Making Cartesian Divers

This is a science experiment that has been around for years, but still so much fun!  It involves looking at buoyance and air pressure.

You will need:

An empty, clean (preferably clear) 1liter or 2 liter plastic pop bottle and lid.  A small tube opened at on end such as an eyedropper or disposable pipet.  A bowl of water.  

Now try this:

• Place the eyedropper into the bowl of water and squeeze just enough air into the bulb to make it neutrally buoyant (where it just barely floats).  There should be a small amount of both air and water in the bulb.  This will be your diver.

• Fill the plastic pop bottle completely to the top with water.

• Put the diver into the plastic pop bottle.  Making sure water completely comes up to the top of the pop bottle, seal it with the lid.

• Now squeeze the bottle and you will see the diver sink to the bottom.  Let go of the squeeze and the diver will come back up to the top.

Explanation:

This activity looks at two science principles:

Pascal's Principal which states that pressure on a fluid is transmitted unchanged throughout the fluid.  Therefore when the bottle is squeezed, it affects the air inside the diver.

Archimedes' Principle which states that when an object displaces a weight of water greater than its weight it floats and is said to be positively buoyant.  The opposite occures when the bottle is squeezed because when the bottle is squeezed the air is compressed.  This allows more water into the diver, making it a greater weight, making it more buoyant and causes it to sink.