Goal: Reason and evaluate statements about a real-world situation.
Source: UMPERG
At the scene of an accident the car causing the crash left skid marks of
a length D. The accident reconstruction team did a test and found that a
police cruiser traveling at the speed limit produces skid marks of
length d < D. Which of the following statements is valid?
- Since D > d the car must have been traveling over the speed limit.
- One cannot conclude that the car was speeding because the test failed
to take reaction time into account. - One cannot conclude that the car was speeding because the test failed
to take the mass of the car into account. - One cannot conclude that the car was speeding because the test failed
to take into account possible differences in the coefficient of friction
for different tires. - Two of the above statements are valid.
- None of the above statements is valid.
- It cannot be determined if any of the statements is valid.
Commentary:
Answer
(4) is valid assuming that the usual kinetic friction model is
applicable. Some students may think that (3) is valid and indicate (3)
or (5). All of the others are definitely invalid. Since (1) is
invalid, (7) is also invalid.
Background
This question seeks to encourage students to reason and analyze the
situation. It offers the opportunity to engage the students in a
discussion of the meaning of validity as well as the physics underlying
the various assertions.
Questions to Reveal Student Reasoning
How would reaction time influence the skid marks?
Suppose the car had several people inside. Would that have affected the
skid marks?
Suppose the test had been made with the same model car as the one in the
accident. Would that make the test more valid?
Suggestions
Allow students to form small groups according to their views and let
them present their arguments to the class. Have student ‘consultants’
suggest appropriate tests to determine if the car was speeding.