Very important basics about third law for a simple problem .
Scene 1: Book resting on the table.
first pair : Gravitational force on book and earth(same type)
second pair : Contact force between book and table(same type)
The forces on the book : Gravitational force ( weight) = Contact force by table on book (normal reaction)
The book is at rest and therefore the Weight and Contact force is equal in magnitude and opposite in direction since the book is in equilibrium.Note: Not a action-reaction pair
Scene 2: The book fallen onto table and had just landed on it. The weight is the same as the above case but the contact force has increased tremendously. The book is decelerating. The contact force by the book on the table has also increased as action reaction pair still holds.
