Fluid Statics
76 Density
Learning Objectives
- Define density.
- Calculate the mass of a reservoir from its density.
- Compare and contrast the densities of various substances.
Which weighs more, a ton of feathers or a ton of bricks? This old riddle plays with the distinction between mass and density. A ton is a ton, of course; but bricks have much greater density than feathers, and so we are tempted to think of them as heavier. (See Figure 76.1.)
Density, as you will see, is an important characteristic of substances. It is crucial, for example, in determining whether an object sinks or floats in a fluid. Density is the mass per unit volume of a substance or object. In equation form, density is defined as
where the Greek letter [latex]\rho[/latex] (rho) is the symbol for density, [latex]m[/latex] is the mass, and [latex]V[/latex] is the volume occupied by the substance.
Density
Density is mass per unit volume. [latex]\rho =\frac{m}{V},[/latex] where [latex]\rho[/latex] is the symbol for density, [latex]m[/latex] is the mass, and [latex]V[/latex] is the volume occupied by the substance.
In the riddle regarding the feathers and bricks, the masses are the same, but the volume occupied by the feathers is much greater, since their density is much lower. The SI unit of density is [latex]{\text{kg/m}}^{3}[/latex], representative values are given in Table 76.1. The metric system was originally devised so that water would have a density of [latex]1\phantom{\rule{0.25em}{0ex}}{\text{g/cm}}^{3}[/latex], equivalent to [latex]{\text{10}}^{3}\phantom{\rule{0.25em}{0ex}}{\text{kg/m}}^{3}[/latex]. Thus the basic mass unit, the kilogram, was first devised to be the mass of 1000 mL of water, which has a volume of 1000 cm3.
As you can see by examining Table 76.1, the density of an object may help identify its composition. The density of gold, for example, is about 2.5 times the density of iron, which is about 2.5 times the density of aluminum. Density also reveals something about the phase of the matter and its substructure. Notice that the densities of liquids and solids are roughly comparable, consistent with the fact that their atoms are in close contact. The densities of gases are much less than those of liquids and solids, because the atoms in gases are separated by large amounts of empty space.
Take-Home Experiment Sugar and Salt
A pile of sugar and a pile of salt look pretty similar, but which weighs more? If the volumes of both piles are the same, any difference in mass is due to their different densities (including the air space between crystals). Which do you think has the greater density? What values did you find? What method did you use to determine these values?
Example 76.1 Calculating the Mass of a Reservoir From Its Volume
A reservoir has a surface area of [latex]\text{50}\text{.}0\phantom{\rule{0.25em}{0ex}}{\text{km}}^{2}[/latex] and an average depth of 40.0 m. What mass of water is held behind the dam? (See Figure 76.2 for a view of a large reser