Here's why:

* Normal Force Definition: The normal force is the force exerted by a surface to prevent an object from passing through it. It acts perpendicular to the surface, hence the name "normal" (meaning perpendicular in this context).

* Sliding Friction: When two surfaces slide against each other, friction acts to oppose the motion. The force of friction is directly related to the normal force. The greater the normal force pressing the surfaces together, the greater the friction.

Example: Imagine a book resting on a table. The table exerts a normal force upwards on the book, equal in magnitude to the book's weight. This normal force keeps the book from falling through the table. If you push the book horizontally, the friction between the book and the table will resist the motion, and this frictional force is directly proportional to the normal force.