Here's why:

* Balanced Forces: When forces acting on an object are equal and opposite, they cancel each other out. This results in no net force, and the object remains at rest or continues moving at a constant velocity.

* Unbalanced Forces: When forces acting on an object are unequal, there's a net force. This net force causes the object to accelerate, meaning it changes its velocity (either speed or direction).

Examples:

* Pushing a heavy box: If you push a box with enough force to overcome friction, the box will move. This is an example of an unbalanced force.

* Pulling a rope in a tug-of-war: If both teams pull with equal force, the rope won't move. This is an example of balanced forces.

Key takeaway: For an object to start moving or change its motion, there needs to be a net force acting on it. This means the forces acting on the object must be unbalanced.