  We consider a self-avoiding walk model of polymer adsorption where the
adsorbed polymer can be desorbed by the application of a force, concentrating
on the case of the square lattice. Using series analysis methods we investigate
the behaviour of the free energy of the system when there is an attractive
potential $\epsilon$ with the surface and a force $f$ applied at the last
vertex, normal to the surface, and extract the phase boundary between the
ballistic and adsorbed phases. We believe this to be exact to graphical
accuracy. We give precise estimates of the location of the transition from the
free phase to the ballistic phase, which we find to be at $y_c=\exp(f/k_B
T_c)=1$, and from the free phase to the adsorbed phase, which we estimate to be
at $a_c=\exp(-\epsilon/k_B T_c)=1.775615 \pm 0.000005$. In addition we prove
that the phase transition from the ballistic to the adsorbed phase is first
order.
