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Chapter 11 | Fluid Statics 447
Figure 11.3 States of Matter: Basics (http://cnx.org/content/m55205/1.2/states-of-matter-basics_en.jar)
11.2 Density
Learning Objectives
By the end of this section, you will be able to:
• Define density.
• Calculate the mass of a reservoir from its density.
• Compare and contrast the densities of various substances.
The information presented in this section supports the following AP® learning objectives and science practices:
• 1.E.1.1 The student is able to predict the densities, differences in densities, or changes in densities under different conditions for natural phenomena and design an investigation to verify the prediction. (S.P. 6.2, 6.4)
• 1.E.1.2 The student is able to select from experimental data the information necessary to determine the density of an object and/or compare densities of several objects. (S.P. 4.1, 6.4)
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 11.4.)
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 (rho) is the symbol for density, is the mass, and 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 , representative values are given in Table 11.1. The
metric system was originally devised so that water would have a density of , equivalent to . 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.
(11.1)
Density
Density is mass per unit volume.
where is the symbol for density, is the mass, and is the volume occupied by the substance.
(11.2)
