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Investigation Question 3:
What can air do when it presses on things?
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What To Do
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Setting the
Scene
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Repeating (or reviewing) the activity from Investigation 2 provides a
way to introduce Investigation 3. The students' attention can be focused
on the water being held out of the cup. The discussion can move to the
air being what keeps the water out, and from there to the idea that the
air is pressing on the water. This to the realization that air presses
on other things, too.
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Presenting the Investigation
Question
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Introduce your students to the investigation question: "What
can air do when it presses on things?"
Have your students discuss the question in pairs, then in groups, and then as a whole class. Record their answers on the flipchart.
Have your students brainstorm ideas about how this investigation question could be investigated.
- Design an experiment that could be used to test the investigation question.
- What materials would be needed?
- What would you have to do?
- What would be measured?
- How long would the experiment take?
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Assessing What Your Students Already Know
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The idea of air pressing on things, or air pressure, may be quite new
to most of your students. They will more likely know that contained air
within a balloon, basket ball, bicycle tire, etc., pushes on the inside
of these inflatables. They may even think of this as air under pressure.
However, the idea that air exerts pressure on the outsides of objects
from every direction is a tough concept to understand.
Spend some time reviewing Investigation 1 and 2. Remind your students
how air was contained with a balloon in Investigation 1 and inside the
underwater cup in Investigation 2. Ask them to consider the following
questions, first in pairs then groups then as a whole class:
- When have you seen air pushing on things? (Some may give air pushing
on boat sails or windmills as examples, which are examples of wind movement
not air pressure. Do not correct them at this stage.)
- How do you know it was air that was pressing on things? (Your students
may come up with a range of possibilities here. Their responses will
alert you to the level of understanding they currently have. Accept
them without judgment at this point.)
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Exploring the Concept
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- First, have everyone put on the safety goggles. Lay one of the strips
of balsa wood on a table edge with about a third of its length sticking
out from the table top.
- Ask your students to predict what they think will happen when you
strike the protruding length of wood with the hammer or mallet. Have
them write or draw this on paper and also write the reasons for their
predictions. (Some may think that the wood will break. Others might
suggest that it will flip up into the air.)
- Give the protruding wood a firm tap with the hammer, and it should
flip the whole piece into the air. (You might want to do this step
to avoid the wood flying too far.)
- Ask for ideas about breaking the wood in two pieces using the hammer.
(Some will suggest that if you hold your hand down on the part lying
on the table, you can break the protruding piece off with a hammer stroke.
Have a volunteer hold the piece down firmly while you tap off the end.)
- Now, using the second strip of balsa, show the students that you
are going to cover the part of the strip lying on the table top with
just 3 sheets of newspaper.
- Have your students first discuss and then predict what will happen
when the wood is struck by the hammer or mallet. They should draw or
write down their predictions and also give the reasons for them. (They
may predict that the paper will tear or that the paper will work like
hands to hold the wood down so that just the protruding end will break
off.)
- Hit the wood with the hammer or mallet. (The air pressure on the
paper holds down the wood on the table enough so that the end can be
lopped off with the hitting stroke.)
- Have your students look again at their predictions. (Did what happened
confirm or refute their predictions? Do the explanations for their predictions
appear to be valid or not?)
- Set three books in a stack on the table. Then, place the balsa wood
on top of the books so that one end protrudes as it did off the bare
table. Set the newspaper on the other end of the balsa. In this case,
the paper will be above the table (although it is likely to sag down
enough to touch the table.) Have students predict what will happen
if you hit the balsa now, and then do it. (The balsa would should
flip the paper up without breaking.) Explain that in this case,
the air was pressing down on the paper as it did before, but because
there was also air below the paper, the air pressure was also pressing
upward, so the pressure was balanced on both sides. When the newspaper
is smooth on the table, there is only air pressure on top, which is
enough to hold the wood down such that the end is broken off.
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