What is the difference between tension and the force that we apply externally. Also when we apply force externally let's say on moleculeA ( connected to other molecules through springs) then by newton third law it also apply force on us then what happens to this force. If possible please someone help me with this doubt it would be of great help if your answer is explained using diagram. ( Since I am still very much confused on what tension really is).
UPDATE :-Also how the force that we apply on moleculeA is transferred to the last molecule because according to newtons 3rd law it should apply opposite force on us which should be cancelled out. Please help.