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
Curriculum Equation Key قوانين نيوتن للحركة - Newton Laws of Motion FormulasFast Exam Solution
F = m × a | W = m × g | F₁₂ = -F₂₁
Word Equation & Conditions: القوة المحصلة = الكتلة × التسارع | Force = Mass × Acceleration
Location & Specifications: الميكانيكا الكلاسيكية والتحصيلي وفيزياء 3 ثانوي و IGCSE Physics.
Section 1 of 4The Law and Its Features
Forces always occur in pairs when two objects interact, and the two forces are equal and opposite.
Newton's third law
- 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
- Equal size The two forces in an action-reaction pair always have the same magnitude.
Opposite direction
- Opposite direction The two forces act in opposite directions along the same line.
Different objects
- Different objects The forces in a pair act on two different objects, so they do not cancel each other out.
Same type
- 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
- 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
- Walking A walker pushes backwards on the ground, and the ground pushes forwards on the walker with friction.
Rocket
- Rocket A rocket pushes gas backwards, and the gas pushes the rocket forwards with an equal thrust.
Earth and an apple
- 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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