In this paper a theoretical model of a collision between a car and a car with a trailer was considered. To properly model the phenomena occurring in this case the principles of mechanics of bodies with variable mass have been used, specifically in case of trailer unhooking and disconnecting from the car. When such a set as a car-trailer is decomposed into two bodies, the loss of mass can be assumed. In the given case the loss of mass related to the car after the trailer separated from it because of the collision. The mass of the trailer was considered as a part of the mass disconnecting from the body taking part in a collision. Of course, the mass of the trailer was assumed to be much lower than the mass of the car. The aim of this paper was to analyze potential applicability of collision mechanics with variable mass to motor vehicle collision analysis in general. From the mechanical point of view, e.g. the Meschersky equation can be used as one of the main tools to solve the problem discussed