Final answer:
The net work done on a book by the gravitational force is positive when the book is moved down and negative when moved up, with the total work being zero over a closed path. The normal force does zero work regardless of the book's displacement because it acts perpendicular to the displacement.
Step-by-step explanation:
During the displacement of a book, the net work done on the book by the gravitational force and the normal force can be understood by examining the work done by each force. If a book is moved horizontally, gravity does zero work on the book because the displacement is perpendicular to the gravitational force. Similarly, the normal force does zero work as well due to it being perpendicular to the displacement. However, if the book is lifted or lowered vertically, gravity does positive or negative work respectively. The normal force does not perform work in the vertical movement since it is always perpendicular to the displacement.
For example, if you lift a library book weighing 20 N, 1 m vertically down from a shelf, the work done by gravity is positive because the force of gravity and the displacement are in the same direction. This positive work is equal to the weight of the book multiplied by the vertical distance, resulting in 20 J. If you move the book 3 m horizontally, gravity does no work since the displacement is perpendicular to the gravitational force. When the book is placed back on the shelf, gravity does negative work, which is equal in magnitude but opposite in sign to the positive work done when the book was lowered.
In summarizing, the total work done by gravity over any closed path (moving the book away and back to its original position) is zero because the positive work done by gravity is balanced by the negative work when the book is returned to its starting position. On the other hand, the normal force always performs zero work because its direction is perpendicular to any displacement.