can be increased from normal to much higher: “What would happen to the scale reading and why?” Another asks students to predict what would happen to a scale reading if we attempted to weigh the solid object in alcohol, which is less dense than water but more dense than air.

Assessment should help the teacher monitor whether students are still operating on the basis of preconceptions, as well as whether they have attained the learning goal(s).

For all these questions, I look for evidence to determine whether the students’ ideas have changed or they are still showing evidence of believing their original idea that fluids only push up or down or that the weight is proportional to air pressure. Thus, I aim to move students’ understanding across the gap from their preconceptions to a more scientific understanding. The assessment allows them to monitor their learning. If there is trouble, they get feedback suggesting they rethink their answer and/or reasoning in light of the class experiences. I thus obtain a report of what sort of problematic thinking students have exhibited and what experiences might help them move farther across the learning gap.

Part B: What Is Gravitational Attraction?
Exploring Similarities and Differences Between Actions at a Distance

In the previous subunit (Part A) the class separated the effects of the surrounding medium from the effects of gravity on static objects. We appear to have taken a bit of a detour into understanding more about the effects of air and water and other fluids on objects submerged in the fluid. Later we will need to return to looking at the effects of the surrounding fluid when we explore falling bodies (Parts C and D). First, however, we explore the concept of “action at a distance,” a key notion in understanding gravity.

Students should be able to see science as involving many questions as yet unanswered.

Although there are still many unanswered questions about gravity, the students do know a great deal about what it does and about the variables on which the strength of the gravity force depends. In Part B, now that the students know about some effects that are not due to gravity, we explore some of the effects that are. Because many effects of gravity are so subtle

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