Introduction
Before any mechanics problem can be solved — whether it involves equilibrium, Newton's second law, or connected particles — you must first identify every force acting on the object of interest. Missing even one force is one of the most penalised errors in 9709 Mechanics papers.
This subtopic introduces the skill of constructing a force diagram (sometimes called a free-body diagram): a clear sketch showing all forces acting on a body, represented as arrows indicating direction, labelled with their magnitudes or standard symbols. Throughout 9709 Mechanics, every object is modelled as a particle — a point mass — meaning we treat all forces as acting at a single point, regardless of the physical size of the real object.
Core Concept
The Particle Model
A "particle" is an idealised object with mass but no size. This means:
- Rotational effects are ignored.
- Every force acts at the same single point.
- A lorry, a box, a person on a slope — all are treated as particles.
Types of Force Encountered in 9709
You must know, recognise, and correctly draw the following forces:
| Force | Symbol | Direction | When it acts |
|---|---|---|---|
| Weight (gravity) | Vertically downwards | Always — on every particle with mass | |
| Normal reaction | or | Perpendicular to the surface, away from it | Particle in contact with a surface |
| Friction | Along the surface, opposing motion (or tendency of motion) | Particle on a rough surface | |
| Tension | Along a string/rod, away from the particle | Particle attached to a string or rod | |
| Thrust (compression) | or | Along a rod, towards the particle | Particle connected by a rod being compressed |
| Applied / driving force | or | As specified in the problem | When stated explicitly |
| Air resistance | Opposing the direction of motion | When stated in the problem |
Drawing a Force Diagram — The Procedure
- Isolate the particle: focus on one object at a time.
- Draw a dot to represent the particle.
- Add each force as an arrow starting at (or passing through) the dot, pointing in the correct direction.
- Label every arrow with its symbol or known magnitude.
- Check: have you included weight? Are reaction forces perpendicular to the surface?
Key modelling point: In 9709, unless told otherwise, strings are light (massless) and inextensible, surfaces are smooth (no friction) unless stated rough, and pulleys are smooth (frictionless).
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