The qualitative strategy is inspired by Reif's work on physics problem solving.¹³ It consists of three steps that promote a clear subdivision of the problem-solving process that are presented separately on colored index cards. One side of the card includes instructions for carrying out the relevant part of the strategy, and the other side includes guiding questions that are designed to assist the students to follow the instructions accurately. Visual representations are used in the strategy as exemplified below.

1. The first step (“system characterization”) consists of two substeps that enable the student to consider the subsystems and all the interactions before focusing on a certain object.

1.a. Representing the situation by a block diagram involving components of the system.

1.b. Constructing a table including all the interactions between objects within the system.

For example, consider the situation presented in Fig. 1. In this step, the student translates the situation, first to a block diagram [Fig. 3(a)] and then to a table of interactions [Fig. 3(b)].

Figure 3a.

Figure 3b.

2. The second step (“from systems to selected objects”) is designed to lead the student to draw a free-body diagram of a selected object. The process is performed in two stages:

2.a. Marking all the pairs of forces in the block diagram using the table of interactions.

2.b. Selecting an object and “gathering” all the forces that act on it using the block diagram.

The guiding questions emphasize Newton's third law (N3) and ensure that all the forces that act on the object appear in the free-body diagram of the step stage. The relative magnitudes of the forces that act on the object are not considered. Note that interactions at a distance are marked by a dotted line.

If we apply the second step in our example (see Fig. 1) we get Fig. 4(a). Isolating the selected object, in this case the dog, and marking the objects that exert forces on it without considering the magnitude of the forces leads to Fig. 4(b).

Figure 4a.

Figure 4b.

3. The third step (“forces and motion”) allows the students to analyze the situation by constructing a complete free-body diagram and relating it to the motion characteristics. The relative length of the arrows that represent these forces in the diagram can be determined based on information given in the problem or can be deduced from the characteristics of the object's motion. This step enables the students to link between forces and motion by delineating the relations between the forces and the observed motion (Newton's second law—N2).

This step allows the student to (a) deduce forces from motion information as described above; (b) deduce motion characteristics from a force diagram; and (c) based on Newton's laws, predict what will happen in a situation, observe the outcome, and explain it (POE¹²— Predict, Observe, and Explain).

In our example, the dog does not move (motion characteristics are known). Thus, the net force along each axis should equal zero. That means that the arrows along each axis have equal length and are in opposite directions (Fig. 5).

Figure 5.

This approach is especially useful in analyzing complex situations, as well as ill-defined problems that characterize authentic situations familiar to students.