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. Groups of three or four students each are assigned to “major” in one of the elaboration activities and then to get around also to investigating each of the other activities more briefly. In every case, they are asked to keep the original bell jar experiment in mind: “How does this activity help us understand the bell jar situation?” With respect to the activity in which they are majoring, they will also be expected to present their results and conclusions to the class.
Elaboration Activity A2: The Inverted Glass of Water. 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. 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. Early in my use of the activity, however, I was surprised by a student who emptied the water and placed the card over the open end of the inverted glass and concluded, “It’s the stickiness of water that holds the card to the glass.” For a moment I was taken aback, but fortunately other students came to my rescue. They said, “At first we thought it might be because the card just stuck to the wet glass, but then we loaded the card with pennies to see how many pennies the card would hold to the empty glass. 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.”