Here's why:

* Newton's First Law of Motion states that an object at rest stays at rest, and an object in motion stays in motion at a constant velocity unless acted upon by an unbalanced force.

* Newton's Second Law of Motion defines the relationship between force, mass, and acceleration. It states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

Therefore, when an unbalanced force acts on an object:

* If the object is at rest, it will start moving.

* If the object is already moving, it will change its speed or direction (or both).

In summary: Unbalanced forces result in a change in the object's motion, which is described as acceleration.