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1.3.3States of Matter

Newton’s Third Law — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 4 parts Lesson 26 of 30 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key Newton Laws of Motion FormulasFast Exam Solution
F = m × a | W = m × g | F₁₂ = -F₂₁
Word Equation & Conditions: Force = Mass × Acceleration
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Section 1 of 4The Law and Its Features

1.3.3 Newton's Third Law

Forces always occur in pairs when two objects interact, and the two forces are equal and opposite.

Newton's third law
  • When object A exerts a force on object B, object B exerts an equal and opposite force on object A.
Equal size
  • The two forces in an action-reaction pair always have the same magnitude.
Opposite direction
  • The two forces act in opposite directions along the same line.
Different objects
  • The forces in a pair act on two different objects, so they do not cancel each other out.
Same type
  • Both forces in a pair are the same kind, for example both are gravitational or both are contact forces.

Section 2 of 4Identifying a Pair

Checking each feature in turn identifies a true action-reaction pair.

1
Identify the object that exerts the first force, and the object it acts on.
2
Identify the second force, which acts in the reverse direction on the first object.
3
Check that the two forces are equal in size and opposite in direction.
4
Check that the two forces act on different objects and are of the same type.

Section 3 of 4Everyday Examples

Everyday Examples
Book on a table
  • The book pushes down on the table, and the table pushes up on the book with an equal normal contact force.
Walking
  • A walker pushes backwards on the ground, and the ground pushes forwards on the walker with friction.
Rocket
  • A rocket pushes gas backwards, and the gas pushes the rocket forwards with an equal thrust.
Earth and an apple
  • The apple is pulled down by gravity, and the apple pulls the Earth up with an equal force.

Section 4 of 4Exam Checklist & Tips

1.3.3 – Key Points for the Exam

  • Action-reaction pair Two equal and opposite forces that act on different objects during an interaction.
  • Newton's third law For every force, there is an equal and opposite force acting on a different object.
  • Normal contact force The force a surface exerts on an object resting on it, perpendicular to the surface.
  • Thrust A driving force produced by pushing gas or water backwards.

Common Mistakes

  • Believing that action-reaction pairs cancel out; they act on different objects, so they cannot cancel.
  • Stating that the reaction force is smaller than the action force, which is never true.
  • Describing one force as causing the other one after a delay; the two forces act at the same time.
  • Confusing an action-reaction pair with two balanced forces acting on the same object.

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