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Allen, George Austin Community Academy
The purpose of these demonstrations is to aid the student in learning basic
principles of air pressure.
Balloon Glass Jar
Balloon In Jar
Inflate the balloon so that it is slightly larger than the mouth of the jar. Try to
force the balloon into the jar; it is difficult, if not impossible. Then slide the
pencil down beside the balloon and the balloon may be pushed into the jar. The
balloon seals the opening of the jar so that air cannot escape around it, and as it
is pushed, it tends to compress the air in the jar slightly. It cannot be easily
pushed against the air pressure.
Rinse the glass with hot water. Leave a little water in it, and invert the glass
onto the smooth surface. The glass will "skitter" around as if on ice, with almost
no friction. As the water is poured out of the glass it is replaced by room air.
Heat stored in the glass and water, heats the air somewhat; it expands and the
pressure lifts the glass a tiny distance from the surface of the table. The glass
floats on a film of water and a cushion of air. This is the same principle used by
the surface-effect vehicles or "hovercraft."
Table Tennis Ball
Soda Straw With Flex
Attach the ball to the end of the string with a tiny piece of tape. Suspend the ball
by the string. Blow upward against the ball through the straw. Blow harder, then
slowly, and reverse. Blow gently up against the ball, just off center, and the ball
will try to "outflank" the air jet by going around it to where it can hang
vertically. The ball's motion will be quite erratic. Blow harder and the ball will
go into the center of the air stream and tend to remain there. Moving air exerts
less pressure than still air, so the ball tends to remain so that the pull of the
moving air around it is nearly equal on all sides.
Gallon Size Plastic Jug
with screw on lid
Put boiling water into the jug and shake it with the lid closed, but loose. When
steam and water stops coming out, screw the lid on tight. The jug will begin to
collapse. The action can be speeded up by using cold water on the jug. As the steam
in the air condenses, the pressure in the jug diminishes. Atmospheric pressure
An Old Long Playing Record
A Wooden Spool
A Candle A Large Balloon
Fix one end of the wooden spool so that the balloon can be slipped over it. Attach
the other end of the spool to the center of the record with candle wax or glue. The
holes in the spool and record should match. Inflate the balloon, slip its mouth over
the spool and place the record on a smooth surface. Release the balloon and the
record will glide over the smooth surface with little friction. When the record
rests on the surface it tends to remain there because of the friction created when
the surfaces move against each other. The air stream from the balloon puts a thin
layer of air between the surfaces, eliminating most of the friction.