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Relative density (also known as specific gravity) is a measure of the density of a material. It is dimensionless, equal to the density of the material divided by the density of water.
Since water's density is one gram per cubic centimeter (under standard conditions), relative
density has the same value as density expressed in grams per cubic centimeter.
Relative density is often used by geologists and mineralogists to help determine the mineral content of a rock or
other sample. Gemologists use it as an aid in the identification of gemstones. The reason that relative density 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
its volume. With an irregularly shaped rock, the volume can be very difficult to accurately measure. The most accurate way is to
put it in a water-filled graduated cylinder and 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 Isaac
Newton's equations yields the following formula for measuring specific gravity:
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where G is the relative density, 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
rather difficult to measure relative densities 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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