b) The variation of atomic mass as a function of atomic mass number A and proton number Z can be approximately described by the semi-empirical mass formula: m(A, Z) = ZmH + (A - Z)mn - aA + as A/ + ac + aa Z (A - 2Z) A/3 A Briefly describe the physical origin of each of the terms in this expression. c) Neglecting the terms with coefficients aa and ap, show that it is energetically possible for an even-even nucleus to fission into two identical fragments if Z A 18.2, given the values of the other coefficients are as FaA-1/2 www 18.34 MeV/c and a = 0.71 MeV/c. -