What is the perpendicular force exerted by a surface on an object?

The perpendicular force exerted by a surface on an object is called the normal force. “Normal” here means perpendicular in the geometric sense, not ordinary or usual.

The normal force is a contact force produced by the interaction between touching surfaces. A book resting on a level table experiences a downward gravitational force and an upward normal force. If the book is stationary, these forces have equal magnitudes, so the net vertical force is zero and the book does not accelerate.

The normal force is not always equal to weight. On an inclined plane, for example, the normal force is typically mg cosθ, where θ is the angle of the incline measured from the horizontal. It can also change when an elevator accelerates or when an object is pressed against a wall. Friction acts parallel to the surface, while the normal force acts perpendicular to it.

A common mistake is to treat gravity and the normal force as a Newton's third-law pair. They are not: both act on the book. The third-law partner of the table's normal force on the book is the book's force on the table.