  We analyze the time evolution of the kinetic properties of nuclear matter
produced in heavy-ion collisions at Fermi energies. The collision system is
simulated using Constrained Molecular Dynamics (CoMD) transport calculations
whose output is the isospin, position, and momentum of the nucleons. Focusing
on central 35$\,$A$\cdot$MeV $^{40}$Ca+$^{40}$Ca collisions we utilize this
information to extract localized momentum distributions in volume elements of
8$\,$fm$^3$ and time steps of 5$\,$fm/$c$. We then parameterize the
single-particle momentum distributions with thermally motivated fit functions
in the local rest frame of each cell. While the transverse-momentum
distributions are well reproduced by thermal ones, the longitudinal ones carry
a marked imprint of the initial nuclear motion which we capture by introducing
a centroid motion into our fit functions. In particular, we find that Fermi
distributions yield significantly better fits than Boltzmann ones, a
consequence of the Pauli blocking implemented in CoMD. From the fits we extract
the time dependence of the thermodynamic and collective properties of the
excited nuclear medium. We find that the transverse temperature gradually rises
to about 6$\,$MeV, which is accompanied by a dissipation of the initial
centroid motion of the incoming nuclei which vanishes at about 100$\,$fm/$c$
after initial impact. We are therefore able to track the transition of beam
energy into random kinetic energy for nucleons, suggesting a three-dimensional
equilibration of energy in the late stages of the collision.
