Why does squeezing the bottle make the Cartesian diver sink?

Squeezing the bottle increases the water pressure throughout the bottle, which pushes water up into the diver and compresses the air bubble inside. This adds mass without adding much volume, raising the diver's average density above that of water, so it sinks.

What happens to the air inside a Cartesian diver when you squeeze the bottle?

The air inside the diver is compressed into a smaller volume because water pressure pushes water into the diver. Since the amount of air stays the same but takes up less space, the diver's overall density increases, causing it to sink.

Does a Cartesian diver work because of buoyancy?

Yes, a Cartesian diver works by changing its buoyancy. The diver sinks when its weight becomes greater than the buoyant force of the water it displaces, and it floats when the buoyant force is greater than its weight.

Why does the Cartesian diver float again when you let go of the bottle?

When you release the bottle, the pressure on the water decreases, so the compressed air inside the diver expands back to its original volume. That pushes some water out of the diver, lowering its average density below that of water, so it floats back up.

How Does a Cartesian Diver Work? Physics of Sink & Float

Q: How does a Cartesian diver work?

A Cartesian diver works by changing the average density of the diver so it becomes greater or less than the surrounding water. When you squeeze the bottle, water is forced into the diver, compressing the air inside and increasing its overall density, so it sinks; releasing the bottle lets the air expand and the diver becomes less dense, so it rises.

A Cartesian diver sinks when you squeeze a sealed bottle because the increased water pressure compresses the air trapped inside the diver. Water enters as the air pocket shrinks, making the diver heavier and reducing its buoyancy. It sinks when its weight exceeds the upward buoyant force. Release the bottle, and the air expands, pushing water back out; the diver becomes more buoyant and rises.

The effect relies on two principles: air is compressible, and buoyancy depends on the volume of water an object displaces. The trapped air changes volume as pressure changes, while water changes volume very little.

What is inside a Cartesian diver?

A typical diver is a small, open-bottom container that traps a pocket of air. An eyedropper is a common choice. The diver sits inside a flexible, sealed bottle filled with water.

At rest, the air pocket makes the diver buoyant enough to float near the surface. It is usually adjusted so that it is almost fully submerged, with only a small part above the water. This leaves it close to the point where a small change in pressure can make it sink.

The diver must be able to take in water as the air pocket shrinks. In an eyedropper, water enters through the open tip. Some traditional divers trap air among fibres, such as wool threads; the trapped gas changes volume as the pressure changes.

What happens when you squeeze the bottle?

Squeezing the bottle raises the pressure in the water. That pressure is transmitted to the air pocket in the diver, compressing it. Water then moves into the diver to occupy some of the space the air has left.

As a result, the diver gains mass while the shrinking air pocket reduces its buoyancy. Archimedes’ principle states that the buoyant force on an object equals the weight of the fluid it displaces (Physics Classroom). When the diver’s weight exceeds that upward force, it sinks.

When you release the bottle, the pressure falls. The air pocket expands and pushes some water out of the diver. Its mass decreases and its buoyancy increases, so it rises.

Why does the diver need an air pocket?

The air pocket is the part that changes volume when pressure changes. As it compresses, water enters the diver; as it expands, water leaves. This changes the diver’s mass and the volume of water it displaces, producing the sinking and rising motion.

A diver with too much air may float high and need a stronger squeeze before it sinks. With too little air, it may sink before you squeeze the bottle. Adjusting the air pocket so the diver is close to neutral buoyancy makes the response easier to see.

Can a Cartesian diver hover in the middle?

A diver can pass through a depth where its weight and buoyant force balance. At that moment, it may appear to hover. But this balance is generally unstable: a small movement can push the diver farther from that depth.

If the diver moves slightly downward, the pressure around it increases and its air pocket compresses further. Its buoyancy decreases, so it tends to sink more. If it moves slightly upward, the pressure falls, the air expands and buoyancy increases, so it tends to rise further.

A diver that seems to stay still may be moving slowly or held in place by friction or other effects. A freely moving diver will generally continue upward or downward once it moves away from the balance point.

What affects how easily it moves?

The size of the air pocket and the diver’s initial buoyancy affect how it responds. A diver adjusted close to the sinking threshold can respond to a relatively small squeeze. One that floats high may need more pressure to take in enough water to sink.

The bottle must be sealed so that squeezing raises the pressure in the water, and its walls must flex under your hand. If the bottle leaks or the diver is poorly adjusted, the motion may be weak or may not work.

Key takeaways

Squeezing a sealed bottle raises the water pressure and compresses the air inside the diver.
As the air pocket shrinks, water enters the diver, increasing its mass and reducing its buoyancy.
The diver sinks when its weight exceeds the buoyant force. It rises when the air expands and its buoyancy increases.
Adjusting the air pocket so the diver floats close to the surface makes the effect easier to control.

How Does a Cartesian Diver Work? The Physics Behind the Sinking and Floating Toy