Question

A book is placed on a level floor. Is it true that the gravitational force (Fg) on the book by the earth and the normal force Fn on the book by the floor constitute an action-reaction force pair by Newton's 3rd law? explain.

Answer 1

Newton's 3rd law applies in all situations and to all types of force. But the pair of forces are always the same type, eg both gravitational or both electrical.

And: If you have a book on a level floor the book is exerted a force on the level floor (weight due to gravity), and the level floor reacts with an equal and opposite force. But the force acting on the level floor is due to gravity (is this the same as a gravitational force?), and the forcing acting from the level floorto the book is a reaction force. So one is a gravitational, and the other is not. Therefore this is not Newton's Third Law as the forces must be of the same type.