Density or specific mass?

Density or specific mass?

 

 

MASS DENSITY

Fluids are materials that can flow, and they include both gases and liquids. Air is the most common gas, and flows from place to place as wind. Water is the most familiar liquid, and flowing water has many uses, from generating hydroelectric power to white-water rafting. The mass density of a liquid or gas is one of the important factors that determine its behavior as a fluid. As indicated below, the mass density is the mass per unit volume and is denoted by the Greek letter rho (p).

Equal volumes of different substances generally have different masses, so the density depends on the nature of the material. Gases have the smallest den-sities because gas molecules are relatively far apart and a gas contains a large fraction of empty space. In contrast, the molecules are much more tightly packed in liquids and solids, and the tighter packing leads to larger densities. The densities of gases are very sensitive to changes in temperature and pressure. However, for the range of temperatures and pressures encountered in this text, the densities of liquids and solids do not differ much from the values.

It is the mass of a substance, not its weight, which enters into the definition of density. In situations where weight is needed, it can be calculated from the mass density, the volume, and the acceleration due to gravity.