Cartesian Diver

Challenge

What makes the diver go up and down?

Supplies

• • 2-liter Soda Bottle with Lid

  • Water

  • Diver (one of the following)

    • • Ketchup or Mustard Packet and 1-2 Paper Clips

      • Eye Dropper

      • 2"x3" Piece of Foil (this is the hardest to make work)

What to do

• • Fill the bottle with water

  • Tune your diver to be only slightly buoyant:

    • • Place paper clips on the bottle of the ketchup packet until it sinks, then remove one

      • Slowly add water to the dropper until it sinks, the let out 5 drops

      • Crumple the foil and test the diver. If it doesn't sink, compress it more

  • Place diver in bottle

  • Make sure bottle is completely full of water and put on lid

  • Squeeze bottle and the diver should sink

What’s happening?

As you squeeze the bottle you're increasing the pressure inside. Water distributes the pressure uniformly and is incompressible. The increased pressure reduces the volume of the tiny bit of air inside the packet. With less air (buoyancy), it sinks. Releasing your grip on the bottle allows the air inside the packet to expand, thus providing more buoyancy and floating the packet.

If you are presenting this to an audience such as a class, don't tell them how it works. Have an audience member point to the diver and "move it with their mind!" As they concentrate, subtly squeeze the bottle. After they sit down, ask how that happened and squeeze the bottle again. Ask how they could devise experiments to determine how it works.