As I know, Newton's third law states that "forces occur in pair between two bodies,a pair of equal and opposite forces on each other and Net force on a system of particles is equal to total external force on the system,since internal forces cancel each other." Let's consider a very simple case.
A man pulls a string of mass 'M' on a frictionless surface with a force "F". Let's divide the string into two equal sections,A and B. Only a part of one end of the section B is in contact with the man.
If we take "section B" as our system, there is an external pulling force of "F" at one end by man, and a force of "F" by section A in opposite direction, due to contact action-reaction pair between sections B and A. Making total external force on System " Section B " zero.
I don't get how it can be. Because total external force on section "B" must be "F" if we consider both sections A and B together as a system. So,What is it that I'm missing in my understanding? Please help! As you can see,I'm not a student of physics,just a curious noob.