Cartesian Diver

Submarines and How They Submerge and Resurface

                                       

Level:                   Elementary / Middle

 

Subject:                Science

 

Objective:             Students will understand the relationship of scientific principles to the ability of submarines to dive and later resurface through construct of the Cartesian Diver.

 

Background:         Buoyancy was discovered by a Greek scientist named Archimedes and it is explained in Archimedes' Principle. Archimedes' Principle states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. This example of buoyancy was first made by a Frenchman named de Cartes and so it has been called a Cartesian Diver.

 

Materials:             Large plastic bottle with screw top lid

                            A pen lid without any holes in the top

                            Lump of plasticine

                            Water to fill bottle

 

Task:                     Students will construct a Cartesian Diver to conduct an experiment which demonstrates how submarines sink and rise.

 

Activity:

1.                     Roll the plasticine in a ball about the size of a marble and stick it on the bottom of the pen lid.

2.                     Drop it gently into a bowl of water so that air is trapped in the lid.

3.                     Adjust the amount of plasticine on the lid until the lid floats upright with just the tip of the lid above the water.

4.                     After you have the right amount of plasticine on the lid, fill up the bottle and carefully drop the lid into the bottle. Screw the lid on tight and your Cartesian Diver is finished.

5.                     To make the diver sink, squeeze the bottle. Release it and it should float again. By squeezing just the right amount, students may be able to have the diver stay in the middle.

6.                     If the diver doesn't sink, you may need more plasticine on the lid.

How does this work? It has to do with the balance of the forces of weight and buoyancy. Most of us know what weight is -- the force which keeps us on the earth and stops us floating off into space, but what is buoyancy?

With the Cartesian Diver, the buoyancy is equal to the weight of the water which is displaced by the pen lid and the air bubble under the lid. When the bottle is squeezed, the air bubble becomes smaller and displaces less water so buoyancy is less and weight pulls the diver to the bottom of the bottle. When it is released, the bubble becomes large again and buoyancy pushes the diver to the top.

How does this relate to submarines? Well, submarines have tanks at each end which they fill with water to reduce the buoyancy and sink, and pump with air to increase the buoyancy and float.

If your diver gets knocked about too much, some of the air may escape from under the lid and the diver will sink to the bottom and not come up. Should this happen, don't despair, just get the diver out and put it back in the bottle again and it should be as good as new.

Circumstances of performance: This student work is to be produced under the following work conditions:

 

__ alone                                  __ in a group

__ in class                               __ as homework

__ with teacher feedback         __ timed

__ with peer feedback              __ opportunity for revision

 

Standards addressed:

S1b. Physical Science Concepts: Motions and Forces. The student produces evident that demonstrates understanding of motions and forces, such as Önet effects of balanced and unbalanced forces.

 

Boy Scout Advancement Requirements:

Oceanography Merit Badge

                  6. Define Physical Chemistry.

(a)  Construct a Cartesian Diver

(b)  Explain why the medicine dropper sinks to the bottom when the sides are squeezed.

Educator's Information Paper -

 

Diving and Surfacing

A submarine or a ship can float because the weight of water that it displaces is equal to the weight of the ship. This displacement of water creates an upward force called the buoyant force and acts opposite to gravity, which would pull the ship down. Unlike a ship, a submarine can control its buoyancy, thus allowing it to sink and surface at will.

To control its buoyancy, the submarine has ballast tanks and auxiliary, or trim tanks, that can be alternately filled with water or air. When the submarine is on the surface, the ballast tanks are filled with air and the submarine's overall density is less than that of the surrounding water. As the submarine dives, the ballast tanks are flooded with water and the air in the ballast tanks is vented from the submarine until its overall density is greater than the surrounding water and the submarine begins to sink (negative buoyancy). A supply of compressed air is maintained aboard the submarine in air flasks for life support and for use with the ballast tanks. In addition, the submarine has movable sets of short "wings" called hydroplanes on the stern (back) that help to control the angle of the dive. The hydroplanes are angled so that water moves over the stern, which forces the stern upward; therefore, the submarine is angled downward.

To keep the submarine level at any set depth, the submarine maintains a balance of air and water in the trim tanks so that its overall density is equal to the surrounding water (neutral buoyancy). When the submarine reaches its cruising depth, the hydroplanes are leveled so that the submarine travels level through the water. Water is also forced between the bow and stern trim tanks to keep the sub level. The submarine can steer in the water by using the tail rudder to turn starboard (right) or port (left) and the hydroplanes to control the fore-aft angle of the submarine. In addition, some submarines are equipped with a retractable secondary propulsion motor that can swivel 360 degrees.

When the submarine surfaces, compressed air flows from the air flasks into the ballast tanks and the water is forced out of the submarine until its overall density is less than the surrounding water (positive buoyancy) and the submarine rises. The hydroplanes are angled so that water moves up over the stern, which forces the stern downward; therefore, the submarine is angled upward. In an emergency, the ballast tanks can be filled quickly with high-pressure air to take the submarine to the surface very rapidly.

 

Taken from "How Submarines Work" by Carl Freudenrich, Ph.D. and Marshall Brain http://www.howstuffworks.com/submarine5.htm