Internal energy is the sum of all the forms of energies that a system can possess. Internal energy is made up of a number of components (a) the transitional energy of molecules (b) the rotational energy of molecules (c) the vibrational energy of molecules (d) the coulombic energy between the electrons and nuclei in the atom (e) the interaction between the component particles of the system. Thus, internal energy does not include energy due to gravitational pull.