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Change a bottle’s shape to make a Cartesian diver rise when the bottle is squeezed in just the right way.
Most Cartesian divers go down when you squeeze them, but this one goes up. Why is that happening?
When you squeeze the narrow sides of the bottle’s major axis you make the bottle’s minor axis a little larger. This causes the volume inside the bottle to increase, lowering the pressure inside.
Your condiment packet contains two things: the condiment sauce and a bubble of air. The air bubble is there to help ensure that the packet doesn’t rupture in transit or from temperature variations. When you squeeze the bottle and increase its volume, the reduced pressure inside the bottle causes the bubble in the condiment packet to expand. Because the mass of the packet stays the same, but the volume increases, the density decreases. This change in density changes the packet’s buoyancy, causing it to rise.
Why does the volume of a non-cylindrical bottle change when you squeeze on it? Here’s a simple way to see:
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Attribution: Exploratorium Teacher Institute