As rockfalls direct strong disasters, they stand as a major natural hazard. In order to manage this phenomenon, it is of great interest to predict as accurate as possible the trajectory of falling blocks. This allows the spatial extension of the risk to be assessed. But the modelling of the trajectory of a falling block requires to take into account the mechanical interaction that occurs between the boulder and natural obstacles, such as the ground or a forest stand. This paper deals with the analysis of the impact of a block onto a rocky ground; the purpose consists in expressing a reflecting law relating both incident and reflected velocities. Designing protective structures require to know the trajectory of the block, as well as the velocity of the block. Thus, the improving in the modelling of the contact between the block and the ground represents an actual challenge.