Cartesian Diver

Milstein Family Hall of Ocean Life Educator's Guide: Activity

Cartesian Diver

In this hands-on experiment, students create a neutrally buoyant "diver" and then observe the effects of increased water pressure.

What you need:

wide-mouth plastic water bottle
tub of water
weights (such as small heavy nuts and bolts)
small balloons
twist tie
rubber band
medicine dropper
a rubber sheet (a cut-up balloon will do)
a tall, narrow jar such as an olive jar

What you do:

Blow up a balloon just a bit so that it can fit through the mouth of the bottle later.
Using the twist tie, attach weights to the balloon. (Congratulate the students ... they just constructed their divers!)
Now, test the diver by floating the diver in the tub of water-adjust the air in the balloon or the weight to get it neutrally buoyant. It should be submerged in the middle of the water.
Fill the water bottle with water, and add the diver.
Put the cap on.
Squeeze the bottle.
Watch the diver sink!

What's the meaning of this?
Ask students if they can explain what makes the diver sink. Ask students: "Does the amount of air in the balloon change?"

This activity demonstrates the property of buoyancy. An object is buoyant in water due to the amount of water it displaces or "pushes aside."

When students squeeze the bottle, they increase the pressure inside the bottle. The air in the balloon is compressed and reduced in volume, the diverbecomes less buoyant and sinks. Stress that the amount of air stays the same, even though its volume has changed.

Fish keep themselves from either sinking or floating to the surface by using muscles to squeeze or relax a small sac (with a small air bubble inside) in their bodies. By squeezing the sac smaller, the fish will sink. By relaxing their muscles, the sac increases in size, displaces more water, and a fish will begin to rise to the surface. This same principle is used to control the buoyancy of a submarine. By pumping water in and out of tanks stored in the submarine, a submarine can be made to rise and sink.

Activity extension:

Instead of a balloon, the "Deep Sea Divers" experiment can be done with a medicine dropper.

What you do:

Float the medicine dropper in the water, bulb up. Squeeze the bulb slightly and then let it go. The object is to take just enough water into the bulb so that it barely remains floating. You may have to try this several times to get it just right. Only the smallest tip of the bulb should be floating above the surface.
After putting the dropper in the water-filled jar, stretch a piece of rubber or cut-up balloon over the bottle top and secure it with a rubber band so that there is a tight seal.
Press down slightly on the rubber seal. Watch what happens to the medicine dropper. The dropper should sink down whenever students press down.
When students release the pressure, the dropper should rise. Pressing down on the balloon should have the same effect as squeezing the bottle in the first experiment.