[en] Momentum dependent interactions may play an important role in the early stages of heavy ion collisions, where large relative momenta are presented. To study the effect of such interactions in a schematic but rather instructive way, the authors propose a generalization of the concept of the nuclear equation of state, which is more appropriate in the situation of two interpenetrating nuclei: As an additional macroscopic (thermodynamic) variable they introduce the relative collective flow of the two nuclei. The dependence of the equation of state on this new variable is investigated. They then investigate the influence of momentum dependent interactions as well as the influence of in medium corrections to the nucleon-nucleon cross sections in the framework of the microscopic quantum molecular dynamics model of heavy ion collisions. Within this model fragment formation can be described in a consistent way. The transverse momentum transfer of medium heavy fragments is an observable of high sensitivity to the equation of state even if in-medium corrections are taken into account. The authors show that medium heavy fragments (A=5-15) exhibit the largest transverse momentum per nucleon with a high correlation in the scattering plane. The time structure of fragment formation is studied in detail