This activity can be used to illustrate Archimedes' principle and the
concept of density. Cute little "divers" can be purchased, but
they make it more difficult to observe the variations.
Before presenting this lesson construct a diver of your own so that you
will be prepared to answer student questions and difficulties.
Materials needed:
A three liter soda bottle, with the label removed.
A dropper (bulb pipette), glass, plastic one float.
A large beaker or other container which can hold water deeper than the
length of the dropper.
A waterproof marker.
Student Introduction:
We will start with a little review. Who can
remind us of what
"density" is? (Hopefully we get some usable responses, or at
least partially correct.)
To make it neat for those who are taking notes: Density is mass
per unit volume. That means, the mass of an object divided by the
volume of the object. So, if you have objects with exactly the
same volume and exactly the same mass, then they have the same density.
The first to understand the nature of density was a ancient Greek,
Archimedes. We refer to his discovery as, "Archimedes'
principle". If an object which is denser than the liquid is
placed into a liquid, the object will displace an amount of liquid with
exactly the same volume as the volume of the object. If an object
which is less dense than the liquid is placed in a liquid, the object
will displace an amount of liquid with exactly the same mass as the
mass of the object.
This is the principle used by submarines. To dive, air is
let out of the top of the ballast tanks letting water in at the bottom,
to increase the mass of
the ship to make it denser than water. To float, compressed air
is forced into the ballast tanks forcing water out at the bottom to,
decrease the
mass of the ship to make it less dense than water.
Now we are going to make a Cartesian diver, a simple device to help us
understand Archimedes' principle.
FAQ's
How can I get the dropper out of the bottle without
dumping all the
water out?
If the dropper is floating, squeeze the bottle
gently until the dropper is up to the mouth of the bottle. It may
help to use forceps (tweezers).
If the dropper is sunk, cover the mouth of the
bottle and turn the bottle over with its mouth below the surface of the
water in the beaker. Uncover the mouth of the bottle, and the
dropper will sink to the bottom of the beaker, then
cover the mouth of the bottle again and set it upright.
Why won't my diver go down? It has too much air in it.
Why won't my diver come back up? It needs another drop of air in
it.
