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M10.1MechanicsCore

Force Diagrams

A force diagram shows every force acting on an object as an arrow. Drawing one correctly — weight, normal reaction, tension, friction — is the essential first step of almost every mechanics problem.

25 min Video by Zeeshan Zamurred Forces and Motion
Edexcel AS Level Maths: 10.1 Force DiagramsWatch the full walkthrough before the notes below.
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What you'll be able to do

  • Identify the common forces
  • Draw a clear force diagram
  • Know the direction of each force
  • Recognise equilibrium
1

The common forces

The forces you meet are: (, always downward), the (, perpendicular to the surface), (along a string, pulling), (along a rod, pushing) and (along the surface, opposing motion).

Weight = mass × gravity, acting straight down.
2

Drawing the diagram

Represent the object as a dot (particle) and draw each force as an arrow the object in the correct direction. Label every arrow. Only include forces actually acting on the object.

Tip — Always include weight (mg) — it’s the most commonly forgotten force.

3

Equilibrium

If an object is in (at rest or moving at constant velocity), the forces balance: they sum to zero. Resolving in perpendicular directions then gives equal-and-opposite force pairs.

Balanced forces ⟶ no acceleration.
1Equilibrium vertically: .
Answer N

Formula recap

Weight (downward).
Normal reaction.
Balanced forces.

Common mistakes to avoid

Forgetting to include the weight on a force diagram.
Weight (mg) always acts — include it pointing straight down.
Drawing the normal reaction vertically on a slope.
The normal reaction is perpendicular to the SURFACE, not always vertical.

Key takeaways

  • Common forces: weight (mg, down), normal reaction (⟂ surface), tension, thrust, friction.
  • Draw each force as a labelled arrow from the particle.
  • Equilibrium means the forces sum to zero.

Test yourself

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