Final answer:
The force of friction acts in the opposite direction of the book's motion, so if the book is pushed to the right, the frictional force is to the left. This is because friction resists the sliding motion between two surfaces.
Step-by-step explanation:
If a book is being pushed across the table to the right, the force of friction will act in the opposite direction of the motion, which is to the left. When you push an object to the right, friction resists this motion by exerting a force in the opposite direction. It does this to oppose the sliding motion between the two surfaces, which in this case is the bottom surface of the book and the tabletop.
The reason for the direction of frictional force is due to the interaction of microscopic contact points found on both surfaces. These points tend to interlock, and sliding motion causes them to get 'dragged' along, which generates the frictional force. For an object in motion, the frictional force is often referred to as kinetic friction.
In terms of net external force, if an object is moving freely (without any additional forces applied), the net external force on the object will be in the same direction as the frictional force. So, if the book was moving to the right without being pushed, the net external force would be to the left, opposing the motion.