A Cartesian diver is a simple science project that lets you make a tiny object sink and rise inside a sealed bottle of water. It is a fun way to see pressure, density, and buoyancy in action. When you squeeze the bottle, the diver sinks, and when you let go, it rises again.
This project matters because it shows how invisible forces in fluids can cause visible motion.
Understanding Make a Cartesian Diver
The important part of the diver is the trapped pocket of air. Air takes up space but has very little mass. This makes the whole object light enough to balance near the surface.
The amount of air must be adjusted carefully. If there is too much air, the diver stays at the top even when the bottle is pressed. If there is too little air, it falls straight away.
A well adjusted diver sits just below the surface or barely floats. This near balance makes a small change in volume produce a clear movement.
Water does not compress much under the pressure made by a hand. The squeeze is therefore passed through the water to every part of the sealed bottle. The air pocket does compress because gases have large spaces between their particles.
As that pocket gets smaller, water enters the diver or pushes farther into it. The diver keeps nearly the same mass, while its total volume decreases. Density means mass in a certain volume.
Packing the same mass into less volume raises its average density. Releasing the bottle allows the air to expand again, which reverses this change.
Buoyancy explains why the direction of motion changes at a particular point. Water pushes upward on an object with a force linked to the water the object displaces. A larger object volume displaces more water, so it gets more upward support.
When the compressed diver has a smaller volume, it displaces less water. Its upward support then becomes too weak to hold its weight. It moves downward.
This is a useful example because sinking does not require adding weight. An object can sink simply because its volume changes while its mass stays nearly constant.
This idea appears in real devices. A submarine changes its average density by taking water into ballast tanks or forcing water out with compressed air. Some fish use a gas filled swim bladder to help control depth.
The classroom model is not exactly the same as either one, but the central density idea is shared. When testing the project, observe the size of the air bubble before and during a squeeze. Try gentle and firm presses, then compare how far the diver travels.
Keep the bottle fully sealed, since escaping air prevents pressure from building. Use a clear bottle and leave little air above the water for a stronger response. Add tiny amounts of weight when adjusting the diver, since one paper clip can make the difference between floating, hovering, and sinking.
Key Facts
- Materials: clear plastic bottle with cap, water, small plastic dropper or condiment packet, and optional paper clips for weight.
- A diver floats when its average density is less than the density of water.
- A diver sinks when its average density becomes greater than the density of water.
- Pressure = force / area, or P = F / A.
- Buoyant force equals the weight of the displaced water.
- Squeezing the bottle increases pressure, compresses the air bubble in the diver, and makes the diver denser.
Vocabulary
- Cartesian diver
- A small object in a sealed bottle that sinks or rises when pressure changes.
- Buoyancy
- The upward force a fluid pushes on an object placed in it.
- Density
- The amount of mass in a given volume of a substance.
- Pressure
- The amount of force pressing on a certain area.
- Compression
- The squeezing of a gas or object into a smaller volume.
Common Mistakes to Avoid
- Filling the diver completely with water makes it unable to rise because there is no air bubble to compress and expand.
- Making the diver too light keeps it floating even when the bottle is squeezed because its density may not become greater than water.
- Leaving the bottle cap loose prevents pressure from building inside the bottle, so the diver may not sink when squeezed.
- Using a bottle that is only partly filled with water can make the results harder to control because extra air in the bottle compresses instead of changing the diver strongly.
Practice Questions
- 1 A diver has a mass of 3 g and a volume of 4 mL. What is its density in g/mL, and would it float in water if water has a density of 1 g/mL?
- 2 A student adds paper clips so the diver has a mass of 5 g and a volume of 4 mL. Calculate its density. Is it more likely to sink or float in water?
- 3 Explain why the diver sinks when the bottle is squeezed and rises when the squeeze is released.