[Mitch]

What assumption(s) were made when deciding the volume term in the semi-emperical mass formula to be equal to a_vA.

[Mitch]

One, is that the radius of the nuclei has a sharp cut-off. Can you think of anything else?

[Grejak]

Two, the nucleus is spherical.

[alkemist]

Well the major assumption here is that the binding energy is linearly dependent on the number of nucleons, which is what you would expect.  It's more of a guess than an assumption.  Basically, it's assumed that no other factors are important in determining the binding energy of a nucleus other then it's nucleon number, which of course it was proven not to be true, and corrected by other terms that correct for surface energy, the coulombic force,  and symmetry.