  We consider a body immersed in a perfect gas, moving under the action of a
constant force $E$ along the $ x $ axis . We assume the gas to be described by
the mean-field approximation and interacting elastically with the body. Such a
dynamic was studied in previous papers In these studies the asymptotic trend
showed no sensitivity whatsoever to the shape of the object moving through the
gas. In this work we investigate how a simple concavity in the shape of the
body can affect its asymptotic behavior; we thus consider the case of hollow
cylinder in three dimensions or a box-like body in two dimensions. We study the
approach of the body velocity $V (t) $ to the limiting velocity $V_{\infty} $
and prove that, under suitable smallness assumptions, the approach to
equilibrium is $| V_{\infty}-V(t)| \approx C t^{-3} $ both in two or three
dimensions, being $C$ a positive constant. This approach is not exponential, as
typical in friction problems, and even slower than for the simple disk and the
convex body in $ \mathbb{R}^2 $ or $ \mathbb{R}^3 $.
