Modern Application for Introductory Physics: Bloodstain Pattern Analysis

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Modern Application for Introductory Physics
Bloodstain Pattern Analysis

• John Eric Goff
• Lynchburg College

2005 Spring Meeting Lynchburg College Lynchburg,
VA 24501
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New Field of InterestForensic 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!

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Forensic Science isVERY Interdisciplinary!!!

• Biology
• Chemistry
• Physics
• Mathematics
• Law
• Psychology
• Sociology

• Biochemistry
• Biophysics
• Physical Chemistry
• Environmental Science
• Handwriting
• Medicine
• Hobbies???

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I got help with this talk!

• http//www.bloodspatter.com/
• (The tutorial is great! However, beware of some
  poor physics in a few places!)

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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.

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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.)

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Active (or Projected) Bloodstains

• Rule of Thumb As impact angle goes down,
  bloodstain shape becomes more elongated.

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Active (or Projected) Bloodstains

• Impact Spatter Blood source is smacked in
  some way and drops fly off in various directions.
• Divide category further by bloods impact speed.

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Low Velocity

• velocity 5 ft/s
• stain size is (relatively) large diameter 4mm
• examples blood drops into blood and footstep
  spatters

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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

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High Velocity

• velocity 100 ft/s (? 68 mph)
• stain size (relatively small) diameter 1mm
• examples gunshots and propellers

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Time for Physics!

• Big Forensic Science Question From where did
  the blood come???

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Try this!
one exception!
tail points in direction of travel
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sin ? W / L
W
L
What does W L mean? sin ? 1 ? ? 90? Drop
is a circle!
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Real World!

• What if blood drop is not a pretty oval?!?

Do the best you can!!!
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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.)

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(No Transcript)
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Look for Convergence!
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Real Crime Scene
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Get Computer Help(Software is available!)
Notice the top view!
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Big Problem!!!

• Neither blood nor anything else travels unaided
  in a straight line through the air!

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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) ? -10?

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What about gravity???
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What about air resistance??? (Drops speed and
size are needed. Tough to know!)
What about gravity???
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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.

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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