Marjie Henderson, Age 10, Of Visalia, Calif., For Her Question:
What is specific gravity?
The force of gravity hugs us c1ose to the ground. It makes our bodies heavy, and if the earth lost its gravity we would float off, lighter than feathers, into the wide blue yonder. The word specific means special and specific gravity is a special way to tell how much heavier one substance is than another.
A sack of stones is too heavy to tote, but a sack of feathers is light enough for a pixie to lift and swing over his shoulder. Wood floats because it is lighter than water, and a pebb1e sinks becat1se it is heavier than water. The air floats above the earth because it is made of gases, and gases are lighter than the solid ground and the watery seas below it.
As a rule we take these weighty matters for granted. But sometimes we wonder how much heavier the air is than the water or how much lighter the water is than the stones. Specific gravity can tell us the answers. Scientists and many other experts have to know exactly how much heavier one substance is than another. A plane designer must know exactly how much lighter a strip of aluminum is than one of steel and whether a seat cover of plastic is heavier than one of cloth anyone knows that lead is heavier than cork. We know because a pound of cork takes up much more room than a pound of lead. If we have a weighing machine and lots of shoe boxes, we can measure how much heavier the lead is than the cork. We would need about 47 Boxes of cork to equal the weight of one box of lead so lead is 47 times heavier Than cork specific gravity is a simpler way to compare the weights of many substances.
When we compare weights, we must think of size. And the size of an object is its volume which is the amount of space it occupies. When we compare the weights of cork and water, we must be sure to have the same volumes of cork and water. Let's use boxy containers a foot high and measuring one foot on each side. Each container holds a volume of one cubic foot. A cubic foot of water weighs almost 62.2 pounds, and a cubic foot of cork weighs about 15 pounds. Water, then, is more than four times heavier than cork. A cubic foot of lead weighs 705 pounds, which means that lead is more than 11 times heavier than water. The specific gravity of lead is 11.34.
The exact specific gravity of many substances had been worked out by experts, checked and rechecked. Scientists can find these special measures of weight on a chart. The specific gravity of iron is 7.87. Iron is almost eight times heavier than water and about three times heavier than aluminum, which has a specific gravity of 2.7.
Finding the exact specific gravity of a substance is a tricky job. Water expands when it becomes warmer, and as it expands it becomes lighter. A thimbleful of hot water weighs a little less than one of cold water. Pure water is heaviest at a temperature of 4 degrees centigrade. So when the expert measurers set about comparing the weight of a substance with an equal volume of water, the water must be exactly 4 centigrade degrees above freezing.