Define the term unbalanced force.

An unbalanced force occurs when two or more forces acting on an object are not equal in size or direction.

It causes a change in the object’s motion—either starting it, stopping it, speeding it up, slowing it down, or changing its direction. For example, when you push a toy car and it moves, your applied force overcomes friction, creating an unbalanced force. 

If all forces were equal and opposite, the object would remain at rest or move at constant velocity. Hence, unbalanced force is the main reason behind acceleration and motion changes according to Newton’s second law.