One of the issues of confusion in some recent online postings was about what distance to use for total travel. In vehicle /pedestrian impacts the distance from the initial point of impact has generally been used as the 'throw' distance. In some instances an occupant may ride on the hood of a vehicle. So confusion came up when a suggestion was made that you ALWAYS make calculations from the separation location?
- NOTE: The confusing posting went on to suggest that you also ALWAYS use the separation location for vehicle to vehicle impacts when applying linear/angular momentum equations.
- That brings up the questions: So what do you do when a side-slap or secondary impact occurs? And how do you accommodate for the energy dissipated during the impact to separation distance? Momentum analysis in accident reconstruction is an approximation technique with several simplifying assumptions such as instantaneous exchange of momentum (so separation is therefore assumed to be impact! and assumption of no external forces, etc.). You generally use the distance from impact to position of rest as the distance travelled.
- An oversimplified analytical relationship that has sometimes been used to approximate the minimum speed of
an ejected occupant or component is based on a ballistic trajectory with the following inherent assumptions:- 1. The optimum launch angle (i.e., 45°) to determine the minimum speed for a given travel distance,
2. A landing at the same elevation as the launch, and
3. No movement on the ground after the landing.
- 1. The optimum launch angle (i.e., 45°) to determine the minimum speed for a given travel distance,
- Distance travelled on the ground (ballistic assumes the ‘bomb’ lands on the target whereas in vehicle/pedestrian impacts the occupant may slide some distance.
Variations in the launch or exit angle (ballistic assumes 45 degrees),
Variations in the coefficient of friction of the ground travel
Landing forces (some assume a bouncing ball type landing)
Summary graphics from the chapter: