I was wondering how mesons or the strong force is accounted for in mass defect. Nucleons individually have more mass then when they are bound together in a nucleus of an atom. I saw a teacher/professor speak on youtube. He said the binding energy (strong force) was calculated as part of the nucleons mass energy when they are seperate. It would almost seem then that mesons or the strong force come out of protons and neutrons. I know that protons and neutrons are made of 3 quarks each; and mesons, being force carriers are made of 2 quarks each. I am a little confused about how the force carrier (meson) or it's energy is calculated as part of the nucleon's mass, when they are seperate and individual.