I don't know if I completely understand mass defect and binding energy so I will write what I understand and hopefully you can explain any misunderstandings I have. I know what mass defect is and that I understand that we need the strong nuclear force to keep the nucleus together etc. but I don't understand where binding energy comes from. So if we have a proton and a neutron and we bring them together to form a deuterium atom, will some of the mass of each become converted into strong nuclear force to keep them together? Where does the energy liberated come from then? Shouldn't there be energy liberated when two nuclei are fused together? Also, can you explain the effect of gravitational potential energy? Is the decrease in GPE the reason why energy is liberated? So basically, where does the energy of mass defect go (becomes strong force?) and where does the energy of fusion reactions come from? If we want to separate a nucleus into its constituents, we have to provide energy equivalent to mass defect, correct? According to E=mc^2, the energy put into the system to separate nucleons is transformed into mass, making the final particles heavier (their actual mass). This creates and is equivalent to the mass defect. Or does nuclear energy come from either splitting high-mass nuclei or fusing lighter nuclei as this increases the binding energy per nucleon? Thank you!