Does the dog in the car have kinetic energy? A multiage case study in the challenges of conceptual change

Kinetic energy is usually the entry point for the study of energy in physics and is often perceived as unproblematic. We present evidence, however, that some learners who seem to have accepted the concept, from elementary school students to college physics majors and in-service teachers, nevertheless do not consistently attribute kinetic energy to moving objects that are being pushed or carried by other objects. Factors that seem to contribute to this idea include that the passive object is not moving “on its own”; the lack of attributes, like wheels, that suggest the ability to move on its own; and the perception that it would stop immediately if the driving object were to stop or disappear. We interpret these observations in terms of a model of a conceptual change via assimilation rather than accommodation and suggest some possible instructional implications.

Figure 1

In questions involving the kinetic energy of an object being pushed or carried by another object, a fraction of students answered that the passive object does or does not have energy as a consequence of its motion, for three levels of prior instruction. While the results are suggestive, they should be viewed with caution. The elementary school data come from 153 students in nine Grade 4 or 5 classes. The data for higher levels come from much smaller numbers of college students, 20 who had taken only high-school physics, and 6 who had taken physics in college. The last group was also responding to a somewhat different prompt. See Table 1 and the text for details and discussion.

Figure 2

Image from the block-push probe video, showing the battery-powered “car” (right) slowly pushing a wooden block at a constant speed to the left.

Figure 3

Reasons given by elementary school students ($N=153$) for their conclusion that the battery-powered car does have energy. While two-thirds mention that the car is moving, a significant fraction gives other reasons. Totals sum to more than 100% because some students offered multiple reasons.

Figure 4

Reasons given by elementary school students for their conclusions regarding whether or not the wooden block has energy. Reasons for concluding the block does have energy are shown on the left side, in green (49% of students); reasons for concluding the block does not have energy are shown on the right side, in red hatching (51% of students). Totals do not add to 100% because some students offered multiple reasons.

Figure 5

The Truck-Crate question given to students who had taken college-level physics.