Final answer:
The book does not slide down when pushed against a wall due to the presence of friction and Newton's laws of motion.
Step-by-step explanation:
When you push a book against a wall with enough force, it does not slide down due to the presence of friction. Friction is the force that opposes the motion of an object when it is in contact with another surface. In this case, the static friction between the book and the wall prevents the book from sliding down.
According to Newton's first law of motion, an object will remain at rest or continue moving at a constant velocity unless acted upon by an external force. In this case, the external force is the friction between the book and the wall, which counteracts the force of gravity pulling the book downwards.
Additionally, Newton's third law states that for every action, there is an equal and opposite reaction. When you push the book against the wall, the wall pushes back with an equal force, contributing to the static friction that keeps the book in place.