Practice question · Sort into groups
A book rests on a table. Sort each force pairing: is it a genuine Newton’s third-law pair?
Groups: Third-law pair · Not a third-law pair
- Table pushes book up / book pushes table down
- Book’s weight / book’s normal force
- Earth pulls book down / book pulls Earth up
- Earth pulls book down / table pushes book up
Hints
- A third-law pair always involves exactly two objects, each acting on the other, with the same type of force.
- Weight and normal force both act on the book: two forces on one object can balance, but they can never be an action–reaction pair.
Show the answer
Third-law pair: Earth pulls book down / book pulls Earth up, Table pushes book up / book pushes table down
Not a third-law pair: Earth pulls book down / table pushes book up, Book’s weight / book’s normal force
Why
Third-law partners swap the roles of the two objects: "A on B" pairs with "B on A", same kind of force. Weight (Earth on book) and normal force (table on book) merely happen to balance while the book is in equilibrium.
Practise Newton's Laws of Motion
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