The Cartesian diver experiment is set up by placing a "diver"—a small, rigid tube, open at one end—in a much larger container with some flexible component; for example, a two litre soft drink bottle. The larger container is filled with water, and must be airtight when closed. The "diver" is partially filled with a small amount of water, but contains enough air so that it is nearly neutrally buoyant, but still buoyant enough that it floats at the top while being almost completely submerged.
The "diving" occurs when the flexible part of the larger container is pressed inward, causing the "diver" to sink to the bottom until the pressure is released, when it floats again.
Explanation
Air in the diver makes it neutrally buoyant and therefore float at the water's surface. As a result of Pascal's principle(pressure exerted anywhere in a confined fluid is transmitted equally in all directions throughout the fluid), when the pressure increases by squeezing the container, the least dense material, the air in the diver, is affected. Thus, the pressure on the water increases the pressure on the air bubble in the diver. The air compresses and reduces in volume, permitting more water to enter the diver. The diver now displaces a lesser weight of water than its own weight and becomes negatively buoyant (i.e. sinks), according to Archimedes principle. When the pressure on the container is released, the air expands again, increasing the weight of water displaced and the diver again becomes neutrally or positively buoyant and floats. If however, the buoyant force is equal to the compressed air force inside the diver, it will either float straight to the top or it will float in the middle of the container.
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