What does the normal force measure in terms of forces acting on an object at rest on a flat surface?

The normal force is defined as the force exerted by a surface to support the weight of an object resting on it, effectively acting perpendicular to the surface. When an object is at rest on a flat surface, the normal force balances the gravitational force acting on the object, which prevents it from accelerating downwards.

In more detail, when an object is placed on a horizontal surface, gravity pulls it downwards, creating a weight force equal to the object's mass multiplied by the acceleration due to gravity. The surface reacts to this force by exerting an equal and opposite normal force. This reaction force keeps the object in a state of static equilibrium, meaning there is no net force acting on the object, allowing it to remain at rest.

Understanding the normal force helps in analyzing various physical scenarios, particularly in problems involving friction, as the maximum frictional force depends on the magnitude of the normal force. Thus, recognizing that the normal force corresponds directly to the reaction from the supporting surface provides clarity on how forces interact in static conditions.