Final answer:
The frictional force acting on the book is 1.5 N, which is equal and opposite to the applied force, according to Newton's first law of motion for an object moving at a constant velocity.
Step-by-step explanation:
Determining the Frictional Force
To determine the frictional force acting on the book, we apply Newton's first law of motion which states that if an object is moving with a constant velocity, the net external force acting on it is zero. In this scenario, where a 1.5-N force is applied to the book to move it at a constant velocity across a desk, the frictional force must be equal and opposite to the applied force to maintain this constant velocity. Hence, the frictional force acting on the book is 1.5 N in the direction opposite to the motion.
Explanation of Frictional Force
When pushing an object across a surface, one often needs to overcome friction. If the object is moving at a constant velocity, it implies that the pushing force and the frictional force are balanced. For example, when pushing a book across a table, applying 2 N of force and sliding it 0.5 m, the work done amounts to 1 J. However, if there is no acceleration, as in the case of the book moving with a constant velocity, the frictional force would be the same as the applied force, resulting in no net force and hence no net work in terms of increasing the book's velocity.
In conclusion, the answer to the student's question is (c) 1.5 N.