Liquid Pressure

Johnson, Leon                       D. H. Williams Elementary School
2605 S. Indiana Ave.                567-7026
Chicago, Illinois 60616
326-1185

Objectives:

To understand Pascal's Law.
To understand Boyle's Law.
To show that water is a virtually incompressible liquid.
To understand Archimedes' Principle.

Apparatus Needed:

Rectangular bottle, 1 cork stopper, 2 rubber stoppers with opening in 
the middle, open ended glass tube, Listerine bottle, gallon jug, 
hammer, clear plastic dishwashing liquid bottles (Ajax or Palmolive) 
with tops, medicine droppers, water, food coloring, hot water bottle, 4 
meters of plastic tubing, C-clamps, metal extension rods, meter stick, 
rope, 30 cm. square board, swing structure approximately 2 meters tall 
made of 2"x4" wood with 60 cm. square 3/4" plywood board at top with 
holes for rope drilled at corners, 4 weights (14.3 lb. exercise weights 
were used in this mini-teach), 20 cm.x50 cm. plywood board. 

Recommended Strategy:   

     Put glass tube through stopper.  Fill rectangular bottle with 
water.  Place a few drops of food coloring in water.  Put stopper in 
top of bottle.  Pass bottle around class.  Have students note what 
happens when the sides are squeezed.  Repeat this procedure with the 
Listerine bottle.  Have the class note the results of squeezing this 
bottle.  Ask questions as to why this happened. 
     Fill gallon jug with water.  Put cork stopper in top of the jug.  
Compress the water by pounding stopper into the jug with a hammer.  The 
increased water pressure should cause the jug to break.  Explain that 
this illustrates Pascal's Law. 
     Show the class a Cartesian Diver model made from plastic soap 
bottle and medicine dropper.  Explain how to make one by filling the 
bottle with water and medicine dropper with just enough water so that 
head of dropper floats.  Dropper should dive when bottle is squeezed.  
It should return to the top when pressure on bottle is released.  Let 
each class member make a Cartesian Diver.  Explain that Pascal's Law, 
Boyle's Law and Archimedes' Principle are illustrated in the Cartesian 
Diver. 
     Fit plastic tubing through rubber stopper.  Fit funnel on other 
end of plastic tubing.  Fill hot water bottle with water.  Color water 
with food coloring.  Secure rubber stopper in opening of hot water 
bottle so that water does not leak and stopper does not pop out.  Place 
bottle on top of 60 cm. square board at top of swing structure.  Run 
rope through 30 cm. square board.  Place it on top of water bottle.  
Run the rope through holes in corners of the 60 cm. square board.  
Place 20 cm.x50 cm. board at end of ropes for the swing.  The C-clamps 
should be attached to the lab table with extension rods holding the 
funnel and plastic tubing about 3 meters above the height of the hot 
water bottle.  Height of water in tube with board on top of water 
bottle should be calibrated as point zero.  Place one weight in swing.  
Measure height of liquid in tube.  Mark off point on tube.  Continue 
placing weights in swing.  Record weight and height of water with each 
addition.  Make a graph of the weight as it changes the water height.  
Have a student sit in the swing.  Measure height of liquid in tubing.  
Put this point on the graph.  Draw a line connecting points to see if 
the line is linear or curved.