Before the Greek mathematician and inventor Archimedes (C.287-212 B.C.), nobody knew why some things float and others don’t. It is said that Archimedes’ king gave a silversmith a certain amount of gold to make him a new crown. When the crown was finished, the king asked Archimedes to figure out a way to secretly decide if the crown was made of pure gold or a mix of gold and cheaper metals, as the king suspected.
Archimedes was puzzled about how to do this, until he stepped into a bathtub and observed that his body displaced (pushed out a place) a certain volume (amount of space a material occupies) of water from the tub. Gold has a specific density, or mass per unit volume. This means that there is a specific amount of gold material in a given volume. Archimedes realized that he could decide the volume of the crown by measuring the volume of water displaced by the crown, and using the volume and the mass of the crown, he could decide its density. It is said that he was so excited about his discovery that he ran naked through the street yelling, “Eureka!” (I have found it!). I have my doubts about this, but who knows.
FYI: The art is from Aryamen’s blog: Density and Buoyancy
Archimedes discovered that the density of the king’s crown was not that of pure gold. The silversmith responsible for making the crown was killed because he stole from the king. Whether this story is true or not, Archimedes did discover a technique for determining density using water displacement.
FYI: The ratio of the density of a solid to the density of water is called specific gravity. The density of water is 1 g/ml. This means specific gravity of a solid is equal to the size of its density. When the specific gravity is greater than 1, the material will sink in water. When the specific gravity is less than 1, the material will float in water. Gold is 19.9 times as dense as water, so it will sink in water.