AIR PRESSURE






Allen, George                 Austin Community Academy
5816 N. Sheridan Rd.(8A)
Chicago, Illinois, 60660
1-312-334 3651

Objective

The purpose of these demonstrations is to aid the student in learning basic 
principles of air pressure. 

Apparatus Needed

Balloon         Glass Jar
Water           Soap
Pencil
                                    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. 


Apparatus Needed

Plastic Glass
Hot Water
Smooth Surface
                                    Gliding Glass

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." 

Apparatus Needed
                                     Erratic Ball
Table Tennis Ball
Thread                 
Scotch Tape 
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. 

Apparatus Needed
                                     Crushed Jug
Gallon Size Plastic Jug
with screw on lid
Boiling Water

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 
crushes it. 

Apparatus Needed
                                    The Hovercraft
An Old Long Playing Record
A Wooden Spool
A Candle     A Large Balloon
Smooth Surface

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.