# Density and Volume - Key terms

### ARCHIMEDES'S PRINCIPLE:

A rule of physics which holds that the buoyant force of an object immersed in fluid is equal to the weight of the fluid displaced by the object. It is named after the Greekmathematician, physicist, and inventor Archimedes (c. 287-212 B.C. ), who first identified it.

### BUOYANCY:

The tendency of an objectimmersed in a fluid to float. This can be explained by Archimedes's principle.

### DENSITY:

The ratio of mass to volume—in other words, the amount of matter within a given area.

### MASS:

According to the second law of motion, mass is the ratio of force to acceleration. Mass may likewise be defined, though much less precisely, as the amount of matter an object contains. Mass is also the product of volume multiplied by density.

### MATTER:

Physical substance that occupies space, has mass, is composed of atoms (or in the case of subatomic particles, is part of an atom), and is convertible intoenergy.

### SPECIFIC GRAVITY:

The density of an object or substance relative to the density of water; or more generally, the ratio between the densities of two objects or substances.

### VOLUME:

The amount of three-dimensional space an object occupies. Volume is usually expressed in cubic units of length.

### WEIGHT DENSITY:

The proper term for density within the British system of weights and measures. The pound is a unit of weight rather than of mass, and thus British units of density are usually rendered in terms of weight density—that is, pounds-per-cubic-foot. By contrast, the metric or international units measure mass density (referred to simply as "density"), which is rendered in terms of kilograms-per-cubic-meter, or grams-per-cubic-centimeter.