Specific gravity
In physics, specific gravity is a dimensionless quantity that was defined by the ratio of the mass or density of a substance to the mass or density of a reference substance of the same given . 

While investigating the explosive rocks on Gamma Trianguli VI in 2268, Spock noted how it had extremely low specific gravity for its size, and along with several other interesting qualities, he should an analysis of the material would prove interesting. ()

After encountering Gem on Minara II in 2269, Spock was able to deduct, based on what he knew of the specific gravity, among other environmental factors, of that planet, that she could not have evolved on the planet she was discovered on. ()

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Physics