Introduction
Blood is among the basic types of physical evidence often encountered at a crime scene. Investigators use blood evidence to identify individuals by both blood type and DNA. By observing and analyzing patterns formed by blood at a scene, they can restage the location, movement, and actions of the crime. Like other physical evidence, blood evidence often links a scene, perpetrator, and victim and can be used to determine a sequence of events.
Activity Details
- Prep Time: 15 minutes
- Class Time: 45-60 minutes
Materials
- Bloodstain Student Investigation Sheet
- Metric Ruler
- Calculator with Trigonometry Functions
Overview
Investigation question
At what angle did blood impact a surface to create the bloodstain?
Investigation objective
Use measurements to determine the angle of incidence of a blood drop.
Standards
This activity addresses the following AP® Biology concepts:
SYI-1
Living systems are organized in a hierarchy of structural levels that interact.
SYI-1.A
Explain how the properties of water that result from its polarity and hydrogen bonding affect its biological function.
SYI-1.A.3
The hydrogen bonds between water molecules result in cohesion, adhesion, and surface tension.
Activity
Procedure
- On the blood spatter card, use a straight edge and draw a straight line bisecting the blood spatter ellipse lengthwise.
- Draw a straight line bisecting the blood spatter ellipse widthwise.
- Measure the lengthwise bisecting line of the blood spatter ellipse and record the spatter’s length in millimeters. (When measuring a spatter drop that has a thin tail, do not include the tail in the length measurement.)
- Measure the widthwise bisecting line and record the spatter’s width in millimeters.
- Repeat steps 1—4 for each example.
- Divide the width measurement by the length measurement and record the answer.
- Calculate the angle of incidence for each drop by using the inverse sine function of a calculator or by looking up the value on a sine function chart. The inverse sine of width/length is the angle of incidence.
Blood spatter card
Analysis
- Assume all of the blood spatters were found on a flat, horizontal surface. Draw an arrow through the blood spatter indicating the direction of blood flow.
- Calculate the sine of each of the angles of incidence for each bloodstain spatter. Add it to your data table.
- What can the angle of incidence tell us about the crime being investigated?
Sample Answers
| Blood Spatter | Width | Length | Width/Length | SIne |
|---|---|---|---|---|
| A | .90 cm | 3.2 cm | .28 | 16º |
| B | 2.0 cm | 1.9 cm | 1.0 | 90º |
| C | 1.3 cm | 3.0 cm | .43 | 26º |
| D | 1.2 cm | 1.9 cm | .63 | 39º |
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