May 18, 2005

Specific Gravity

A Greek mathematician named Archimedes was called in by a wealthy man who thought he'd been cheated. The man showed Archimedes a golden crown he suspected was made of cheaper silver and only coated with gold. The wealthy man wanted to know if Archimedes could prove the crown wasn't pure gold without damaging it.

Archimedes thought it over, and while he was sitting in the public baths the idea came to him. He noticed that as more of his body went into the bath, more of the water ran over the top. He rushed home shouting, "Eureka!" (which means "I have found it!" in Greek).

Try his experiment yourself.

Materials

• Postal scale
• 4 quarters
• 5 chocolate kiss candies
• Glass jar
• Water
• Tape or marker

Use a postal scale to prove that the 4 quarters weigh 1 ounce. The 5 candies weigh 1 ounce, too.

Fill a glass jar half full of water. Mark the level of the water with tape or a marker. Drop the quarters in the water and mark the new water level. Take the quarters out and drop the candy in the water; again, mark the new water level. You'll see that the water levels are different. The coins and the candies both weigh the same (1 ounce), but the candies have more mass and they displaced more water. It proves that items with the same weight can be of different size. Some substances are heavier than others.

Back to Archimedes and the crown. He weighed the crown, and then he formed a piece of gold and a piece of silver, both with the same weight. He put all three in the same amount of water and saw that they had different densities. The gold piece displaced less water than the silver, because gold is denser than silver. He tested the crown and it failed to displace the same amount of water as the solid gold piece. He proved the crown was made of a substance that had a lower specific weight than real gold.

Archimedes wrote it all down, but he didn't say if the wealthy man got a refund or not.

Reprinted with permission from Classical Kids: An Activity Guide to Life in Ancient Greece and Rome by Laurie Carlson. Published by Chicago Review Press, distributed by Independent Publishers Group (www.ipgbook.com). Copyright © 1998 by Laurie Carlson.

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