Here's why:

* Newton's Second Law of Motion: This law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Mathematically, this is expressed as:

F = ma

where:

* F is the net force

* m is the mass

* a is the acceleration

* Zero Net Force: If the net force acting on an object is zero, then the acceleration must also be zero. This means the object will either be at rest or moving at a constant velocity.

Examples:

* A book sitting on a table: The force of gravity pulling the book down is balanced by the normal force from the table pushing up. The net force is zero, so the book doesn't accelerate.

* A car moving at a constant speed: The engine's forward force is balanced by air resistance and friction. The net force is zero, so the car doesn't accelerate.

Important Note: It's important to remember that "no acceleration" doesn't necessarily mean "at rest." An object can be moving at a constant velocity without accelerating.