Here's why:

* Newton's First Law of Motion: An object at rest stays at rest, and an object in motion stays in motion at a constant velocity (speed and direction), unless acted upon by an unbalanced force.

* Newton's Second Law of Motion: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This means:

* More force: More acceleration

* More mass: Less acceleration

In other words:

* If the forces are balanced, they cancel each other out, resulting in zero net force. This means no acceleration, and the object either stays at rest or continues moving at a constant velocity.

* If the forces are unbalanced, there's a net force acting on the object. This net force causes the object to accelerate, meaning its speed and/or direction of motion changes.