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Eureka! Buoyancy Experiments
by Anjali Amit
Archimedes faced a difficult problem. He had been asked to determine whether the sovereign's new crown was indeed solid gold, with no silver added. Mulling over the problem, Archimedes stepped into his bathtub. He noticed that as he entered the tub some water spilled over. The lower he sat in the tub the more water spilled out.
"Eureka!" he shouted.
He could now find out what the crown was made of without destroying it. The story goes that he was so excited by his discovery that he ran down the city streets shouting, "Eureka! Eureka!"
Buoyancy:
What had Archimedes discovered that gave him the answer to his problem? The answer lies in the principle of buoyancy. Some objects float in water (positively buoyant) and some objects sink (negatively buoyant). Other objects remain submerged, neither floating on the surface, nor sinking to the bottom (neutrally buoyant). Buoyancy is the upward force exerted on an object by the fluid or gas in which it is immersed. This force is equal to the weight of fluid the object displaces.
Archimedes realized just that. He noticed that whenever an object is placed in water some water is displaced. The amount of water displaced equals the volume of the object. Archimedes had to figure out if the crown was made of solid gold. Let us say the crown weighed two pounds. All that he had to do was to take a cube of gold that weighed two pounds, place it in a container of water, and see how much water was displaced. If the crown then displaced the same amount of water, he would prove that it was made of solid gold. If not, then there was a mixture of baser elements.
Here are some experiments to demonstrate buoyancy.
| Experiment One: |
| What you need: |
| |
An orange |
| |
A glass and a marker that can write on glass or a measuring cup |
| What to do: |
| |
Fill the glass half-way with water |
| |
Mark the water level |
| |
Drop the orange in the water |
| What Happens: |
| |
The orange floats |
| |
The water level rises |
| |
Mark the new water level |
| Why: |
|
| |
Buoyancy. The orange pushes water out of the way. The water pushes back with a force equal to the weight of the water. This creates the up thrust which holds the orange suspended in the water. |
| |
| Extension of the Experiment: |
| |
Peel the orange. |
| What Happens: |
| |
The orange sinks. |
| Why: |
|
| |
The peel contains air bubbles that help it to stay afloat. Once the peel is gone the orange sinks. |
| |
| Experiment Two: |
| What you need: |
| |
A glass |
| |
A fresh egg |
| |
Salt |
| What to do: |
| |
Fill the glass half-way with water. |
| |
Gently lower the egg into the glass. |
| What Happens: |
| |
The egg sinks. |
| Why: |
| |
The egg is denser than the water. |
| |
| Extension of the Experiment: |
| |
Add ten to twelve tablespoons of salt to the water. |
| What Happens: |
| |
The egg slowly rises to the top. |
| Why: |
| |
Salt water is denser than fresh water so it is able to support the egg. What will happen if you add fresh water? |
Conclusion:
Buoyancy explains how a block of steel sinks, but a ship of steel floats. A concrete ship would float as long as it was buoyant—having enough air in the hull to make it less dense than the surrounding water. Fish have swim bladders that help them to adjust their buoyancy. Even astronauts, going into outer space, prepare for zero-gravity by training in the Neutral Buoyancy Lab near Houston, Texas.
Bibliography:
Farndon, John. Science Experiments Buoyancy. NY: Marshall Cavendish, 2003.
Sullivan, Navin. Measure Up! Weight. NY: Marshall Cavendish Benchmark, 2007.
Interesting Web Sites:
http://www.pbs.org/wgbh/nova/lasalle/buoybasics.html
http://www.seed.slb.com/en/scictr/watch/bathroom/duck.htm
http://theory.uwinnipeg.ca/mod_tech/node67.html
http://www.wisegeek.com/what-is-buoyancy.htm
Anjali is a children's book author whose first book,"Folk Tales From Rajasthan", was published when she was in college. Upon graduation she "sold her soul to Mammon"—went to work for a bank. She writes fiction, non-fiction, and edits technical documents. Her articles have appeared in various magazines. "Bedtime Stories from Around the World", her second book, was published a few years ago.
