Suppose a book is resting on a table. A free body diagram for the book will show the weight acting downward and and the normal force acting upward. These forces are equal in magnitude and opposite in direction. Is this an example of action and reaction, the Third Law? The answer is no, they are not a third law action/reaction pair. So then why are they equal in magnitude and opposite in direction? And what would be the reaction to the gravitational force on the book? What would be the reaction to the normal force