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

* Mass as Inertia: Mass is a fundamental property of matter that represents its resistance to changes in motion. The more massive an object is, the more difficult it is to accelerate it.

In simpler terms:

Imagine pushing a small box and a large, heavy box. The small box will accelerate easily, while the large box will be much harder to move. This is because the large box has more mass and therefore more resistance to acceleration.