Think of a force diagram as a translator. The real world is messy — cars, ropes, slopes — but physics needs clarity. A force diagram strips everything down to a dot (the particle) and labelled arrows showing every force acting on it.
In the model, every object — no matter how big — is treated as a single point in space. A car, a block on a ramp, a hanging mass — they're all just dots. This is called the particle model. Every force acting on that object acts at that same dot.
Types of Force You'll Encounter
| Force | Symbol | Direction | What causes it? |
|---|
| Weight | W N | Vertically downward | Gravity pulling the object's mass |
| Normal Reaction | R N | Perpendicular to surface | Surface pushing back on the object |
| Friction | F N | Parallel to surface, opposing motion | Rough surface resisting sliding |
| Tension | T N | Away from particle (along string) | String/rope being pulled taut |
| Thrust | T N | Towards particle | Rod or strut pushing on the object |
The key rule for direction
Tension pulls — so its arrow points away from the particle, along the string. Thrust pushes — so its arrow points toward the particle. Weight always points straight down. Normal reaction always points away from the surface, at 90°.
Reading a Force Diagram — Block on a Rough Slope
↗ R N (normal reaction, ⊥ to slope)
/
━━━━━━━━━━━
| BLOCK |——→ F N (friction, up the slope, opposing motion)
━━━━━━━━━━━
\
↓ W N (weight, vertically down)
Note: Weight W = mg acts downward from the centre of the particle.
Normal Reaction R is perpendicular (⊥) to the slope surface.
Friction F acts parallel (∥) to the slope, opposing motion.
Common Mistake
Students often draw the normal reaction vertically upward on an inclined plane. It is NOT vertical — it is always perpendicular to the surface the object is resting on. On a slope at angle θ, R is at angle θ from the vertical.
Practice Question 1
A block rests on a rough horizontal table. A horizontal string pulls the block to the right with tension T N. The block is on the verge of moving but is still at rest. List ALL the forces acting on the block and state the direction of each.