Unbalanced forces cause accelerations.

In this case, the unbalanced force is directed opposite the book's motion and will cause it to slow down. To verify if the forces acting upon an object are balanced or unbalanced, an analysis must be conducted to verify what forces are acting upon the object and in what direction. If two individual forces are of equivalent magnitude and opposite direction, the forces are balanced. An object is said to be acted upon by an unbalanced force when there is an individual force that is not being balanced by a force of euqivalent magnitude and in the opposite direction