Science Research Program Guide - Secondary Programs Home ...

The plan:
o make your very own diver
CARTESIAN DIVER
Materials:
o 2 liter soda bottle and its cap or some other 'squeezable' clear plastic bottle
o small container such as a large water glass or bowl
o glass or plastic medicine dropper (one that sinks in water)
Procedures:
o Take the empty soda bottle and fill it completely with water.
o Fill the water glass /bowl with water
o Place the medicine dropper in the glass
o Put some water inside the dropper
o You want to get the dropper to just barely float upright in the water.
o Place the dropper in the soda bottle and screw on the cap tightly.
o Don't allow much air to be between the top of the bottle and the cap.
o Gently squeeze the bottle.
o As you squeeze, the diver will dive (sink) to the bottom of the bottle.
o When you stop squeezing, the diver floats back to the top.
Buoyancy:
The weight of an object pulls it down toward the Earth, but if the object is placed in a liquid, a
force called buoyancy acts in the opposite direction. The buoyant force is equal to the weight of
liquid that the object displaces, or pushes aside.
When a well-made Cartesian Diver floats, only a small portion is above water. The part that is
below water, along with its trapped air, displaces enough water to create a buoyant force exactly
equal to the weight of the diver.
The Cartesian Diver moves up and down due to changes in the balance of weight and buoyancy.
Squeezing on the sides of the bottle pushes on the water inside but does not compress it. The
water pushes on the air in the diver and that does get compressed. If pen lid or medicine dropper
is clear, you should be able to see the water level changing inside.
When the air trapped in the diver is compressed, its volume is reduced. The diver together with
its trapped air then displaces less water. As a result, the upward buoyant force is reduced. When
the decreasing buoyant force becomes less than the weight of the diver, the diver sinks.
If you let go of the bottle, the original pressure in the water is restored. The trapped air in the
diver expands to its original volume, displacing more water as it does so. The increased amount
of water displaced by the diver and its trapped air makes the buoyant force greater. When the
buoyant force is greater than the weight of the diver, the diver rises. At the surface, the diver
floats high enough out of the water for the buoyant force to balance its weight exactly.
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