  We solve numerically the equations of motion for the collapse of a shell of
baryonic matter, made of galaxies and substructure of $ 10^{6} M_{\odot}-
10^{9} M_{\odot}$, falling into the central regions of a cluster of galaxies
taking account of dynamical friction. We calculate the evolution of the
expansion parameter, a(t), of the perturbation using a coefficient of dynamical
friction, $ \eta_{0}$, calculated for a perturbation in which clustering is
absent and a coefficient $ \eta_{cl}$ obtained from a clustered one. The effect
of the dynamical friction is to slow down the collapse (V. Antonuccio-Delogu \&
S. Colafrancesco 1994, hereafter AC) producing an observable variation of the
parameter of expansion of the shell. The effect increases with increasing $
\eta $ and with the increasing of clustering. Finally, we show how the collapse
time depends on $ \eta_{0}$ and $ \eta_{cl}$. keywords: {cosmology: theory-
galaxies: formation}
