The article describes a theoretical approach to determining the mass defect of a tangible object and cohesive energy of its constituent parts. It is shown that the mass defect emergence at the interacting objects’ fusion into a unified whole is a common phenomenon regardless of the fusion process spatial scale. The mass defect emergence regularities are true for all cases of exchange interactions regardless of their intensity. In particular, for strong interaction, for the verification purposes, the derived equations are used to calculate a tangible object’s mass defect in approximation of pairwise and triple interactions of particles comprising the object as a whole. The determined values are: the cohesive energy of protons in nucleus  and neutrons in nucleus  as well as the distance between protons in nucleus

interacting components, fusion, mass, Maclaurin’s series, exchange interactions, mass defect.