Bell Assignment Monday 10/27 List 5 common types of physical evidence. In order to identify a substance, what must be done to it? When comparing a piece of evidence, it must be compared to what 2 things? Since you can’t calculate the exact probability, what do forensic scientists try to do with the evidence?
Blood-Drop Analysis
Why Analyze the Drops Patterns left by falling or projected drops of blood can help determine where a crime took place Blood drops and stains examined closely before evidence is collected to keep the perspective
Shape of Drops Indicates Distance from which the blood fell Angle of impact While droplet is falling, it is primarily spherical in shape. Not tear-shaped like in cartoons Smaller the drop, the more spherical the shape during fall
Drop Acceleration As a drop falls it accelerates until it reaches a constant or terminal velocity Measurements have shown that a blood drop, resulting from dripping at a height of 15 ft, has a volume of about 0.05mL and falls at a velocity of ~ 25ft/s. Smaller drops have a terminal velocity less than 25 ft/s and larger drops are greater than 25 ft/s
Drop Acceleration, cont. Individual blood droplet can give an investigator the following information: The droplet’s speed at time of impact Direction droplet travelled Approximate size of blood drop What could the droplet’s size tell an investigator about the wound it came from?
Round Drops If you examine a blood droplet that struck a surface straight-on (90-degree angle from the surface), the droplet is generally round Straight-on impacts on hard, smooth surfaces produce droplets with smooth edges Higher velocities and rougher surfaces produce drops with more ragged edges
Elongated Drops Angle of impact affects the droplet’s shape Droplets falling on surfaces at an angle that is greater than 90 degrees have elongated shapes. The larger the angle, the more elliptical the droplet
Dripping and Spraying A droplet that forms slowly, as in a dripping wound, has a volume of about 0.05 mL. Smaller droplets are produced during active situation – fight or beating Blood droplets as small as an aerosol spray indicate the wound was produced by a powerful force – gunshot or explosion
Bloodstain Pattern Analysis Tutorial J. Slemko Forensic Consulting http://www.bloodspatter.com/BPATutorial.htm