How can the length of skid marks be used to determine the initial velocity of a car?

Have students find articles about major car accidents, hand out an article, or find a website to be read and discussed in class. Preliminary assignment should include information and images involving skids.

1. Display an image of tire marks in class (overhead, monitor, projection). Other possibilities: go outside and observe tire marks made by police. Watch video clip of police making the skids marks. Have students list observations of examples given.

2. Discuss homework and ask students to generate a list of variables that affect car accidents especially rapid braking events (RBE’s).

3. Have the students consider the following question:

"If I showed you a set of skid marks and asked you to tell

me how fast the car was traveling that made them?

what information would you want to have?"

Use this question to narrow the list to variables so that it only includes factors related to mass of car, coefficient of friction and the car’s velocity.

4. Review connections between initial velocity, acceleration, constant acceleration and Newton’s Laws as well as forces and coefficient of friction (road conditions, type of tire, etc). Review concepts and equations and point out connections between these.

5. Students work in small lab groups to develop a qualitative understanding of the relationships between the three variables from Step 1 and their role in answering the question posed. In other words, how will these three variables affect the length of skid marks? At the end of their discussion, they should be able to state the relationship in terms of a hypothesis between the following: initial velocity to skid mark length, coefficient of friction to length and mass to length. Through class discussion and topics previously covered, the class should be able to come to consensus on the relationships between the variables.

6. At this point, you may want to have an accident reconstructionist (most likely a police officer) visit the class and talk to the students about the mathematics and physics of accident reconstruction and the techniques he/she uses to analyze accidents. Students should ask questions!

7. Students work in small groups to develop a flow chart which shows the relationships between variables. On this flow chart, students should write out matching equations for each variable in the flow sheet.

8. Introduce EXCEL as a way to model what happens in a car crash. Show examples of effective Excel sheets to the students. Discuss rudiments of cells, labels and equations. Then have students take their flow chart and begin thinking through a worksheet on paper for practice in class. Be sure to cover the following:

9. Students now take their practice spreadsheets and create a computer based EXCEL sheet. Their spreadsheet should enable them to answer the initial question. (Example) Students who feel comfortable should try inserting pictures and comments.

10. Students present spreadsheet models to the class for discussion and evaluation. This is the time to fix and refine the models. Be sure to allow time for students to revisit and change their models. Eventually, students should arrive at some kind of consensus regarding spreadsheet procedures and equations.

11. Now (police) mark out an "accident" on the parking lot in preparation for the final assessment of the module.

12. Send small groups of students out to the site to measure the skid marks and test their model. Tell students to be sure to save their results. After all groups have measured and tested, students will share their models and results with the class.

13. You might want to have the accident reconstructionist return at this point to "grade" the students’ work and do a wrap-up discussion.