I start the pump.
Keeping the Faith with a High-Quality Curriculum
It is important to give students opportunities to apply without being told, if possible ideas learned earlier. The result surprises many students. The scale reading does not appear to change at all. Some students give a high five. It could be a tiny bit more than it was. It could be a tiny bit less, or it might be exactly the same.
Maybe if we had a really, really accurate scale we could tell. I also want the students to see that conclusions are different from results, so I often guide them carefully to discuss each. What did happen? What did we observe? We observed no apparent change in the scale reading. Students should be provided opportunities to differentiate between summarizing observable results and the conclusions generalized from those results. Did we learn anything? Conclusions are about the meaning of the. So, what can we conclude? What do these results tell us about the effects of the air?
And apparently gravity is not caused by air pressure pressing things down. Activity A1 is a simple worksheet asking students to review their answers to questions about their initial ideas, other ideas that have come out in discussion, and the results and conclusions from the preceding benchmark lesson. Typically, I hand this summary sheet out as homework and collect it at the beginning of the next class.
By reviewing what students have written, I can identify related issues that need to be discussed further with certain students. Alternatively, I may ask students to check and discuss their answers with each other in groups and to add a page of corrections to their own answers before handing in their original responses. One purpose of this activity is to encourage students to monitor their own learning.
Progressing from the preinstruction question through the benchmark discussion takes about one class period. In showing that gravity is not caused by air pressure, we have generated questions about the effects of the surrounding air. Students now want to know the answer to the original question. I used to end the investigations of the surrounding air at this point and move on to investigating factors affecting gravity, but I discovered that students slipped back to believing that air pressed only down or only up.
Therefore, we redesigned the curriculum activities to include more time for investigation into the effects of surrounding fluids. Doing so also allows us to incorporate some critical introductory experiences with qualitative ideas about forces on objects. This experience helps lay the groundwork for the later unit on forces, when we will revisit these ideas and experiences. Revisiting ideas in new contexts helps organize them in a rich conceptual framework and facilitates application across contexts.
In the benchmark lesson, several ideas were raised that need further testing. Some students suggested air only pushed up, others that air only pushed down, still others that air pushed equally or did not push at all. Some suggested that air was like water; others contested that idea. Each of the following activities is intended to give students opportunities to test these ideas in several contexts, recognizable from their everyday world.
That is, each activity could easily be repeated at home; in fact, some students may have already done them. One goal of my class is for students to leave seeing the world differently. This activity was derived from a trick sometimes done at parties.
A glass of water with a plastic card over the opening is inverted. If this is done carefully, the water stays in the glass. Students are asked to do the activity and see what they can learn about the directions in which air and water can push.
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They are also given the opportunity to explore the system and see what else they can learn. Allowing students freedom to explore may give teachers opportunities to learn. Teachers need to allow themselves to learn. My purpose here is to help students see that air can apparently push upward on the card sufficiently to support the card and the water. That is usually one conclusion reached by some students. We found it would only hold about three pennies before the card would drop off. The water we had in the glass weighs a lot more than three pennies. Stickiness might help, but it is not the main reason the card stays on.
The main reason must be the air below the card. This was such a nice example of suggesting and testing alternative explanations that I now bring up the possibility of the stickiness being all that is needed if this idea does not come up in the group presentation. When they fill the glass, put on the card, and invert the glass, they put their finger over the hole.
When they move their finger off the hole, the water and card fall. They conclude that the air rushing in the hole pushes down on the water and that the air pushing from under the card is not providing sufficient support. After making these observations, students are ready to draw the tentative conclusion that the upward push by the air on the card must be what is supporting most of the weight of the water on the card.
They note the water must push down on the card, and since the stickiness of the water is not enough to hold the card, there must be a big push up by the air. This conclusion is reached more easily by more mature students than by middle-level students. The latter need help making sense of this argument. Most are willing to say tentatively that it makes sense that the air pushes up and are more convinced after they see the various directions in which air pushes in the other activities.
This activity was derived from some students describing observations they had made while hand-washing dishes. They had observed what happened when an inverted glass was submerged in a dishpan of water. In activity A3, a narrow cylinder e. Again, students are asked to see what they can learn about the directions that air and water can push.
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I want students to see that air and water can push up and down, and that the deeper one goes in a fluid, the greater is the push in any direction. While doing this activity, students observe that the farther down one pushes the floating cylinder, the more difficult it is to push. Thus, they conclude that the water is pushing upward on the air in the small cylinder, and the push is greater the deeper one goes.
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Typically, some students cite as additional evidence the observation that the water level in the small cylinder rises within that cylinder the farther down one pushes the small cylinder, thus compressing the air. I commend these students for their careful observation and suggest that other students observe what happens to the level of the water in the inner cylinder.
The more the air is compressed, the harder the water must be pushing upward on the air to compress it, and the more the compressed air must be pushing upward on the inside of the small cylinder. The students appear to have reached the conclusions I hoped for. Although I primed them with relevant questions, they made the observations and reached the conclusions.
Elaboration Activity A4: Leaky Bottle.
This activity, like the others, came from experiences students had suggested helped them with their thinking about fluids. A 2-liter plastic soda bottle with three holes in it at three different heights is filled to the top with water and allowed to leak into a basin. Again, students are asked to see what they can learn about the directions in which air and water can push. Here I want students to learn that air and water can push sideways as well as up and down and again, that the push of air and water is greater the deeper one goes.
In what direction would the air push on it?
In what direction would the water in the container push on the droplet?