Since water's density is one gram per cubic centimeter (under standard conditions), specific gravity has the same value as density expressed in grams per cubic centimeter.
Specific gravity is often used by geologists and mineralogists to help determine the mineral content of a rock or other sample. Gemmologists use it as an aid in the identification of gems. The reason that specific gravity is measured in terms of the density of water is because that that is the easiest way to measure it in the field. Basically, density is defined as the mass of a sample divided by it's volume. With an irregularly shaped rock, the volume can be very difficult to accurately measure. About the most accurate way would be to put it in a graduated cylinder that has water in it an see how much water it displaced. Even this method can be rather inaccurate, though, since it is easy to accidentally spill some water. It is far easier to simply suspend the sample from a spring scale and weigh it under water. Solving Newton's equations yields the following formula for measuring specific gravity:
where G is the specific gravity, W is the weight of the sample (measured in pounds, Newtons, or some other force), and F is the force measured while the sample was submerged. Note that it is tricky to measure specific gravities less than one because in order to do so the sign of F must change, meaning that you would have to supply a downward force to keep the sample underwater.
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