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

Time for Physics!

Big Forensic Science Question: “From where did the blood come???”

tail points in direction of

travel

one exception!

sin θ = W / L

W

L

What does W = L mean?sin θ = 1 → θ = 90Drop 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

θ = -10

v = 20 ft/s

parallel to ground

1 m

ground

What about gravity???

What about gravity???

What about air resistance???(Drop’s speed and size areneeded. Tough to know!)

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