The idea
Start by identifying the object and removing its surroundings from the diagram. Include weight, contact forces, tension or drag when they act on it. Weight acts downward; the normal contact force acts perpendicular to the surface. Resolve angled forces along chosen axes before finding the resultant. A zero resultant means zero acceleration, which allows either rest or constant velocity.
Worked example
A 2 kg box rests on a horizontal table with no other vertical forces. Use g = 9.8 m/s². Find the normal force.
- The box has weight mg = 2 × 9.8 = 19.6 N downward.
- Its vertical acceleration is zero, so the upward and downward forces balance.
- The normal force is 19.6 N upward.
A mistake to avoid
Do not add a separate “force of motion”. Velocity is not a force, and an object does not need a forward resultant to keep moving at constant velocity.
Check your understanding
Is the normal force always equal to the weight?
Show the explanation
No. An angled surface, another vertical force or vertical acceleration can change the normal force.
Explore the lesson
This guide connects to Forces & Free-Body Diagrams in the IB Physics course. The first two published lessons per subject are free previews; further interactive lessons require an account.