Blood Splatter: The Point of Origin
|Topic||-Forensic Science -|
|Program||Brown Science Prep|
|Developed by||Joseph Paliotti|
Overview / Purpose / Essential Questions
- How can physics and image analysis tools be used
to solve crimes?
- Can the physical properties of a fluid be used
to track where it came from?
Performance / Lesson Objective(s)
The goal of these two laboratory lessons is to learn how knowledge of mathematics, physics, and image analysis software can be used in combination to investigate evidence at a crime scene, like blood splatter. By the end of this activity, students will be able to:
Determine the direction of blood flow based on
the shape of the droplet splatter.
Calculate the angle of impact for individual
drops of blood spatter.
Use lines of convergence to help determine the
position of the victim when the wound was inflicted.
Use the Law of Tangents to calculate the height
above floor level where the wound was inflicted.
- Activity Sheet for Activity 7-6
- 1 metric ruler
- 1 colored pencil or marker
- 1 pencil
- calculator with tangent function
- tangent tables (optional)
Using only blood-spatter analysis, you may be able to recognize the events leading up to the crime. Although crime scene investigators may arrive at the crime scene after the victim and witnesses are no longer present, they still need to determine what happened. Often several witnesses give different accounts of the crime. Which witness is providing an accurate description of what really happened?
During the investigation, the crime-scene investigators need to determine if the evidence, in this case the blood spatter, matches the description given by the witnesses, the suspect(s), and the victim(s). In domestic abuse cases, the victim of domestic abuse may tell a false story to try to protect the abusing partner. A victim may state that a head injury occurred as a result of falling down stairs. However, if the blood-spatter patterns are inconsistent with this type of injury, then what type of injury did cause the blood spatter? What actually happened? Is a witness lying? Further investigation is required when the blood-spatter evidence tells a different story than the witness’s account of the incident.
In this activity, students analyze blood spatter in three dimensions. By noting the shape of the droplet of blood, the direction in which the blood was moving can be determined. The size of the blood spatter will provide some indication of the velocity of the blood when it hit a surface. By examining at least two drops of blood spatter, the location of the injured person can be determined so long as the injury occurred in two dimensions (lines of convergence). The distance from the area of convergence to the drop of blood can easily be measured. To determine the point of origin, or height from the impact surface, further calculations are necessary. By measuring the width and length of a single drop of blood, the angle of impact can be assessed. By using the Law of Tangents, the height from which the blood fell, or the point of origin for the blood, can be calculated.
Math Review: A right triangle
• Contains one 90-degree angle.
• The hypotenuse is the longest side of a triangle, opposite the 90-degree angle (right angle).
• The opposite side to an angle is the side directly opposite the angle of interest.
• The adjacent side to an angle is the side closest to the angle that is not the hypotenuse.
Laboratory exercise 1: Examine the relationship between height and velocity as well as height and blood drop diameter for falling blood drops, both vertically and at an angle.
Procedure – Using a dropper, allow a single drop of blood to drop from five different heights: 5 cm, 25 cm, 50 cm, 75 cm, and 100 cm onto a sheet of paper. Record the following information in the data table below:
1. Sketch of the drop.
2. Time it takes from releasing the drop to impact the ground.
3. Final velocity of the drop.
a. Use the equation vf = vi+at, where vf is final velocity, vi is initial velocity (0 m/s),
b. a is acceleration (9.8 m/s2), and t is the time it took the drop to fall.
4. Use Image-J to determine the diameter of the drop.
See the attached document here
Wrap up / Conclusion
Ask students whether this forensic evidence is compelling and conclusive.
Ask students what disciplines (e.g., biology, math, physics, computer science, etc.) are used to properly assess blood splatter.
Supporting Web Information
Pre Assessment Plan
Ask how the shape of a blood droplet can be used to determine where it came from.
What scientific disciplines would be useful for analyzing blood splatter?
Post Assessment Plan
Repeat questions of pre-assessment planAsk how this same approach could be used for analyzing splatter paths in three dimensions
|Audience(s)||High school students
|Grade Level(s)||High School
|Created||08/22/2017 03:38 PM|
|Updated||08/22/2017 03:54 PM|