An analysis of the pattern of blood stains at a crime scene can determine the position of the source of the blood stain and hence whether a person was standing or sitting when the blood left their body. Currently the angle at which the blood stain hits the ground is determined from the stain’s width to length ratio and a straight line drawn back from the stain in the direction of impact. (The longer the stain, the shallower the angle of impact). Where the lines from several stains intersect is regarded as the position of the source of the blood.
However this neglects the effect of gravity on the flight of the blood droplet. Using Newton’s equations, the same equations as those used to describe the movement of a non-spinning ball through the air, it can be shown that gravity’s effect on the blood droplet’s flight can be included if the impact velocity is known.
Looking at a stain, not only can we measure the stain’s length and width to give the impact angle, but also the stain’s size and the number of fingers or spines around the edge of the stain. Carrying out experimental work with several Forensic Engineering Science students, Dr Clare Knock has shown that for a given surface, the stain size and number of fingers depends on the drop size, impact velocity and impact angle. We can already measure the impact angle and so we have two unknowns; the drop size and impact velocity and two knowns; the number of fingers and the stain’s size. Hence from the experimental data an equation can be obtained to calculate the impact velocity from the stain size and number of fingers and so more accurately determine the position of the source of the blood stain.
For further information see:
Knock, C. & M. Davison (2007). Predicting the position of the source of blood stains for angled impacts. J. Forensic Science, 52(5):1044-1050.