Unbalanced forces do not cancel. The net force is nonzero. An object accelerates in the direction of the greater force. Push a box with more force than friction and it moves. The net force is the difference. The box accelerates. That is Newton's second law in action.

Unbalanced forces change motion. They start objects moving. They stop them. They change direction. They change speed. A car accelerating has unbalanced forces. The engine force is greater than friction and drag. A car braking has unbalanced forces. The brake force is greater than the engine force. A car turning has unbalanced forces. The friction from the tires provides centripetal force toward the center of the turn.

Unbalanced force examples

• Car accelerating: engine force greater than friction.
• Car braking: brake force greater than engine force.
• Ball thrown upward: gravity unbalanced, decelerating.
• Skydiver falling: gravity greater than air resistance at first.
• Rocket launching: thrust greater than gravity.

Unbalanced forces are the cause of all acceleration. Without them, objects would either stay at rest or move in a straight line at constant speed forever. That is inertia. Unbalanced forces break inertia. They change velocity. The direction of the net force determines the direction of acceleration. The magnitude determines how quickly the velocity changes. A small net force produces a small acceleration. A large net force produces a large acceleration. Mass resists acceleration. A heavy object needs more force to achieve the same acceleration as a light one. Unbalanced force is the engine of change in mechanics. It is what makes things happen.