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.
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.