Claim for balanced and unbalanced forces

Balanced force:

Consider a book kept on a table. Two forces act on the book. One is the gravitational pull that acts downwards and second is the upward normal force. These two forces act in opposite direction and balances each other due to which the book on the table is at equilibrium and maintains its motion.

Unbalanced force:

Consider a book sliding on the table from left to right. The force of gravity pulling downward and the force of the table pushing upwards on the book are of equal magnitude and opposite directions. These two forces balance each other. Yet there is no force present to balance the force of friction. As the book moves to the right, friction acts to the left to slow the book down. There is an unbalanced force; and as such, the book changes its state of motion. The book is not at equilibrium and subsequently accelerates. Unbalanced forces cause accelerations.

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me