Trajectory of Blood Drops in an Experimental Model with the Use of a Firearm
*Correspondence to: Richard Billich, Ph. D., MSc. Department of Anatomy and Biomechanics, Faculty of Physical Education and Sports, Charles University in Prague, Jose Martiho 31, 162 52 Praha 6-Veleslavin, Czech Republic, Europe, phone: +420 732 208 067; e-mail: firstname.lastname@example.org
Blood marks analysis is a common technique used in forensics. However, current methods used have their limitations and deviations. The most commonly used trigonometric model is based on the linear movement of a blood drop, which disregards air resistance and gravitational force which affects all mass points in an area of space.
The aim of this paper is to map the flight trajectory of blood drops in an experimental environment with the use of a firearm and to evaluate the option of replacing the real world flight trajectory by a more accurately definable parabolic approximation.
An experimental model was created which simulates the real world dispersion of blood in the case of firearm use. Blood samples with a volume of 100 ml were placed into a plastic bag and were shot at. For the purpose of the experiment was prepared wooden chamber (dimensions: 2 x 2 x 2 m). Plastic bags containing blood samples were hanged in the chamber and positioned into the chamber's center. Blood samples were shot at from a distance of two meters by a Taurus.357 magnum handgun. Magnum.357 FMJ ammunition was used. Each shot was recorded by a high speed camera under a 90° angle placed on the sample's level in a 2 m distance from the chamber.
Based on the physical and experimental models it was determined that the real flight trajectory of blood drops may be defined by a parabolic approximation while maintaining validity for 90% blood drops. The parabolic approximation does not significantly differ from the ballistic curve and real flight trajectories in cases of lower flight velocities and blood drop sizes of less than 0, 5 mm, meaning blood drops located in a distance of less than 0, 5 m from the point of origin. From the progression of the curves capturing the flight of blood drops it was ascertained that for the purpose of using trigonometric models, it is best to use bloodstains located less than 1m from the convergence point, the reason being the fact that the deviation caused by the use of the parabolic approximation is minimal and doesn't cross 3 mm. The use of the parabolic approximation with distances of over 2 m leads to deviations of more than 3 mm when compared to reality.
The outcome of these comparisons shows that the real world flight trajectory differs very little from the ballistic curve and parabolic approximation under the given experimental conditions. In crime reconstructions the angle of incidence (angle of impact) of a blood drop impacting a horizontal surface plays an important role. In this area the outcomes of the ballistic curve and the parabolic approximation are nearly identical, especially for blood drops with low initial identical, especially for blood drops with low initial speeds, which constituted the majority under the specific experimental conditions.
Blood samples, analysis, parabolic approximation, flight trajectory, forensic ballistics, biomechanics.