1. John Eric Goff Lynchburg College
Modern Application for Introductory Physics: Bloodstain Pattern Analysis
John Eric Goff
Lynchburg College
2005 Spring Meeting
Lynchburg College
Lynchburg, VA

2. 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!

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!
(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. Try this!
one exception!
tail points in
direction of
travel

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

25. 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.

26. 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