SEED Science

Cartesian Diver

Cartesian Diver

The human body is less dense than water; thus people float. So how do scuba divers stay under the water and control their depth? A diver wears two pieces of equipment that helps him or her stay underwater: a weight belt and a buoyancy compensator. The heavy metal weights on the belt enable the diver to sink. The buoyancy compensator can be inflated with air to increase buoyancy or deflated to reduce it.

Here’s an experiment to help you see how this works.

Tools and Materials

  • A 1 L (0.26 gal) plastic water or soda bottle
  • Cap from a ballpoint pen, with no holes in the top
  • Oil-based modeling clay
  • Water
  • Bowl or bucket

What to Do

  1. Fill the bowl with water to the halfway level.
  2. Roll some clay into a ball the size of a marble.
  3. Stick the pointed end of the pen lid into the clay. This is your “diver.”
  4. Gently lower the ‘diver’ into the bowl of water so that the lid remains full of air.
  5. If the diver sinks, remove some clay. If it floats, add some more clay. Adjust the amount of clay until the top of the pen lid only just sticks out of the water. The experiment will not work unless the amount of clay is just right.
  6. Next, fill the plastic bottle to the brim.
    Lower the diver
  7. Gently lower the diver into the bottle.
  8. Screw on the lid.
  9. Check that the only air inside the bottle is in the diver.
  10. Squeeze the sides of the bottle—the diver should sink. If you stop squeezing, the diver should float back to the top of the bottle. If the diver does not sink when you squeeze really hard, add more clay

If you are careful, you may be able to make the diver sink to the middle of the bottle, then just hover in the water.




Try These Ideas

  • What happens if you add salt to the water?
  • What happens if you use a bottle with an oval cross section and squeeze across the widest part of the bottle?
  • Instead of using a pen lid and clay, use a medicine dropper. Fill it with enough water so that it just floats.

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