Newton's third law — Science definition
Defined in 2 courses:
- For every action force, there is an equal and opposite reaction force. (Physical Science: Matter, Forces & Energy)
- For every force one object exerts on a second, the second exerts an equal and opposite force back on the first. (Physics & Environmental Science)
Where it is taught
- Newton's Three Laws of Motion — Motion, Physical Science: Matter, Forces & Energy, Grade 8
- Newton's Three Laws of Motion — Forces, Rotation & Equilibrium, Physics & Environmental Science, Grade 12
In use
Newton's third law delivers the famous line: for every action force, there is an equal and opposite reaction force — and, crucially, those two forces act on different objects.
Newton's Three Laws of Motion
Questions using newton's third law
Problem 1
A book rests on a table. Earth pulls the book down with the force of gravity. According to Newton's third law, what is the reaction force to this pull?
- The book pulling up on the Earth
- The book's inertia
- The table pushing up on the book
- The book pushing down on the table
Answer and explanation
The book pulling up on the Earth
A third-law pair acts on two different objects. If Earth pulls the book down, the reaction is the book pulling Earth up. (The table's push is a separate force, not the reaction to gravity.)