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cartesian project sans questions-2


                         An object floats in a liquid when it is less dense than the liquid. Do you think that you could
                         make a floating object sink without changing the density of the liquid? Does your answer
                         change when you consider an object that includes a pocket of trapped air? Try the following
                         activity and see what you find.

                         Imagine entering a freshwater pond or lake. Take a deep breath and you're likely to float.
                         Exhale, and you'll probably find yourself sinking. These "ups and downs" depend upon the
                         amount of air in your lungs. As the volume of this gas increases, you become more buoyant. As
                         the gas volume drops, you lose buoyancy and begin to sink.

                        In this main activity, you'll construct a device called a Cartesian diver. Like a floating person,
this diver has a buoyancy that depends upon its volume of trapped air. As you explore its behavior, you'll uncover the
relationship between pressure and volume.

Cartesian divers were first noted by a student of Galileo Galilee! Some people back in those days thought its
mysterious dives and accents in the water smacked of dark magic, as witnessed by the name it was sometimes called:
Devil's Diver.

Density isn't just about weight. It is just as much about how much space something takes up. Density is the ratio of
weight divided by volume. If the density is greater than water, it sinks. If the density is less than water, it is buoyant.

Submarines and submersibles have ballast tanks that fill up with water to make them dive. When it's time to surface,
air is pumped into the tanks, forcing the water out and making the sub float to the top. Scuba divers wear heavy belts
of lead to make them sink in the water, but they also have a buoyancy compensator. This is a bag that they inflate with
air from their oxygen tank. When it is inflated, it causes them to float up to the surface. While underwater they'll put
just enough air in the bag to keep them from floating or sinking.

        •   beral pipet                                           • water
        •   hex nut                                               • plastic soft drink bottle with cap
        •   scissors                                              • super glue or hot melt glue gun and
        •   tall glass or cup                                     glue

1. Screw the hex nut onto the pipet up to the bulb.
2. Cut off the pipet stem below the hex nut, creating a “Cartesian diver.” (See Figure 1.)
3. Place the Cartesian diver in a tall glass full of water.
4. Adjust the amount of water in
the diver so it floats with its top
just at the water line. (You can
control how much the diver sinks
or floats by squeezing the bulb
to draw in or release water.)

5. Fill the soft drink bottle with water to within 2 or 3
cm of the top. Place the diver in the bottle and screw the
cap on tight. (See Figure 2.) Note the location of the
diver. Squeeze the bottle firmly.

6. Remove the diver from the bottle. Place the diver in
the glass of water and adjust it so it just barely sinks.
Place it in the completely filled bottle.

7. Find a way to make the diver float to the surface. (There are several ways that you can
do this.) Describe what you did and why it worked.

8. Remove the diver from the bottle. Place the diver in the glass of water and adjust it so
it floats.

9. Seal the open end of the diver with a minimum of super glue, hot melt glue (if these are
available), or tape. Allow the glue to dry. Place the diver in the completely filled bottle, and
screw on the cap.

Extension: Sunken Treasure or Diver Rescue….
Pick two of the following to complete this project.

A. The Sunken Diver. Adjust a diver’s density so that it just barely sinks and then
put it in a bottle of water. Try to find a way to make the diver ascend to the
surface. Some ideas might include: taking the cap off and heating the bottle, or
placing it in a vacuum jar, or perhaps adding a solute to increase the density ot the
surrounding water.

B. Cartesian Retrievers. Place two divers in the same bottle—one that barely
floats and one that barely sinks, but with mechanisms or devices attached to them
that will enable the floating one to dive down and retrieve the sunken one off the
bottom. Use magnets, chewing gum, Velcro, a suction cup, a net, a hook and handle—
whatever works! See Figure 3 and 4.
C. Cartesian Counters & Messages. Place several numbered divers together in one
bottle, but all with different densities, so they descend in order—1, 2, 3… (or
letter the divers to spell out a secret message!) See Figure 5.

D. Diving Whirligigs. Cut a small sheet of plastic into a pinwheel. Punch a hole in
the center and fit it into the stem of the pipet, just above the hex nut. Now the
diver will spin gracefully as it sinks, and reverse its spin on the way up. Attach
pipe-cleaner arms and legs to make an unusual diving ballerina!

E. Closed-system Divers. After the density has been adjusted, try sealing the
mouth of the diver with a drop of hot-melt glue. Now, when the bottle is squeezed,
instead of water being forced into the diver’s mouth, the sides of the diver are
forced noticeably inward (Figure 6). This closed system allows the use of colored
water inside the diver and results in divers that can be stored and transported
outside the bottle. What’s more, the shape distortion may be used in several ways:
for instance, wires may be attached at the sides of the diver and fashioned into
“jaws” that hang downward. Then, when the middle gets pushed inward, the jaws
spring open and a ferocious Cartesian shark dives downward with his mouth open to
snatch a unsuspecting diver off the bottom!

F. Density Column Divers. Make a density column inside the bottle. This can consist
of anything that forms layers (half oil/half water, for example). Use several
divers—all adjusted to suspend themselves at different levels throughout the

G. Remote Controlled Divers. Use airline tubing (aquarium tubing) or Tygon® tubing
to connect two plastic bottle caps together. Screw one cap onto a water-filled
bottle containing a standard diver (or any of the variations listed above) and screw
the other cap onto a second bottle (the remote control) which just contains water.
When the remote control is squeezed, the diver in the first bottle will descend,
even from across the room through several meters of tubing! As mentioned above,
it helps to have the bottles as full as possible and have the tubing completely filled
with water as well. Try replacing the water-filled bottle with a bottle of soda.
Instead of squeezing, just shake! Or use the carbon dioxide- producing reaction
between baking soda and vinegar to create the pressure in the remote control
H. The Cartesian Sea-Saw. Try to construct two Cartesian divers and attach them
to the ends of a see-saw structure which alternately tips back and forth as you
squeeze and release the bottle. At first, this might seem impossible, for both
divers would increase in density as the bottle is squeezed. But by varying the
length of the lever arms or by making one diver more sensitive than the other (by
using, for example, a regular diver on one end and a closed-system diver on the
other), such an underwater see-saw feasible!

I. Concentric Divers. Make a diver small enough to fit inside another one, so as the
little one dives inside the bigger one, the bigger one dives inside the bottle!

J. The Electric Diver. Build a diver with a built-in circuit that causes a light to go
on or a bell to ring when the diver descends.

You will be working in groups of 3-4 to build 2 of the extension projects listed
above. You will bring those two divers to class on the due date.

DUE DATE: ______________________

Rubric to come…

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