Here's why:

* Balanced forces: When forces are balanced, they cancel each other out, resulting in no net force. This means an object at rest stays at rest, and an object in motion continues at a constant velocity.

* Unbalanced forces: When forces are unbalanced, there is a net force acting on an object. This causes the object to accelerate (change its speed and/or direction).

In the case of a car accelerating, the force of the engine propelling the car forward is greater than the forces opposing its motion (friction, air resistance). This unbalanced force causes the car to speed up, demonstrating that the forces are not balanced.