Make a Cartesian Diver with a Ketchup Packet

Want a submarine? A new military submarine costs about $4 billion, but if you're on a tight budget you can make your own for next to nothing. This article describes how to make a brilliant "Cartesian diver," a very simple submarine, using only water and condiment packets. While you can't explore the ocean in it, it’s a great project to learn more about the principle of buoyancy.

Steps

  1. Get three or more unopened condiment packets. Small Open a Ketchup Packet or mayo packets such as those you get at fast food restaurants are perfect. Make sure they are full and have no tears or holes.
  2. Choose a packet. Drop all the condiment packets into a pint glass of water. They will generally all float at somewhat different levels, and one or more may sink. Choose a packet that floats with its tip just barely above the water.
  3. Drop the chosen packet into a nearly full bottle of water (made from plastic, not glass!). You may need to bend the packet a bit to get it through the neck of the bottle. Be careful not to break the packet open when doing so.
  4. Fill the bottle completely with water. Add more water to the bottle until it is completely full. It is essential that the bottle be full.
  5. Screw on the bottle cap as tightly as possible.
  6. Squeeze the sides of the bottle. Observe what the packet does. It should sink to the bottom of the bottle.
  7. Let go of the bottle. When you stop squeezing, the packet will rise back up to the top.
  8. Vary how hard you squeeze the bottle. You will find that the harder you squeeze the bottle, the faster the packet will dive to the bottom.

Alternate method

  1. Place the sealed bottle in a pan of water and warm the pan slowly on the stove. Heating the water causes it to expand, increasing the pressure on the inside of the bottle, which will then make the diver sink to the bottom.
  2. Allow the bottle to cool, and the water contracts, pressure drops, and the diver will rise to the surface again. Be sure to warm the sealed bottle in a pan of water - NOT directly on the stove or in the microwave. Bottles made for carbonated drinks can withstand great pressures (well over 130 psi), so the chance of bursting is small, but reasonable care should be used.
  3. Do not continue to heat the bottle after the diver sinks - take it off the heat and allow it to cool and you should be fine.

Tips

Warnings

  • In order to prevent burns, children should not attempt the alternate method without adult supervision.

Why It Works

  • Whether an object sinks or floats depends on if the total weight of the object is greater or less than the weight of the water it displaces. When a packet is just barely floating the parts that are less dense than water (air) are just barely canceling the parts that are more dense than water (package and ketchup). The Cartesian diver works because the pressure you exert on the bottle is transferred to the air bubble within the packet, making it smaller, thus increasing its density. As this occurs, the packet sinks as the air no longer cancels the ketchup. When you release the pressure, the bubble expands again, and the packet rises. Fish have an air sac within their bodies, and by controlling the amount of air in the sac, they use the same principle to rise and sink. Submarines also use this principle to dive and surface.
  • Also, according to Pascal's Law (also called Pascal's Principle), pressure in a fluid is exerted equally throughout the fluid. No matter where you squeeze the bottle, at the top or at the bottom, the pressure will be exerted equally and the same diving effect will occur.
  • The same principle is at work in a diver's buoyancy compensation device (BC) or even your lungs at depth underwater. Release air from the BC (or your lungs), it will compress the remaining volume, and you sink. Add some volume (by adding more air to your BC or your lungs), and you will rise to the surface. Mastering these two skills are the ABCs of diving.

Things You'll Need

  • 3 or more condiment packets (e.g. ketchup or mayonnaise)
  • 1 pint glass full of water
  • 1 large bottle of water
  • 1 jug of water (to top off the bottle)

Related Articles

Sources and Citations

  • VideoJug.com A video on how to make a Cartesian diver. The original source of this article. Shared with permission.
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