Density is defined as the mass divided by the volume of an object or substance. The metric unit is kilograms per cubic centimeter, and the customary unit is pounds per cubic foot. The common symbol for density is $\rho$.


Things can float if their density is less than the substance they are in. For example, if you put a piece of cork in some water, the cork will float. This is because cork is less dense than water, namely, $\frac{\rho_{\text{cork}}}{\rho_{\text{water}}}=\frac15$.

Relative Density

Relative density compares the density of an object or substance to water's. The density of an object is expressed as its fraction or multiple of water's density. Thus, in the example above, the relative density of cork is $\frac15$. If an object's relative density is less than 1, it floats in water, since it is less dense than water. On the other hand, if an object's relative density is greater than 1, it will sink in water, since it is more dense than water.

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