Newton's second law of motion involves the effect of force acting on a mass. It has to be understood that this actually means the combination of any and all individual forces acting on it, with their individual strengths and directions all added together.
When all the individual forces acting on an object add up to zero, then the whole group of forces is said to be 'balanced'.
When all the individual forces acting on an object don't add up to zero, then the whole group of forces is said to be 'unbalanced'.
Notice that there's no such thing as 'a balanced force' or 'an unbalanced force'. It's a group of two or more forces that's balanced or unbalanced.
