Newton's Third Law Action-Reaction Pair

Newton's Third Law Principle

Newton's Third Law states that forces occur in pairs. For every action force, there is an equal and opposite reaction force. Crucially, these forces act on different objects.

Analyzing the Forces on the Book

When a book rests on a table, two primary vertical forces act on it:

• Gravitational Force: The Earth exerts a downward force (weight) on the book.
• Normal Force: The table exerts an upward force on the book, perpendicular to the surface.

Identifying the Action-Reaction Pair

Let's evaluate the options based on Newton's Third Law:

• Options 1 & 4: These options suggest the normal force and gravitational force acting on the book form an action-reaction pair. This is incorrect because both forces act *on the same object* (the book). They are balanced forces that keep the book at rest, not an action-reaction pair.
• Option 2: This option correctly identifies an action-reaction pair. The action is the gravitational force the Earth exerts on the book (pulling it down). The reaction is the equal and opposite gravitational force the book exerts on the Earth (pulling the Earth up). These forces act on different objects (book and Earth) and are a direct consequence of Newton's Third Law applied to gravity.

Conclusion

The correct action-reaction pair involving the book and the Earth, according to Newton's Third Law, is the mutual gravitational attraction between them.