Answer
Force (a) and force (b) must be equal in magnitude and opposite in direction even though they are not an interaction pair.
Work Step by Step
In this situation, there are two forces acting on the book.
(a) the force of the Earth pulling on the book. Note that this force is directed straight down.
(b) the force of the table pushing on the book. Note that this force is directed straight up.
Since the book is not moving, the net force on the book must be zero. Therefore, force (a) and force (b) must be equal in magnitude and opposite in direction even though they are not an interaction pair.
