S.O.S. Mathematics CyberBoard

Would anyone agree that the third sentence does not make sense with regards to the overall explanation? Thanks.


"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."

It looks like the third sentence is on a different subject, namely if there are three forces, you can't analyse the problem as if there are only two forces.