical energy by heating. Air resistance depends on factors including speed and exposed shape.
For a falling body, weight initially exceeds air resistance, so it accelerates. As speed increases, air resistance increases. When air resistance equals weight, the resultant is zero and the body continues at terminal velocity.
Formulae and relationships
| Relationship | Use |
|---|---|
| Relate weight, mass and gravitational field strength. | |
| Define gravitational field strength. | |
| Relate resultant force to acceleration. |
Worked examples
Example 1: A car has a driving force of and resistive force of . Find its acceleration.
- Choose the forward direction as positive.
- Resultant force: .
- Use .
Answer: forwards.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Draw the object boundary and every external force before calculating a resultant.
- Connect resultant force to acceleration and keep mass separate from weight.
- Use momentum and impulse with a sign convention that makes direction explicit.
Official outcome coverage
K323 P3: 13 mapped outcomes, references P3(a), P3(b), P3(c), P3(d), P3(e), P3(f), P3(g), P3(h), P3(i), P3(j), P3(k), P3(l), P3(m). Check the official K323 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Use force and motion data to test how acceleration changes with resultant force or mass while keeping the other variable controlled.
Exam traps and retrieval check
Avoid these traps
- Using a single force instead of the resultant in .
- Drawing the force that the object exerts on something else.
- Saying terminal velocity means there are no forces rather than balanced forces.
Check from memory
What remains constant when resultant force is zero?
Velocity, which may be zero or non-zero.
Where do Newton third-law forces act?
On two different interacting bodies.
Why does a falling object approach terminal velocity?
Increasing air resistance reduces the resultant until it reaches zero.
Pure versus Combined scope
Combined Physics revisits part of this core under CP4 Dynamics, with force also used in CP3 Force and Pressure, but with reduced outcome scope. Combined students should follow the component checklist rather than assume every K323 outcome is assessable.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K323. Open the related IP explanation.