Question:

How does this trick work?

(Make sure you watch the previous video on the classic cartesian diver first)

Answer:

Because of the unique shape of the dettol bottle, whether the water becomes more or less pressurized depends which sides are being squeezed.

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When the bottle is squeezed on the longer sides, the cross-sectional area becomes smaller (because we are making it more elongated).

This causes the water in the bottle to rise and press against the bottle cap, thus increasing the water pressure. The increased water pressure forces more water into the diver. Its weight of the diver overcomes the upthrust. And it sinks! This is the classic cartesian diver.

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However, when the bottle is squeezed on the shorter sides, the cross-sectional area becomes larger (because we are making it more circular).

The water level in the bottle drops, causing the water pressure to decrease. The decreased water pressure draws water out of the diver (pushed out by the air pocket at the top of the diver). The weight of the diver drops below the upthrust, and it floats! This is the reverse cartesian diver.

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