ABSTRACT

In reconstructing vehicle crashes, Equation 18.7 is integrated over the duration of the crush phase. This results in a computation of the total impulse delivered over that phase, and a calculation of the momentum change that has occurred during the interval, as seen in Equation 18.22. The velocities at impact are V1 and V2 for vehicles 1 and 2, respectively, and the velocities at separation are and . When both vehicles are taken together as a single system, there is no external impulse delivered to the system during the crash phase as long as tire/roadway friction forces can be ignored. In that event, Equation 18.22 then becomes

m1V1+m2V2 = m1V'1+m2V'2 (19.1)

Note that nothing has been said (or assumed) about a collision having occurred. The only thing for sure is that momentum has been conserved. It follows that Equation 19.1 applies as well when there is no collision (or when there is no transfer of momentum between the vehicles). It is also the case that the crush phase does not have to be infinitesimally short. Finite time intervals (and finite configuration changes in the vehicles) are admissible, as long as the external impulse delivered to the system during that time interval is negligible.