The Cartesian Diver experiment involves a bottle of water (preferably plastic!) and a glass test tube. It demonstrates simple principles of pressure.

  1. Fill your bottle completely with water.
  2. Fill the test tube partway with water.
  3. Carefully submerge the test tube into the bottle (opening face down).
  4. Make sure the bottle is completely filled and then secure the cap.

  • You may now 'operate' the experiment by squeezing the bottle with varying degrees of force.
  • When you do so the pressure inside the bottle will increase (as its volume decreases).
  • The water is incompressible, and the only substance less dense that the water is the air inside the test tube.
  • Hence it is this water that is compressed into a smaller volume.
  • Less of the test tube is filled with air, and more of it is filled with water!

From here we have two ways of consideing the situation:

  1. Consider the test-tube and air as your system.
    • Weight stays the same.
    • As you squeeze the bottole, the volume of water displaced decreases.
    • Thus upthrust decreases.
    • Take into account that: Resultant force = Weight - Upthrust
    • Weight is constant as mentioned above.
    • Therefore the (downward) resultant force increases!
    • BING: Your diver sinks lower!
  2. Consider the entire test-tube (with air and water) as your system.

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