  We consider the energy weighted sums (EWS) for isovector giant dipole
resonances (IVGDR) in finite nuclei within Landau kinetic theory. The
dependence of both IVGDR energy, $E_{IVGDR}$, and the EWS enhancement factor,
$\kappa (A)$, on the mass number $A$ occurs because of the boundary condition
on the moving nuclear surface. The values of $E_{IVGDR}A^{1/3}$ and $\kappa
(A)$ increase with $A$. The obtained value of the enhancement factor is about
10% for light nuclei and reaches approximately 20% for heavy nuclei. A fit of
the enhancement factor to the proper experimental data provides a value for the
isovector Landau amplitude of $F'_1\simeq 1.1$.
