Notes for the Instructor Copy

In this lesson, students learn that tracks and tread left at a crime scene can reveal significant information at an investigation. Even a partial impression can provide information. In the first activity, students will observe a simple method of creating a tread impression, making a mold, and casting it with Plaster of Paris. Students will compare the impression vs. the cast to understand how positive and negative are reversed in the impression vs. cast image. They will have an opportunity to make their own impressions of the tire and other found objects such as a key. They will record their observations in a data table and analyze their information to make comparisons. In the second activity, students see the crime scene evidence. They analyze a print of the bicycle’s tread pattern from the crime scene and match it to the tread patterns of bicycles belonging to the four suspects.

Tracks refer to the path left on or in a surface by any travel method. Automobile tracks, for example, can reveal the direction of travel as well as changes in direction. Car tracks reveal track width, which is the distance between the two front and two rear tires. This width can be used to identify the vehicle make, such as a Honda Civic. The tread pattern, which is the pattern left by the tire, can determine which particular Honda Civic model!

With an automobile, the tread width is indicated by measuring the distance across the tread pattern. Both front and back tread widths are measured, because manufacturers use different patterns on the front and the rear of the car.

At the crime scene, prints can appear two dimensional if made on a hard surface, or three dimensional if they were left in snow or mud. When tire prints are evidence, a number of steps must be followed to record them as evidence:

1. The prints must be protected. The location of the tracks or prints may need to be covered or roped off.
2. Two types of photographs must be taken: a photo of the general area (which includes other objects and a ruler to help with scale) and a close up that shows more details (the close up also contains a ruler).
3. The location of the track must be plotted on the crime scene map.
4. Any debris within the print must be removed carefully.
5. To keep a track from deteriorating on fragile surfaces, such as mud or snow, a special spray is used to coat, seal, and protect the surface and keep it relatively firm.
6. A form or frame is placed around the area so a cast can be created. The form must be high enough to allow the casting mixture to capture the deepest part of the impression.

Once a cast has been made, the cast is examined in a lab for specific cuts, markings, or lettering that help in comparing it to other prints, either visually or by using a database.

Track and tread patterns are both physical evidence and class evidence. The table included in the note from Mr. Mugg offers information about physical and class evidence, as well as several examples.

The activities in this lesson address Next Generation Science Standards practices of Asking Questions and Defining Problems, Planning and Carrying Out Investigations, and Engaging in Argument from Evidence. In addition, they address Common Core State Standards CCSS.ELA-Literacy.CCRA.SL.1, CCSS.ELA-Literacy.CCRA.SL.2, and CCSS.Math. Content.3.MD.B.4. See the Standard Matrix included on page 111 for more detailed information.