and the table pushes upwards on the book.Moreover, the forces acting on the book 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 any number of forces acting on the same object is covered by considering the sum of all forces.

A possible cause of this problem is that the third law is often stated in an abbreviated form: For every action there is an equal and opposite reaction, without