Introduction: The Clean Getaway Lab
This is a lesson I use to introduce my physics students to the process of scientific investigation including writing a problem statement, generating a hypothesis, collecting and organizing data, analyzing data, writing a conclusion that is supported by data, and communicating results in a lab report. It also serves as a motivational activity to get students excited about investigating the world around them.
I present the class with the challenge of determining what happened at the scene of the "Clean Getaway." I provide them with a series of digital photos taken of parts of the floor at the crime scene. Each photo includes a protractor for scale.
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Step 1: Required Materials
Materials for making fake blood:
shampoo (this is why its called the clean getaway!)
a generous amount of red food coloring
a few drops of blue food coloring
The amount of coloring you need depends on the original color of the shampoo. Once you think you have a good color, smear a little on white paper to see the color.
disposable gloves for students
lab aprons for students
large knife smeared with fake blood and put inside evidence bag (I used a plastic baggie with a zipper closure.)
supply of fake blood for students
prepared crime science blood spatters
access to a computer
photo analysis software
Step 2: Preparing the Crime Scene Splatters
To prepare the blood splatter for the students to analyze, smear the fake blood on the knife and use several different motions to create splatter on a piece of graph paper.
Once the fake blood has dried a little, place a protractor on the paper and photograph it from directly above the paper.
I created six different crime scene photographs so that each of my lab groups had a separate photograph to analyze during the activity.
Step 3: Instructions for Student Exploration of Blood Spatter Behavior
The task presented to students is to use the materials provided to determine what caused the splatter pattern. Digital photographs of the splatter patterns are available as well as photo analysis software that allows you to quickly and accurately measure features in a photograph.
As students work they should be maintaining a record of their procedure and measurements in a lab notebook. Someone reading the lab notebook should be able to reproduce exactly the measurements, so they should be sure to include sufficient detail in each explanation. Students should also explain each calculation in their data analysis and make certain that their conclusion provides a clear link between the data collected, the analysis, and the proposed explanation of what transpired at the crime scene.
As students begin to brainstorm, different groups will develop different approaches to the problem such as measuring drops from different heights, different types of motion, and/or drops landing on boards placed at different angles. I monitor their progress but I did not offer advice or steer them towards any particular approach. After the class has completed their investigations we discuss the positive contributions from each group, and also what each group could have improved to get better or more complete results. If time allows you can then allow students to take more data to improve their conclusion about the activity at the crime scene.
Step 4: Photo Analysis
Once students have completed their investigations (which hopefully include changing one variables at a time and taking clear photographs of their results) they are ready to do a photo analysis of their data and the crime scene data.
I example I showed in the photograph uses Logger Pro software which includes both photo and video analysis features and can be obtained from Vernier Software at http://www.vernier.com. This technique is also available in many different software packages including open source programs. A quick web search for software compatible with the operating system of your computer will reveal many options.
The steps to follow are:
1. Open the photo analysis software and import your photograph into the program.
2. Establish the scale for your photograph using the protractor. This is done by clicking and dragging across one inch of the protractor scale visible in the photograph and entering the length of that region as one inch, then repeating for a vertical portion of the protractor that you can measure and then enter to establish a scaled distance.
3. Set the units of measure you wish to use for your measurements.
4. Click and drag across any distances you want to measure, such as diameters of blood droplets, lengths of blood trails, etc. depending on the research question you are attempting to answer. For example, how is the diameter of the droplet affected by the height from which the droplet is released.
5. Add text annotations to the photograph to explain the known blood splatter patterns were created.
6. Save analysis for use in the final report.
Step 5: Preparation of Lab Report
After completing their investigation the students will have their analysis of their own blood splatters and the crime scene. I ask them to prepare a lab report. I have attached two rubrics.
One rubric I use if students have an opportunity to research the analysis of blood splatter before beginning their investigation, and the other is a shorter form if time did not permit that time for background research.
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