How to Draw Force Diagrams in IGCSE Physics

Force diagrams are required in many exam questions. A correct diagram earns marks; an incomplete one loses them.

What is a free-body diagram

A free-body diagram shows all the forces acting on a single object. Each force is represented by an arrow. The arrow shows:

• The direction of the force
• The point of application (where the force acts)
• The relative magnitude (longer arrow = larger force)

Steps to draw one

1. Identify the object you are drawing forces on.
2. Draw the object as a simple shape (a box or a dot).
3. Identify every force acting on the object (not forces the object exerts on other things).
4. Draw each force as a labelled arrow starting from the object.
5. Make the arrows proportional to the forces (a 20 N arrow should be roughly twice as long as a 10 N arrow).

Forces to consider

• Weight (W): always acts downward towards the centre of the Earth. W = mg.
• Normal contact force (N): perpendicular to the surface the object rests on.
• Friction (f): parallel to the surface, opposing motion or tendency to move.
• Air resistance (drag): opposing the direction of motion through air.
• Tension (T): along a rope or string, pulling the object.
• Applied force (F): any push or pull applied to the object.
• Upthrust: on an object in a fluid, acting upward (equal to the weight of fluid displaced).

Common mistakes

• Drawing the reaction force as weight on a different object (Newton’s third law pair), instead of the normal contact force on the same object.
• Missing air resistance for a moving object.
• Drawing forces that do not act on the object in question.

In exam questions

Label every arrow with the force name. If values are given, write the value next to the arrow. Draw the diagram large enough that all arrows and labels are readable.