Bloodstain Pattern Analysis (CS-AAPT).ppt

ANALISIS DE PATRONES DE MANCHAS DE SANGRE

1. Modern Application for
Introductory Physics:
Bloodstain Pattern Analysis
John Eric Goff
Lynchburg College

2. New Field of Interest:
Forensic Science
•Television shows and movies influence student
interests.
•Forensic Sciencecourses, minors, and majors are
springing up around the country. (New course here
at LC!)
•Watch for some very BADscience on TV and in
movies!

3. Forensic Science is
VERY Interdisciplinary!!!
•Biology
•Chemistry
•Physics
•Mathematics
•Law
•Psychology
•Sociology
•Biochemistry
•Biophysics
•Physical Chemistry
•Environmental Science
•Handwriting
•Medicine
•Hobbies???

4. I got help with
this talk!
http://www.bloodspatter.com/
(The tutorial is great! However,
beware of some poor physics in a
few places!)

5. 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.

6. 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.)

7. Active (or “Projected”)
Bloodstains
Rule of Thumb: As impact angle goes down,
bloodstain shape becomes more elongated.

8. 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.

9. Low Velocity
•velocity ≤ 5 ft/s
•stain size is (relatively) large: diameter ≥ 4mm
•examples: blood drops into blood and footstep
spatters

10. 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

11. High Velocity
•velocity ≥ 100 ft/s (68 mph)
•stain size (relatively small): diameter ≤ 1mm
•examples: gunshots and propellers

12. Time for Physics!
Big Forensic Science Question: “From
where did the blood come???”

13. tail points in
direction of
travel
one exception!

14. sin θ= W / L
W
L
What does W = L mean?
sin θ= 1 → θ= 90
Drop is a circle!

15. Real World!
What if blood drop is not a pretty oval?!?
Do the best you can!!!

16. Physics and Math
•Work backwards from blood spatter to
determine “launch position” of blood.
•BE AWAREof all approximations that
are used in analysis! (This is a great
example for teaching students about
approximations in physics.)

18. Look for Convergence!

19. Real Crime Scene

20. Get Computer Help
(Software is available!)
Notice the “top view”!

21. Big Problem!!!
Neither blood nor anything else
travels unaided in a straight line
through the air!

22. 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

23. What about gravity???

24. What about gravity???
What about air resistance???
(Drop’s speed and size are
needed. Tough to know!)

25. Moral of the Story
Calculation using “straight-line”
trajectory (no gravity and no air
resistance) predicts a “launch” point
higherthan 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.

26. Forensic Science Literature
The Directional Analysis of Bloodstain
Patterns: Theory and Experimental
Validationby 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