Video · Teachers Media
Cartesian Diver Lesson Idea
A Cartesian diver is a classic physics demonstration: a small figure or capsule in a sealed bottle of water rises and falls when you squeeze the bottle. Simple to build, unfailingly compelling to pupils, and a genuine window into pressure, density and buoyancy.
About this video
The diver is a small object trapped inside a sealed water-filled bottle, with an air pocket that keeps it just buoyant. When you squeeze the bottle, the increased pressure compresses the air pocket, reducing the diver's overall volume. Less volume means less water displaced, which means less upward buoyancy force — and the diver sinks. Release the squeeze, the air expands, buoyancy rises, and the diver bobs back up.
The physics packed into that ten-second event is dense. Pressure transmits equally through the water (Pascal's principle). Air is compressible in a way water isn't. Buoyancy depends on how much fluid is displaced (Archimedes' principle). The diver floats when displaced water weight equals its weight, sinks when it doesn't. Any of these can be the focus of a lesson.
The demonstration also lends itself beautifully to a pupil-design task. Give small groups a plastic bottle, a straw, a paperclip and some Blu-Tack. Their job: build a working diver. The engineering iteration — too heavy sinks, too light floats, just right hovers — is doing physics through hands.
What to teach with a diver
- Buoyancy. An object sinks or floats depending on whether it displaces enough fluid to match its weight.
- Compressibility. Air compresses under pressure. Water doesn't (much). This asymmetry is why the diver works.
- Pressure. Pressure applied at one place in a connected fluid transmits everywhere.
- Balance of forces. The diver hovering is a great example of two equal and opposite forces in equilibrium — weight down, buoyancy up.
Discussion prompts
- What's the difference between a demonstration you show pupils and one they build themselves? Which physics do they retain better?
- How do you use the diver to build toward the more formal equations pupils meet later (P = F/A, upthrust = ρVg)?
- Where does this demonstration connect to submarines, hot-air balloons, fish swim bladders — the applied physics of buoyancy?
Try this in your classroom
- Have pupils build their own divers in pairs. Constraint: it must hover when you don't squeeze, sink when you do, rise when you release.
- Once they have working divers, ask them to write an explanation as if they were teaching a Year 6 pupil. What language works, what doesn't?
- Extend: how would you build a diver that hovers at exactly the middle of the bottle? What does that tell you about density?