Transcript Bloodstain Pattern Analysis

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 is
VERY Interdisciplinary!!!
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Biology
Chemistry
Physics
Mathematics
Law
Psychology
Sociology
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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
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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
Time for Physics!
Big Forensic Science Question: “From
where did the blood come???”
one exception!
tail points in
direction of
travel
sin θ = W / L
W
L
What does W = L mean?
sin θ = 1 → θ = 90
Drop is a circle!
Real World!
What if blood drop is not a pretty oval?!?
Do the best you can!!!
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.)
Look for Convergence!
Real Crime Scene
Get Computer Help
(Software is available!)
Notice the “top view”!
Big Problem!!!
 Neither blood nor anything else
travels unaided in a straight line
through the air!
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
parallel to ground
θ = -10
1m
v = 20 ft/s
ground
What about gravity???
What about air resistance???
(Drop’s speed and size are
needed. Tough to know!)
What about gravity???
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.
Forensic Science Literature
The Directional Analysis of Bloodstain
Patterns: Theory and Experimental
Validation by A. L. Carter
“Therefore, the best one can do here is to
estimate an upper limit for the height of the
source.” (p. 181)
Can. Soc. Forens. Sci. J. Vol. 34. No. 4 (2001) pp. 173-189