Virginia Tech® home

Cartesian Divers Project

The Cartesian Diver demonstrates the principles of buoyancy, pressure, and forces. Like a submarine, the diver changes its density to enable it to float. Instead of tanks of compressed air, the cartesian diver uses the incompressibility of water to create an object that will sink when the pressure around it is increased. To create the diver, the students must find a way to balance the buoyant force from the trapped air in a pen cap, and the force of gravity.

Subjects covered

  • Buoyancy
  • Forces
  • Pressure
  • Incompressibility of water


Provided by requester

  • 20oz bottle (rinsed out soda bottles work perfectly!)
  • Water

Provided by us

  • Consumed supplies
    • Pen caps
    • Clay
    • Paper towels
  • Reusable supplies
    • 20 oz cups


The diver is made by taking a pen cap and sealing the hole in the top with clay. The students then add clay to the bottom of the cap to balance it.  To test if the diver is correctly balanced before putting it in the bottle, the students place it in the large cup of water and push it below the surface gently. The diver should just barely float. If it sinks to the bottom, some clay must be removed. If the diver floats up very quickly, more clay should be added to the bottom of the cap. Once the divers are balanced, the 20oz bottles should be filled completely with water and the cap should be gently placed in. As the diver is being inserted, some water will spill out, so this part is best done over a tub, sink, or paper towels. Care should be taken to prevent the cap from losing its air while being inserted. Once the diver is inside, screw the cap on the bottle and squeeze the sides. If it is filled enough; and the diver balanced correctly,  a small squeeze of the bottle should cause the diver should sink. When squeezed, the pressure inside the bottle forces water into the cap making it heavier and sink. When the pressure is released, the air in the diver forces the water out and the diver rises again. Once completed, the student has its own diver to take home.