This is the fundamental definition of force, known as Newton's Second Law of Motion:

Force (F) = Mass (m) x Acceleration (a)

Let's break down why this is:

* Mass: Represents the amount of matter in an object. The more massive an object, the more resistant it is to changes in motion.

* Acceleration: Is the rate of change of velocity over time. It indicates how quickly an object's speed or direction is changing.

Force is the push or pull that causes an object to accelerate. Therefore, a greater force is needed to accelerate a more massive object, or to cause a greater acceleration of the same object.

Example: Pushing a heavy box requires a greater force than pushing a lighter box to achieve the same acceleration.