Density and Pressure

Density is the amount of matter in a volume of a substance. The symbol for density is the Greek letter rho ( r) and the formula for density is

or

The units (SI) are kg/m3. Density is also commonly expressed in

grams/cubic  centimeter

grams/cc

grams/milliliter

These are all the same unit. The density of water is 1000 kg/m3, or 1 g/cc at 4oC. Here are some common densities that will be useful for homework. Others can be found in the links below, or by looking them up.

Material

Density

(in kg/m3)

Material

Density

(in kg/m3)

Material

Density

(in kg/m3)

Water (at 4oC)

1000

Air

1.29

Helium

0.179

Aluminum

2700

Iron/Steel

7800

Copper

8900

Gold

19300

Lead

11300

Wood

300 - 900

Ice

917

Mercury

13600

Sea Water

1024

 

Specific gravity is the ratio of the density of a substance to the density of water at 4oC.  A hydrometer (right) is used to measure specific gravity.  For example, ethyl alcohol has a density of 790 kg/m3. It's specific gravity is 790 kg/m3/1000kg/m3 or .790. For some materials (solids and liquids under low pressures) we can assume the density is constant under a range of pressures. Temperature can cause changes in the density of these materials. For gases, however, density can change easily with pressure and temperature. We'll discuss this further in other lessons.

Pressure is defined as the Force per unit area on an object, or:

The SI unit for pressure is the Pascal (Pa), or Newton/meter2

 

For Practice Problems, Try:

Giancoli Multiple Choice PracticeQuestions (It will be a few lessons before all of this is covered)