In his $Thermodynamics$, Fermi mentions the following:
“The entropy of a system composed of several parts is very often equal to the sum of the entropies of all the parts. This is true if the energy of the system is the sum of the energies of all the parts and if the work performed by the system during a transformation is equal to the sum of the amounts of work performed by all the parts.”
I understand that the sum of energies might not equal the total because of the inter-particle attractions.
I suspect that the presence of forces which violate Newton’s third law are sufficient to make the sum of works not to equal to the total work.
Am I correct?