Try These At Home 2: Exploring Buoyancy

The Cartesian Diver: this is a classic demo named after the17th-century philosopher and mathematician René Descartes.

Buoyancy is the force that decides whether an object will sink or float, and has had a long and colorful history. As the story goes, the Greek thinker Archimedes was sitting in his bathtub one day when he noticed how the water around him rose when he got in. Suddenly, he realized that he could use the water level rise to measure an object’s volume. Shouting “Eureka!” he burst out of the tub and ran out into the streets stark naked.

Fascination with buoyancy continues into modern times. Astronauts have exploited buoyancy to simulate being in space. Scuba divers use it to turn the underwater world into their playground. And then of course there is David Letterman’s Will it Float?, an entire sketch dedicated to watching what happens when something is dropped into a giant pool of water. Demonstrations at home of buoyancy are easy to come by, too. Below are two of my favorites.

Cartesian Diver

This is a classic demo named after the17th-century philosopher and mathematician René Descartes, although curiously, no one seems to know why. Build a miniature model of a submarine here and take control of an object’s depth.


What to do: You need a 2-liter plastic soda bottle, the cap to a ballpoint pen, water, and a lump of clay. Drain the soda bottle and refill with water. Attach a lump of clay to the bottom of the pen cap (enough to weigh it down but not quite to sink it). Drop the cap into the filled soda bottle and seal the bottle’s top. Squeeze! Depending on the amount of pressure you apply, you should be able to make your pen cap dive to the bottom of the bottle and resurface at will.

What’s going on? An object will float or sink depending on how its density (its mass divided by its volume) compares with that of the surrounding liquid. For example, a steel rod is heavy for its size so it sinks. However, if you increase the rod’s volume by trapping an air bubble inside or reshaping the steel into a boat, then you can make it float. In the case of the Cartesian diver there is an air bubble trapped beneath the pen cap. When you squeeze the bottle you compress the air bubble into a smaller volume, and while the diver still weighs the same it now sinks. This is exactly how the ballast tanks of a submarine work, and many fish have an organ called a swim bladder that uses the principle to control their depth.

Layered Liquids

Awesomeness ensues when you mix the effects of buoyancy with liquids that don’t mix. You may already be familiar with the fancier versions of this demo in the form of lava lamps or the oil drop toys you can buy in many trinket stores.

What to do: You need corn syrup, water, vegetable oil, a clear container, and some food coloring. Use the food coloring to dye the corn syrup, water, and oil different colors for increased effect. Pour about an inch of corn syrup into the bottom of the clear container, then gently pour about an inch of water above it, and finally pour the oil atop the water. Each liquid will float atop the one beneath it.

What’s going on? Density can affect whether or not a liquid will float in exactly the same way that it can determine whether a solid object floats. In this example water is less dense than corn syrup, so it floats on top. Oil is less dense than both corn syrup and water, so it floats highest of all. Such layering of liquids can also happen between salt water and fresh water in underwater caves, sometimes dangerously tricking divers into believing there is an air bubble over their heads when in fact there is just a different kind of water.

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