DAY 1 - THE EXPERIMENT
As I entered Mr. Bool’s seventh grade science classroom, each student was holding and manipulating a plastic bag with a straw sticking out one side. I asked one student what the class doing, and she explained they were making “pneumatic bags.” She showed me that they had taken a flimsy supermarket plastic bag, taped all the edges closed, and inserted a plastic straw halfway into one of the sides. They were now blowing on the straw to inflate the bags and then slowly deflating them being careful not to break the bag or cause a leak in any of the sides. She explained they were going to use the air pressure in the bags to push up some weight.
I was struck by how each student was intently crafting his/her bag, sealing the sides, and alternatively showing the success to their classmates or asking for guidance from Mr. Bool or a fellow student. These were scientists at work preparing their apparatus for the experiment soon to be enacted.
Mr. Bool asked several students to help him balance one of the tables on a floor scale. He then asked one of the students to read and record the weight on the white board in front of the science lab. The students then prepped a second table that had been placed in the middle of the room. They placed their bags along the side length of the table, three on each end, with the straws facing out. Once this was done, they helped Mr. Bool lift the other table upside down, with the legs pointing to the ceiling, and placed it on top of the plastic bags.
Impulsively I signaled to the teacher if I might climb onto the table. The students had all crouched down poised by their bags with the straws extending outward, ready to blow. He nodded and up I climbed. He asked me one last question before giving the instruction to blow. “How much do you weigh?” “One eighty-five,” I answered and the number was written on the white board under the weight of the table.
As the students began to blow, the bags inflated, and both the table and I began to rise! One student was ready with a cm ruler and called out the rise of the table, “Five, nine, twelve, fourteen centimeters.” “POP!” One bag’s seal suddenly gave way. “Oh no!” moaned another as her bag slipped off the edge, pushed out by the pressure. I remained floating on air, slightly tilted to my left, for a few more minutes until the students stopped blowing and the table and I settled back down.
Mr. Bool asked several questions about what had happened, focusing on the terms “force, weight, mass, and pressure.” All the data was recorded for next day’s discussion.
DAY 2 – THE LECTURE
When I returned the next day, an outline of a lab report and the data from the previous day were written on the white board. A standard lecture / discussion unfolded, with Mr. Bool discussing how the force of the air pressure contained in the plastic bags had lifted the mass of the table and me. He explained how the invisible molecules of oxygen and hydrogen in the bags, moving incessantly at high rates of speed, were bouncing off the plastic walls and thereby creating the pressure to push up the table.
It was, of course, the real experience of seeing and feeling the table rise as the bags inflated that made it possible for the students to understand the physical reality of invisible, tiny air molecules. The scientific words and abstract concepts became a physical reality for the students.
Mr. Bool then asked the students to calculate the area of the tabletop in square inches. He then showed them how to calculate the mathematical measurement of pressure by dividing the weight of the table by the area to get the pounds per square inch.
DAY 3 – THE LAB REPORTS
When I returned the third day, the students were scattered around the room writing their formal report. Herb provided the classic structure: Procedure, Results, and Discussion/Explanation. The students were filling in the chart with numbers, calculating the pounds per square inch, and composing narrative sentences to explain the results.
Here is one student’s explanation of air pressure from her lab report. I know I could not have explained what happened to me with this degree of detail. Could you?
As I entered Mr. Bool’s seventh grade science classroom, each student was holding and manipulating a plastic bag with a straw sticking out one side. I asked one student what the class doing, and she explained they were making “pneumatic bags.” She showed me that they had taken a flimsy supermarket plastic bag, taped all the edges closed, and inserted a plastic straw halfway into one of the sides. They were now blowing on the straw to inflate the bags and then slowly deflating them being careful not to break the bag or cause a leak in any of the sides. She explained they were going to use the air pressure in the bags to push up some weight.
I was struck by how each student was intently crafting his/her bag, sealing the sides, and alternatively showing the success to their classmates or asking for guidance from Mr. Bool or a fellow student. These were scientists at work preparing their apparatus for the experiment soon to be enacted.
Mr. Bool asked several students to help him balance one of the tables on a floor scale. He then asked one of the students to read and record the weight on the white board in front of the science lab. The students then prepped a second table that had been placed in the middle of the room. They placed their bags along the side length of the table, three on each end, with the straws facing out. Once this was done, they helped Mr. Bool lift the other table upside down, with the legs pointing to the ceiling, and placed it on top of the plastic bags.
Impulsively I signaled to the teacher if I might climb onto the table. The students had all crouched down poised by their bags with the straws extending outward, ready to blow. He nodded and up I climbed. He asked me one last question before giving the instruction to blow. “How much do you weigh?” “One eighty-five,” I answered and the number was written on the white board under the weight of the table.
As the students began to blow, the bags inflated, and both the table and I began to rise! One student was ready with a cm ruler and called out the rise of the table, “Five, nine, twelve, fourteen centimeters.” “POP!” One bag’s seal suddenly gave way. “Oh no!” moaned another as her bag slipped off the edge, pushed out by the pressure. I remained floating on air, slightly tilted to my left, for a few more minutes until the students stopped blowing and the table and I settled back down.
Mr. Bool asked several questions about what had happened, focusing on the terms “force, weight, mass, and pressure.” All the data was recorded for next day’s discussion.
DAY 2 – THE LECTURE
When I returned the next day, an outline of a lab report and the data from the previous day were written on the white board. A standard lecture / discussion unfolded, with Mr. Bool discussing how the force of the air pressure contained in the plastic bags had lifted the mass of the table and me. He explained how the invisible molecules of oxygen and hydrogen in the bags, moving incessantly at high rates of speed, were bouncing off the plastic walls and thereby creating the pressure to push up the table.
It was, of course, the real experience of seeing and feeling the table rise as the bags inflated that made it possible for the students to understand the physical reality of invisible, tiny air molecules. The scientific words and abstract concepts became a physical reality for the students.
Mr. Bool then asked the students to calculate the area of the tabletop in square inches. He then showed them how to calculate the mathematical measurement of pressure by dividing the weight of the table by the area to get the pounds per square inch.
DAY 3 – THE LAB REPORTS
When I returned the third day, the students were scattered around the room writing their formal report. Herb provided the classic structure: Procedure, Results, and Discussion/Explanation. The students were filling in the chart with numbers, calculating the pounds per square inch, and composing narrative sentences to explain the results.
Here is one student’s explanation of air pressure from her lab report. I know I could not have explained what happened to me with this degree of detail. Could you?
After doing this experiment, I can easily see that air can exert a force or pressure against the wall of its container. This can be seen because all that air was able to lift 285 pounds. This was only possible with the Kinetic Molecular Theory, which states that air molecules are always moving, and since they are always moving, the air molecules are colliding.
There are far too many air molecules in our atmosphere to count, which is why collisions between those molecules are so frequent. There are 4 sextillion molecules of air in every cubic inch of normal air (air that is not in a container). One air molecule experiences about ten billion collisions per second while traveling at 1,000 miles per hour. Since these molecules are always colliding, when air is compressed in a container (in this example, a bag) they collide against the wall of the container. When they collide against the wall they create a force (because the molecules of air have mass), and since that force is spread out all along the walls of the container, it is called pressure.
As we blew into the pneumatic bags, more and more air molecules were in the bag, and they collided more creating more pressure on the wall of the bag. As the bag inflated, it applied pressure against the underside of the table and lifted it. To be able to lift the table the air molecules need to be compressed. This means that the molecules are being squeezed together as tightly as possible so that more molecules can enter the bag. Since the room between air molecules is larger than the molecules themselves, they can be compressed and expanded easily. The more molecules there are in the bag, the more pressure is applied.
Pressure is measured in psi (pounds per square inch). To find out the psi in this example, you find the force (285 lbs.) and find the area of the table, (2160 sq. in.). Then divide the force by area, and you get the pressure (Force divided by area=pressure). In this instance, the psi equals .13194. This means that for every square inch on the table there’s .13194 pounds pressing on it. That pressure raises the table, so this is how air creates a force.
This series of learning activities is a model of thoughtful instructional design. Note the variety of activities,
- Crafting of scientific equipment by each and every student, requiring precision and care,
- Enacting, demonstrating, measuring, and recording a surprising result - Lifting a Table – requiring the participation of all students,
- Listening and note taking to learn specific scientific terms and abstract concepts, and understanding HOW they described the physical events,
- Applying a mathematical calculation to more precisely describe physical events,
- Composing on a blank page a coherent and integrated written document that shows the connections between physical event, abstract concepts, and mathematical symbols, in your own words
In-depth learning can only occur when all of these different modalities are offered to students in a purposefully designed sequence with sufficient time devoted to each activity. Too often in schools time is not allowed for the students to conduct the experiment, AND to discuss the concepts in words, AND to apply mathematical symbols to the words and events, AND to put in all together in a coherent report. Teachers are often pressured in many ways to compact the process. They might demonstrate the event instead of having every student do it, combine the lecture/discussion and writing, or give a short answer test. That would result in superficial learning. Without time for each phase the students won’t integrate in their minds the events, the concepts and the symbols.
Understanding how abstract concepts and mathematical symbols represent and describe physical phenomena is the fundamental process of human intelligence. By structuring a sequence of learning activities, we can enable students to develop their capacity for conceptual thinking and problem solving.
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