They can exert pressures in all directions, and they appear to have this increase in pressure with depth.

After students have had experiences and come up with ideas to summarize those experiences, it makes sense to introduce a technical term for ease of communication.

What are the differences? With some guidance, students suggest that air is “squishable” and water is not. They know that water is denser, heavier for the same volume, for we studied density earlier in the year. Students also may talk about the stickiness of water to itself and to other things, like the containers it is in. Since the students have summarized the ideas, I can now introduce the technical terms “cohesion” and “adhesion.” Now they are ready for another elaboration activity that more closely approximates the initial benchmark activity.

Elaboration Activity A6: “Weighing” an Object in a Fluid Medium. In this activity, I weigh a solid cylinder suspended by a string and ask, “What will happen to the scale reading if I attempt to weigh this object while it is under water?” Virtually all students suggest the scale reading will be lower than when the object is weighed out of the water. They are given an opportunity to test their predictions and are then encouraged to explain the results.

When complex explanations involving several factors are needed for their reasoning, students need more time to put the pieces together.

The scale reading is lower. Some students conclude that the water is pushing up by an amount that is the difference between what the object weighed when out of the water and when in the water. Note, however, that this is going back to the conclusion that water pushes up, with no mention of any downward push. Many textbooks let students off the hook at this point: “This upward force by the water is called the buoyant force.” But this prevents a deeper, more useful understanding involving the resolution of the up and down forces, so I press for more: “Tell me about the pushes by the water on this solid, metal cylinder.” Several students jump in with claims based on their previous experiences. They introduce their earlier conclusion that the water is pushing on the top and sides as well as on the bottom. I probe for more: “In what directions are those pushes?” Now students are even more eager to apply the ideas that have emerged in the last few days.

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement