Understanding Density and Specific Gravity
Mass Divided by Volume is a Property of the Type of Material
Jan 18, 2009
It is relatively easy to memorize and repeat the definition of density, however understanding what it means is somewhat more difficult.
Definition and Meaning of Density
Density is defined as mass divided by volume.
The mass of an object is a measure both of its gravitational effects and of how much force is required to accelerate it. The mass of an object is a property of how much material there is.
The volume of an object is a measure of its size, or how much space it takes up. Notice that the mass and size (or volume) are two distinctly different properties of an object. People often use the words mass and size interchangeably, but this is not correct. Like mass, an object's volume is a property of how much material there is.
The density of an object measures how much the material is compressed or how tightly it is squeezed together. Think of it as how many kilograms of mass are squeezed into a cubic meter of volume.
When dividing numbers, numbers that are the same cancel out. For example, 3X2 divided by 3X4 equals 2 divided by 4 because the 3 cancels out. The same principle applies to the amount of material when calculating the density of an object. When the mass is divided by the volume, the amount of material, which affects both the mass and volume, cancels out.
Therefore density is not a property of the amount of material. The density is a property of the type of material regardless of the amount.
Think about a stick broken off from a very large tree. They are both made of the same type of wood. The tree will have a much larger mass and a much larger volume than the stick because the tree contains much more wood than the stick. However when dividing the mass by the volume to find the density the amount of wood cancels out. The stick and tree will therefore have the same density. The density is a property of the type of material, not the amount.
Units for Density and Specific Gravity
Because mass and volume are usually measured in kilograms and cubic meters, density is measured in kilograms per cubic meter. It is also common to measure mass in grams, volume in cubic centimeters, and density in grams per cubic centimeter. The conversion factor is 1 gram per cubic centimeter equals 1000 kilograms per cubic meter.
Density is also often measured in terms of the density of water which is 1000 kilograms per cubic meter or 1 gram per cubic centimeter. A rock with a density of 2300 kilograms per cubic meter would then have a density 2.3 times the density of water. This is called the specific gravity.
Applications of Density
Because of Archimedes' principle, an object will float in a fluid if its density is less than the density of the fluid. A ship will float in water only if its density is less than the density of water.
Because density is a property of the type of material, the density can be used as a clue in deducing the unknown composition of an object. For example it is possible to tell if a piece of jewelry is solid gold or merely gold plated by comparing its density to the density of gold.
Scientists use the density of Earth and other planets as a clue to help deduce the interior composition of the planet.
Density is a very useful fundamental scientific concept that measures how compressed matter is.
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