Changes in fluid pressure affect the buoyancy of a Cartesian diver made from a condiment packet. The diver floats, sinks, or hovers in response to pressure changes.
- Fill a cup with water and drop in your packets one by one to see how they’ll work (see photo). The best packets to use in this activity are ones that just barely float.
- After you’ve found the proper packet, fill an empty, clear plastic bottle to the very top with water. Shove your unopened condiment packet into the plastic bottle. Replace the cap and you’re done!
Squeeze the plastic bottle to make the diver sink, and release it to make it rise.
The key to making this work is to choose a condiment-packet diver that barely floats. This works because, while many sauces are denser than water, it’s actually the air bubble sealed inside the packet that determines whether it will sink or swim.
Squeezing the bottle increases pressure on the condiment packet, compressing the air bubble inside. When the higher pressure compresses the air in the packet, the packet displaces less water, thus decreasing its buoyancy and causing it to sink. When you release the sides of the bottle, the pressure decreases and the air inside the packet expands once again. The packet’s buoyancy increases and the diver rises.
The Greek philosopher Archimedes was the first person to notice that the upward force that water exerts on an object, whether floating or submerged, is equal to the weight of the volume of water that the object displaces. That is, the buoyant force is equal to the weight of the displaced water.
Change the shape of the diving bottle and you can get an additional effect. If you use a thin, flat plastic bottle (like from shampoo or mouthwash), squeezing the differently shaped sides changes what happens inside the bottle.
Choose a diver that just barely floats, put it in a bottle filled completely with water, and put the cap on. Squeeze on the wide sides of the bottle and you make the volume inside smaller—the pressure inside the bottle goes up, compressing the air bubble in the diver, and the diver sinks and remains on the bottom (as it does in the preceding activity). Squeeze on the narrow sides of the bottle and you make the volume inside larger: The pressure inside the bottle goes down, allowing the air bubble in the diver to expand, and the diver rises.
With a round bottle, you won’t be able to get the diver to rise again once it’s on the bottom.