The Pressure Of A Liquid






Bradley, Patricia            John Hope Community Academy
8005 S. Eberhart
Chicago, IL  60619
1-312-538-8066

Objectives

1. Students will see what happens when the pressure of a liquid is not the same on 
   all sides of a submerged object. 
2. Students will find out how the pressures of a liquid (water) works. 

Equipment and Materials

Part 1                                       Part 2       

empty milk carton                       hollow tubes open on both 
a pen or pencil                         sides (small orange juice 
a piece of tape the length of carton    can)
two liter plastic bottle                bowl or basin
(put hole in bottom of bottle)          metal jar lid
2 cans (one large, one small)           water
put hole in each can 2 cm from          drinking glass
bottom, fill each can 5cm high          fish tank
drinking glass 

Recommended Strategies

     Begin lesson by using phenomenological approach in demonstrating how water 
pressure works.  Using an empty milk carton, a pen or pencil, and a piece of adhesive 
tape. The length of the carton will help show how water pressure works. Use the 
pencil to punch three holes, one above the other, on one side of the carton.  Make 
the top hole at least 3cm (1 1/4 inches) from the top of the carton.  Cover the holes 
with the adhesive tape.  Fill the carton with water.  Put the carton in the sink or 
bathtub.  Strip off the tape quickly.  Which stream of water travels farthest?  How 
does it work?  WATER PRESSURE is the key to the answer.  The water near the bottom of 
the carton has the force of all the water above it pushing it out.  The water near 
the top has very little water-and therefore pressure-above it.  This demonstrates 
water pressure from the sides. Next, punch a hole in the bottom of each can (small 
and large) 2cm from the bottom of the cans.  Cover the holes with a strip of tape.  
Fill each can with water to the same depth (e.g. 5cm, 2 inches, deep in each can).  
The larger can will hold more water than the small can.  Put both cans in the sink or 
bathtub.  Pull off the tape at the same time.  Which stream goes farther?   (The 
streams should be the same length.)  The depth of the water makes the pressure 
greater, but the amount of water doesn't.  This demonstrates water pressure in 
downward a direction. 
     Next, using a fish bowl filled with water, demonstrate water pressure in upward 
direction by using a two liter bottle with hole in the bottom.  Stick bottle in fish 
tank and watch the water flow upward. 
     Now you are ready to begin your lesson on water pressure because you have 
demonstrated that a fluid exerts a pressure in all directions.  If  an object is 
placed in a bowl of water, pressure is exerted on all sides of the object.  If, 
however, one side of the object is not in contact with the water, pressure is not 
equal on all sides.  In this activity you will see what happens when the pressure of 
a liquid is not the same on all sides of a submerged  object.  To do this activity 
follow the steps below to learn more about the pressure of a liquid.  1.  Fill a 
drinking glass with water.  2.  Fill a bowl or basin with water.  3.  Place the metal 
jar lid under the hollow tube.  Now, keeping both objects together, place them in the 
bowl or basin of water.  (Be sure to hold them together, otherwise, the can will turn 
on its side.  4.  Does the plate sink to the bottom?  Explain  your answer.  5.  
Slowly pour water from the drinking glass into the hollow tube.  At what point does 
the jar lid sink to the bottom? 
     ANSWERS 4.  NO.  Pressure exerted by the fluid (water) pushing upward on the jar 
lid is greater than pressure pushing down on it.  Because there is no water in hollow 
tube, there is no water pressure.  Water pressure on all sides of the object isn't 
equal because one side isn't in contact with the water.  5.  The jar lid sinks to the 
bottom when the level of the water in the hollow tube is equal to the level of water 
in the bowl or basin.