Forces and their interactions
What is a force?
A force is a push or a pull on an object, measured in newtons (N) with a force meter (newtonmeter). Force is a vector — it has both size and direction. It can change an object's speed, direction or shape.
Contact vs non-contact forces
| Contact (touching) | Non-contact (no touching) |
|---|---|
| Friction | Gravity |
| Air resistance / drag | Magnetic |
| Normal contact / reaction | Electrostatic |
| Tension | |
| Upthrust |
Newton's third law
Every action has an equal and opposite reaction. When you push down on a chair (action), the chair pushes up on you (reaction). The pair acts on different objects and they are the same type of force.
Free-body diagrams
A free-body diagram represents the object as a dot or box and shows all forces acting on it as labelled arrows. Length of the arrow = magnitude; direction of the arrow = direction of the force.
Resultant force
When two forces act in the same direction, add them. If they act in opposite directions, subtract them.
For two forces at right angles (Higher), use Pythagoras:
F_resultant = √(F_x² + F_y²)
If the resultant force = 0, the object is in equilibrium — it stays at rest or continues at constant velocity (Newton's first law).
✦Worked example
A box on the floor: weight 200 N down, normal contact 200 N up → resultant = 0 → object at rest. Push horizontally with 50 N, friction 30 N → resultant = 20 N forward → box accelerates.
Edexcel exam tip
Always state the direction of a resultant force ("20 N to the right"), not just the size. Direction is worth a separate mark.
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