The Pressure Of A Liquid

Bradley, Patricia John Hope Community Academy

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.
Return to Physics Index