Within a book on a table, there are billions of forces pushing and pulling on all the molecules. Why is it that these forces never by chance add up to a net force in one direction, causing the book to accelerate “spontaneously” across the table?
The external force is needed to displace or accelerate the object. We know that newton's first object will retain at rest until external forces acting on it. Here only force pairs are internal to the book, and exert no net force on the book. An external net force is necessary to accelerate the book.