3.4.1.5
Newton's third law: forces arise in equal and opposite pairs on different objects
Newton's Laws & Momentum — AQA A-Level Physics
Key Definition
Newton's third law — If Object A exerts a force on Object B, then Object B exerts a force on Object A that is equal in magnitude but opposite in direction.
- Third-law pairs must be: the same type of force, equal in magnitude, opposite in direction, and acting on different objects.
- Weight and normal reaction on the same object are NOT a third-law pair -- they are different types of force acting on the same object.
- Example: foot pushes ground backwards (contact force), ground pushes foot forwards (contact force). Same type, same magnitude, opposite directions, different objects.
Common Mistake
MEDIUM
Students often: Don't say weight and normal reaction on a book are a Newton's third lawWhen two objects interact, the forces they exert on each other are equal in magnitude, opposite in direction, and act on different objects. pair.
Instead: They act on the same object (the book) and are different types of force. The third-law pair for the book's weight is the gravitational pull of the book on the Earth. The third-law pair for the normal reaction from the table is the contact force of the book pushing down on the table.
Instead: They act on the same object (the book) and are different types of force. The third-law pair for the book's weight is the gravitational pull of the book on the Earth. The third-law pair for the normal reaction from the table is the contact force of the book pushing down on the table.
Examiner Tips and Tricks
Use a three-part label: 'Object A pushes/pulls Object B, and Object B pushes/pulls Object A.' If both forces are the same type, the same magnitude, opposite directions, and on different objects, they form a third-law pair.