A Cartesian Diver – Lab Activity
Disclaimer: This activity is a customized version of the experiment found in the textbook with permission from Wiley
Learning Goals:
Students will be able to:
⦁ Improvise and build small physics experiment from household items
⦁ Observe and describe physical phenomena
⦁ Describe what Buoyancy is and how it is affected an object’s change in density
⦁ Relate everyday life events with physical theories
Experiment:
The concept of buoyancy: An object immersed in a fluid experiences an upward force from that fluid. This buoyant force is what lifts a helium balloon into the sky and suspends a boat on the surface of water. How the object responds to buoyancy depends on the relative densities of the object and the fluid surrounding it, where density is the ratio of mass to volume. An object that is denser than its surrounding fluid sinks, while one that is less dense than its surrounding fluid floats.
To see the importance of density in determining whether an object sinks or floats, you can construct a simple toy called a Cartesian diver. This once-popular parlor gadget consists of a small air-filled vial floating in a sealed container of water. Normally, the air bubble inside the vial keeps it floating at the surface of the water, but whenever you squeeze the container, the vial sinks.
To make a Cartesian diver, you’ll need only a plastic soda bottle and a small vial that’s open at one end. The vial can be made of almost anything—plastic, metal, or glass—as long as it’s dense enough to sink in water. Fill the plastic soda bottle with water and float the vial in it upside down; air trapped inside the vial should keep the vial afloat. Now slowly reduce the size of the air bubble inside the vial until the vial barely floats. You can make this adjustment by tipping the vial to let some of its air escape or by removing it from the bottle and pouring water into it. Once you have the vial floating only a few millimeters out of the water at the top of the soda bottle, cap the bottle and prepare to test your diver.
Before you squeeze the soda bottle, think about how squeezing it will affect the air bubble. Now squeeze the bottle gently and watch the air bubble.
⦁ How does the bubble’s size depend on how hard you squeeze the bottle?
⦁ Why and how should the two be related?
Squeeze the bottle hard enough that the vial sinks.
⦁ Why is there a relationship between the size of the air bubble and the diver’s height in the water?
As you release the pressure on the bottle, the diver will float back up to the surface.
⦁ Why does the sunken diver suddenly become buoyant again?
By carefully squeezing the bottle, you can even make the diver hover in the middle of the bottle. Try making the diver hover while your eyes are closed.
⦁ Why is hovering so difficult to sustain?
⦁ Why must you watch the diver to make it hover?
Concluding Everyday Life Task:
⦁ Name two examples from your personal daily life where buoyancy plays a role. Please try not to give standard examples as given in the textbook but make it a personal challenge to discover buoyancy in your daily life!
Directions:
⦁ Carefully build the experimental setup as instructed
⦁ Perform the experiment by following the given instructions
⦁ Record your observations and conclusions in a brief written report by answering the given questions
⦁ Include pictures or a short video of your experiment into your report in order to support your findings
⦁ Submit the report including photos or videos before the deadline
