Focus question

What causes the diver to go up and down in the water?

Essential materials

• plastic drink bottle (we recommend 1 liter or larger)
• bowl, bucket, beaker, or other large-mouth container
• detachable top or lid from a ballpoint pen (make sure there is no hole in the top or lid)
• waterproof modeling clay (such as Plasticine)
• water

Main ideas and background information for both activities

• The density of an object is the mass of the object divided by its volume.
• An object with a density less than that of a fluid will float in that fluid.
• An object with a density more than that of a fluid will sink in that fluid.
• An object with the same density as that of a fluid will neither sink nor float in that fluid. This state is called neutral buoyancy. Some scientists also call this special state “flinking,” a combination of floating and sinking. An object with neutral buoyancy is able to remain stationary at any depth in that fluid.
• When the inverted pen top is submerged in the water, it displaces an amount of water equal to the volume of the material in the top plus the volume of the air trapped in the top.
• Squeezing on the bottle increases the pressure in the water and compresses the air inside the pen top. This decreases the volume of water displaced by the top with its air.
• Because the top with air is totally submerged, a decrease in the volume of the trapped air is also a decrease in the total volume of water displaced by the pen top. This results in an increase in the density of the pen top with its air.

Procedural tips If you cannot find waterproof modeling clay, two or three sticks of chewing gum will work about as well. Select a small pen top or lid over a large one for better results. If your pen top or lid has holes in it, plug the holes with clay or gum.
When you are testing the top in the bowl, add or take away clay or gum from the attached blob until the top just barely floats in the water. After lowering the pen top and attached blob into the bottle of water, add more water to fill the bottle completely before screwing on the lid. Attach a ruler to the side of the bottle so that students can measure the movement or move the diver to a particular depth.

Discussion

1. How did your observations compare with what you thought would happen?
2. What do you think would happen if you repeated this experiment with a larger pen top? …a smaller pen top? Explain your reasoning.
3. What do you think would happen if you repeated this experiment with a smaller blob of clay or gum, so that the pen top floated higher up in the water initially?
4. When operating underwater, a submarine allows water to come into special tanks. Later, the submarine will pump the water out of the tanks. What is the purpose of doing this? What happens to the submarine and why? (This is how a submarine is able to go down and up in the ocean. Letting water into the tanks reduces the buoyant force on the submarine, so it sinks. Pumping water out of the tanks increases the buoyant force on the submarine, so it rises.)
5. What other situations can you think of in which buoyancy makes a significant difference? How does it affect your everyday life? (examples: swimming, scuba diving, submarines, pontoons on bridges, boats, airplanes …)
6. Did you attach a ruler to the bottle? If so, what did you have to do to make the diver go to a specific level without overshooting? How easy or difficult was it to keep the diver at that level for a period of time?

Assessment

Are students able to describe what causes the diver to go up and down in the water? (When you squeeze the bottle, the increased pressure in the water compresses the air trapped in the floating diver. This reduces the amount of water displaced by the diver together with its air, and also reduces the buoyant force on the diver. When the buoyant force becomes less than the weight of the diver, it sinks. When you release the bottle, the opposite happens, and the diver rises.)

Extensions and further investigations

• What other objects could be made into divers? Repeat the activity using one or more of these objects. (This could include a medicine dropper, an inverted test tube, etc.)
• Instead of water, use other common but safe liquids to see if the diver behaves differently. (This could include vinegar, rubbing alcohol, cooking oil, shampoo, etc.)

Career connections

Students may wonder how buoyancy relates to their lives now and in the future. The concepts addressed in this activity are significant in any career field that involves immersion in a fluid. Scuba diving immediately comes to mind as a popular example. Scuba divers sometimes wear belts of weights to help regulate their buoyancy. They also use a device called a buoyancy compensator to help them change their buoyancy and regulate their depth more easily. (See the sidebar Similarities to Scuba Diving.)

People in many other career fields need to take buoyancy into account in their regular activities. These careers include:

• Underwater divers (e.g., snorkelers, scuba divers, deep sea divers)
• Submariners and sailors
• Hot air balloonists

Correlations with Standards

United States: This activity correlates with portions of NSES Content Standard A, Science as Inquiry, and Content Standard G, History and Nature of Science, Grades 5-8 and 9-12, and with the following additional standards.

Grades 5-8
Standard B - Physical Science: B1, B2

Grades 9-12
Standard B - Physical Science: B2, B4

Britain: This activity correlates with the English National Curriculum Sc1, Science Enquiry, and the following additional standards:

KS2:
Sc1, BoS, 1a, 1d, 1e

KS3:
Sc1, Sc3, 1a; Sc4, 2b, 2c; BoS, 1a, 1d, 1e, 1f

KS4:
Sc1, Useful Background; BoS, 1a, 1d, 1e, 1f

Glossary/vocabulary

buoyancy
buoyant force
density
fluid

Resource links

Archimedes Principle on Scubatoys.com
Explanation of buoyancy from a scuba perspective, with examples of applications.

The ABCs of BCs
For an explanation of buoyancy compensators.

Buoyancy Brainteasers from NOVA

Cartesian Divers from Venture Engineering and Science Camp
Why is the diver called Cartesian? No one really knows, but this site has some information.

Cartesian Diver
For an interesting related activity.