  We study a two dimensional collision problem for a rigid solid immersed in a
cavity filled with a perfect fluid. We are led to investigate the asymptotic
behavior of the Dirichlet energy associated to the solution of a Laplace
Neumann problem as the distance $\varepsilon>0$ between the solid and the
cavity's bottom tends to zero. Denoting by $\alpha>0$ the tangency exponent at
the contact point, we prove that the solid always reaches the cavity in finite
time, but with a non zero velocity for $\alpha <2$ (real shock case), and with
null velocity for $\alpha \geqslant 2$ (smooth landing case). Our proof is
based on a suitable change of variables sending to infinity the cusp
singularity at the contact. More precisely, for every $\varepsilon\geqslant 0$,
we transform the Laplace Neumann problem into a generalized Neumann problem set
on a domain containing a horizontal strip $]0,\ell_\varepsilon[\times ]0,1[$,
where $\ell_\varepsilon\to +\infty$.
