This is incorrect; the third law refers to forces on two different objects. For example, a book lying on a table is subject to a downward gravitational force (exerted by the earth) and to an upward normal force by the table. Since the book is not accelerating, these forces must be exactly balanced, according to Newton's first or second law. They are therefore 'equal and opposite'. However, these forces are not always equally strong; they will be different if the book is pushed down by a third force, or if the table is slanted, or if the table-and-book system is in an accelerating elevator. The case of three or more forces