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Modern Application for Introductory Physics:
Bloodstain Pattern Analysis
John Eric Goff
Lynchburg College
New Field of Interest:Forensic Science
• Television shows and movies influence student interests.
• Forensic Science courses, minors, and majors are springing up around the country. (New course here at LC!)
• Watch for some very BAD science on TV and in movies!
Forensic Science isVERY Interdisciplinary!!!
• Biology• Chemistry• Physics• Mathematics• Law• Psychology• Sociology
• Biochemistry• Biophysics• Physical Chemistry• Environmental Science• Handwriting• Medicine• Hobbies???
I got help with this talk!
http://www.bloodspatter.com/(The tutorial is great! However,
beware of some poor physics in a few places!)
Bloodstain Patterns
• Physical Evidence• Usually Found at Violent Crime Scenes• Gives Clues as to WHAT Happened• May Suggest Sequence of Events• Analysis MUST be placed in context of
all aspects of crime scene investigation in order to reconstruct (possible) criminal events.
Different Types of Bloodstain Patterns
• Passive Bloodstains (drops, pools, etc.)
• Transfer Bloodstains (wipe a weapon, etc.)
• Active (or “Projected”) Bloodstains (bullets, stepping in blood, etc.)
Active (or “Projected”) Bloodstains
Rule of Thumb: As impact angle goes down, bloodstain shape becomes more elongated.
Active (or “Projected”) Bloodstains
Impact Spatter – Blood source is “smacked” in some way and drops fly off in various directions.
Divide category further by blood’s impact speed.
Low Velocity• velocity ≤ 5 ft/s• stain size is (relatively) large: diameter ≥ 4mm• examples: blood drops into blood and footstep
spatters
Medium Velocity• 5 ft/s ≤ velocity ≤ 25 ft/s• stain size : 1 mm ≤ diameter ≤ 4mm• examples: blood flicked off finger and blunt
object used on victim
High Velocity• velocity ≥ 100 ft/s ( 68 mph)• stain size (relatively small): diameter ≤ 1mm• examples: gunshots and propellers
Physics and Math
• Work backwards from blood spatter to determine “launch position” of blood.
• BE AWARE of all approximations that are used in analysis! (This is a great example for teaching students about approximations in physics.)
Example• Consider “medium velocity” blood drop
of diameter 3 mm “launched” at a position 1 m off the ground.
• Numbers: v = 20 ft/s (6 m/s) & θ = -10
θ = -10
v = 20 ft/s
parallel to ground
1 m
ground
Moral of the Story Calculation using “straight-line”
trajectory (no gravity and no air resistance) predicts a “launch” point higher than actual point.
“Straight-line” trajectory is reasonable for high “launch” velocities and/or stain and “launch” points “close” together. (The idea is to keep the flight time as short as possible.)
More accuracy requires a better model and more specialized work.