Final answer:
The book on the car seat continues to move forward due to inertia and only comes to a stop when friction acts upon it.
Step-by-step explanation:
When a car suddenly stops at a red light, the book on the car seat continues to move forward due to inertia. Inertia is the tendency of a body to resist changes in its state of motion.
As the car stops, the seat exerts a backward force on the book. However, the book tends to stay in motion due to its inertia and continues moving forward until a force, such as friction, stops it.
Friction between the book and the seat will eventually bring the book to a stop, but initially, the book keeps moving forward because of its inertia.